© ISO 2012 Dosimetry for exposures to cosmic radiation in civilian aircraft — Part 1 Conceptual basis for measurements Dosimétrie pour l’exposition au rayonnement cosmique à bord d’un avion civil — Pa[.]
General
2.1.1 calibration operation that, under specified conditions, establishes a relation between the conventional quantity, H 0 , and the indication, G
A calibration can be represented through various forms, including a statement, calibration function, diagram, curve, or table It may also involve an additive or multiplicative correction to the measurement indication, accompanied by the associated measurement uncertainty.
Note 2 to entry: Calibration should not be confused with adjustment of a measuring system, often mistakenly called “self-calibration”, or with verification of calibration.
Note 3 to entry: Often, the first step alone in the above definition is perceived as being calibration.
The N coefficient represents the ratio of the conventional quantity value to the corrected indication of the instrument It is important to note that the calibration coefficient is calculated by multiplying the calibration factor by the instrument constant Additionally, the reciprocal of the N coefficient serves as the response of the instrument.
For the calibration of certain instruments, such as ionization chambers, the instrument constant and calibration factor are combined and referred to as the calibration coefficient.
To prevent confusion, it is essential to specify the quantity being measured, such as the calibration coefficient for fluence (\$N_{\Phi}\$), kerma (\$N_{K}\$), or absorbed dose (\$N_{D}\$).
Gquantity value provided by a measuring instrument or a measuring system
Note 1 to entry: An indication can be presented in visual or acoustic form or can be transferred to another device
Indicators can be represented by various forms, such as the position of a pointer on an analogue display, a numerical value shown or printed for digital outputs, a specific code pattern for coded outputs, or a designated quantity for material measurements.
Note 2 to entry: An indication and a corresponding value of the quantity being measured are not necessarily values of quantities of the same kind.
2.1.4 reference conditions conditions of use prescribed for testing the performance of a detector assembly or for comparison of results of measurements
Note 1 to entry: The reference conditions represent the values of the set of influence quantities for which the calibration result is valid without any correction.
The measurand's value can be selected based on the characteristics of the detector assembly being calibrated While the quantity measured is not an influence quantity, it can affect the calibration outcome and the detector's response.
Rquotient of the indication, G, or the corrected indication, G corr , and the conventional quantity value to be measured
To prevent confusion, it is essential to clarify which quotient from the definition of the response (either to G or to G corr) is being utilized Additionally, it is important to specify the quantity being measured, such as the response concerning fluence (R Φ), kerma (R K), or absorbed dose (R D).
Note 2 to entry: The reciprocal of the response under the specified conditions is equal to the calibration coefficient N coeff
The response value of the detector assembly can change depending on the magnitude of the quantity being measured, indicating a non-constant response.
Note 4 to entry: The response usually varies with the energy and direction distribution of the incident radiation
It is, therefore, useful to consider the response as a function, R(E,Ω), of the radiation energy, E, and of the direction,
The incident monodirectional radiation is characterized by two key responses: R(E), which indicates energy dependence, and R(Ω), which reflects angle dependence The angle Ω can be defined as the angle α between the reference direction of the detector assembly and the direction of the external monodirectional field.
Quantities and units
2.2.1 particle fluence fluence Φnumber, dN, at a given point of space, of particles incident on a small spherical domain, divided by the cross-sectional area, da, of that domain: Φ =d d
Note 1 to entry: The unit of the fluence is m −2 ; a frequently used unit is cm −2
The energy distribution of particle fluence, denoted as \$\Phi_E\$, is defined as the quotient of the differential fluence, \$d\Phi\$, by the differential energy, \$dE\$, representing the fluence of particles with energy between \$E\$ and \$E + dE\$ Similarly, the direction distribution of particle fluence is represented as \$\Phi_\Omega\$ The complete expression for the double differential particle fluence is given by \$\Phi_{E,\Omega}(E,\Omega)\$, where the subscripts indicate the variables for differentiation and the symbols in brackets specify the values of these variables For instance, the energy distribution at a specific energy \$E = E_0\$ is expressed as \$\Phi_E(E_0)\$ If no specific values are provided, the brackets can be omitted.
2.2.2 particle fluence rate fluence rate Φ rate of the particle fluence expressed as Φ= Φ ⋅ d d d t d d
2 where dΦ is the increment of the particle fluence during an infinitesimal time interval with duration dt
Note 1 to entry: The unit of the fluence rate is m −2 s −1 , a frequently used unit is cm −2 s −1
The energy imparted (\(\varepsilon\)) for ionizing radiation within a specified three-dimensional domain is calculated as the sum of the energies deposited in individual interactions This can be expressed as \(\varepsilon = \sum \varepsilon_i\), where \(\varepsilon_i\) represents the energy deposited during a single interaction The energy deposited in each interaction is determined by the equation \(\varepsilon_i = \varepsilon_{\text{in}} - \varepsilon_{\text{out}} + Q\) Here, \(\varepsilon_{\text{in}}\) is the energy of the incident ionizing particle, excluding its rest energy, while \(\varepsilon_{\text{out}}\) is the total energy of all ionizing particles that exit the interaction, also excluding rest energy.
Q is the change in the rest energies of the nucleus and of all particles involved in the interaction
Note 1 to entry: Energy imparted is a stochastic quantity.
Note 2 to entry: The unit of the energy imparted is J.
2.2.4 mean energy imparted εmean energy imparted to the matter in a given domain, expressed as ε = R in − R out +∑ Q © ISO 2012 – All rights reserved ``,,,``,,`,```,,,,`,```,```,,,-`-`,,`,,`,`,,` - 3 where
R in is the radiant energy of all those charged and uncharged ionizing particles that enter the domain,
R out is the radiant energy of all those charged and uncharged ionizing particles that leave the domain, and
∑Q is the sum of all changes of the rest energy of nuclei and elementary particles that occur in that domain
Note 1 to entry: This quantity has the meaning of expected value of the energy imparted.
Note 2 to entry: The unit of the mean energy imparted is J.
2.2.5 specific energy imparted specific energy zfor any ionizing radiation, z = ε m where ε is the energy imparted to the irradiated matter, m is the mass of the irradiated matter
Note 1 to entry: Specific energy imparted is a stochastic quantity.
In a small domain, the mean specific energy imparted is equivalent to the absorbed dose This specific energy can arise from one or multiple energy-deposition events The unit for measuring specific energy is joules per kilogram (J⋅kg⁻¹), commonly referred to as gray (Gy).
D=dm d ε where dε is the mean energy imparted by ionizing radiation to an element of irradiated matter of mass dm, where ε =∫ D m d
Note 1 to entry: In the limit of a small domain, the mean specific energy is equal to the absorbed dose.
Note 2 to entry: The unit of absorbed dose is J kg −1 , with the special name gray (Gy).
For uncharged particles that indirectly ionize, the average total of the initial kinetic energies (\$dE_{tr}\$) of all charged ionizing particles generated by these uncharged particles in a specific mass element (\$dm\$) of matter is calculated.
Note 1 to entry: Quantity d E tr includes the kinetic energy of the charged particles emitted in the decay of excited atoms or molecules or nuclei.
Note 2 to entry: The unit of kerma is J kg −1 , with the special name gray (Gy).
2.2.8 unrestricted linear energy transfer linear energy transfer
For an ionizing charged particle, the mean energy, denoted as \( dE \Delta \), is defined as the energy imparted locally to matter along a small path, minus the total kinetic energies of all electrons released with energies exceeding \( \Delta \), all divided by the path length \( dl \).
The quantity in question remains undefined without specifying Δ, which represents the maximum kinetic energy of secondary electrons considered to be "locally deposited." Δ can be expressed in electronvolts (eV).
Note 2 to entry: Linear energy transfer is often abbreviated LET, but to which should be appended the subscript Δ or its numerical value.
Note 3 to entry: The unit of the linear energy transfer is J m −1 , a frequently used unit is keV μm −1
If no energy cut-off is applied, the unrestricted linear energy transfer, denoted as L ∞, is equivalent to the linear electronic stopping power, S el, and can be simply referred to as L.
Hat the point of interest in tissue,
Q is the quality factor at that point, and
Q is defined by the unrestricted linear energy transfer (LET) of charged particles traversing a small volume element The LET value is typically provided for charged particles in water, with minimal differences when considering tissue The dose equivalent at a specific point in tissue is calculated using the formula where \( D_L = \frac{dD}{dL} \) represents the absorbed dose distribution in relation to LET at that point.
Note 2 to entry: The relationship of Q and L is given in ICRP Publication 103 (ICRP, 2007) [2]
Note 3 to entry: The unit of dose equivalent is J kg −1 , with the special name sievert (Sv).
2.2.10 single-event dose-mean specific energy dose-mean specific energy per event z D expectation z D= zd z z
( )d where d 1 (z)is the dose probability density of z
The dose probability density of z is represented by \( d_1(z) \), indicating the fraction of the absorbed dose delivered in individual events with specific energies within the range from \( z \) to \( z + dz \).
2.2.11 lineal energy yquotient of the energy, ε s , imparted to the matter in a given volume by a single energy deposition event, by the mean chord length, l, in that volume: y=εl s
Note 1 to entry: The unit of lineal energy is J m −1 , a frequently used unit is keV μm −1
2.2.12 dose-mean lineal energy y D expectation y D= y d y y( ) d
0 where d(y)is the dose probability density of y
The dose probability density of \( y \), denoted as \( d(y) \), represents the fraction of absorbed dose delivered in single events with lineal energy within the range from \( y \) to \( y + dy \).
Note 2 to entry: Both the dose-mean lineal energy and distribution d ( y ) are independent of the absorbed dose or dose rate.
The H*(10) dose equivalent in a radiation field is defined as the value that would be generated by the corresponding expanded and aligned field within the ICRU sphere at a depth of 10 mm, specifically on the radius opposite to the direction of the aligned field It is important to note that the unit of ambient dose equivalent is joules per kilogram (J kg\(^{-1}\)), which is commonly referred to as the sievert (Sv).
2.2.14 particle-fluence-to-ambient-dose-equivalent conversion coefficient h∗ Φ quotient of the particle ambient dose equivalent, H*(10), and the particle fluence, Φ: h∗ = ∗ Φ H Φ
Note 1 to entry: The unit of the particle-fluence-to-ambient-dose-equivalent conversion coefficient is J m 2 kg −1 with the special name Sv m 2 , a frequently used unit is pSv cm 2
X v mass of a unit-area column of air above a point in the atmosphere
Note 1 to entry: The unit of atmosphere depth is kg m −2 ; a frequently used unit is g cm −2
Standard barometric altitude refers to the altitude measured by a barometric altimeter that is calibrated according to the International Standard Atmosphere (ISA), as defined in ISO 2533 This measurement is taken when the altimeter's reference datum is set to 1,013.25 hPa.
Flight levels, such as FL 350, indicate pressure altitude in multiples of 100 feet based on the International Standard Atmosphere (ISA) and a datum setting of 1,013.25 hPa In certain countries, flight levels are expressed in meters, necessitating appropriate conversions before utilizing the data provided in this International Standard.
Pmomentum per charge (of a particle in a magnetic field), given by
P p=Ze where p is the particle momentum,
Z is the number of charges on the particle, and e is the charge on the proton
Note 1 to entry: The base unit of magnetic rigidity is the tesla metre (T⋅m) ( = V⋅m –1 ⋅s) A frequently used unit is
In a unit system where the speed of light, \( c \), and the charge of the proton, \( e \), are both set to 1, the magnetic rigidity is defined as \( \frac{pc}{Ze} \).
Magnetic rigidity defines the paths of charged particles within magnetic fields, indicating that particles with the same magnetic rigidity will follow identical trajectories regardless of their mass or charge.
2.2.18 geomagnetic cut-off rigidity cut-off rigidity r c minimum magnetic rigidity an incident particle can have and still penetrate the geomagnetic field to reach a given location above the Earth
Atmospheric radiation field
Cosmic radiation consists of high-energy particles, primarily fully ionized atoms, originating from extraterrestrial sources These cosmic rays interact with the atmosphere and other matter, generating additional particles and ionizing radiation.
2.3.2 primary cosmic radiation primary cosmic rays cosmic radiation incident from space at the Earth’s orbit
2.3.3 secondary cosmic radiation secondary cosmic rays cosmogenic particles particles which are created directly or in a cascade of reactions by primary cosmic rays interacting with the atmosphere or other matter
Key particles relevant to radiation protection and measurements in aircraft include neutrons, protons, photons, electrons, positrons, and muons, with pions and nuclear ions heavier than protons being of lesser significance.
2.3.4 galactic cosmic radiation galactic cosmic rays
GCRcosmic radiation originating outside the solar system
2.3.5 solar particles solar cosmic radiation solar cosmic rays cosmic radiation originating from the sun
SPElarge fluence rate of energetic solar particles ejected into space by a solar eruption
Note 1 to entry: Solar particle events are directional.
A sudden increase in cosmic radiation has been detected on the ground, with at least two neutron monitor stations recording a simultaneous rise of over 1% in the five-minute-averaged count rate linked to solar energetic particles.
Note 1 to entry: A GLE is associated with a solar-particle event having a high fluence rate of particles with high energy (greater than 500 MeV).
Note 2 to entry: GLEs are relatively rare, occurring on average about once per year GLEs are numbered; the first number being given to that occurring in February 1942.
2.3.8 solar modulation change of the GCR field (outside the Earth’s magnetosphere) caused by change of solar activity and consequent change of the magnetic field of the heliosphere
2.3.9 solar cycle period during which the solar activity varies with successive maxima separated by an average interval of about 11 years
The reversal of the Sun's magnetic field polarity every 11 years contributes to a longer solar cycle, averaging approximately 22 years, known as the Hale cycle.
The sunspot cycle, indicated by the Wolf number, typically lasts around 11 years, though it can range from 7 to 17 years This approximate 11-year cycle is also observed in geomagnetism, aurora frequency, and various ionospheric traits Notably, the u index of geomagnetic intensity variation exhibits a strong correlation with solar activity.
The Wolf number quantifies sunspot activity using the formula \( k(10g + f) \), where \( f \) represents the count of individual sunspots, \( g \) denotes the number of sunspot groups, and \( k \) is a variable factor influenced by the observer's experience and the specific observatory's location and equipment.
2.3.11 solar maximum time period of maximum solar activity during a solar cycle, usually defined in terms of relative sunspot number
2.3.12 solar minimum time period of minimum solar activity during a solar cycle, usually defined in terms of relative sunspot number © ISO 2012 – All rights reserved ``,,,``,,`,```,,,,`,```,```,,,-`-`,,`,,`,`,,` - 9
2.3.13 cosmic ray neutron monitor ground level neutron monitor cosmic radiation neutron monitor
The GLNMlarge detector is designed to measure the time-dependent relative fluence rate of high-energy cosmic radiation, specifically focusing on the secondary neutrons produced in the atmosphere Additionally, it is capable of detecting protons, other hadrons, and muons.
Cosmic radiation neutron monitors are installed globally at various locations and altitudes, including on ships and aircraft, to conduct studies on cosmic radiation and assess solar modulation.
General description of the cosmic radiation field in the atmosphere
Galactic cosmic radiation (GCR) and energetic solar particles interact with atmospheric atomic nuclei, leading to a cascade of secondary reactions that result in cosmic radiation exposure, which diminishes from aviation altitudes to sea level GCR can reach energies up to \$10^{20}\$ eV, although lower energy particles are more common Upon penetrating the solar system's magnetic field, GCR exhibits a peak energy distribution between a few hundred MeV and 1 GeV per nucleon, influenced by solar magnetic activity The energy spectrum follows a power function of \$E^{-2.7}\$ eV up to \$10^{15}\$ eV, after which it steepens to \$E^{-3}\$ eV The fluence rate of GCR entering the solar system remains relatively constant over time, with these energetic ions approaching Earth isotropically.
The Earth's and sun's magnetic fields influence the ratio of galactic cosmic ray (GCR) protons and heavier ions that reach the atmosphere During periods of low geomagnetic cut-off and solar activity, the GCR ion composition consists of about 90% protons, 9% helium ions, and 1% heavier nuclei At a vertical cut-off of 15 GV, this composition shifts to approximately 83% protons, 15% helium ions, and nearly 2% heavier ions.
The variation of ambient dose equivalent due to different secondary cosmic radiation components in the atmosphere changes with altitude, as depicted in Figure 1 At sea level, muons are the primary contributors to ambient dose equivalent and effective dose However, at aviation altitudes, neutrons, electrons, positrons, protons, photons, and muons become the most significant components Additionally, at higher altitudes, nuclear ions heavier than protons begin to play a role Representative normalized energy distributions of fluence rates for these key particles at various altitudes and solar activity levels are presented in Annex A.
The Earth experiences bursts of energetic protons and heavier particles from solar magnetic disturbances and coronal mass ejections (CMEs), which can accelerate these particles These solar particle events (SPEs) typically have lower energy levels, generally below 100 MeV, and are short-lived, lasting from a few hours to a few days On average, only one SPE per year results in significant high-energy particles, known as ground-level events (GLEs), which can be detected by neutron monitors and cause notable dose rates at high altitudes For aircraft crews, the cumulative dose from galactic cosmic rays (GCR) is significantly higher than that from SPEs, although intense SPEs can influence GCR dose rates by altering the Earth's magnetic field and affecting the intensity of galactic particles reaching the atmosphere.
Y ambient dose equivalent rate (μSv/h)
Conditions: 1 GV cut-off and solar minimum (deceleration potential, ϕ, of 465 MV) [9]
Figure 1 — Calculated ambient dose equivalent rates as function of standard barometric altitude for high latitudes at solar minimum for various atmospheric cosmic radiation component particles
General calibration considerations for the dosimetry of cosmic radiation fields
The general approach necessary for measurement and calibration is given here Details of calibration fields and procedures are given in ISO 20785-2.
Ambient dose equivalent cannot be directly measured using conventional dosimetric techniques, making its experimental determination in complex radiation fields particularly challenging An approximate method involves using a tissue equivalent proportional counter (TEPC) to assess the dose equivalent for a small mass of tissue by measuring the absorbed dose distribution in lineal energy, which serves as an approximation for linear energy transfer (LET) Although corrections are applied and the LET-dependent quality factor is used, this approach still does not accurately capture the ambient dose equivalent.
Measuring and calculating the radiation field dosimetry in aircraft necessitates specialized techniques The ideal method involves using devices that provide an ambient dose equivalent response unaffected by the energy and direction of the total field or the specific field component being assessed Typically, corrections must be applied based on energy and directional data.
``,,,``,,`,```,,,,`,```,```,,,-`-`,,`,,`,`,,` - characteristics of the field and the energy and angle ambient dose equivalent response characteristics of the device.
3.2.3 Considerations concerning the radiation field
The field primarily consists of photons, electrons, positrons, muons, protons, and neutrons, with minimal dose equivalent contribution from energetic primary heavy charged particles (HZE) or their fragments Electrons, positrons, and muons are forms of directly ionizing radiation, while indirectly ionizing photons and secondary electrons interact with matter through the electromagnetic force Neutrons, along with a minor contribution from pions, engage via the strong interaction, generating directly ionizing secondary particles Protons exhibit both direct ionization through the electromagnetic force and indirect ionization via neutron-like strong interactions.
The non-neutron component consists of directly ionizing particles and secondary electrons generated by indirectly ionizing photons, while the neutron component includes neutrons and neutron-like interactions of protons For dosimetric purposes, the field is categorized into low-LET (less than 10 keV/μm) and high-LET (10 keV/μm or greater) components, reflecting the quality factor's dependence on LET, which is unity below 10 keV/μm This classification can be applied to TEPCs and other detectors, although the low-LET/high-LET threshold may range from 5 to 10 keV/μm The low-LET component encompasses directly ionizing electrons, positrons, muons, secondary electrons from photon interactions, and energy deposition from protons and secondary particles In contrast, the high-LET component arises from short-range secondary particles resulting from strong interactions of protons and neutrons While the contributions of low-LET and non-neutron components, as well as high-LET and neutron components, to the total ambient dose equivalent may differ, they are generally comparable in magnitude.
The ambient dose equivalent, crucial for radiation measurements, can be effectively estimated using calibrated tissue equivalent proportional counters, recombination ionization chambers, or semiconductor spectrometers Low-LET or non-neutron energy deposition is measurable with ionization chambers, silicon-based detectors, scintillation detectors, or passive luminescence detectors In contrast, high-LET or neutron components can be assessed using extended range neutron survey meters, multi-sphere spectrometers, or passive detectors like etched track, bubble, or fission foil detectors By summing the low-LET and high-LET components, along with proper calibration, the total ambient dose equivalent is obtained It is vital that these instruments are fully characterized at national standards laboratories to ensure traceability in measuring complex radiation fields.
Calibration and usage of measurement devices are defined in various ISO and ICRU documents, such as ISO 4037-3, ISO 8529-3, ISO 20785-2, and ICRU Report 66:2001 Determining uncertainties in measurement is crucial in dosimetry, as these uncertainties are often not statistically independent Even when they are independent, the overall uncertainty is not merely the root mean square of individual uncertainties; it is influenced by the measurement and analysis procedures Further details can be found in ISO/IEC Guide 98-3.
At aviation altitudes, the ambient dose equivalent is primarily influenced by neutrons (ranging from a few hundred keV to a few GeV), protons (from a few tens of MeV to a few GeV), and other particles such as electrons, positrons, and photons (also from a few MeV to a few GeV) To accurately assess the response characteristics of devices measuring ambient dose equivalent in cosmic radiation fields, testing should ideally be conducted using ISO reference radiations However, these ISO standards do not encompass the full energy spectrum of photons, neutrons, and electrons, which are crucial for a comprehensive understanding of total ambient dose equivalent Consequently, there is a need for additional calibration fields, including proton radiation fields for certain devices.
To assess the response characteristics of devices like the tissue equivalent proportional counter (TEPC) to high-energy low-LET radiation where reference fields are lacking, measurements and calculations reveal that the energy deposition distribution in the device's sensitive volume closely resembles that of the ISO high-energy photon reference field This is crucial for addressing challenges related to establishing the low-LET threshold for TEPCs and similar devices Quasi-monoenergetic neutron fields are available for energies up to approximately 200 MeV For evaluating neutron response characteristics at higher energies, measurements can be conducted using mono-energetic proton beams alongside calculations, or within broad energy distribution neutron fields, also supplemented by calculations.
In non-ISO fields, it is essential to employ a traceable technique for measuring particle fluence, which can then be converted to ambient dose equivalent using appropriate fluence-to-dose conversion factors.
CERN has developed the CERF (CERN-EU high-energy Reference Field facility) to facilitate instrument response measurements and inter-comparisons in a simulated cosmic radiation neutron field This facility, created in collaboration with the European Commission, generates neutron fields using high-energy proton and pion beams with momenta of 120 GeV/c or 205 GeV/c directed at a copper target The setup includes substantial concrete shielding at the beam's target positions, with additional iron or concrete shields above, ensuring that the areal mass of the 80 cm concrete shields is comparable to the air layer at altitudes between 10 km and 15 km Well-characterized neutron fields are present both beside the target area and on the roof shields, with the neutron component and other hadrons calculated using the Monte Carlo code FLUKA Although multi-sphere spectrometry measurements have been conducted, the metrology of the field currently lacks traceability to national standards.
3.2.5.2 Cosmic radiation fields on mountains
Cosmic radiation fields at high elevations closely resemble those found in aircraft, but differences exist that may impact instrument evaluations Even at altitudes of 4 km, the composition and spectral fluence of cosmic radiation on the ground differ from that at aviation altitudes, with a higher fraction of dose from muons at lower elevations The neutron spectrum on the ground contains more high-energy neutrons above 10 MeV, fewer neutrons in the 1 eV to 2 MeV range, and an increased presence of thermal energy neutrons Additionally, various ground materials—such as soil, water, snow, and concrete—scatter neutrons differently, influencing the neutron spectrum's shape.
Conversion coefficients
The conversion coefficients from fluence to ambient dose equivalent vary based on the type and energy of the particles involved Established data for energies up to 20 MeV are included in international guidelines from organizations such as ISO, IEC, and ICRU A comprehensive compilation of these conversion factors for all relevant particles and energy levels is available in reference [29].
Introduction
Detectors used for measuring ambient dose equivalent on aircraft are akin to those found in accelerator laboratories These devices can be classified as active detectors.
``,,,``,,`,```,,,,`,```,```,,,-`-`,,`,,`,`,,` - or passive, or by the component of the field measured (see, for example, Reference [30] ) This standard gives the basis for instruments’ use in the determination of ambient dose equivalent.
Active devices
4.2.1 Devices to determine all field components
The two main types of energy deposition spectrometers are gas-filled devices, in particular tissue equivalent proportional counters (TEPCs), and solid state (normally silicon) devices.
A tissue equivalent proportional counter (TEPC) is designed to detect both directly and indirectly ionizing particles by utilizing a gas with a chemical composition similar to tissue, simulating a biological site at low pressure Typically cylindrical, TEPCs collect electrons generated by incident radiation on a central anode when an electric field is applied Each detected event produces an output signal proportional to the initial energy deposited, which is analyzed through pulse height analysis to create a lineal energy distribution spectrum, denoted as d(y) This spectrum approximates the linear energy transfer (LET), while the absorbed dose is calculated by dividing the total deposited energy by the mass of the gas Additionally, the dose equivalent can be determined by incorporating the quality factor into the absorbed dose distribution.
Ambient dose equivalent is calibrated using reference fields, but when assessing cosmic radiation, a correction for the device's lower-LET threshold is typically required Additionally, an internal alpha particle source, such as 244 Cm, can be utilized for calibration in terms of y For further details, refer to References [31] and [32].
4.2.1.1.2 Solid-state energy deposition spectrometers
Solid-state energy deposition spectrometers are utilized to measure the energy deposited in silicon detectors, either through a single detector that records pulse height distribution or multiple detectors with varying LET thresholds The total dose and its distribution in LET can be correlated with the dose and dose equivalent to tissue, enabling the determination of ambient dose equivalent through appropriate characterization and calibration.
4.2.1.2 Devices based on the determination of absorbed dose and mean quality factor
Dose equivalent can be assessed when a device measures both the absorbed dose and the mean quality factor in a radiation field This method encompasses the use of a TEPC functioning in variance/co-variance mode, as well as the recombination chamber.
The variance method is a technique that utilizes a TEPC or other microdosimetric detectors to measure the dose-mean lineal energy (\$y_D\$) of a radiation field without the need to analyze the \$y\$ spectrum This approach involves assessing the fluctuations in specific energy values as energy is deposited by multiple independent particles within equal time intervals The single-event dose-mean specific energy is calculated as the product of the relative variance of the measurements and their mean, where the mean represents the absorbed dose in the detector.
In environments with pulsed or strongly varying radiation fields, variance corrections can be achieved through the use of dual detectors or by analyzing the variance of sequential measurements For broader applications, a mean quality factor is derived from a linear function of the variable \( y_D \), with the slope and intercept coefficients established through instrument characterization across different photon and neutron conditions.
In certain applications, such as cosmic radiation measurements, single high-LET events exceeding 150 keV/μm can be identified within the multiple-event spectrum A quality factor for this fraction can be established in accordance with ICRP 60 guidelines.
The recombination chamber leverages the relationship between local ionization density and the initial recombination of ions within the gas cavity of the ionization chamber, which is influenced by the linear energy transfer (LET) and provides insights into the radiation quality of the fields being studied The saturation current in the recombination chamber is directly proportional to the total absorbed dose (D) By measuring the ionization current at a specifically selected "recombination" voltage, it is possible to determine a recombination index that reflects radiation quality, leading to the assessment of dose equivalent and, following proper characterization and calibration, the ambient dose equivalent.
The integration of ionization chamber data with scintillation detector measurements in current mode follows the same principle as the recombination method The ionization chamber quantifies the absorbed dose, while a mean quality factor is established by assessing the reduction in scintillation yield of organic scintillators as the linear energy transfer (LET) of energy-transferring particles increases.
Organic and inorganic scintillators are effective in detecting photons and charged particles due to their high detection efficiency Large-volume organic scintillators, rich in hydrogen and carbon, serve as neutron detectors, with their detection efficiency influenced by the material's thickness While small scintillation counters, typically around 1 cm thick, lack adequate efficiency for neutron dosimetry, larger organic scintillators, with thicknesses of several tens of centimeters, are suitable for spectrometric applications.
Scintillation counters used as dosemeters in low-energy photon fields require a specific arrangement of absorber materials to achieve a flat energy-dependent response in terms of H*(10) Despite being calibrated with reference photon fields, the readings in cosmic radiation fields can significantly differ from true values or those obtained from ionization chambers due to the limited energy range these devices measure High-energy charged particles can deposit energy exceeding a few MeV, resulting in reduced detection efficiency Therefore, it is essential to correct the dosemeter readings to account for this discrepancy.
4.2.2 Devices for low LET/non-neutron
Ionization chambers function by collecting electrons and ions generated by radiation in a gas, with each electron-ion pair requiring approximately 30 eV of energy An electric field between electrodes facilitates the collection of these charges, and the resulting current, measured by an electrometer, is proportional to the energy deposited by ionizing radiation Typically operated in current mode as DC devices, ionization chambers are often housed in airtight containers, usually spherical or cylindrical For enhanced sensitivity in low-dose-rate measurements, these chambers are filled with inert gas at higher pressures In mixed radiation scenarios, a common configuration features an 8-liter sensitive volume filled with argon at 2.5 MPa, constructed from welded stainless steel for durability.
Geiger Müller (GM) counters, commonly available on the market, are frequently used to measure low-LET radiation in aircraft These devices are user-friendly and deliver reliable results.
GM counters provide high statistical accuracy measurements of natural radiation levels up to high altitudes, effectively registering events from directly ionizing particles and photons, while showing minimal response to neutrons (less than 5%) When calibrated against a 60 Co standard radiation field, GM counters exhibit a response to cosmic radiation fields that is comparable to that of ionization chambers.
Real-time electronic personal dosemeters, primarily utilizing silicon-based detectors, are widely available, with one variant incorporating a gas-filled ion chamber paired with a semiconductor non-volatile memory cell These devices are mainly engineered to detect photon and beta radiations, with several featuring tissue-equivalent encapsulation suitable for assessing the low-LET component of radiation fields in aircraft Additionally, a select few are specifically designed to measure neutron fields up to certain levels.
10 MeV, either separately or as combined neutron–photon devices Such dosemeters would require a full characterization of the proton and high-energy neutron response before use.
4.2.3 Devices for high-LET/neutron component
Passive devices
Passive detectors effectively measure ambient dose equivalent over single or multiple flights, taking into account sensitivity and intrinsic background The primary passive devices for high-LET/neutron components include track etch detectors and superheated emulsions, also referred to as bubble damage or superheated drop detectors Both detector types are responsive to high-energy protons, with their expected contribution to the readings remaining minimal.
The ambient dose equivalent response of neutron passive detectors in aircraft typically ranges from 5% to 10%, which is lower than the ISO reference fields extending up to 20 MeV To minimize the significant uncertainty in measurements from single-track etch detectors, utilizing stacks is recommended Additionally, track detectors can effectively be employed with fission foils, provided they have adequate sensitivity.
Passive detectors for low-LET/non-neutron radiation include thermoluminescent (TL), optically stimulated luminescent (OSL), and photoluminescent (RPL) detectors Common materials used for onboard exposure level determination are LiF:Mg,Ti; LiF:Mg,Cu,P; Al2O3:C; CaSO4:Dy; and Al-P glasses Recent advancements in TL materials have enhanced sensitivity, enabling the measurement of ambient dose equivalents as low as 1 μSv It is also important to consider the response of these detectors to high-energy neutrons.
Secondary particles generated by charged particles in detector materials leave tracks that are recorded through chemical etching of damage trails For detailed properties of etched track detectors, refer to References [63] and [64] Poly allyl diglycol carbonate (PADC), commonly known as CR–39, is the primary material used for measuring galactic cosmic rays (GCR) Additionally, these detectors can also be utilized to determine particle spectra with linear energy transfer (LET) ranging from approximately 5 keV/μm to 10 keV/μm.
The absorbed dose distribution in water ranges from 500 keV/μm to 1,000 keV/μm, allowing for the measurement of linear energy transfer (LET) and the determination of dose equivalent By analyzing the dimensions of numerous tracks created by secondary particles, primarily protons, researchers can characterize track parameters in samples of track etch material exposed to proton and heavy charged-particle beams.
Polycarbonate is notable for its low background levels and reliable response, making it a valuable material in various applications It can be utilized in the form of bottles or test tubes as track detectors, including bubble detectors These polycarbonate bottles serve as effective backup detectors for bubble devices, particularly during intense solar flare events when the primary detectors may saturate Additionally, challenges related to limited response and low signal-to-noise ratios have been addressed by counting coincident tracks on matched surfaces of paired detectors.
Since 1970, the registration of holes created by fission fragments in thin plastic films, such as polyethylene terephthalate and polycarbonate, has been utilized for neutron detection and dosimetry Neutron-induced fission cross-sections for heavy nuclei like \$^{235}U\$, \$^{238}U\$, \$^{232}Th\$, and \$^{209}Bi\$ are recommended as secondary standards for monitoring neutron flux above 20 MeV Among these, \$^{209}Bi\$ is particularly valuable for high-energy neutron studies due to its lack of reactions with low-energy neutrons, smooth cross-section variation with neutron energy, and non-radioactive nature, which simplifies its use and transport Various fission-fragment detectors, including fission chambers, thin film breakdown counters, and fission-fragment damage track detectors, can effectively exploit the advantages of fission reactions for neutron detection and dosimetry Although low sensitivity can limit the application of fission-type detectors in high-energy scenarios, advanced spark counters and specialized counting procedures can mitigate this issue, even for low fission cross-sections like that of bismuth.
4.3.4 Superheated emulsion neutron detectors (bubble) detectors
Superheated emulsion neutron detectors, also known as neutron bubble detectors, are effective passive devices for monitoring neutron radiation at aircraft altitudes They provide direct readings and are completely insensitive to gamma radiation, capable of estimating neutron ambient dose equivalents as low as 1 μSv These detectors consist of a small polycarbonate bottle filled with superheated liquid droplets within a visco-elastic medium When neutrons interact with the detector, they can generate recoil charged particles that lead to bubble nucleation and growth The detector's performance is influenced by the temperature and pressure of the host medium, but normal aircraft pressure variations do not significantly affect sensitivity Additionally, the detector can be reset by applying pressure to the visco-elastic medium, allowing it to be reused for measurements Temperature compensation can be achieved by adding an expandable material on top of the gel, and detectors can be designed with varying sensitivities.
When using sets of detectors with differing energy dependences of response, an estimation of neutron fluence energy distribution spectra can be made.
Thermoluminescence, or radiothermoluminescence, refers to the light emitted when irradiated crystalline materials are heated, as they store energy in metastable "traps." Thermoluminescent detectors (TLDs) are primarily effective in measuring low LET or non-neutron radiation from directly ionizing radiation and secondary particles While some materials and techniques can estimate high-LET or neutron components, TLDs typically focus on low-LET measurements, introducing uncertainty regarding neutron responses Neutrons account for approximately 50% of the ambient dose equivalent in the GCR field on aircraft, yet only contribute about 10% to the total absorbed dose in tissue Additionally, TLDs exhibit lower neutron kerma compared to tissue, and their relative light conversion efficiency for secondary charged particles is generally low, limiting unwanted neutron response to about 5% of total H*(10) Utilizing multiple detectors can help reduce the uncertainty in TLD measurements.
High-sensitivity materials used in TL dosimetry face a significant drawback due to their dependence on heating rates, which leads to thermal quenching of luminescence efficiency This issue can be addressed by employing photoluminescent detectors (PLDs), such as radiophotoluminescent (RPL) glass or optically stimulated luminescence (OSL) detectors like Al₂O₃:C These detectors utilize laser light to stimulate the radiation-induced luminescence signal, and various effective photoluminescence readout modes have been developed However, PLDs exhibit low light conversion efficiencies when dealing with high-LET energy deposition.
Representative particle fluence rate energy distributions for the cosmic radiation field at flight altitudes for solar minimum and maximum conditions and for minimum and maximum vertical cut- off rigidity [80]
Figures A.1 to A.6 illustrate the calculated energy distributions of particle fluence rates for neutrons, protons, pions, electrons, photons, and muons under extreme conditions of solar activity, geomagnetic cut-off, and altitude relevant to civilian aircraft Each figure presents the particle fluence rates, denoted as \$\frac{d^2 \Phi}{dt \cdot dE}\$ (cm\$^{-2}\$ s\$^{-1}\$ GeV\$^{-1}\$), which represent the fluence per time interval \$dt\$ and energy interval \$dE\$ These rates are multiplied by the energy \$E\$ and normalized to the energy-integrated fluence rates, with normalization factors provided for each curve in the figures.
Y normalized fluence rate of neutrons
Figure A.1 — Normalized energy distribution of neutron fluence rate
Y normalized fluence rate of protons
Figure A.2 — Normalized energy distribution of proton fluence rate
Y normalized fluence rate of pions
Figure A.3 — Normalized energy distribution of fluence rate of charged pions © ISO 2012 – All rights reserved 21
Y normalized fluence rate of electrons
Figure A.4 — Normalized energy distribution of electron fluence rate
Y normalized fluence rate of photons
Figure A.5 — Normalized energy distribution of photon fluence rate
Y normalized fluence rate of muons
Figure A.6 — Normalized energy distribution of muon fluence rate © ISO 2012 – All rights reserved 23
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