1. Giới thiệuGammagamma (gg) là sự trùng hợp ngẫu nhiênkỹ thuật xác định và hoặc định lượng phân rã hạt nhânsự kiện dựa trên sự quan sát của nhiều gray duy nhấtchữ ký. Tính trùng hợp ngẫu nhiên, vì nó thườnggọi, sử dụng ít nhất hai máy dò gray để đocác chùm đồng hồ được phát ra trong mỗi sự kiện phân rã. Trongnguyên tắc, gg đếm sự trùng hợp có thể đạt được một mức cao hơnmức độ phân biệt đối xử hơn không ngẫu nhiên ( single )quang phổ vì nó áp dụng một định nghĩa nghiêm ngặt hơnnhững gì tạo thành một sự kiện hợp lệ, cụ thể là quan sáthai tia g tương quan phân rã trong một khoảng thời gian nhất định.Yêu cầu này rất hữu ích trong việc tách các sự kiện quan tâmtừ số lượng lớn hơn của trở lại không tương quanmặt đất trong một khoảng thời gian tính.Tính trùng lặp được sử dụng thường xuyên bởi hạt nhâncác nhà quang phổ học để làm sáng tỏ các chương trình phân rã phức tạp. Cácáp dụng sự trùng hợp ngẫu nhiên với kích hoạt neutronphân tích (NAA) đã được báo cáo trong tài liệu trên 40 nămtrước 13. Kể từ đó, một số giấy tờ đã đượcxuất bản mô tả việc sử dụng sự trùng hợp ngẫu nhiên trongNAA (sau đây gọi là cNAA) 47, nhưng cNAA chưa trở thànhcông cụ phổ biến của hóa học phân tích hạt nhân. Có thểgiải thích cho sự khác biệt này trong ứng dụng là hạt nhâncác nghiên cứu cấu trúc đòi hỏi một sự xác định sự trùng hợp ngẫu nhiênmối quan hệ giữa các tia g để xây dựng hạt nhâncấp bậc; trong khi quang phổ hạt nhân khi sử dụngtrong NAA phụ thuộc vào kiến thức về nguồn gốc của các nguyên tốvà ở mức độ thấp hơn, cường độ của tia g, nhưng khôngđòi hỏi một sự hiểu biết thân mật về sự trùng hợp graycác mối quan hệ. Vì vậy đã có ít động lực để áp dụngkỹ thuật để đo lường phân tích hơn là cóđã được cho các nghiên cứu cấu trúc hạt nhân.Việc thực hiện cNAA cũng được giới hạn ở mức độbởi cơ cấu hạt nhân. Để một phần tử đượcđược xác định bởi cNAA nó phải có một đồng vị dân cưbằng cách bắt giữ nơtrôn phát ra một thác nước gquang hợp lýcường độ. Nhiều nuclit có gray cascades, nhưng thường làb thức ăn nhánh là yếu hoặc cường độ tuyệt đối của tia gammanhỏ. Đồng thời, có một số đồng vị (ví dụ: 203Hgvà 51Cr) quan trọng đối với NAA phát thải chỉ có một btrì hoãngray. Kết quả ròng là số lượng các yếu tốcó thể được xác định bởi cNAA nhỏ hơn sốcó thể được xác định bằng NAA thông thường
Trang 1Nuclear Instruments and Methods in Physics Research A 589 (2008) 243–249
gg coincidence spectrometer for instrumental neutron-activation analysis
B.E Tomlin , R Zeisler, R.M Lindstrom Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8395, USA
Received 21 February 2008; accepted 28 February 2008
Available online 6 March 2008
Abstract
Neutron-activation analysis (NAA) is an important technique for the accurate and precise determination of trace and ultra-trace elemental compositions The application of gg coincidence counting to NAA in order to enhance specificity was first explored over 40 years ago but has not evolved into a regularly used technique A gg coincidence spectrometer has been constructed at the National Institute of Standards and Technology, using two HPGe g-ray detectors and an all-digital data-acquisition system, for the purpose of exploring coincidence NAA and its value in characterizing reference materials This paper describes the initial evaluation of the quantitative precision of coincidence counting versus singles spectrometry, based upon a sample of neutron-irradiated bovine liver material Published by Elsevier B.V
PACS: 82.80.Jp; 29.30.Kv; 82.80.Ej
Keywords: Instrumental neutron-activation analysis; Gamma–gamma coincidence; g-Ray spectrometry; Digital data acquisition
1 Introduction
Gamma–gamma (gg) coincidence spectrometry is a
technique for identifying and/or quantifying nuclear decay
events based on the observation of unique multiple g-ray
signatures Coincidence counting, as it is commonly
referred, employs at least two g-ray detectors to measure
the coincident g-rays emitted in each decay event In
principle, gg coincidence counting can achieve a higher
degree of discrimination than noncoincidence (‘‘singles’’)
spectrometry since it applies a more stringent definition of
what constitutes a valid event, namely the observation of
two decay-correlated g-rays within a specified time window
This requirement is useful in separating events of interest
from the much larger number of uncorrelated
‘‘back-ground’’ events in a counting period
Coincidence counting is used routinely by nuclear
spectroscopists to elucidate complex decay schemes The
application of coincidence counting to neutron-activation
analysis (NAA) was reported in the literature over 40 years
ago [1–3] Since then, a number of papers have been
published describing the use of coincidence counting in NAA (hereafter cNAA)[4–7], yet cNAA has not become a common tool of nuclear analytical chemistry One possible explanation for this difference in application is that nuclear structure studies require a determination of coincidence relationships between g-rays in order to construct nuclear level schemes; whereas nuclear spectrometry as employed
in NAA depends on a knowledge of the elemental origins and, to a lesser extent, the intensities of g-rays, but does not require an intimate knowledge of g-ray coincidence relationships Thus there has been less motivation to apply the technique to analytical measurements than there has been for nuclear structure studies
The implementation of cNAA is also limited to a degree
by nuclear structure In order for an element to be determined by cNAA it must have an isotope populated
by neutron capture that emits a g-ray cascade of reasonable intensity Many nuclides have g-ray cascades, but often the b-branch feeding is weak or the g-ray absolute intensities are small Also, there are a number of isotopes (e.g.203Hg and51Cr) important for NAA that emit only one b-delayed g-ray The net result is that the number of elements that can be determined by cNAA is smaller than the number that can be determined by conventional NAA Cooper has
www.elsevier.com/locate/nima
0168-9002/$ - see front matter Published by Elsevier B.V.
Corresponding author Tel./fax: +1 301 975 6283.
E-mail address: bryan.tomlin@nist.gov (B.E Tomlin).
Trang 2provided a reasonable list of nuclides that may benefit
from cNAA[8] Given the aforementioned restrictions, it is
clear that cNAA is best described as a niche application
Despite the apparent reasons for a lack of cNAA
development, this technique does indeed offer worthwhile
advantages to the nuclear analytical community The
higher level of discrimination that can be obtained with
coincidence counting has been demonstrated in the past to
yield improved sensitivity and selectivity in the
determina-tion of trace elements by NAA[8] This paper describes a
gg coincidence spectrometer constructed at the National
Institute of Standards and Technology (NIST) using
state-of-the-art digital data-acquisition electronics and reports
on the initial evaluation of background-suppression
characteristics of the spectrometer
2 Experimental
2.1 Spectrometer characteristics
The NIST cNAA spectrometer has been constructed
using two coaxial p-type high-purity germanium (HPGe)
g-ray detectors with resistive-feedback preamplifiers The
detectors have efficiencies of 63% and 31%, respectively,
relative to NaI(Tl) at 1.33 MeV, and energy resolutions of
1.8 keV at the same g-ray energy A schematic view of the
coincidence spectrometer is shown in Fig 1 The HPGe
detectors are oriented at 1801 in axial alignment with their
endcaps facing each other; the endcap-to-endcap distance
is continuously adjustable from approximately 0 to
200 mm For counting, a radioactive sample is positioned
on a phenolic spacer that sits directly on the endcap of the
uplooking 31% detector Layered shielding sufficient to
cover the entire lengths of both detector capsules has been
incorporated Thicknesses of at least 50 mm of Pb and
6 mm of Cu are present at all points in the shielding in
order to attenuate environmental background
contribu-tions, as well as Pb X-rays generated inside the shielding
The data-acquisition system for the NIST cNAA
spectrometer utilizes all-digital electronics, based on the
XIA LLC Pixie-41module[9] The Pixie-4 is a four-channel
digital pulse-processing module deployed in CompactPCI
for Instrumentation (PXI) architecture The waveform of
an input signal, taken directly from an HPGe preamplifier
output, is continuously sampled and digitized by a flash
ADC at a rate of 7:5 106samples=s The signal pulse
height is determined by a programmable, digital
trapezoi-dal filter implemented in a field-programmable gate array
(FPGA) Preamplifier pulse heights are determined to
16-bit resolution Event timing and pulse-pileup inspection
is also carried out in the FPGA by a ‘‘fast’’ programmable
trapezoidal filter Events are time-stamped at the full ADC rate of 75 MHz In the present system the Pixie-4 resides in
a 3U PXI crate, and a host desktop PC controls the pulse-processing module and performs data readout via a PCI-PXI fiber-optic bridge All operating parameters, including the filter values, are user-adjustable in software on the host PC; the coincidence time window is also set in software with a granularity of 13.33 ns and a lower limit of 170 ns
as presently employed Spectral data are recorded in list mode such that for each event the pulse height, time of detection (to 13.33 ns resolution) and a bit-mask indicating which detectors triggered is stored sequentially in on-board memory and written to the host PC hard disk in periodic blocks This method of data acquisition, as opposed to the
Pb Al Cu
63% HPGe
31% HPGe
30-L LN2 Dewar
6-L LN2 Dewar
Sample Position
Fig 1 Cross-sectional schematic of the NIST gg coincidence spectro-meter Objects are not drawn to scale.
1
Certain commercial equipment, instruments, or materials are identified
in this paper in order to specify the experimental procedure adequately.
Such identification is not intended to imply recommendation or
endorsement by the NIST, nor is it intended to imply that the materials
or equipment identified are necessarily the best available for the purpose.
Trang 3traditional multichannel-analyzer approach, preserves the
timing information for each detected g-ray Pulse-height
analysis is performed offline A schematic of the
data-acquisition system is given inFig 2a
The advantages that digital signal processing offers to
g-ray spectrometry have been described by a number of
authors[10–13] Perhaps the most significant contribution
that digital pulse processing makes is the ability to perform
sophisticated pulse-shape analysis on the raw preamplifier
signal, either in realtime or offline With regard to the
present work, a pulse-shape analysis algorithm is employed
in the Pixie-4 in order to execute ballistic deficit correction
[11] which can be a serious problem in coaxial HPGe
detectors [14], such as those employed in the NIST
coincidence spectrometer Additionally, a digitally
con-trolled signal-processing module does not suffer from drift
of shaping and timing parameters that often plagues
analog modules This is an especially attractive attribute
for a device like the NIST cNAA spectrometer that will be
used for long individual counts or many short counts over
several weeks or months Finally, the high level of
integration that is possible in a digital pulse-processing
network allows for a device with a compact form factor
The simplicity of the present data-acquisition system is
especially apparent in contrast to previously reported
coincidence counting systems that incorporate many
NIM-based analog processing modules (cf Refs [5,8]
andFig 2b)
2.2 Data reduction The NIST coincidence spectrometer data-acquisition system offers the advantage of simultaneous acquisition
of singles and coincidence data, allowing for potentially higher sample throughput Also, a dual analysis (conven-tional singles and coincidence) can be performed on each sample count, thus facilitating the comparison of these two methods under identical counting conditions Data reduc-tion and analysis are accomplished using a combinareduc-tion of in-house software that utilizes the ROOT object-oriented data analysis framework [15], as well as commercial g-ray spectral analysis packages
During offline analysis, list-mode events are sorted into one of three categories according to which detectors fired in
a given event: detector ‘‘A’’ only (singles), detector ‘‘B’’ only (singles), or detectors ‘‘A’’ and ‘‘B’’ (coincidences) Each singles event is added to a 32k histogram based on the recorded pulse height; a coincidence count is added to a 32k 32k two-dimensional histogram (seeFig 3) accord-ing to the pulse height in each detector A one-dimensional coincidence g-ray spectrum is produced by setting a gate on
a range of channels encompassing approximately 3s on each side of a given g-ray peak and projecting out the spectrum of all g-rays that were recorded in coincidence with the gross peak counts Thereafter, standard g-ray peak integration techniques are used to obtained net peak counts and background continuum counts
CFD = constant-fraction discriminator GDG = gate and delay generator
TAC = time-to-amplitude converter TSCA = timing single channel analyzer
COM FPGA
SP FPGA Flash
ADC HPGe preamp
HPGe preamp
Flash ADC
host PC
fiber-optic/
PCI bus Pixie-4 Module
DSP (energy, time)
SP FPGA = signal processing field-programmable gate array COM FPGA = communication FPGA
DSP = digital signal processor PCI = peripheral component interconnect
PCI I/O SP
FPGA
Memory
Clock
delay
TAC stop
start
TSCA
ADC energy
ADC time
ADC energy
gate
gate
CFD
CFD fast amp
fast amp HPGe
HPGe
shaper
shaper
host PC
ethernet
preamp
preamp
GDG
Fig 2 (a) Schematic of the digital coincidence-counting system as implemented in the present work (b) Representative analog coincidence-counting data-acquisition system (adapted from Ref [5] ).
Trang 42.3 Performance test
A sample of desiccated, jet-milled bovine liver tissue,
from part of a reference material certification program, was
obtained for use in the evaluation of the coincidence
spectrometer The sample, weighing 390.08 mg, was sealed
in a high-purity quartz vial and irradiated in the RT-4
pneumatic irradiation channel of the NIST reactor at 3:4
1013cm2s1 neutron flux The short-lived and
intermedi-ate activities were allowed to decay away over the course of
80 d before counting was commenced The quartz sample
vial was placed horizontally in the spectrometer chamber
on top of a 1 cm phenolic spacer, such that the vial was
approximately centered vertically between the two detector
faces with the sample powder evenly distributed along the
length of the vial The sample was counted for 109 h at
average input count rates of 1960 and 1660 Hz in the lower
and upper detectors, respectively, with an average
frac-tional deadtime of 1.5%
3 Discussion
A representative singles g-ray spectrum recorded for the
irradiated bovine liver sample by the upper detector is
shown in Fig 4 Also shown in the figure is the
corresponding total projection (i.e., gated on all energies)
of coincident g-rays recorded by the upper detector One
feature of the coincidence spectrum that is immediately
noticeable is the presence of distinctive Compton scattering
‘‘peaks’’ at around 200 and 800 keV These structures are a
result of the geometry of the detectors in the spectrometer
The face-to-face alignment enhances the probability of
recording coincident backscattered g-rays, such that when the intense g-rays above 1000 keV are Compton scattered through 180they deposit 800 keV in one detector and the remainder of their energy ( 200 keV) in the other detector in a true coincidence event The 180 orientation was chosen to permit easy adjustments of the source– detector distances which is a valuable feature when dealing with samples of widely varying activity This orientation also provides a large geometric efficiency when the endcap-to-endcap distance is made small A known drawback to this configuration is this detector cross-talk The present configuration does seem to yield a higher continuum for some regions of the spectrum, especially for peaks in the 800–900 keV range Future improvements to the setup may include a 90 orientation with shielding between the detector capsules, or perhaps a Pb sample collimator placed between the detector endcaps
In the portion of the coincidence spectrum below
1000 keV, the emergence of several peaks above the Compton continuum demonstrates qualitatively the back-ground reduction which can be obtained even with an unrestricted coincidence requirement It is worth noting that a number of these peaks like the110mAg 657 keV line are barely discernible above the level of the continuum in the singles spectrum The75Se 400 keV g-ray comes from a noncoincident transition and is, therefore, effectively suppressed by the coincidence requirement However, in the case of the 65Zn 1115 keV peak, which is also noncoincident, the suppression is not perfect due to the presence of random coincidences between the abundant
65
Zn decays and all other decay events
The coincidence spectrum shown inFig 4represents the broadest coincidence requirement, that is any energy in coincidence with any other energy This picture was presented to illustrate some of the general features of the coincidence spectrometer In practice a much higher degree
of discrimination is obtained by setting a restrictive energy gate.Fig 5 shows a portion of the same singles spectrum given inFig 4, as well as the coincidence spectrum gated
on a narrow range of energies encompassing the 657 keV
Detector “A” Energy
536 keV 569 keV 640 keV
795 keV
801 keV
1 10
102
10 3
0 604 1167 1400 1969
801
563 604
569
795
Fig 3 A representative portion of a two-dimensional gg coincidence
spectrum for a sample containing 134 Cs A partial decay scheme
highlighting the relevant transitions is provided in the inset The vertical
and horizontal lines trailing from each peak are due to random
coincidences with Compton-scattered g-rays; the diagonal lines result
from backscattered g-ray coincidences.
1 10 100 1000
104
10 5
10 6
10 7
0
Energy [keV]
Fig 4 Upper spectrum: singles Lower spectrum: a total projection (i.e., a gate on all energies).
Trang 5peak Three coincident 110mAg peaks appear along with
134
Cs and 60Co peaks The 134Cs and 60Co g-rays are
present as Compton coincidences The gated gross 657 keV
peak includes Compton contributions from 1173, 1332,
and 1365 keV (134Cs) g-rays, so any g-ray which would
occur in coincidence with a full-energy event will also occur
in coincidence with these partial-energy scattered events
The characteristic of the gated spectrum which should be
highlighted is the high-degree of background reduction
that has been achieved In fact, it is the thorough
suppression of the background continuum that even makes
it possible to discern the 884 and 937 keV peaks, which
cannot be seen at all in the singles spectrum The example
of the 657 keV gate demonstrates qualitatively that the
NIST coincidence spectrometer is capable of the significant
background suppression that is beneficial in analyzing
trace elements, such as Ag in the bovine liver sample
As an adjunct to NAA, coincidence counting is of value
if it satisfies any of the following conditions: (i) if it can provide an analytical result with greater precision than conventional singles counting; (ii) if it can resolve the direct interference of two overlapping g-ray peaks; or (iii) if it can reveal the presence of an analyte which is below some minimum detectable activity limit in conventional NAA In the present work, the performance of the NIST coincidence spectrometer was evaluated based on the first criterion, since this arguably may have the widest impact Future analyses will investigate the capabilities with regard to the latter two criteria
In instrumental NAA the predominant contribution of uncertainty to a determined concentration is usually the uncertainty in the g-ray peak counting statistics [16], especially for very low analyte concentrations; therefore, the performance of the NIST cNAA spectrometer was evaluated on whether it could produce g-ray peak areas with smaller relative standard deviations than the corre-sponding singles g-ray peaks Other means of comparing coincidence and singles spectra, such as peak-to-back-ground ratios, which may be interpreted as a form of
‘‘signal-to-noise ratio’’ for g-ray spectra, have been presented by other authors [17]; however, while such measures often reveal significant enhancement of the specified ratio, they do not necessarily translate to any improvement in the analytical result This point is illustrated by the values in Table 1 One can see that across the board improvements in the peak-to-background ratios (A=b) are obtained in the coincidence-gated spectra relative to the singles spectra Examination of the relative standard deviations, however, reveals that in only two cases, i.e., Ag and Sc, is any enhancement obtained by the coincidence method
1
10
100
1000
10 4
105
106
107
600
Energy [keV]
Fig 5 Upper spectrum: singles Lower spectrum: a projection gated on
the110mAg 657 keV line.
Table 1
Counting statistics and calculated parameters for relevant coincidence peaks in bovine liver sample
657 (110mAg) 3436 238 475 26 497.2 0.13 25.7 69.4 84 10 0.71 117.6 13.7
884 ( 110m Ag) 4277 464 862 21 130.1 0.20 29.9 109 102 3 0.23 442.0 10.6
1173 (60Co) 870 129 22 597 836.9 1039.7 0.1 0.026 22 705 200 8.70 2611.1 0.7
1332 (60Co) 784 729 15 710 654.6 1198.8 0.1 0.020 22 534 252 10.08 2235.5 0.7
604 (134Cs) 84 757 317 184 26 432.0 3.21 1.4 3.74 2999 16 1.00 2999.0 1.9
795 ( 134 Cs) 59 103 367 007 33 364.3 1.77 2.0 6.21 3024 55 3.44 879.7 1.9
121 ( 75 Se) 413 361 2 247 271 249 696.8 1.66 0.7 5.44 34 531 1746 134.31 257.1 0.6
136 ( 75 Se) 1 370 003 2 206 599 220 659.9 6.21 0.2 1.61 139 536 5427 387.64 360.0 0.3
264 ( 75 Se) 1 032 122 907 182 75 598.5 13.7 0.2 0.88 141 726 3093 193.31 733.1 0.3
279 ( 75 Se) 489 549 712 424 64 765.8 7.56 0.4 1.46 35 655 953 59.56 598.6 0.6
192 ( 59 Fe) 74 706 890 447 127 206.7 0.59 2.1 11.9 2388 246 35.14 68.0 2.3
1099 ( 59 Fe) 537 278 140 343 8255.5 65.1 0.2 0.26 2696 75 3.75 718.9 2.1
Note: A ¼ net peak counts; B ¼ integral continuum counts; b ¼ avg continuum counts; and RSTD ¼ relative std dev of net peak counts.
Trang 6The failure of an increased peak-to-background ratio to
correlate with an improved peak area uncertainty is
explained by the significantly reduced peak counts in the
coincidence spectrum As the net area of a given peak is
reduced, eventually a point is reached where the
uncer-tainty due to the net counts is more significant than the
uncertainty contribution due to the continuum counts
Since coincidence counting efficiency is roughly equal to
the product of the efficiencies of each individual detector,
then it stands to reason that a coincidence spectrum will
always contain fewer counts than the corresponding singles
spectrum Thus, the application of coincidence counting to
NAA is only of value in situations where spectral
background can be reduced without too great a loss of
net peak counts In other words, coincidence detection
efficiency is as important as background reduction In the
examples ofTable 1, Sc and Ag started out with rather low
peak-to-background ratios and the background reduction
was more than enough to compensate for the smaller
coincidence peaks; the Co peaks, on the other hand, started
with much larger peak-to-background ratios, so even if the
background in the coincidence spectrum were reduced to
zero, the loss of peak counts would increase the relative
peak area uncertainties
One of the goals in evaluating the behavior of the cNAA
spectrometer was to establish, at least empirically, a means
of deciding when it is of value to use coincidence gating
Since peak-to-background increases do not necessarily
correlate with improved analytical results, an alternative
parameter was sought to predict the effectiveness of
coincidence counting for analytical purposes Using the
relation given in Eq (1) as the basis of a quantitative
benchmark, the expression in Eq (2) was derived:
ðsA=AÞcoincidence
where sAis the standard deviation of the net peak counts A,
and
Bs
2As
þ1
Ac
As
where As and Ac are net peak counts for singles and
coincidence spectra, respectively, and Bs is the integrated
continuum counts under the given singles peak Eq (2)
illustrates the contributions of both background reduction
(in the B=A term) and coincidence efficiency (in the Ac=As
term) towards enhanced precision, in agreement with the
general conclusion of Galloway[18] For the detectors and
source geometry presently employed, the quantity Ac=As
has been empirically determined to fall within the range of
0.03–0.05 Thus, it may be conjectured that, in the singles
spectrum, a background-to-peak ratio (Bs=As) of 40 or
more will yield a coincidence-gated peak with a lower
relative standard deviation Inspection of the B=A values
given in Table 1 reveals that this correlation is generally
borne out—small improvements in the relative standard deviations are observed for B=A around 10 and significant improvements when B=A is above 60 The ratio B=A provides a convenient means for evaluating the utility of coincidence gating, since a singles spectrum is acquired simultaneously with a coincidence spectrum and can be quickly analyzed before further analysis
Based on the analysis of the bovine liver material, it may
be concluded that in certain cases, as demonstrated by Ag and Sc, quantitative gg coincidence spectrometry as employed in the NIST coincidence spectrometer can be
an effective means of obtaining greater precision in NAA elemental determinations While greater precision is not the only reason to apply coincidence counting to NAA, it does potentially have a bearing on the greatest number of elements For the NIST spectrometer, the primary condi-tion which indicates that a given peak could benefit from coincidence counting is the presence of a very high background continuum as indicated by background-to-peak ratios determined from the corresponding singles spectrum In activated materials, primary contributors to the g-ray spectrum continuum include Bremsstrahlung and Compton scattering Sample matrices, such as biological materials, which are rich in P and Zn, tend to display high background levels due to relatively long-lived 32P Brems-strahlung and65Zn Compton events; therefore, cNAA may prove useful in the analysis of such materials
4 Conclusions
A gg coincidence spectrometer has been constructed at NIST, using two HPGe g-ray detectors and an all-digital data-acquisition system, for the purpose of exploring coincidence NAA and its value in characterizing reference materials The present work demonstrates the successful substitution of advanced digital signal processing for conventional analog electronics in quantitative gg coin-cidence spectrometry An initial evaluation of quantitative coincidence counting in comparison to singles spectro-metry, based upon a sample of neutron-irradiated bovine liver material, corroborated the feasibility of this approach
It has been determined that in this case only those peaks in the singles spectrum that have very large background-to-peak ratios display any enhancement in background-to-peak area uncertainties by means of coincidence gating A simple empirical criterion for identifying these g-ray peaks was deduced The value of coincidence counting in resolving direct g-ray peak interferences or in determining elements which fall below minimum detectable activity levels in conventional spectrometry will be considered in concert with future work on the characterization of reference materials by gg coincidence NAA
Disclaimer Contribution of the National Institute of Standards and Technology, not subject to copyright in the United States
Trang 7[1] J.I Kim, A Speecke, J Hoste, Anal Chim Acta 33 (1965) 123.
[2] E.T Bramlitt, Anal Chem 38 (1966) 1669.
[3] W.D Ehmann, D.M McKown, Anal Lett 2 (1969) 49.
[4] M Vobecky´, et al., Anal Chim Acta 386 (1999) 181.
[5] J Jaku˚bek, et al., Nucl Instr and Meth A 414 (1998) 261.
[6] Y Hatsukawa, et al., J Radioanal Nucl Chem 272 (2007) 273.
[7] H Huber, Ch Koeberl, I McDonald, W.U Reimold, J Radioanal.
Nucl Chem 244 (2000) 603.
[8] J.A Cooper, Anal Chem 43 (1971) 838.
[9] W Hennig, et al., Nucl Instr and Meth B 263 (2007) 175.
[10] W.K Warburton, M Momayezi, B Hubbard-Nelson, W Skulski,
Appl Radiat Isot 53 (2000) 913.
[11] W Skulski, M Momayezi, B Hubbard-Nelson, P Grudberg,
J Harris, W Warburton, Acta Phys Pol B 31 (2000) 47.
[12] M Bolic´, V Drndarevic´, Nucl Instr and Meth A 482 (2002) 761 [13] B Hubbard-Nelson, M Momayezi, W.K Warburton, Nucl Instr and Meth A 422 (1999) 411.
[14] G Knoll, Radiation Detection and Measurement, third ed., Wiley, New York, 2000.
[15] R Brun, F Rademakers, Nucl Instr and Meth A 389 (1997) 81 [16] E.G Moreira, M.B.A Vasconcellos, M Saiki, J Radioanal Nucl Chem 269 (2006) 377.
[17] P.P Ember, T Belgya, G.L Molna´r, Appl Radiat Isot 56 (2002) 535.
[18] B.B Galloway, Nucl Instr and Meth 55 (1967) 29.