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Gamma and X-Ray Detection

Phone contact information

Benelux/Denmark (32) 2 481 85 30 • Canada 905-660-5373 • Central Europe +43 (0)2230 37000 • France (33) 1 39 48 52 00 • Germany (49) 6142 73820 • Japan 81-3-5844-2681 •

Russia (7-495) 429-6577 • United Kingdom (44) 1235 838333 • United States (1) 203-238-2351

Figure 1.1 Efﬁciency Calibration

The choice of a particular detector type for an application depends

upon the X-ray or gamma energy range of interest and the

applica-tion’s resolution and efﬁciency requirements Additional

consider-ations include count rate performance, the suitability of the detector

for timing experiments, and of course, price.

DETECTOR EFFICIENCY

The efﬁciency of a detector is a measure of how many pulses occur

for a given number of gamma rays Various kinds of efﬁciency

deﬁni-tions are in common use for gamma ray detectors:

a Absolute Efﬁciency: The ratio of the number of counts

pro-duced by the detector to the number of gamma rays emitted

by the source (in all directions).

b Intrinsic Efﬁciency: The ratio of the number of pulses

pro-duced by the detector to the number of gamma rays striking

the detector.

c Relative Efﬁciency: Efﬁciency of one detector relative to

an-other; commonly that of a germanium detector relative to a

3 in diameter by 3 in long NaI crystal, each at 25 cm from a

point source, and speciﬁed at 1.33 MeV only.

d Full-Energy Peak (or Photopeak) Efﬁciency: The efﬁciency

for producing full-energy peak pulses only, rather than a

pulse of any size for the gamma ray.

Clearly, to be useful, the detector must be capable of absorbing a

large fraction of the gamma ray energy This is accomplished by

us-ing a detector of suitable size, or by choosus-ing a detector material of

suitable high Z An example of a full-energy peak efﬁciency curve

for a germanium detector is shown in Figure 1.1.

DETECTOR RESOLUTION

Resolution is a measure of the width (full width half max) of a

single energy peak at a speciﬁc energy, either expressed in

ab-solute keV (as with Germanium Detectors), or as a percentage of

the energy at that point (Sodium Iodide Detectors) Better (lower

FWHM value) resolution enables the system to more clearly

sep-arate the peaks within a spectrum Figure 1.2 shows two

spec-tra collected from the same source, one using a sodium iodide

(NaI(TI)) detector and one using germanium (HPGe) Even though

this is a rather simple spectrum, the peaks presented by the

so-dium iodide detector are overlapping to some degree, while those

from the germanium detector are clearly separated In a complex

spectrum, with peaks numbering in the hundreds, the use of a

germanium detector becomes mandatory for analysis.

GAS-FILLED DETECTORS

A gas-ﬁlled detector is basically a metal chamber ﬁlled with gas and

containing a positively biased anode wire A photon passing through

the gas produces free electrons and positive ions The electrons are

attracted to the anode, producing an electric pulse.

At low anode voltages, the electrons may recombine with the ions

Recombination may also occur for a high density of ions At a

sufﬁ-ciently high voltage nearly all electrons are collected, and the

detec-tor is known as an ionization chamber At higher voltages the trons are accelerated toward the anode at energies high enough to ionize other atoms, thus creating a larger number of electrons This detector is known as a proportional counter At higher voltages the electron multiplication is even greater, and the number of electrons collected is independent of the initial ionization This detector is the Geiger-Mueller counter, in which the large output pulse is the same for all photons At still higher voltages continuous discharge occurs The different voltage regions are indicated schematically in Figure 1.3 The actual voltages can vary widely from one detector to the next, depending upon the detector geometry and the gas type and pressure.

elec-IONIZATION CHAMBER

The very low signal output for the ionization chamber makes this detector difficult to use for detecting individual gamma rays It finds use in high radiation fluxes in which the total current produced can

be very large Many radiation monitoring instruments use ionization chambers Absolute ionization measurements can be made, using

an electrometer for recording the output 1

A detector is typically speciﬁed in terms of its physical size, fective window size and gas path length, operating voltage range and resolution for the 5.9 keV X ray from a 55 Fe source (Mn X ray) Typical resolutions are about 16 to 20% full-width at half maximum (FWHM).

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ef-Operating voltages depend upon the ﬁll gas as well as the

geom-etry For X rays, noble gases are often used, with xenon, krypton,

neon and argon common choices Xenon and krypton are selected

for higher energy X rays or to get higher efﬁciencies, while neon

is selected when it is desired to detect low energy X rays in the

presence of unwanted higher energy X rays Sometimes gas

mix-tures are used, such as P-10 gas, which is a mixture of 90% argon

and 10% methane Gas pressures are typically one atmosphere

The 2006 preampliﬁer available for proportional counters is shown

in Figure 1.4.

GEIGER-MUELLER COUNTER

The Geiger-Mueller counter produces a large voltage pulse that is

easily counted without further ampliﬁcation No energy

measure-ments are possible since the output pulse height is independent

of initial ionization Geiger-Mueller counters are available in a wide

variety of sizes, generally with a thin mica window The operating

voltage is in the plateau region (see Figure 1.3), which can be

rela-Figure 1.2

Figure 1.3 Gas Detector Output vs Anode Voltage

tively ﬂat over a range of bias voltage The plateau is determined by measuring the counting rate as a function of the anode voltage The discharge produced by an ionization must be quenched in order for the detector to be returned to a neutral ionization state for the next pulse This is accomplished by using a ﬁll gas that contains

a small amount of halogen in addition to a noble gas The voltage drop across a large resistor between the anode and bias supply will also serve to quench the discharge since the operating voltage will

be reduced below the plateau.

The Geiger-Mueller counter is inactive or “dead” after each pulse until the quenching is complete This dead time can be hundreds

of microseconds long, which limits the counter to low count rate applications.

Figure 1.4 Proportional Counter and Preampliﬁer

SCINTILLATION DETECTORS

A gamma ray interacting with a scintillator produces a pulse of light, which is converted to an electric pulse by a photomultiplier tube The photomultiplier consists of a photocathode, a focusing electrode and

10 or more dynodes that multiply the number of electrons striking them several times each The anode and dynodes are biased by a chain of resistors typically located in a plug-on tube base assembly Complete assemblies including scintillator and photomultiplier tube are commercially available from CANBERRA.

The properties of scintillation material required for good detectors are transparency, availability in large size, and large light output proportional to gamma ray energy Relatively few materials have good properties for detectors Thallium activated NaI and CsI crystals are commonly used, as well as a wide variety of plastics LaBr 3 (Ce) crystals are a newer type of scintillation detector material offering better resolution, but otherwise, similar characteristics to

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Figure 1.5 57 Co Spectrum from Counter

SEMICONDUCTOR DETECTORS

A semiconductor is a material that can act as an insulator or as a conductor In electronics the term “solid state” is often used inter- changeably with semiconductor, but in the detector ﬁeld the term can obviously be applied to solid scintillators Therefore, semiconductor is the preferred term for those detectors which are fabricated from either elemental or compound single crystal materials having

a band gap in the range of approximately 1 to 5 eV The group IV elements silicon and germanium are by far the most widely-used semiconductors, although some compound semiconductor materials are ﬁnding use in special applications as development work on them continues.

Table 1.1 shows some of the key characteristics of various ductors as detector materials:

semicon-Table 1.1 Element vs Band Gap

pair (eV)

Si Ge CdTe HgI 2 GaAs

14 32 48-52 80-53 31-33

1.12 0.74 1.47 2.13 1.43

3.61 2.98 4.43 6.5 5.2 Semiconductor detectors have a p-i-n diode structure in which the intrinsic (i) region is created by depletion of charge carriers when

a reverse bias is applied across the diode When photons interact within the depletion region, charge carriers (holes and electrons) are freed and are swept to their respective collecting electrode by the electric ﬁeld The resultant charge is integrated by a charge sensitive preampliﬁer and converted to a voltage pulse with an amplitude proportional to the original photon energy.

Since the depletion depth is inversely proportional to net electrical impurity concentration, and since counting efﬁciency is also dependent on the purity of the material, large volumes of very pure material are needed to ensure high counting efﬁciency for high energy photons.

NaI detector crystals NaI is still the dominant material for gamma

detection because it provides good gamma ray resolution and is

economical However, plastics have much faster pulse light decay

and ﬁnd use in timing applications, even though they often offer little

or no energy resolution.

NaI(Tl) SCINTILLATION DETECTORS

The high Z of iodine in NaI gives good efﬁciency for gamma ray

detection A small amount of Tl is added in order to activate the

crystal, so that the designation is usually NaI(Tl) for the crystal

The best resolution achievable ranges from 7.5%-8.5% for the 662

keV gamma ray from 137 Cs for 3 in diameter by 3 in long crystal,

and is slightly worse for smaller and larger sizes Figure 1.7 shows,

respectively, the absorption efﬁciencies of various thicknesses

of NaI crystals and the transmission coefﬁcient through the most

commonly used entrance windows Many conﬁgurations of NaI

de-tectors are commercially available, ranging from crystals for X-ray

measurements in which the detector is relatively thin (to optimize

resolution at the expense of efﬁciency at higher energies), to large

crystals with multiple phototubes Crystals built with a well to allow

nearly spherical 4π geometry counting of weak samples are also a

widely-used configuration A typical preamplifier and amplifier

com-bination is shown in Figure 1.6.

Figure 1.6 NaI(Tl) Detector Electronics

The light decay time constant in NaI is about 0.25 microseconds,

and typical charge sensitive preampliﬁers translate this into an

output pulse rise time of about 0.5 microseconds For this reason,

NaI detectors are not as well-suited as plastic detectors for fast

coincidence measurements, where very short resolving times are

required LaBr3 (Ce) detectors have a light decay time constant of

0.03 microseconds making them another possible solution for

coin-cidence measurements.

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Prior to the mid-1970’s the required purity levels of Si and Ge could

be achieved only by counter-doping p-type crystals with the n-type

impurity, lithium, in a process known as lithium-ion drifting Although

this process is still widely used in the production of Si(Li) X-ray

detectors, it is no longer required for germanium detectors since

sufﬁciently pure crystals have been available since 1976.

The band gap ﬁgures in Table 1.1 signify the temperature

sensitiv-ity of the materials and the practical ways in which these materials

can be used as detectors Just as Ge transistors have much lower

maximum operating temperatures than Si devices, so do Ge

detec-tors As a practical matter both Ge and Si photon detectors must

be cooled in order to reduce the thermal charge carrier generation

(noise) to an acceptable level This requirement is quite aside from

the lithium precipitation problem which made the old Ge(Li), and to

some degree Si(Li) detectors, perishable at room temperature.

The most common medium for detector cooling is liquid nitrogen,

however, recent advances in electrical cooling systems have made

electrically refrigerated cryostats a viable alternative for many

detector applications.

In liquid nitrogen (LN 2 ) cooled detectors, the detector element (and

in some cases preampliﬁer components), are housed in a clean

vacuum chamber which is attached to or inserted in a LN 2 Dewar

The detector is in thermal contact with the liquid nitrogen which

cools it to around 77 °K or –200 °C At these temperatures, reverse

leakage currents are in the range of 10 -9 to 10 -12 amperes.

Figure 1.7

In electrically refrigerated detectors, both closed-cycle mixed frigerant and helium refrigeration systems have been developed to eliminate the need for liquid nitrogen Besides the obvious advantage of being able to operate where liquid nitrogen is unavailable or supply is uncertain, refrigerated detectors are ideal for applications requiring long-term unattended operation, or applications such as undersea operation, where it is impractical to vent LN 2 gas from a conventional cryostat to its surroundings.

re-A cross-sectional view of a typical liquid nitrogen cryostat is shown

in Figure 1.8.

DETECTOR STRUCTURE

The ﬁrst semiconductor photon detectors had a simple planar ture similar to their predecessor, the Silicon Surface Barrier (SSB) detector Soon the grooved planar Si(Li) detector evolved from attempts to reduce leakage currents and thus improve resolution The coaxial Ge(Li) detector was developed in order to increase overall detector volume, and thus detection efﬁciency, while keep- ing depletion (drift) depths reasonable and minimizing capacitance Other variations on these structures have come, and some have gone away, but there are several currently in use These are illustrated in Figure 1.9 with their salient features and approximate energy ranges.

struc-For more information on speciﬁc detector types refer to the Detector Product Section of this catalog.

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DETECTOR PERFORMANCE

Semiconductor detectors provide greatly improved energy

resolu-tion over other types of radiaresolu-tion detectors for many reasons

Fun-damentally, the resolution advantage can be attributed to the small

amount of energy required to produce a charge carrier and the

con-sequent large “output signal” relative to other detector types for the

same incident photon energy At 3 eV/e-h pair (see Table 1.1) the

number of charge carriers produced in Ge is about one and two

or-ders of magnitude higher than in gas and scintillation detectors

re-spectively The charge multiplication that takes place in proportional

counters and in the electron multipliers associated with scintillation

detectors, resulting in large output signals, does nothing to improve

the fundamental statistics of charge production.

The resultant energy reduction in keV (FWHM) vs energy for ous detector types is illustrated in Table 1.2.

vari-Table 1.2 Energy Resolution (keV FWHM)

vs Detector Type

Proportional Counter X-ray NaI(Tl)

3 x 3 NaI(Tl) Si(Li) Low Energy Ge Coaxial Ge

1.2 3.0

— 0.16 0.14

—

— 12.0 12.0

— 0.5 0.8

—

— 60

—

— 1.8

At low energies, detector efﬁciency is a function of cross-sectional area and window thickness while at high energies total active detector volume more or less determines counting efﬁciency Detectors having thin contacts, e.g Si(Li), Low-Energy Ge and Reverse Elec- trode Ge detectors, are usually equipped with a Be or composite carbon cryostat window to take full advantage of their intrinsic energy response.

Coaxial Ge detectors are specified in terms of their relative full- energy peak efficiency compared to that of a 3 in x 3 in NaI(Tl) Scintillation detector at a detector to source distance of 25 cm De- tectors of greater than 100% relative efficiency have been fabricated from germanium crystals ranging up to about 75 mm in diameter About two kg of germanium is required for such a detector.

Curves of detector efﬁciency vs energy for various types of Ge detectors can be found in the Detector Product Section of this catalog.

Figure 1.8 Model 7500SL Vertical Dipstick Cryostat

Figure 1.9 Detector Structures and Energy Ranges

1 A.C Melissinos, Experiments in Modern Physics, Academic Press, New York (1966), p 178.

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Charged Particle Detection

SILICON CHARGED PARTICLE DETECTORS

Silicon Charged Particle detectors have a P-I-N structure in which

a depletion region is formed by applying reverse bias, with the

re-sultant electric ﬁeld collecting the electron-hole pairs produced by

an incident charged particle The resistivity of the silicon must be

high enough to allow a large enough depletion region at moderate

bias voltages A traditional example of this type of detector is the

Silicon Surface Barrier (SSB) detector In this detector, the n-type

silicon has a gold surface-barrier contact as the positive contact,

and deposited aluminum is used at the back of the detector as the

ohmic contact.

A modern version of the charged particle detector is the CANBERRA

PIPS ® detector (Passivated Implanted Planar Silicon) This

detec-tor employs implanted rather than surface barrier contacts and is

therefore more rugged and reliable than the Silicon Surface Barrier

(SSB) detector it replaces.

At the junction there is a repulsion of majority carriers (electrons in

the n-type and holes in p-type) so that a depleted region exists An

applied reverse bias widens this depleted region which is the

sensi-tive detector volume, and can be extended to the limit of breakdown

voltage Detectors are generally available with depletion depths of

100 to 700 µm.

Detectors are speciﬁed in terms of surface area and alpha or beta

particle resolution as well as depletion depth The resolution

de-pends largely upon detector size, being best for small area

detec-tors Alpha resolution of 12 to 35 keV and beta resolutions of 6 to

30 keV are typical Areas of 25 to 5000 mm 2 are available as

stan-dard, with larger detectors available in various geometries for

cus-tom applications Additionally, PIPS detectors are available fully

depleted, so that a dE/dx energy loss measurement can be made

by stacking detectors on axis Detectors for this application are

sup-plied in a transmission mount, (i.e with the bias connector on the

side of the detector).

A chart of the energies of various particles measured at several

depletion depths is shown in Table 1.3 Note that even the thinnest

detector is adequate for alpha particles from radioactive sources,

but that only very low energy electrons are fully absorbed However,

for a detector viewing a source of electron lines, such as conversion

electron lines, sharp peaks will be observed since some electron

path lengths will lie fully in depleted region Figure 1.10 shows

rang-es of particlrang-es commonly occurring in nuclear reactions.

Table 1.3 Particle Ranges and PIPS Depletion Depth

Maximum Particle Energy Depletion

7

15

21

27 33

15

55

85

105 130 Since charge collected from the particle ionization is so small that it

is impractical to use the resultant pulses without intermediate

am-pliﬁcation, a charge-sensitive preampliﬁer is used to initially prepare

the signal.

Figure 1.11 illustrates the electronics used in single-input alpha spectroscopy application Note that the sample and detector are located inside a vacuum chamber so that the energy loss in air is not involved.

LIQUID SCINTILLATORS

Two very important beta-emitting isotopes, tritium and 14 C, have very low energy beta rays These are at 19 and 156 keV respectively, too low to detect reliably with solid scintillators The liquid scintillation technique involves mixing a liquid scintillator with the sample, and then observing the light pulses with one or more photomultiplier tubes The efﬁciency of such a counter is virtually 100% – essentially 4π geometry with no attenuation between source and detector Pulse processing of the resultant Photomultiplier outputs allows the rejection of cosmic events, and the separation, if desired,

of alpha and beta events The increased sensitivity of the Liquid Scintillation counter, coupled with advances in sample preparation techniques, has led to its increasing use for low-level alpha and beta measurements.

Figure 1.10 Range-Energy Curves in Silicon

Figure 1.11

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Basic Counting Systems

PULSE ELECTRONICS

The nuclear electronics industry has standardized the signal

deﬁ-nitions, power supply voltages and physical dimensions of basic

nuclear instrumentation modules using the Nuclear Instrumentation

Methods (NIM) standard initiated in the 1960s This

standardiza-tion provides users with the ability to interchange modules, and the

ﬂexibility to reconﬁgure or expand nuclear counting systems, as

their counting applications change or grow CANBERRA is a

lead-ing supplier of Nuclear Instrumentation Modules (also called NIM),

which are presented in greater detail in Section 1 of this catalog In

the past several years, the industry trend has been to offer modular

detector electronics with the multichannel analyzer (MCA) and all

supporting instrumentation for spectroscopy with a single

detec-tor combined in a compact, stand-alone enclosure These modular

MCAs are smaller, lighter and use less power than the NIM-based

counting systems that preceded them However, their performance

is equal to, or greater than, comparable NIM-based systems

CANBERRA is also a leading supplier of these modular detector

electronics which are described in the Multichannel Analyzers

Sec-tion of this catalog Depending on the applicaSec-tion and budget, NIM

or modular electronics may be the best counting equipment solution

for the user, and CANBERRA supports both of these form factors

with a wide variety of products.

Basic electronic principals, components and conﬁgurations which

are fundamental in solving common nuclear applications are

discussed below.

PREAMPLIFIERS AND AMPLIFIERS

Most detectors can be represented as a capacitor into which a charge is deposited, (as shown in Figure 1.12) By applying detector bias, an electric ﬁeld is created which causes the charge carriers

to migrate and be collected During the charge collection a small current ﬂows, and the voltage drop across the bias resistor is the pulse voltage.

The preamplifier is isolated from the high voltage by a capacitor The rise time of the preamplifier’s output pulse is related to the collection time of the charge, while the decay time of the preamplifier’s output pulse is the RC time constant characteristic of the preamplifier itself Rise times range from a few nanoseconds to a few microseconds, while decay times are usually set at about 50 microseconds Charge-sensitive preamplifiers are commonly used for most solid state detectors In charge-sensitive preamplifiers, an output voltage pulse is produced that is proportional to the input charge The output voltage is essentially independent of detector capacitance, which is especially important in silicon charged particle detection (i.e PIPS ® detectors), since the detector capacitance depends strongly upon the bias voltage However, noise is also affected by the capacitance.

To maximize performance, the preampliﬁer should be located at the detector to reduce capacitance of the leads, which can degrade the rise time as well as lower the effective signal size Additionally, the preampliﬁer also serves to provide a match between the high impedance of the detector and the low impedance of coaxial cables

to the ampliﬁer, which may be located at great distances from the preampliﬁer.

The ampliﬁer serves to shape the pulse as well as further amplify it The long delay time of the preampliﬁer pulse may not be returned

to zero voltage before another pulse occurs, so it is important to shorten it and only preserve the detector information in the pulse rise time The RC clipping technique can be used in which the pulse

is differentiated to remove the slowly varying decay time, and then integrated somewhat to reduce the noise The unipolar pulse that results is much shorter The actual circuitry used is an active ﬁlter for selected frequencies A near-Gaussian pulse shape is produced, yielding optimum signal-to-noise characteristics and count rate performance.

Figure 1.12 Basic Detector and Ampliﬁcation

Figure 1.13 Standard Pulse Waveforms

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A second differentiation produces a bipolar pulse This bipolar pulse

has the advantage of nearly equal amounts of positive and negative

area, so that the net voltage is zero When a bipolar pulse passes

from one stage of a circuit to another through a capacitor, no charge

is left on the capacitor between pulses With a unipolar pulse, the

charge must leak off through associated resistance, or be reset to

zero with a baseline restorer.

High performance gamma spectrometers are often designed today

using Digital Signal Processing (DSP) techniques rather than

ana-log shaping ampliﬁers The shaping functions are then performed in

the digital domain rather than with analog circuitry This is discussed

later in this section.

Typical preampliﬁer and ampliﬁer pulses are shown in Figure 1.13

The dashed line in the unipolar pulse indicates undershoot which can

occur when, at medium to high count rates, a substantial amount of

the ampliﬁer’s output pulses begin to ride on the undershoot of the

previous pulse If left uncorrected, the measured pulse amplitudes

for these pulses would be too low, and when added to pulses whose

amplitudes are correct, would lead to spectrum broadening of peaks

in acquired spectra To compensate for this effect, pole/zero

cancel-lation quickly returns the pulse to the zero baseline voltage.

The bipolar pulse has the further advantage over unipolar in that

the zero crossing point is nearly independent of time (relative to the

start of the pulse) for a wide range of amplitudes This is very useful

in timing applications such as the ones discussed below However,

the unipolar pulse has lower noise, and constant fraction

discrimina-tors have been developed for timing with unipolar pulses.

For further discussions on preampliﬁer and ampliﬁer characteristics,

please refer to each applicable module’s subsection.

Figure 1.14 Multichannel Analyzer Components with Analog Signal Processing

PULSE HEIGHT ANALYSIS AND COUNTING TECHNIQUES

Pulse Height Analysis may consist of a simple discriminator that can be set above noise level and which produces a standard log-

ic pulse (see Figure 1.13) for use in a pulse counter or as gating signal However, most data consists of a range of pulse heights of which only a small portion is of interest One can employ either of the following:

1 Single Channel Analyzer and Counter

2 Multichannel Analyzer The single channel analyzer (SCA) has a lower and an upper level discriminator, and produces an output logic pulse whenever an input pulse falls between the discriminator levels With this device, all voltage pulses in a speciﬁc range can be selected and counted If additional voltage ranges are of interest, additional SCAs and counters (i.e scalers) can be added as required, but the expense and complexity of multiple SCAs and counters usually make this con- ﬁguration impractical.

If a full voltage (i.e energy) spectrum is desired, the SCA’s nators can be set to a narrow range (i.e window) and then stepped through a range of voltages If the counts are recorded and plotted for each step, an energy spectrum will result In a typical example

discrimi-of the use discrimi-of the Model 2030 SCA, the lower level discriminator (LLD) and window can be manually or externally (for instance, by

a computer) incremented, and the counts recorded for each step However, the preferred method of collecting a full energy spectrum

is with a multichannel analyzer.

The multichannel analyzer (MCA), which can be considered as a series of SCAs with incrementing narrow windows, basically consists of an analog-to-digital converter (ADC), control logic, memory and display The multichannel analyzer collects pulses in all voltage ranges at once and displays this information in real time, providing

a major improvement over SCA spectrum analysis.

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Figure 1.14 illustrates a typical MCA block diagram An input energy

pulse is checked to see if it is within the selected SCA range, and

then passed to the ADC The ADC converts the pulse to a number

proportional to the energy of the event This number is taken to be

the address of a memory location, and one count is added to the

contents of that memory location After collecting data for some

pe-riod of time, the memory contains a list of numbers corresponding

to the number of pulses at each discrete voltage The memory is

accessed by a host computer which is responsible for spectrum

dis-play and analysis as well as control of the MCA Depending on the

speciﬁc model MCA, the host computer may be either a dedicated,

embedded processor or a standard off-the-shelf computer.

PULSE HEIGHT ANALYSIS WITH DIGITAL SIGNAL

PROCESSORS

Today’s high performance Multichannel Analyzer systems are

de-signed using Digital Signal Processing (DSP) techniques rather

than the traditional analog methods DSP ﬁlters and processes the

signals using high speed digital calculations rather than

manipula-tion of the time varying voltage signals in the analog domain The

preampliﬁer signal ﬁrst passes through an analog differentiator,

then is delivered to a high speed digitizing ADC (Figure 1.15) The

output of the ADC is a series of digital values that represent the

dif-ferentiated pulse Those signals are then ﬁltered using high-speed

digital calculations within the Digital Signal Processor.

For optimal speed and accuracy in signal processing, a trapezoidal

ﬁlter algorithm is deployed in the DSP implementation Trapezoidal

ﬁltering has been shown to allow for the highest throughput

perfor-mance with the least degradation of spectral resolution

Addition-ally, the DSP based design is intrinsically more stable, resulting in

better performance over a range of environmental conditions.

COUNTERS AND RATEMETERS

Counters and ratemeters are used to record the number of logic

pulses, either on an individual basis as in a counter, or as an

aver-age count rate as in a ratemeter Counters and ratemeters are built

with very high count rate capabilities so that dead times are

mini-mized Counters are usually used in combination with a timer (either

Figure 1.15 Multichannel Analyzer Components with Digital Signal Processing

Figure 1.16 NaI Detector and Counter/Timer

with Alarm Ratemeter

built-in, or external), so that the number of pulses per unit of time are recorded Ratemeters feature a built-in timer, so that the count rate per unit of time is automatically displayed Whereas counters have

an LED or LCD for the number of logic pulses, ratemeters have a mechanical meter for real-time display of the count rate Typically, most counters are designed with 8-decade count capacity and offer

an optional external control/output interface, while ratemeters are designed with linear or log count rate scales, recorder outputs and optional alarm level presets/outputs Additional information may be found in the Counters and Ratemeters Introduction.

SIMPLE COUNTING SYSTEMS

As related above, pulse height analysis can consist of a simple gle channel analyzer and counter, or a multichannel analyzer Gen- erally, low resolution/high efﬁciency detectors (such as proportional counters and NaI(Tl) detectors) are used in X ray or low-energy gamma ray applications where only a few peaks occur An example

sin-of a simple NaI(Tl) detector-based counting system sin-of this type is illustrated in Figure 1.16.

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In this conﬁguration, a Model 2015A Ampliﬁer/SCA is used to

gen-erate a logic pulse for every ampliﬁed (detector) pulse that falls

within the SCA’s “energy window” The logic pulse is then used as

an input to the Model 512 Counter/Timer which provides the user

with a choice of either preset time or preset count operation The

Model 512 is equipped with an RS-232 interface, which enables

it to be controlled and read out to a computer for data storage or

further analysis.

Alternatively, Model 1481LA Linear/Log Ratemeter is used as the

counter, with an alarm relay that will trigger if the count rate exceeds

a user preset value.

Although counters are still used in some applications, most of

today’s counting systems include a multichannel analyzer (MCA)

Besides being more cost effective than multiple SCA-based

sys-tems, a MCA-based system can provide complete pulse height

analysis such that all nuclides, (i.e., even those not expected), can

be easily viewed and/or analyzed.

NaI(Tl) DETECTORS AND MULTICHANNEL ANALYZERS

The need for a single-input Pulse Height Analysis system for use with

a Sodium Iodide detector is served most simply by a photomultiplier

tube (PMT) base MCA such as the uniSpec (Figure 1.17) The

uni-Spec MCA includes a high voltage power supply, preampliﬁer,

am-pliﬁer, spectrum stabilizer and ADC in addition to its MCA functions,

and thus, there is no need for any NIM modules or a NIM Bin All of

Figure 1.18 HPGe Detector and Analog MCA Conﬁguration

this capability is provided in an enclosure no larger than a standard tube base preampliﬁer, and the computer interface is via a USB port

on the host computer or a USB hub Further technical discussions concerning multichannel analyzers and multichannel analysis systems (including spectroscopy software) may be found in the Multi- channel Analyzers and Counting Room Software sections.

GERMANIUM DETECTORS AND MULTICHANNEL ANALYZERS

A typical analog HPGe detector-based gamma spectroscopy tem consists of a HPGe detector, high voltage power supply, preamplifier (which is usually sold as part of the detector), amplifier, ADC and multichannel analyzer As will be discussed in more detail later, DSP configurations replace the amplifier and ADC with digital signal processing electronics.

sys-The analog system components are available in several different types, allowing the system to be tailored to the precise needs of the application and the available budget For example, low-end ampli- ﬁers such as the Model 2022 offer basic capabilities, but users with higher count rate or resolution requirements may consider the Mod-

el 2026 or 2025 with Pileup Rejection/Live Time Correction (PUR/ LTC) feature and both Gaussian and triangular shaping Similarly, the ADC chosen for a system including a 556A NIM MCA could be either an economical Wilkinson ADC like the Model 8701 or a faster Fixed Dead Time (FDT) ADC like Model and 8715 For more Figure 1.17 NaI Detector and MCA Conﬁguration

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information about selecting speciﬁc modules, refer to the

introduc-tion secintroduc-tions for those speciﬁc components.

For applications requiring security of the signal processing,

CANBERRA offers a variety of computer controlled electronics

which require access via a host computer, rather than unprotected

front panel for adjustment For example, the AIM/ICB NIM family is

a network based, computer controlled signal processing line that

can be controlled remotely by a Genie 2000 or Genie-ESP

spec-troscopy workstation.

Spectroscopy systems based on Digital Signal Processing (DSP)

have been widely accepted as the state of the art In a DSP based

system, the ampliﬁer and ADC are replaced by a set of digital

cir-cuits which implement the ﬁltering functions in high speed digital

calculations CANBERRA offers several DSP based products, all

of which offer superior environmental stability, higher count rate

throughput performance and better resolution over a range of count

rate conditions Models 2060, 9660, DSA-1000, DSA-2000 and the

InSpector 2000 all employ this advanced DSP technology.

Figures 1.18, 1.19 and 1.20 show several of the available

Germanium Detectors/MCA conﬁgurations Optional LN2 Monitors,

Level Alarms, and Control Systems are available for most types of

be obtained with the Model 2025 AFT Research Ampliﬁer Besides allowing the user to select a long shaping time constant, the Model

2025 features an enhanced baseline restorer which is ideal for set preampliﬁers Any of the CANBERRA Digital Signal Processing MCAs or components can be used with these detectors and provide even better throughput and resolution performance.

re-MULTIPLE INPUT SYSTEMS

CANBERRA offers two solutions for multiple input counting systems which process the ampliﬁed signals from a number of detectors A multiple input scenario would typically be considered six or more detector inputs – or the point at which multiple independent MCA systems become cost prohibitive for a given counting application The Multiport II (Figure 1.18) is the ﬁrst solution and, also, the more robust of the two It offers the capability for up to six totally independent MCAs and ADCs housed in one double-wide NIM Because the MCAs and ADCs are separate from each other, any combination of detectors and channel number settings may be used for each input.

Figure 1.20 HPGe Detector with DSA-2000 Digital Signal Processor (DSP) Figure 1.19 HPGe Detector with DSA-1000 or InSpector 2000 Digital Signal Processor (DSP)

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The second solution employs a Model 8224 Multiplexer (or Mixer/

Router) to route the signals from multiple detectors to a single ADC

for digitizing and on to a 556A MCA for processing as shown in

Figure 1.21 Since this conﬁguration shares the MCA and ADC

among the detectors, it has a lower cost per input than the

Multi-port II – particularly for large numbers of detectors However, the

Multiplexer conﬁguration has a major drawback due to the single

ADC; the count rate of the individual detectors must be relatively

low to avoid excessive signal pileup Additionally, a Multiplexer must

allocate the memory of the MCA to its various inputs (same amount

for each input), which decreases the number of channels available

for each individual detector Within these constraints, Multiplexers

can be quite efﬁcient for applications such as low-level

environmen-tal alpha spectroscopy in which multiple low-intensity inputs are

col-lected in MCA memory segments of 512 channels or less Low-level

gamma counting with NaI detectors, which typically don’t need more

than 1024 channels, is another application that can make use of a

Multiplexer An example conﬁguration is depicted in Figure 1.21.

It should be noted that the Multiport II and the 8224 Multiplexer do

not include spectroscopy ampliﬁers or detector bias supplies These

components must be supplied by other parts of the signal chain

Also, these two solutions do not include the beneﬁts of Digital

Sig-nal Processing

Advances in electronics technology have dramatically lowered the

cost of MCAs, so that today, it is frequently more effective to use

multiple complete MCA systems (or the Multiport II) in place of a

Multiplexer.

LOW LEVEL GAMMA RAY COUNTING

Large volume HPGe detectors have become dominant over other

detector types for low level gamma ray spectroscopy because of

their inherently good resolution and linearity It is only in the

analy-sis of single radionuclides that NaI(Tl) detectors can compare in

sensitivity with HPGe detectors Since the majority of all gamma

spectroscopy applications require the analysis of more complex,

multi-radionuclide samples, the following discussion will be limited

to the application of HPGe detectors to low level counting.

The sensitivity of a HPGe spectrometer system depends on

sev-eral factors, including detector efﬁciency, detector resolution,

background radiation, sample constituency, sample geometry and

counting time The following paragraphs discuss the role these

fac-tors play in low level gamma ray counting.

1 EFFICIENCY: Generally, the sensitivity of a HPGe system will be in direct proportion to the detector efficiency HPGe detectors are almost universally specified for efficiency relative to a 3 in NaI(Tl) at 25 cm detector-to-source distance at 1.33 MeV, and from this benchmark one may roughly com- pute the efficiency at lower energies However, for the cus- tomer who is counting weak samples with lower gamma energies, for instance 100-800 keV, the following subtle con- siderations to the detector design are important to system performance:

a The detector should have an adequate diameter This sures that the efﬁciency at medium and low energies will

as-be high relative to the efﬁciency at 1.33 MeV, where it is bought and paid for.

b The detector-to-end-cap distance should be minimal – ﬁve millimeters or less The inverse square law is real and will affect sensitivity.

c The detector should be of closed end coaxial geometry, to assure that the entire front face is active.

2 RESOLUTION: Generally, the superior resolution of a HPGe detector is sufﬁcient enough to avoid the problem of peak convolution, (i.e., all peaks are separate and distinct) The sensitivity of a system improves as the resolution improves because higher resolution means that spectral line widths are smaller, and fewer background counts are therefore involved in calculating peak integrals.

Since the sensitivity is inversely related to the square root of the background, that is,

improvements in resolution will not improve sensitivity as dramatically as increased efﬁciency.

Figure 1.21 Multiple Input NaI Detector System

Sensitivity = 1

Bkg

√

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3 BACKGROUND RADIATION AND SAMPLE

CONSTITU-ENCY: Interfering background in gamma spectra originates

either from within the sample being counted

(Compton-pro-duced) or from the environment If the sample being

ana-lyzed has a high content of high-energy gamma emitting

radioisotopes, the Compton-produced background will

eas-ily outweigh the environmental background For extremely

weak samples, the environmental background becomes

more signiﬁcant Obviously, massive shielding will do little to

improve system sensitivity for low energy gamma rays in the

presence of relatively intense higher energy radiation

How-ever, Compton-suppression can be very effective in reducing

this background.

4 SAMPLE GEOMETRY: An often overlooked aspect of HPGe

detector sensitivity is the sample geometry For a given

sam-ple size (and the samsam-ple size should be as a large as

prac-ticable for maximum sensitivity), the sample should be

dis-tributed so as to minimize the distance between the sample

volume and the detector itself.

This rules out analyzing “test tube” samples with non-well

type detectors, or “large area ﬂat samples” with standard

de-tectors It does rule in favor of using re-entrant or

Marinelli-beaker-type sample containers, which distribute part of the

sample around the circumference of the detector.

GERMANIUM DETECTORS WITH INERT SHIELDS

There are many different types of shield designs that are available,

and because of the difﬁculty in determining the background

contri-bution of the materials used in a given shield, it is difﬁcult to assign

performance levels to various types of shields However, some

crite-ria for shield designs have evolved over the years, such as:

1 The shield should not be designed to contain unnecessary

components like the Dewar It will only contribute to increased

background if it is within the walls of the shield, as well as

unnecessarily increase the shield’s size, weight and cost.

2 The detector should be readily installed and removable from

the shield.

Pity the person who has to move lead bricks (at 12 kg each)

to disengage a HPGe detector A HPGe detector and shield

system should have a liquid nitrogen transfer system to avoid

removing the detector for the weekly ﬁlling.

3 Sample entry should be convenient to the operator.

4 The shield should accommodate a variety of sample sizes

and conﬁgurations.

The HPGe detector should be located in the center of the

shield so as to minimize scatter from the walls In this

posi-tion, the shield must accommodate the largest sample that

is anticipated Also, sample placement should be accurately

repeatable and easily veriﬁed by the operator.

The shield design that has all these features and is moderately

priced is the CANBERRA Model 747 Lead Shield illustrated in

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The performance of the shield using a CANBERRA HPGe detector

Resolution 12%1.95 keV FWHM at 1.33 MeV

0.90 keV FWHM at 1.22 keV Background

Count: 2.25 counts/second in the 50 keV–2.7 MeV range

Sensitivity: Table 1.4 lists the sensitivities of several single

radioisotopes, assuming a counting time of

50 000 seconds, a 50% error and a

2

3

6 10

LOW BACKGROUND CRYOSTATS

The design or conﬁguration of the cryostat is another factor in

sys-tem performance Some cryostat/shield designs do not prevent

streaming from the outside environment, nor do they provide

self-shielding from their own relatively hot components Through an

improper choice of material types and/or thicknesses, the cryostat

may appreciably contribute to the background CANBERRA has

developed sources for select, low-background, materials, and has

invested in the design and fabrication of low-background cryostats,

as described in the Introduction to the Cryostats and Cryostat

Options Section.

HPGe COMPTON SUPPRESSION SPECTROMETER

When the ultimate in low level counting is required, a Compton Suppression Spectrometer, in conjunction with an appropriate low- background shield/cryostat design, is the answer In this conﬁgura- tion, the HPGe detector is surrounded by an active NaI(Tl) or plastic scintillation guard detector (also known as an annulus detector), with the electronics conﬁgured in an anticoincidence counting mode The Compton continuum, which is primarily caused by gamma rays which sustain one or more inelastic collisions and escape (i.e scatter out of) the germanium detector material without imparting their full energy, can lead to concealment of low activity peaks Since this is undesirable in low level counting applications, a Compton Suppression Spectrometer can be used to gate (i.e turn off) data acquisition whenever one of the incompletely absorbed photons escapes the germanium detector and is “seen” by the annulus detector When acquisition is complete, the resultant spectrum contains only peaks attributed to gamma rays which have imparted their full energy within the detector material.

It should be pointed out that some radioisotopes (those having incident gamma rays) such as 60 Co, will not be analyzed properly

co-by the anticoincidence spectrum from a Compton Suppression tem Therefore, two spectra are usually obtained from such a spectrometer – one in the anticoincidence mode, and the other in the normal (ungated) mode.

Sys-Figure 1.24 illustrates a typical example of a Compton Suppression System.

One type of annulus has six (6) 5.08 cm (2-inch) diameter multiplier tubes (PMTs) on one end, and a 7.62 cm (3-inch) diameter NaI(Tl) plug with one PMT (which is operated in parallel with the other PMT) on the other end A simpler type of annulus detector uses a 15.24 cm (6-inch) diameter NaI(Tl) well detector on a single PMT In either conﬁguration, the annulus must be large enough to allow the insertion of the HPGe detector’s endcap along with the sample.

photo-Figure 1.24 Compton Suppression System

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While some endorse the use of a fairly complex Timing Chain to

derive the anti-Compton gate signal, CANBERRA has found that

the simpliﬁed circuit shown in Figure 1.24 yields equivalent results 2

The “Incoming Count Rate” signals from the Spectroscopy

Ampli-ﬁers are checked for coincidence, and, if it exists, the 2040

Co-incidence Analyzer’s output is used as an anti-coCo-incidence input

to the ADC’s Gate When coincidence occurs, this gate “turns off”

the delayed unipolar signal from the Spectroscopy Ampliﬁer Typical

Compton Suppression Spectrometer results are illustrated in Figure

1.25 It can be seen that the ‘ﬁgure of merit’ – the value of the 137 Cs

peak at 662 keV divided by the average contents of the Compton

continuum (the energy range 358-382 keV) – is on the order of

1000:1.

HIGH COUNT RATE GAMMA RAY SYSTEMS

High count rate applications require special techniques to assure

good resolution and/or good throughput In general, “high count

rate” is used to refer to incoming count rate, that is, the number

of events seen by the detector The term “throughput rate” may be

of more interest to the researcher, being a measure of the rate at

which the system can accurately process these incoming counts

In high count rate HPGe detector applications, problems such as

the loss of resolution, excessively long counting times, erroneous

peak to background ratios, inaccurate counting statistics or system

shutdown due to overload and saturation begin to appear In some

experiments, the solution to these problems merely lies in reducing

the incoming count rate to the detector, or by employing

electron-ics which inhibit the processing of pulses through the electronelectron-ics

when events are occurring so fast that they are overlapping (pulse

pileup) In this latter solution, system throughput will of course be

reduced, but parameters such as resolution will be enhanced Table

1.5 indicates the throughput limitations of the major components of

a spectroscopy chain Note that the term “energy rate limited” refers

to the fact that the component’s performance is not only affected

by the incoming count rate, but by the relative energy (amplitude)

of the incoming counts as well Each element in the chain can be

2 Compton Suppression Made Easy, Application Note

Figure 1.25 Ge Spectra with Compton Suppression

optimized for high count rate performance.

or more sophisticated peak shapes.

Some ways to address high count rate in the detector include ing the detector farther away from the source, or collimating the detector – which in both cases reduces the number of events seen by the detector – or using a detector of lesser efﬁciency The detector

mov-in the latter case will ‘see’ fewer events, and furthermore will have a lower charge collection time.

THE PREAMPLIFIER

Most Germanium detectors in use today are equipped with feedback, charge sensitive preamplifiers In the RC-feedback preamplifier, a feedback resistor discharges the integrator, typically in one or two milliseconds If the incoming energy rate (count rate X energy/count) produces a current that exceeds the capability of the resistor to bleed it off, the output will increase until, in the extreme, the preamplifier saturates and ceases to operate This limit occurs

RC-at approximRC-ately 200k MeV/s The sRC-aturRC-ated condition remains til the count rate is reduced The saturation limit is dependent on both energy and count rate and is usually speciﬁed in terms of the

un-“energy/rate limit” The energy/rate limit can be increased by ing the value of the feedback resistor, but this in turn allows more noise to pass through the preampliﬁer, resulting in a degradation in

lower-Table 1.5 Major System Components and their Throughput Limitations

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When a Coaxial Germanium detector is used in applications

requir-ing high throughput, the Model 2101 Transistor Reset Preampliﬁer

(TRP) is favored over traditional RC feedback Preampliﬁers The

higher cost of the TRP is justiﬁed by its much higher energy rate

ca-pacity, an enhancement obtained by replacing the Feedback

Resis-tor of a typical RC feedback preampliﬁer with a special reset circuit

This circuit monitors the dc level of the preampliﬁer and discharges

the feedback capacitor whenever the output reaches a

predeter-mined reset threshold At moderate to high count rates (i.e above

20 000 cps), the absence of the feedback resistor and its attendant

noise and secondary time constant contributions lead to: 1) lower

preampliﬁer noise contributions, 2) inherently better resolution and

reduced spectrum broadening vs count rate, 3) elimination of the

need for pole/zero cancellation, and 4) elimination of ‘lock-up’ due

to saturation Figure 1.26 illustrates the throughout performance of

the two preampliﬁer styles.

Although the Model 2101 TRP virtually never shuts down due to

saturation, its reset process and the ampliﬁer overload which it

causes does induce intervals of dead time into the counting system

The Model 2101 has been designed with a small Charge Gain (50

mV/MeV) and a wide Dynamic Range (4 V) to signiﬁcantly reduce

the dead time due to resets in comparison to competitive units.

DIGITAL SIGNAL PROCESSOR

As we described in an earlier section, Digital Signal Processors

(DSP) have come to replace the analog shaping ampliﬁer and ADC

in most high performance gamma spectroscopy systems It is in

applications involving high count rate performance where the

advantages of DSP become most pronounced.

In gamma spectroscopy systems, the DSP replaces the

functional-ity of both the shaping amplifier and the ADC The DSP first filters

the signal for optimum signal to noise ratio and to provide gain It

then detects the peak amplitude of the ﬁltered pulse to calculate the

memory address of the MCA channel into which the event is to be

stored.

In the DSP, the analog signal from the preampliﬁer is ﬁrst

differenti-ated in the analog domain to provide a rapid return to baseline This

is depicted in Figure 1.27 The resulting time varying voltage signal

is sampled by a high speed sampling analog to digital converter This

results in a digitized proﬁle of the differentiated preampliﬁer signal

represented in internal memory of the DSP From this point on, the

signal is processed in the digital domain by the DSP – essentially

a high speed digital computer executing calculations as opposed

to analog circuits manipulating time varying voltage signals.

Figure 1.26 Throughput vs Count Rate:

Throughput Optimization

Figure 1.27 Typical Ampliﬁer Pulses

Figure 1.28 Trapezoidal Pulse Waveform as processed in DSP

Processing the signals digitally allows more sophisticated ﬁltering functions to be applied to the signal It also allows greater ﬂexibility

to the user in terms of adjusting ﬁltering parameters – more ble settings are available because they are handled as digital com- mands, not the selection of discrete analog components Finally, the use of high speed digital electronics allows the signals to be processed more rapidly, thus contributing further to the count rate performance of the system.

possi-CANBERRA’s DSP products deploy a trapezoidal ﬁltering algorithm

as shown in Figure 1.28 Two parameters are available for user adjustment – the rise/fall time of the trapezoid (hereafter referred to

as rise time) and the ﬂat top time

Adjusting the rise time changes the filter characteristics to optimize for noise characteristics The larger the rise time, the better the signal to noise ratio Shorter rise times will adversely affect signal to noise ratio and degrade the resolution of the system Flat top ad- justments are made to accommodate the variations in pulse rise time which in turn is proportional to the charge collection time in the detector Larger detectors tend to have a larger number of long rise time (large charger collection time) events, thus requiring a longer flat top time Failure to set the DSP rise time long enough to accommodate the longest charge collection time events results in degraded resolution, an effect known as ballistic deficit Note that for some types of smaller detectors, the flat top time can be set near or very close to zero, resulting in a triangular shape

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Figure 1.29 A comparison of the system throughput as a function

of input count rate for a DSP and an analog system optimized for

high throughput for a small detector (11%)

Figure 1.30 A comparison of the system resolution as a function

of input count rate for a DSP and an analog system optimized for maximum throughput for a small detector (11%)

Figure 1.31 A comparison of the system throughput as a function

of input count rate for an analog system optimized for maximum throughput with a DSP system set for a similar throughput

These two parameters together control the total event processing

time The total processing time for an event processed with the DSP

trapezoidal algorithm is deﬁned by the equation:

T p = (2T r ) + T ﬂat top

We see that the settings for both parameters effect the total

pro-cessing time, which in turn effects the count rate throughput of the

system As we noted earlier, setting either parameter too fast can

result in lost resolution Increasing the settings improve resolution,

but lengthen processing time and sacriﬁce throughput A tradeoff

exists (as it did in analog systems) between count rate throughput

and resolution Higher throughput can be attained with a loss of

resolution and better resolution can be attained at a loss of

through-put – up to the limits imposed by the performance of the detector

and preampliﬁer components.

These tradeoffs also existed in traditional analog systems, but the

tradeoffs can now be made at a higher level – the DSP provides

both improved throughput and improved resolution as compared to

analog This is due to a number of factors First, the trapezoidal

algorithm is simply more efﬁcient and can process the signals more

accurately and rapidly than analog electronics.

Secondly, the user has much more ﬂexibility to vary the components

of the processing time In analog systems, the processing was

con-trolled by a single parameter – the shaping time Now with DSP, two

parameters are available – one to accommodate noise level and

one to accommodate detector pulse rise time By adjusting these

two separately, optimum settings can more readily be attained

re-sulting, generally, in shorter total processing time to reach the same

resolution result Additionally, the analog ampliﬁers typically were

limited to six or fewer shaping time selections If, say, 2 µs shaping

was too short, the next available selection was usually 4 µs – twice

the processing time With the CANBERRA DSP products, the user

can typically select from 35 to 40 rise times and 21 ﬂat top times

Again, this greater granularity of adjustment makes it possible to

more closely optimize the performance.

Note that the CANBERRA DSP products also implement Pile Up

Rejection/Live Time Correction (PUR/LTC) Earlier products

imple-mented this feature with analog circuitry, but in the DSP this is

in-corporated into the digital domain functions Pulse pileup occurs

when a new pulse from the preampliﬁer reaches the input stages

of the DSP before the previous pulse is fully processed In such

cases, the PUR/LTC function a) inhibits the processing of any

inval-id, composite pulses and b) turns off the live time clock during the

time pulse processing is gated off In this manner, piled up events

– which would serve only to distort the spectrum – are rejected

before storage by the MCA and the actual live counting time of the

MCA remains correct.

The improved performance of the DSP as compared to analog

systems is shown in Figures 1.29 to 1.34 Figures 1.29 and 1.30 show

real performance data collected with a DSP and an analog gated

integrator and fast ADC (the fastest available using analog

technol-ogy) For this experiment, a Model 2060 DSP was set for rise time

of 0.72 µs and ﬂat top time of 0.68 µs The analog gated integrator

ampliﬁer (Model 2024) was set for shaping of 0.25 µs and paired with a

800 ns Fixed Dead Time ADC These settings were chosen for

op-timal throughput with a relatively small (≈11% efﬁcient) germanium

detector.

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As we can see from Figure 1.29, the DSP based system provides

higher throughput by approximately 50% Figure 1.30 shows the

resolution comparison for the same experiment and demonstrates

that the DSP also provides signiﬁcantly better resolution once the

input count rate exceeds approximately 150 kcps Note that the

shape of the resolution curve in Figure 1.30 is also much ﬂatter,

indicating that widely varying count rates can be accommodated at

a relatively constant resolution.

Note that with these settings chosen for highest throughput, the

res-olution performance at lower count rates is actually slightly worse

with the DSP However, in an application involving those count rates,

it is unlikely those settings would be used Figures 1.31 and 1.32

show the same analog data compared to the DSP system with the

rise time extended to 1.24 µs This reduces the throughput of the

DSP system although it is still superior to that of the analog Further,

we see now that with these settings, the resolution of the DSP is

su-perior to the analog across the full range of incoming count rates.

Figures 1.33 and 1.34 compare a Model 2060 DSP to a

Gauss-ian analog system consisting of a Model 2025 ampliﬁer and Model

8715 ADC In this case, the settings of both systems were chosen

to provide optimal resolution under the high incoming count rates

Analog systems were set for 2 µs and 4 µs Gaussian shaping times

while the DSP settings were 5.6 µs rise time and 0.8 µs ﬂat top

Fig-ure 1.33 shows that, with these settings, the throughput of the DSP

system is approximately equal to that of the 2 µs Gaussian system

Yet Figure 1.34 shows the resolution of the DSP system is superior

to the 4 µs Gaussian system Again, the DSP allows the

spectros-copist to achieve a signiﬁcantly better tradeoff between throughput

and resolution.

LOSS FREE COUNTING APPLICATIONS

The correction of the Live Time Clock as described above,

effec-tively extending the counting time to account for those periods when

the system could not accept an input, is adequate for most samples,

in particular those for which the count rate is relatively constant

However, for short half-lived samples, or samples whose

constitu-ents change (as in a ﬂow monitoring application), this method will

not be accurate In addition, even if the “counts per unit time” are

accurate using the traditional method for dead time correction, the

“real” counting time will have been extended by an amount equal to

the dead time, which may in fact increase the actual collection time

to an undesirable length.

The principal goal of Loss Free Counting (LFC) is to insure that at

the end of any data acquisition interval, the electronics have

accu-mulated all of the events that occurred regardless of any dead time

that may have been present in the system LFC is based on the

concept that by adding “n” counts per event to an MCA’s channel

register, rather than digitizing and storing a single count at a time,

a “zero dead time” energy spectrum can be accumulated that

as-sures all counts are included in the spectrum Assuming that “n” is

correctly derived, (“n” should equal “1” plus a “weighting factor”

rep-resenting the number of events that were lost since the last event

was stored), and the data is truly random in nature, the concept is

statistically valid The factor “n” is derived on a continuous basis by

examining the state of the Ampliﬁer and ADC every 200 ns The

pro-portion of time during which the Ampliﬁer and ADC are processing a

pulse provides a measure for the magnitude of the weighting factor

“n”, which is updated every 20 µs Loss free counting requires that

the MCA support “add-n” or multiple “add-one” data transfer; consult

the factory for details.

Figure 1.32 A comparison of the resolution between an analog system optimized for maximum throughput and a DSP system

set for a similar throughput

Figure 1.33 A comparison of the system throughput as

a function of input count rate for a DSP and two analog systems optimized for resolution

Figure 1.34 A comparison of the system resolution as

a function of input count rate for a DSP and two analog systems optimized for resolution

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Unfortunately, counting statistics in a Loss Free Counting system

cannot be calculated from the corrected spectrum One basic

as-sumption used by all peak ﬁtting algorithms is that of Poisson

count-ing statistics That is, the uncertainty of the counts is proportional

to the square root of the number of counts While this assumption

is true for traditional “add-1” front-ends, it is not true of the “add-n”

Loss Free Counting front-end This assumption is especially poor

as the weighting factor becomes large To properly quantify the

un-certainty in each channel’s contents, the peak ﬁtting program must

have access to both the corrected “add-n” and the uncorrected

“add-1” spectra Therefore, CANBERRA also offers a “Dual-LFC”

hardware option for the Model 599 Loss Free Counting Module

which allows the collection of both of these spectra so that

statisti-cally correct peak ﬁlling can occur Note that the correction software

for the “Dual-LFC” system is only available for VMS-based Genie

Systems.

PIPS DETECTORS AND MULTICHANNEL ANALYZERS

Alpha spectroscopy measurements of low-level samples require

long counting times A large area PIPS detector, when conﬁgured

Figure 1.35 Example Large Scale Alpha Spectroscopy System based on the Alpha Analyst

with a CANBERRA alpha spectrometer and multichannel analyzer, provides a high resolution, low background, counting system that will satisfy a multitude of alpha spectroscopy applications.

An example of a single chamber alpha spectroscopy system (that can easily be upgraded) is illustrated in Figure 1.11 Note that the Model 7401 Alpha Spectrometer is a complete, self-contained, double-width NIM module that contains a vacuum chamber, vacuum gage, detector bias supply, preampliﬁer/ampliﬁer, SCA, counter/tim-

er and pulser for setup and test Multiple Model 7401 Alpha trometers can be conﬁgured with a vacuum system that allows individual vacuum chambers to be opened and loaded without affecting the vacuum or data acquisition of the other spectrometers.

Spec-However, where numerous samples are counted simultaneously, it

is more cost effective and user efﬁcient to select a system based on the Alpha Analyst (Figure 1.35) This turn-key system supports multiple detectors in a comprehensive software environment featuring full computer control of all vacuum elements and acquisition electronics To learn more about CANBERRA’s Alpha Analyst, please refer to Section 1 of this catalog.

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Timing and Coincidence Counting Systems

COINCIDENCE TECHNIQUES

There are many applications that require the measurement of events

that occur in two separate detectors within a given time interval, or

the measurement of the time delay between the two events These

two approaches are used in gamma-gamma or particle-gamma

co-incidence measurements, positron lifetime studies, decay scheme

studies and similar applications, and are titled coincidence or timing

measurements.

A coincidence system determines when two events occur within

a certain ﬁxed time period However, in practice it’s not possible

to analyze coincidence events with 100% conﬁdence due to the

uncertainties associated with the statistical nature of the process

Statistical timing errors may occur from the detection process and

uncertainties in the electronics resulting from timing jitter, amplitude

walk and noise, which lead to statistically variable time delays

be-tween processed events A simple coincidence circuit solves this

problem by essentially summing the two input pulses, passing the

resultant sum pulse through a discriminator level, and generating

an output pulse when the two input pulses overlap Figure 1.36

illustrates this process Note that the period of time in which the two

input pulses can be accepted is deﬁned as the resolving time, which

is determined by the width of the pulses, τ, such that the resolving

time is equal to 2τ.

The 2040 Coincidence Analyzer uses a more sophisticated scheme

allowing analysis of several input signals It produces a logic pulse

output when the input pulses, on the active inputs, occur within the

resolving time window selected by the front panel control.

Since detector events occur at random times, accidental

coinci-dences can occur between two pulses which produce background

in the coincidence counting The rate of accidental or random

coin-cidences is given by:

N acc =N 1 N 2 (2τ)

Where:

N 1 = Count rate in detector number 1

N 2 = Count rate in detector number 2 2τ = The resolving time of the coincidence circuit The number of counts in the detectors depends upon the experiment and the detectors, so the best way to reduce accidental coincidences

is to make the resolving time as small as possible However, the resolving time cannot be reduced below the amount of time jitter in the detector pulses without losing true coincidences, so the type of detector determines the minimum resolving time usable.

A coincidence setup with NaI detectors is shown in Figure 1.37 The unipolar pulse from the 2022 Ampliﬁer is processed by a Model 2037A Constant Fraction Timing SCA to produce a standard NIM logic pulse for the 2040 Coincidence unit The 814FP Pulser is used

to set up delays and test operation.

Figure 1.36 Coincidence Pulses

Figure 1.37 Coincidence Electronics

Trang 21

In order to properly operate the system, a delay curve is obtained

in which coincidences are measured as a function of relative delay

between the two detectors In the ideal case of no time jitter in either

detector, the solid curve in Figure 1.38 is obtained However, real

detectors will produce the dashed curve, and the minimum

resolv-ing time settresolv-ing is where there is a ﬂat region (indicatresolv-ing all true

coincidences are collected) Thus, the proper relative delay is the

value for the center of the ﬂat region.

Typical resolving times are 10 nanoseconds or better, for an energy

range of 0.1 to 1 MeV Shorter resolving times are possible for

plas-tic scintillators and silicon charged parplas-ticle detectors, even down to

less than 1 nanosecond In general, the shorter the rise time of the

preampliﬁer pulse, the smaller the resolving time This is discussed

further with fast discriminators.

Another coincidence technique involves the direct measurement of

the time delay between two pulses A time-to-amplitude converter

(TAC) converts a time difference between two input pulses to a 0 to

10 volt pulse This analog pulse can then be used as an input to an

SCA, or ADC and MCA The Model 2145 TAC provides an integral

SCA capability as well as an output pulse, gate delays and other

features.

Figure 1.39 Coincidence Electronics with TAC Unit

The use of a 2145 TAC with NaI detectors in exact analogy with the

2040 Coincidence unit is shown in Figure 1.39 There are several advantages to the use of a TAC First, no delay curve needs to be taken since all relative decays occurring are recorded, and second,

no resolving time setting is involved.

The natural time spectrum of the two detectors can be stored directly in an MCA, and if a window is set very narrow, as in Figure 1.40, then there are a minimum number of accidental coincidences recorded The 2058 Nanosecond Delay is required to delay one detector pulse from the other so that a measurable time difference occurs.

TIMING DISCRIMINATORS

A crucial part of any coincidence system is the timing tor used to determine when a pulse occurs There are two general categories:

discrimina-• Slow (or energy) Timing

• Fast Timing The timing single channel analyzers used in Figures 1.37 and 1.39 are examples of slow timing The single channel analyzer operates with shaped pulses to select the range of energies involved in the coincidence, and produces an output logic pulse that is, ideally, independent of input pulse amplitude Fast timing uses pulses directly from the detector, without regard to speciﬁc energy.

Three basic techniques are used in both fast and slow modes for acquiring timing information:

• Leading Edge

• Crossover

• Constant Fraction CANBERRA provides electronic modules for performing any of these techniques, and the proper choice depends upon the detector and application, as discussed below.

Figure 1.38 Resolving Time Curve

Trang 22

The most fundamental timing circuit generates a logic signal when

the leading edge of an input pulse crosses through a discriminator

level as shown in Figure 1.41.

The main problem is that the time of the output pulse varies

mark-edly with amplitude, as can be seen by comparing the two signals

shown This effect can be reduced by setting the discriminator at a

very low level, such as just above noise The Model 2037A Timing

SCA sets the discriminator up to a maximum of 200 millivolts.

Crossover timing relies on the fact that the zero-crossing point in a

bipolar pulse is very nearly independent of pulse amplitude (See

Figure 1.42) The Model 2037A Timing Single Channel Analyzer

offers a crossover mode of operation for bipolar input pulses The

crossover technique has some limitations in that there is still time

dependence or “walk” for different amplitudes, and that signals with

varying rise times from the same detector (such as occurs with

germanium detectors), will produce walk.

Figure 1.40 TAC Spectrum

Figure 1.41 Leading Edge Timing

The constant fraction technique will eliminate most of the

short-comings of the two former methods of timing The constant fraction

timing technique is similar to a discriminator, but with a threshold

that is a constant fraction of the signal amplitude A discussion of

the constant fraction technique, as implemented in the Model 2126

Constant Fraction Discriminator, is given in the Timing Section.

This module can be connected to NaI or fast plastic detectors with negative amplitude output signals ranging from –5 mV to –5 V, and rise times down to 1 nanosecond The 2126 performs no signal processing on the input and is very often attached directly to the anode

of a photomultiplier tube As mentioned above, constant fraction discrimination is a method that offers a timing output signal when

a constant ratio of the pulse height is reached This ratio, once set,

is consistent from pulse to pulse, thus removing the amplitude and rise time errors that arise The main problem with this method is that it is still sensitive to pulse shape distortion, exhibiting poor time resolution for energies less than 200 keV and for poorly shaped or noisy pulses.

Whenever noise or low amplitude is a significant characteristic of a detector, a filter network is required between the signal source and amplifier to alleviate the noise distortion The Model 2111 Timing Fil- ter Amplifier has a built-in filter that attenuates the noise component before amplification of a low signal When used with a constant fraction discriminator such as the 2126, a stable time reference can be derived The 2111/2126 combination has widespread application in gamma-gamma coincidence and lifetime studies, offering excellent time resolution, which in connection with the high resolution of large germanium detectors, increases the rate of useful data collection.

The 2111 Timing Filter Ampliﬁer is applicable for both surface rier and germanium timing applications Both of these detectors produce signals of low amplitude, distorted with noise, and in the case of germanium detectors, poorly shaped rise times A gamma- gamma coincidence system with NaI and germanium detectors is shown in Figure 1.43 This is an example of a “fast-slow” coincidence system in which Model 2111 and 2126 constant fraction discriminators are used to indicate the presence of a pulse, and an energy range on the NaI detector is selected The energy spectrum of the germanium detector is stored in the MCA if the ADC gate is opened

bar-by a coincidence pulse representing a combination of proper germanium timing and the selected NaI energy A Model 2145 TAC

NaI-is used to set the true coincidence range because of the set-up convenience offered, as described above The TAC SCA output and the Timing SCA output of a Model 2015A Ampliﬁer/Timing SCA are placed in coincidence with Model 2040 Coincidence unit If desired,

an energy requirement could be placed on the germanium tor by adding a Model 2037A Timing SCA on the 2025 ampliﬁer’s bipolar output, and feeding the 2037A’s output to the Reset/Inhibit input on the 2145 TAC.

detec-Anticoincidence systems are occasionally required, as mentioned above for Compton suppression in germanium detectors (see Figure 1.24) or cosmic ray suppression in alpha/beta counting.

Figure 1.42 Bipolar Pulses and Crossover Timing

Trang 23

Figure 1.43 NaI-Ge Fast-Slow Coincidence Electronics

Trang 24

Spectrum Analysis

CANBERRA offers a variety of nuclear systems which perform

data analysis as well as data acquisition These systems range

from small stand alone systems to more sophisticated

conﬁgura-tions involving a variety of computer platforms Typical applicaconﬁgura-tions

include Environmental Monitoring, Body Burden Analysis, Nuclear

Waste Assay, Safeguards and other applications Details of these

systems are provided later in this catalog, or in various brochures

that are available from CANBERRA The following section presents

some of the typical procedures and calculations involved in nuclear

applications.

COUNTING STATISTICS

Radioactive decay occurs randomly in time, so the measurement of

the number of events detected in a given time period is never

ex-act, but represents an average value with some uncertainty Better

average values can be obtained by acquiring data over longer time

periods But, since this is not always possible, it is necessary to be

able to estimate the accuracy of any given average.

Nuclear events follow a Poisson distribution which is the limiting

case of a binomial distribution for an inﬁnite number of time

inter-vals, and closely resembles a Gaussian distribution when the

num-ber of observed events is large The Poisson distribution for

observ-ing N events when the average is N, is given by:

PN =

and has standard deviation σ (sigma) equal to √N A graph of P N for N equal to 3 and to 10 is shown in Figure 1.44 The curves are asymmetric and have the property that N is not exactly the most probable value but is close to it However, as N increases the curve becomes more symmetric, and approaches the Gaussian distribution:

PN = • e –x 2 / 2N = • e –x / 2σ 2

Where: x = N – N The integral of the area under the Gaussian curve is often used to report errors in terms of a conﬁdence level in percent For example,

in the value reported as 64 ± 8, 8 is equal to σ and represents 68%

of the area under the appropriate Gaussian curve for N=64 It may

be stated as the value one is 68% confident of obtaining if the surement is repeated Traditionally, many of CANBERRA’s MCAs have used 1.65 σ, which corresponds to a 90% confidence level Probable error is often used, which corresponds to a 50% confidence level These can be user-set to other values, such as:

0.68 1.0 1.15 1.65 1.96

50 68.3 75 90 95

Figure 1.44 Poisson Distributions for N=3 and 10

NNe –N

N!

1 √2πN

1 √2πα

Trang 25

Since the uncertainty depends upon the square root of the counts,

improvements in accuracy by counting longer, or by using a more

efﬁcient detector, only increase as the square root For example, if

564 counts are obtained in an hour for σ ≈ √564 ≈ 24 for a 24/564

= 4.3% accuracy, counting for two hours to get 1133 counts with σ

≈ 34 only gives an improvement to 3.0% In other words, counting

twice as long gives an improvement of √2 = 1.4, or 40%.

Examples of data in which counting statistics apply include: the

counts in a counter, the counts in a single channel of an MCA

trum, or the sum of counts in a group of channels in an MCA

spec-trum The situation becomes even more complicated when

subtract-ing a background as shown in the followsubtract-ing separate, but frequent,

cases.

• Subtracting background counts, as in one counter’s value

from another, or for each channel (when subtracting one

spectrum from another).

• Subtracting a straight line background from a peak on top of

the background in a spectrum, such as a HPGe peak on top

of Compton pulses from higher energy gamma rays.

The error in adding or subtracting two Poisson distributed numbers

with errors, as in:

Ntotal = (N1 ± √N 1 ) ± (N2 ± √N 2 )

is given by:

σ N total = √(√N 1 ) 2 + (√N 2 ) 2

Consider a low level counting situation in which 56 counts are

ob-tained in 10 minutes, and a background of 38 counts in 10 minutes

was measured without the sample The result is 56–38 = 18 counts,

with an error of √56 + 38 = √94 = or approximately 9.7, a σ value

of 54%.

A better procedure is to measure the background over a longer

period of time to obtain a small percentage error and factor to the

appropriate time for each sample analyzed Using the same

exam-ple as above, but with a 100 minute background of 380 counts, the

result would be 56–(380/10) = 18 counts, with an error of

√ 56 + ([ 10 / 100 ] 2 x 380) = √56 + 38 = √59.8

or approximately 7.7, a σ value of 43%.

NET AREA CALCULATION

For the case in which a peak lies on a background that cannot be

subtracted by a background spectrum, such as shown in Figure

1.45 for an MCA spectrum from a HPGe detector:

The area above the background represents the total counts between

the vertical lines minus the trapezoidal area below the horizontal

line If the total counts are P and the end-points of the horizontal line

are B 1 and B 2 , then the net area is given by:

A = P – (B1 + B2)

Where: n = The number of channels between B 1 and B 2

It is tempting to calculate the uncertainty by just using the formula

for subtracting two numbers, with standard deviations of:

σ N = √P + (B1 + B2)

However, this is incorrect because the trapezoidal area is not son distributed and its error is not just the square root of the counts, but depends upon how the errors in B 1 and B 2 affect the horizontal line across the entire region The proper procedure, which is implemented in CANBERRA MCAs and in analysis of peak areas in various HPGe software packages, is derived as follows:

Pois-The standard deviation in a function A is given by:

A = f (N1 N2 Nn) where Nn is the counts in channel N.

Figure 1.45 Net Area Determination

n 2

= (P1 + Pn) +

2 (√B 1 )2 +

2 (√B 2 )21/2

= P +

2 (B 1 + B 2 )

A = P – +

and the standard deviation is:

σ(A) = P +

2 +

1/2

B2

n2

Trang 26

Most CANBERRA MCAs and analysis software packages perform

end-point averaging with a user-selectable number of end-points.

There are many ways of calculating the net counts under a peak

The method described above is a valid, common method, provided

that there are no interferences from photopeaks adjacent to the

peak of interest, and assuming that the background continuum

var-ies linearly from one side of the peak to the other.

However, if interferences exist, other methods of calculating the

net area of a peak must be employed which can include, (but are

not limited to), the use of parabolic or step background algorithms,

as well as non-linear ﬁtting algorithms, etc For further discussions

concerning these techniques and others, the reader is referred to

more detailed texts and formal spectroscopy training courses.

ENERGY CALIBRATION

Many nuclear applications require a means for determining the

en-ergy at a particular channel location of a spectrum To meet this

need, CANBERRA has implemented various techniques which are

brieﬂy discussed.

In some MCAs, a simple two-point energy calibration is used to

determine both the offset and slope by the equation:

E = A (ch) + B

Where: ch = channel number

Thus, the energy vs channel number can be directly read out

How-ever, the more advanced MCA Systems, such as those based on

Genie 2000 or Genie-ESP, allow users to choose between

ﬁrst-order (i.e linear) or second-ﬁrst-order (i.e quadratic) equations that use

a least squares ﬁt to multiple data points.

Most preampliﬁer, ampliﬁer and ADC systems are very linear and

ﬁrst-order energy calibration can properly describe the data For

ex-ample, a CANBERRA germanium detector with 2002 Preampliﬁer,

2025 Ampliﬁer and 8701 ADC has nonlinearities less than 0.05%

for the preampliﬁer and ampliﬁer, and 0.025% for the ADC The

combined nonlinearity is then:

± √(0.05%) 2 + (0.05%) 2 + (0.025%) 2 = ±0.075%

This is still a very small number, but for a spectrum of 4000

chan-nels, the low and high energy channels may be correct and leave

a 0.00075 x 2000 = 1.5 channel uncertainty at channel 2000 A

second-order term in the energy calibration can remove this in order

to provide very accurate energy-channel calibration over the entire

range, according to the equation:

E = A(ch) 2 + B(ch) + C

Where: ch = channel number

A further reﬁnement is provided by using least-squares techniques

to determine the equation that best ﬁts the data, when more than

the minimum number of points is available, (2 for ﬁrst-order, 3 for

second-order) The Genie 2000 and Genie-ESP MCA Systems use

this technique.

NUCLIDE IDENTIFICATION AND QUANTITATIVE ANALYSIS

Many applications with high purity germanium (HPGe) detector

spectra involve identifying particular gamma rays with speciﬁc

nu-clides The sharp peaks in the HPGe spectra, coupled with a

care-ful precise energy calibration, can be used for generally unique

determinations of the nuclides in a sample If an automatic peak

search capability is provided then a complete sample analysis can

be accomplished without operator intervention, which is ideal for analyzing large numbers of samples All CANBERRA HPGe/computer-based gamma spectroscopy systems provide nuclide identiﬁ- cation through peak searches of spectra and scans of standard and user-generated nuclide libraries A sample printout of a Genie 2000 nuclide identiﬁcation report is shown in Figure 1.46.

In the Genie software platforms, the peak search locates peak centroids and then enters a region of interest about each peak This is especially useful for observing the quality of data obtained CANBERRA analysis software provides the additional capability of resolving overlapping peaks into individual components.

Figure 1.46 Isotope ID

Figure 1.47 88 Y Decay Scheme

Trang 27

The ﬁnal step in nuclide analysis is to determine the intensity of

the radioactivity corresponding to each isotope The net area of the

peak is directly related to the intensity, but it is also necessary to

correct for the efﬁciency of the detector, the branching ratio of the

source, and the half life (if it is desired to relate the activity to an

ear-lier or later time) The efﬁciency was discussed earear-lier and has an

energy dependence such as shown in Figure 1.1 The branching

ra-tio (or yield) is used to correct the number of gamma rays observed

to obtain the number of disintegrations of the source Figure 1.47

shows the decay scheme for 88 Y and the percent of disintegrations

leading to the various gamma rays.

The activity of a particular isotope is given in microcuries as:

A(µCi) =

Where yield is the branching ratio fraction and live time is the actual

ADC live time in seconds Half-life corrections are made by

multiply-ing the activity by an exponential factor.

A(at time t o ) = Ae

Where decay time and half-life must be in the same units (seconds,

minutes, hours, or years).

Further speciﬁc data analysis is highly dependent upon the

appli-cation, detector and electronics conﬁguration, and is beyond the

scope of this brief presentation.

EFFICIENCY CALIBRATION

In the equation for activity cited above, the value for efﬁciency is

dependent on the geometry of the sample – size, density, and

dis-tance from detector For the detectors used in gamma analysis,

ef-ﬁciency varies signiﬁcantly with energy (see Figure 1.1) Therefore,

each counting geometry requires an efﬁciency calibration, using a

known standard in the same geometry which includes multiple

en-ergies A series of data pairs of efﬁciency vs energy are generated

from the relationship:

Efﬁciency =

Where Activity is the strength (in Bq) of the standard source (at

col-lection time) at the given energy, yield is the branching ratio fraction

and live time is the actual ADC live time in seconds.

In the Genie software system, the calibration data from the standard

are entered into a “Certiﬁcate File”, with these data being used for

subsequent efﬁciency calibrations The software will automatically

correct for source decay by the formula:

A(at count time) = A(at certiﬁcate time)e –

Where decay time and half-life are in the same units (seconds,

min-utes, hours, or years).

In(2)xDecay Time Half Life

Net Area (Live Time)(Activity)(Yield)

In(2)xDecay Time Half Life

CANBERRA has recently introduced Mathematical Efficiency Calibration products (S573, S574) that do not require radioactive sources for efficiency calibrations These new products (ISOCS™, LabSOCS™) rely on fundamental physical measurements and nuclear constants to accurately determine the energy-efficiency pairs From the series of data pairs, a curve of efficiency versus energy is generated, with the user having a choice of fitting paradigms Thus, the software can calculate efficiency at any energy in the calibrated energy range when analyzing an unknown spectrum.

MINIMUM DETECTABLE ACTIVITY

The calculation of Minimum Detectable Activity for a given nuclide,

at the 95% conﬁdence level, is usually based on Currie’s

deriva-tion (Currie, L.A (1968) Anal Chem 40:586.), with one simpliﬁed

T over the energy range of interest

T is the collect time (sec) EFF is the efﬁciency at the energy of interest

Y is the Branching Ratio

wt is sample weight This formulation takes into account both kinds of errors – false positive and false negative, and yields the smallest level of activity which can be detected with 95% conﬁdence, while also having 95% conﬁdence that “activity” is not detected falsely from a null sample When the measurement is made on a ‘blank’, with no activity, but with the same form and density as an actual sample, the calculated MDA

is an a priori estimate of the best sensitivity that can be expected

from sample measurements When the calculation is applied to a spectrum collected from an actual sample, the background at the energy of interest will in most cases be higher, due to interference and Compton scattering from other nuclides in the sample Thus, the

MDA for an actual sample, computed a posteriori, will be somewhat higher than the a priori estimate.

The MDA – also referred to as Lower Limit of Detection (LLD) – can

be improved by increasing the efﬁciency of detection, decreasing the background, or, for a given experimental setup, by increasing the collect time or the sample size It is frequently necessary to select the appropriate collect time to ensure that the measured MDA will be below the action level mandated by the count-room procedures.

The above formula for MDA, generally accepted in the United States and many other countries, is implemented in a more complete form

in CANBERRA Analytical software Some CANBERRA software packages, such as Genie 2000, offer the user a choice of additional formulas required in other countries.

2.71 + 4.66(σ)

T • EFF • Y • wt Net Area

(Live Time)(Efﬁciency)(Yield)(3.7x10 4 )

Trang 28

Neutron Detection and Counting

SOURCES OF NEUTRONS

There are several methods by which neutrons may be produced in

the fuel cycle principle amongst these are:

1 Alpha Particle Induced Reactions

Plutonium and uranium isotopes decay by alpha particle

emission The alpha particle is absorbed by the nuclei of the

low atomic number elements (Li, B, Be, O, F, C, Si, etc.) and

a neutron is produced The yield depends upon the chemical

composition of the matrix and the alpha production rate for

plutonium and uranium Neutrons from (α,n) reactions are

produced randomly (not time-correlated) and they exhibit a

broad energy spectrum Other α-emitting nuclides can also

make important contributions, for example 241 Am.

2 Spontaneous Fission

The even-numbered isotopes of plutonium ( 238 Pu, 240 Pu,

and 242 Pu) spontaneously ﬁssion (SF) at a rate of 1100,

471, and 800 SF/gram-second respectively Like (α,n)

trons, SF neutrons have a broad energy spectrum SF

neu-trons are time-correlated (several neuneu-trons are produced at

the same time), with the average number of neutrons per

ﬁssion being between 2.16 and 2.26 Uranium isotopes and

odd-numbered plutonium isotopes spontaneously ﬁssion

at a much lower rate (0.0003 to 0.006 SF/gram-second) In

spent fuel Cm and Cf isotopes may be signiﬁcant.

3 Induced Fission

Fissions can be induced in 239 Pu, 235 U, and 238 U by neutron

interrogation of the sample with an external neutron source

Like SF neutrons, they have a broad energy spectrum and

are time-correlated.

NEUTRON DETECTION

Neutrons have mass but no electrical charge Because of this they

cannot directly produce ionization in a detector, and therefore

can-not be directly detected This means that neutron detectors must rely

upon a conversion process where an incident neutron interacts with

a nucleus to produce a secondary charged particle These charged

particles are then directly detected and from them the presence of

neutrons is deduced.

The most common reaction used for high efﬁciency thermal neutron

detection today is:

n + 3 He → p + 3 H + 765 keV

where both the proton and the triton are detected by a gas ﬁlled

proportional counters using 3 He ﬁll gas Quench gas is also added

to control the ionization process.

Another common method uses BF 3 ﬁlled detectors that utilize the

ﬁssion of the 10 B atom to provide the charged particle.

n + B →

A typical instrument conﬁguration that can be used with either of

these detectors can be seen in Figure 1.48.

Gas filled proportional counters offer low noise amplification of the ionization event producing a charge pulse processed by the attached nucleonics chain They offer high detection efficiency with excellent gamma discrimination They also provide a cost effective and stable means of constructing detectors for a wide range of applications.

Gas proportional detectors such as these are efﬁcient only for mal (low energy) neutrons; for high energy neutrons their capture cross sections are very small, making it very unlikely that a neutron will interact with the ﬁll gas and cause the necessary detection reaction Because of this it is necessary to slow the neutrons down to increase the probability of interaction.

ther-This is usually done by surrounding the detector and/or the sample being counted by a hydrogen-rich material (moderator) such as high density polyethylene Typically 10 cm (4 in.) of polyethylene surround the detector with a few cm being between the proportional counters and the neutron source.

The pulse height spectrum from the interaction of a thermal neutron

in a typical 3 He neutron detector will look as shown in Figure 1.49 Perhaps the most important point to note is there is no information about the primary neutron spectrum Because all of the neutrons which are detected have been moderated to reduce their energy

to the thermal level, all neutron energy information is lost All of the events of interest fall into one peak, which is the reaction energy (765 keV) Once a discriminator has been set to eliminate the gam-

ma interference and those events produced by interactions with the walls of the detector tube, simple gross counting is all that is required.

NEUTRON COINCIDENCE COUNTING

One of the more common applications of neutron detection and counting is the assay of fissile materials via the SF signature of the fertile nuclides When the fissile material is contained in a high density matrix or a matrix which includes fission products, the interfering gammas from those fission products may very well overwhelm the weak gammas emitted from the fissile material, making neutron counting the only viable method available for performing the assay

In general neutron and gamma techniques are complementary HRGS may provide relative isotopic information, for example, and the neutron assay bulk non destructive quantiﬁcation.

Figure 1.48 Neutron Counter Electronics

Trang 29

sample matrix For many materials of interest, the vast majority of

the neutrons that are detected will be from the (α,n) reaction and not

from the spontaneous ﬁssion of the ﬁssile material.

It is not possible to use energy discrimination to distinguish

neu-trons from different sources, therefore, traditional spectroscopy

techniques cannot be used However, there is a characteristic time

distribution difference between (α,n) neutrons and those neutrons

produced as the result of a ﬁssion event that can be exploited

Speciﬁcally, ﬁssion events will produce multiple neutrons –

usu-ally two and sometimes three – simultaneously; (α,n) neutrons,

on the other hand, are produced individually and randomly This

allows coincidence counting techniques to be used to distinguish

the prompt ﬁssion neutrons from the random (α,n) neutrons.

A neutron coincidence system is shown schematically in Figure

1.50 The outputs from the 3 He tubes are ﬁrst processed by fast

pre-ampliﬁer/ampliﬁer/discriminator (PAD) circuits, and logically ORed

to the input of the neutron coincidence analyzer for processing.

The coincidence logic identiﬁes those neutron counts that occur within

a short time of each other: ﬁssion neutrons, closely spaced (α,n)

neu-trons, and counts due to “accidental coincidences” Once one neutron

has been detected, the probability of detecting another neutron from

the same ﬁssion decreases approximately exponentially with time

according to the following equation:

P(t) = exp (–t/t d )

where

P(t) = Probability of detecting coincidence neutrons in time t

t d = die-away time of the moderated detector assembly

The die-away time is the characteristic time a neutron will

survive before it is absorbed in the 3 He tubes or escapes the

counter The neutron die-away time normally ranges from 10 to

128 µs depending upon the counter geometry.

Figure 1.49 Thermal Neutron Induced Pulse Height Spectrum

from a Moderated 3 He Detector

The probability for detecting random neutrons from an item is stant with time To determine whether the neutron events which are detected are time-correlated, two equal time periods are sampled

con-by the coincidence logic for each neutron that is detected The first gate or counting window is opened for a time period equal to about 1.267 t d after a neutron is detected Other counts within this time period are due to multiple fission neutrons from the triggering event, other fissions, and (α,n) reactions After a delay of approximately

4000 µs, the second gate is opened and random neutron events are counted The 4000 µs delay assures no time correlation with the neutron which triggered the count The difference in the two time- gated counts (Reals+Accidentals and Accidentals, respectively) is the net Real coincidence count, or Reals The net Reals count is related to the ﬁssile material in the sample by a calibration constant Modern CANBERRA neutron analysis instruments are based

on shift register counting The shift register preserves the pulse quence allowing coincidence data to be evaluated for each neutron event.

se-MULTIPLICITY COUNTING

Neutron coincidence counting provides two measured values (Reals and Totals) while in some cases there are three (or more) unknown variables which need to be determined: mass of

240 Pu-effective, (α,n)-to-(SF,n) ratio, and multiplication factor This condition arises, for example, when impurities in the material under analysis preclude the estimation of alpha ratio from the isotopics

In Multiplicity Counting, a third measured parameter – the tion of multiple counts – is derived, and thus the three unknowns may be calculated A special version of the shift register neutron coincidence analyzer performs the necessary data collection and software reduces the histogram of multiplicity events recorded to singles (Totals or Gross), Doubles (or Reals) and Triples event rates These are used in conjunction with interpretational models to ex- tract the unknown variables for product material m eff , α and M L are normally extracted as discussed already for low level waste where

distribu-ML ~1 then m eff , α and ε, the detection efﬁciency, can be extracted Alternatively, special algorithms can be invoked to reduced detection limits and low level bias caused by cosmic-ray induced spallation events in the waste.

Figure 1.50 Schematic Arrangement of a Thermal Well Coincidence Counter

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System Selection Guide

Gamma

Spectroscopy Multi-Station/Multi-Input DSA-2000AIM/ICB NIM Genie-ESP orGenie 2000 Section 1

DSA-1000, Multiport II Genie 2000 Standalone/Single Input DSA-2000

DSA-1000 Multiport II Eagle Plus

Genie 2000 Section 1

Sample Changer Based Gamma Analyst Genie-ESP

or Apex (Gamma)

Section 1

(HPGe) Portable/Battery Powered InSpector 2000 Genie 2000 Section 1(NaI) required

Portable/No AC Power uniSpecInSpector 1000 Genie 2000Genie 2000 optional Section 1Alpha

Spectroscopy Multi-Station/Multi-Input or

Single Station/Multi-Input

Alpha Analyst Genie-ESP/Apex-Alpha

Client/Server or

Apex-Alpha Desktop

Section 1

Single Station/Limited Number of Inputs Alpha Analyst Apex-Alpha Desktop Section 1Alpha/Beta

Counters Short Count Times Computer Based: Series 5 XLB

Embedded: Series 5 E

iMatic

Eclipse SW LB-Link optional

iLink optional

Section 1

Transportable iSolo

Series 5 APC iLink optionalLB-Link optional Section 1Neutron

Counting Neutron Coincidence Counting JSR-12, JSR-14, 2150, Genie 2000/NDA-2000 Section 3

Alpha/Beta and Gamma

Distributed Alpha Sentry Sampling Heads ASM-1000

Gamma only G64 Area Gamma Monitor RADACS Section 2

PASSIVE NEUTRON COUNTERS

When a neutron coincidence counter is used for the assay of 238 Pu,

240 Pu, and 242 Pu, the neutrons from the spontaneous ﬁssion of

these isotopes are detected and counted Since no external neutron

source is required to induce ﬁssion, assay systems of this type are

known as Passive Neutron Counters.

ACTIVE NEUTRON COUNTERS

235 U, 238 U, and 239 Pu do not spontaneously ﬁssion at a high enough rate to allow passive assay techniques to be used For this reason, uranium assays utilize an external neutron source to induce ﬁssion

in the sample Assay systems using this technique are known as Active Neutron Counters Various kinds exist; e.g Active Well Co- incidence Counter which makes use of Am Li sources; 252 Cf shuf- ﬂer; and the Differential Die Away techniques which uses a pulsed neutron generator CANBERRA offers all of these methods and also undertakes special commissions.

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Introduction

Standard Canberra cryostats and

shields are built with materials which

are screened for extraordinary levels

of natural and man-made radioactive

impurities By this means we are

assured that our standard systems

have relatively low background

levels and are suitable for routine use

in normal applications such as

Radiochemistry, Internal Dosimetry,

Activation Analysis, Waste Assay

and Environmental Counting

Ordinary construction and shielding

materials, however, do contain trace

amounts of naturally occurring and

man-made radionuclides which result

in prominent peaks and an elevated

background continuum, with a

resultant compromise in Minimum

Detectable Activity (MDA)

The design or configuration of

cryostat-shield systems is another

factor in system performance Some

cryostat and shield designs do not

adequately prevent streaming from

the outside environment nor do they

provide self-shielding from their own

relatively “hot” components Worst

of all, through an inappropriate choice

of material types and/or thicknesses

they may actually contribute to the

background or compromise system

performance and/or reliability

Canberra has many years of

experience in materials selection and

in the design and fabrication of low

level counting systems We have

found and developed reliable sources

for select materials and we have

invested in the laboratory facilities

necessary to screen materials and to

test complete systems This effort has

led to the development of standard

Ultra Low-Background Detector

Systems which are described herein

While these standard systems do not

entirely eliminate the need for customdesign systems, they provide predict-able high performance at modest costand can save users the immenseexpense and struggle associated withdesigning, specifying, and testing ofcustom systems

Apologies are due those who havepushed detector system backgrounds

to the utmost limits These systemsbetter deserve the “Ultra” superlativebut they are far beyond practicallimits for commercial applications

Cryostat Design

Five factors are of paramount concern

in the design of a low backgroundcryostat:

1) Background from materials

in close proximity todetector element must beminimized

2) There must be self-shieldingfrom “hotter” materials used

in construction

3) Streaming from the outsideenvironment must bereduced or eliminated.4) Materials having highcross sections for cosmicneutrons with attendantgamma emission should

be kept away from thedetector element

5) The design should notcompromise performanceand reliability

Canberra has two cryostat designswhich satisfy these five concernsadequately They are the Model7500SL-RDC and the 7915-30.Refer to Figure 1 and consider thefollowing explanation of the benefits

of these designs

A The detector chamber is simple

and of low mass The variety ofmaterials is kept to a minimum

so that background contributioncan be controlled

Application

NoteULTRA LOW-BACKGROUND DETECTOR SYSTEMS

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Figure 1

Cryostat designs suitable for low-background detector systems

B The preamplifier body which

contains fiberglass

printed-circuit boards, aluminum

hardware, and a wide variety

of components, is located

remotely and is shielded from

the detector by the full shield

thickness – not by some token

internal lead shield of

inadequate thickness and

compromising location

C Offset between the detector

element and the outside world

prevents streaming and reduced

background from the hotter

cryostat materials such as

molecular sieves or activated

charcoal This offset is common

to U-style cryostats like the

7915 Among dipstick cryostats,

only the Canberra 7500SL is

built with this offset Internal

lead shields are sometimes

proposed to overcome this

fault but they are usually of

inadequate thickness, they can

produce unwanted X rays in

close proximity to the detector

element, and they can

compro-mise reliability of the detector

system

Cryostat Materials

The difficulty in building low

background detectors is nowhere

more evident than in the choice of

materials offered by some detector

manufacturers for various detectortypes Canberra too, has struggled

to find materials which provide lowbackgrounds without compromisingnormal detector performance andwithout compromising long-termreliability Our choices, andcomments on the alternatives,are illustrated in Figure 2

content of less than one part perbillion This material is much moreexpensive than the high purity magne-sium which has been used for endcaps.Corrosion problems with magnesium,however, are endemic and use of thismaterial in thin cross-section forvacuum systems is pure folly Inaddition, magnesium with guaranteedlimits of primordial radionuclides isnot available

B Vacuum Chamber

The vacuum chamber is fabricatedfrom selected stainless steel or fromhigh purity copper

Post World War II steel is nated by 60Co which was widely used

contami-in blast furnace crucible lcontami-iners tomonitor wear or breakthrough Sincestainless steel is a heavily recycledmaterial there is virtually no such thing

as virgin stainless By batch testing,however, we can select stainless withdiminishingly small 60Co content.The alternative to stainless steel is highpurity copper Canberra uses copperwhich is 99.99% pure Standard OFHC(Oxygen-Free, High-Conductivity)copper, which is often mentioned as amaterial of choice for low-backgrounddetectors, can have up to five times theimpurity concentration of the Canberracopper

It is Canberra’s choice to use stainlesssteel for the detector chamber when-ever the material supply allows and

to substitute high purity copper onlywhen a supply problem exists Theadvantage of stainless steel is that nopassivating coatings are necessary toprevent corrosion

C Detector Holder

The detector holder has two tions: It must physically contain thedetector element in good thermalcontact with the cold finger and it mustprovide an infrared radiation shield forthe detector element to reduce IRgenerated leakage current and noise.For the latter function, the detectorholder or a separate IR shield mustsurround the detector element

func-High purity copper is a good choice of

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holder material for Coaxial, XtRa, and

Well detectors because these detectors

have a substantial Ge dead layer on

the cylindrical surface which limits

low-energy response from the side

These Canberra detectors generally

have a dead layer of less than 0.5 mm

which stops 60% of 50 keV

photons Much thicker dead layers

have been found on detectors made by

other manufacturers

The additional attenuation by the

copper holder, while not insignificant

(75% at 50 keV) is not prohibitive

in the energy range where these

detectors are most often used

Canberra provides an appropriately

thin IR shield at the normal entrance

window in any case

For reverse-electrode (REGe)

detectors, however, copper is not the

best choice of holder material These

detectors are most often purchased

because of their thin dead layer for

low-energy response and/or for

Compton-Suppressed Spectrometers

which require efficient capture of

escaping scattered photons Low

energy efficiency for the Marinelli

(re-entrant) beaker counting geometry

is compromised by the use of

a copper holder

Low-Energy Ge (LEGe) detectors,

optimized for very low energies, are

designed to detect radiation entering

the window (front) only

For these reasons Canberra uses

high-purity aluminum for REGe detectors

and the less expensive high-purity

copper for other types If your

application warrants the use of

high-purity aluminum with other detector

types it can be supplied as

an option

D Entrance Window

For Coaxial detectors the aluminum

end-cap, which is made seamless and

thus demonstrates excellent vacuum

integrity, is the obvious choice

The aluminum window, which can be

made as thin as 0.5 mm on request,

transmits 60% of 20 keV photons but

only 2% of 6 keV photons Our

low-energy detectors, the XtRa, REGe andLEGe types, are normally equippedwith a beryllium window which, at0.5 mm thickness, transmits 65% of 6keV photons Beryllium, however,even in the purest grade available, isnot low background

The best commercially available highpurity beryllium foil is 99.8% pureand contains significant level ofprimordial radionuclides (severalparts per million) While it is farbetter than commercial grades ofberyllium foil which may have up

to 100 PPM uranium content, it isnot a good choice of Ultra Low-Background systems

Faced with this dilemma, Canberrahas mastered the use of space-agecomposite materials in this applica-tion The resultant window is acarbon-fiber composite which is lightyet strong It is conductive andtherefore provides EMI resistance It

is helium leak-tight and it is virtuallyfree of radioactive contaminants

it can be supplied as an option

Shields

As in the case of cryostats, shielddesign and material choices have adrastic effect on system performancewhen it comes to low-level counting.Shield design also has a big impact onease of detector installation and use.And, as the most massive objectsfound in most counting systems,shields affect the overall appearance

of the laboratory where they arelocated

Canberra shields provide a balancedcombination of performance,ergonomics, and appearance TheUltra Low-Background versions

of the standard shields differ only

in the materials used in construction

so both standard and low-levelshields are compatible with awide variety of standard andlow-background cryostats

Figure 3

Window transmission characteristics

In the standard 0.5 mm thickness ittransmits 80% of 10 keV photons

Finally, this window does not corrode,

is virtually unbreakable, and unlikeberyllium, is non-toxic

For all these reasons Canberra’scarbon-composite window should

be used whenever possible in lowenergy, low background systems

If an alternative window is necessary,

Materials

Bulk - The best choice for the bulkshielding for low-level counting islead Canberra has a source of 60Cofree steel and this material is suitablefor many applications, but for ultralow-level counting it is a poor choicebecause of the increased Comptonscatter and resultant continuum ofbackground counts in the range of

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is designed to stop the K-shell X raysand everything below them in energy,there is no huge advantage to old lead

in applications where backgroundcontinuum from cosmic interactionsdominates, as is the case for mostabove-ground systems having inertshielding only For systems operatingunderground or with active cosmicguard detectors, the beta bremsstrahl-ung contribution to background maybecome significant

If the effects of 210Pb are stood, the solutions proposed for leadbackground range from the sublime

misunder-to the ridiculous “Virgin” lead issuggested but in this case the 210Pbwill have had little or no chance todecay “Doe Run Mine” lead issometimes specified but this mineclosed around the turn of the century.Indeed some lead deposits may beless active in primordial radionuclides,but no “clean” virgin lead is known

to exist so selection is the rule for thelead bulk

If the operating conditions warrantthe use of lead with certified low

210Pb content, we can supply shieldsmade from selected lead Depending

on the 210Pb content, this can drasticallyincrease the cost

Figure 4

Background continuum vs shield material

100-300 keV This effect is shown in

Figure 4 which compares the

back-ground continuum from a detector

operating in a 15 cm thick steel shield

to that of the same detector operating

in a 10 cm thick lead shield

This continuum can be reduced by

adding a lead liner to the steel shield

Our experiments have shown that

such a liner, even as great as 25 mm

in thickness, does not reduce the

continuum to the level obtained with

lead bulk

Lead contains 210Pb in varying

concentrations The refining process

does not separate 210Pb from the stable

isotope, but since 210Pb has a half life

of 22 years, “old” lead can be notably

lower in 210Pb than “new” lead The

source of the lead ore

is also a big factor in 210Pb

concentration

Now the 46.5 keV gamma ray from

210Pb is readily stopped by the graded

liner used to suppress lead K-shell

X rays However, the 210Bi daughter

of 210Pb is a beta emitter with an

end-point energy of 1161 keV It has been

suggested that bremsstrahlung from

this beta leads to a significant increase

in the background continuum up to

several hundred keV The normal

graded liner would be ineffective in

this energy range To check this

theory we tested samples of lead

with varying 210Pb content on a

low-background LEGe detector with

210Pb levels (the beta obviously excitesthe K-shell X rays) However, thecontinuum differences are fairly small,even with the roughly 3:1 difference

in 210Pb content Above 500 keV, there

is no difference in backgrounds Sincethe graded liner

Figure 5

Background spectra from lead samples Top - 210 Pb content ≈ 20 mBq/gm Bottom - 210 Pb content ≈ 60 mBq/gm

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Liner

Again there are trade-offs between

background continuum and lead

X-ray peaks The graded liners

typically used to suppress the lead

X rays (75-85 keV) consist of 0.5 to

1.5 mm thick layers of cadmium and

copper The cadmium is an effective

filter for lead X rays while the copper

attenuates the cadmium X rays and

prevents personnel exposure to the

toxic cadmium

This graded liner has the undesirable

effect of increasing the background

continuum however This effect is

illustrated in Figure 6 which shows

that copper alone in the thickness

necessary to stop lead X rays (5 mm

for 98%) will almost double the

background continuum in the 200

keV range

If the lead X-rays were of no concern

in the application, and if toxicity and

decontamination were of no concern,

shields would perform better without a

liner Generally this is not the case,

however, so Canberra shields are

equipped with graded liners – but

with a difference Canberra shields

are built with a tin and copper liner

In the interest of the environment

and in the safety of our workers

and customers we have eliminated

cadmium entirely

Many shields are equipped with

0.5 mm of cadmium but this will stop

only about 70% of the lead X rays

One mm of tin will stop about 95%

of the lead X rays With an additional

1.5 mm of copper, the total attenuation

of lead X rays in the Canberra shields

is about 98.5% Another disadvantage

of cadmium is high cross-section for

neutrons from cosmic radiation For

example, the 113Cd (η, γ) 114Cd reaction

results in a prominent background

peak at 558.2 keV and a lesser peak at

651 keV

Important features of the detector/

shield system are illustrated in Figure 7

A Door closes tightly against

shield body to prevent

streaming and to allow

shielding against radon

Figure 6

Background continuum as a function of copper liner thickness

B Shielding materials chosen

for attenuation, backgroundcontribution, and scatteringproperties

C Shield penetration for

detector entry held to aminimum

D Preamplifier and Dewar

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Canberra Industries Inc., Nuclear Products Group, 800 Research Parkway, Meriden, CT 06450 U.S.A.

Tel: (203) 238-2351 Toll Free 1-800-243-4422 FAX: (203) 235-1347 With Offices In: Australia, Austria, Belgium, Canada, Denmark, France, Germany, Italy, Netherlands, Russia, United Kingdom.

System Performance

These overlapped spectra (Figure 8)

show backgrounds from a 40% coaxial

detector (1) unshielded (2) with standard

cryostat in 4 inch thick shield and

(3) with Ultra Low-Background

7500SL-RDC cryostat in 4 inch thick

shield Virtually no peaks due to

cryostat materials appear in spectrum (3)

Further reduction in the continuum

and in the 40K and 208Tl (Nat Th) high

energy lines require thicker shielding,

active shielding (cosmic guard detectors)

or subterranean operation

Cryostat Options

Table 1 lists the Ultra Low-Background

cryostat hardware options and model

numbers The base cryostat and

hardware options are the same for all

detector types The materials options

are different for different detector

types Consult the factory for a

specific proposal for detectors,

cryostats and shields

CAN0012 8/95 Printed in U.S.A.

For example a coaxial detector in a vertical dipstick cryostat requires the following items:

Cryostat: 7500SL, Remote Detector Chamber: RDC, Low-Background materials: ULB - GC

Table 1 - Model List For Ultra Low-Background Cryostat Options

Cryostat Type

Base Cryostat

Hardware Option

Ultra Low-Background Material Vertical Dipstick

U-Style

7500SL 7915-30

RDC SL

ULB-GC (Coaxial) ULB-GR (REGe) ULB-GL (LEGe) ULB-GW (Well)

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Germanium Detectors

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The Company

Canberra Industries is the world's largest manufacturer of nuclear radiation detection and analysis systems In order to better serve our customers worldwide, Canberra operates detector manufacturing facilities in the U.S and in Europe Canberra Semiconduc- tor, N.V., which is located in Olen, Belgium, provides Ge detectors to the European market In addition, C.S.N.V manufactures a semiconductor detector for charged particles known as the PIPS (Passivated Implanted Planar Silicon) Detector Detector Products Division, which shares the home plant in Meriden, Connecticut, provides Ge detectors to the world market outside Europe, and manufactures Si(Li) X-ray Spectrometers as well as a comprehensive line of shields and accessories for detectors and detector systems.

The cover shows a few of the many types of detectors, shields and accessories manufactured by Canberra Our facility in Olen, Belgium and our home plant in Meriden, Connecticut are shown on the back.

Consult the catalog or your Canberra sales office for information on other Canberra products, including Si(Li) detectors, PIPS tors, NaI (TI) detectors, shields, and accessories.

detec-Germanium Detectors

Germanium detectors are semiconductor diodes having a P-I-N structure in which the Intrinsic (I) region is sensitive to ionizing radiation, particularly X-rays and gamma rays Under reverse bias, an electric field extends across the intrinsic or depleted region When photons interact with the material within the depleted volume of a detector, charge carriers (holes and electrons) are produced and are swept by the electric field to the P and N electrodes This charge, which is in proportion to the energy deposited in the detector

by the incoming photon, is converted into a voltage pulse by an integral charge sensitive preamplifier.

Because germanium has a relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers (thus reverse leakage current) to an acceptable level Otherwise, leakage current induced noise destroys the energy resolution

of the detector Liquid nitrogen, which has a temperature of 77°K is the common cooling medium for such detectors The detector is mounted in a vacuum chamber which is attached to or inserted into an LN2 dewar or an electrically powered cooler The sensitive detector surfaces are thus protected from moisture and condensable contaminants.

The Best Detector for Your Application

Canberra offers the widest choice of detector types in the industry Employing the appropriate technology in both materials and processing techniques, Canberra can offer the optimum detector for a wide range of applications We use both p-type and n-type germanium and we use diffused, implanted, and barrier contacts to achieve this product variety.

The illustrations and charts below depict the various detector geometries that are available from Canberra, the energy range they cover, and their salient performance characteristics Consult the individual specification sheets for detailed descriptions, performance ranges, and model availability of each type.

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Your Choice of Cryostat Type

Canberra Cryostats are manufactured in flanged and slimline

styles, with configurations to suit virtually any application Salient

features of both types of cryostats are shown in the illustrations

below Slimline Cryostats are available in the Canberra patented

convertible version.

Consult the individual cryostat spec sheets for more details on the complete product line Special cryostats are our forte Consult your Canberra salesman for information on:

• Low Background Cryostats and Systems

• Multi-Element Detector Telescopes

• Remote Detector Chamber Cryostats for Compton Suppression and Low Background Counting

• Cryostat Arrays for Pu and U Lung Burden Systems

• Electrically Cooled Detectors

• X-ray and γ -ray Array Detectors

Standard Cryostat Configurations

• Vertical Dipstick

• Horizontal Dipstick

• Horizontal Integral

• Vertical (Down-looking) Integral

• Portable Multi-Attitude (MAC and Big MAC)

• U-Type Integral (Fixed and Swivel-head)

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