ASTM E1894-24

Standard Guide for Selecting Dosimetry Systems for Application in Pulsed X-Ray Sources

SIGNIFICANCE AND USE
4.1 Flash X-ray facilities provide intense bremsstrahlung radiation environments, usually in a single sub-microsecond pulse, which often fluctuates in amplitude, shape, and spectrum from shot to shot. Therefore, appropriate dosimetry must be fielded on every exposure to characterize the environment, see ICRU Report 34. These intense bremsstrahlung sources have a variety of applications which include the following:
(1) Studies of the effects of X-rays and gamma rays on materials.
(2) Studies of the effects of radiation on electronic devices such as transistors, diodes, and capacitors.
(3) Computer code validation studies.  
4.2 This guide is written to assist the experimenter in selecting the needed dosimetry systems for use at pulsed X-ray facilities. This guide also provides a brief summary on how to use each of the dosimetry systems. Other guides (see Section 2) provide more detailed information on selected dosimetry systems in radiation environments and should be consulted after an initial decision is made on the appropriate dosimetry system to use. There are many key parameters which describe a flash X-ray source, such as dose, dose rate, spectrum, pulse width, etc., such that typically no single dosimetry system can measure all the parameters simultaneously. However, it is frequently the case that not all key parameters must be measured in a given experiment.
SCOPE
1.1 This guide provides assistance in selecting and using dosimetry systems in flash X-ray experiments. Both dose and dose rate techniques are described.  
1.2 Operating characteristics of flash X-ray sources are given, with emphasis on the spectrum of the photon output.  
1.3 Assistance is provided to relate the measured dose to the response of a device under test (DUT). The device is assumed to be a semiconductor electronic part or system.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Apr-2024
• ICS: 17.240 - Radiation measurements
• Technical Committee: E10 - Nuclear Technology and Applications
• Drafting Committee: E10.07 - Radiation Dosimetry for Radiation Effects on Materials and Devices

Overview

ASTM E1894-24: Standard Guide for Selecting Dosimetry Systems for Application in Pulsed X-Ray Sources is a comprehensive guide developed by ASTM International to assist users in selecting appropriate dosimetry systems for experiments involving pulsed (flash) X-ray sources. These specialized sources deliver intense bremsstrahlung radiation, typically in single, sub-microsecond pulses that can vary in amplitude, shape, and spectrum from shot to shot. Due to this variability, precise and reliable radiation dosimetry is essential for characterizing radiation environments and ensuring the validity and reproducibility of experimental results in various scientific and industrial applications.

Key Topics

• Characteristics of Flash X-Ray Sources: Covers the operating principles of pulsed X-ray facilities, typically producing high-energy photon pulses using bremsstrahlung mechanisms and high-voltage electron beams.
• Dosimetry Requirements: Addresses why accurate, real-time dosimetry is critical given shot-to-shot variability. Reliable dosimetry is required for every exposure to properly characterize the radiation field.
• Selection of Dosimetry Systems: Explains the criteria for choosing dosimetry systems, considering parameters such as dose, dose rate, spectral energy distribution, and pulse width.
• Relationship to Device Under Test (DUT): Describes methods to relate measured dose to the response and potential changes in the performance of electronics and materials subjected to irradiation.
• Types of Dosimeters: Summarizes various dosimetry systems, including thermoluminescence dosimeters (TLDs), calorimeters, and other detectors, highlighting their advantages and limitations.

Applications

The guidance in ASTM E1894-24 is valuable for a range of applications, particularly where precise radiation measurement under pulsed X-ray conditions is required. Key areas include:

• Material Testing: Assessing the effects of X-rays and gamma rays on different materials, crucial for developing radiation-resistant materials and components.
• Radiation Effects on Electronics: Evaluating the impact of high-dose rates and rapid irradiation on electronic devices such as semiconductors, transistors, diodes, and capacitors.
• Validation of Simulation Codes: Providing empirical data to validate and calibrate computational models and simulation codes used in radiation transport and effects prediction.
• Quality Assurance in Radiation Facilities: Ensuring compliance with international standards and best practices for accurate, reproducible dosimetry in pulsed X-ray environments.

Practical Value

• Systematic Selection Process: The standard offers a systematic approach for scientists and engineers to select and implement the most suitable dosimetry system for specific pulsed X-ray applications.
• Improved Experiment Reliability: By following the guide, users can achieve more reliable, consistent, and interpretable results across different test setups and laboratories.
• Enhanced Device Qualification: Accurate dose measurement and correlation with device performance allow for more precise qualification of electronic components and advanced materials for use in radiation-rich environments.
• International Compatibility: Developed in accordance with globally recognized standardization principles, ASTM E1894-24 supports international collaboration and comparison of experimental data.

Related Standards

Several referenced documents provide additional detail and context. Key related standards include:

• ASTM E170 - Terminology Relating to Radiation Measurements and Dosimetry
• ASTM E666 - Practice for Calculating Absorbed Dose from Gamma or X Radiation
• ASTM E668 - Practice for Application of Thermoluminescence Dosimetry (TLD) Systems
• ASTM E1249 - Practice for Minimizing Dosimetry Errors in Radiation Hardness Testing
• ISO/ASTM 51261, 51275, 51310 - Practices for Calibration and Use of Dosimetry Systems
• ICRU Reports 14, 17, 34, 51, 60, 76, 77, 80, 85a - Authoritative guidance on dosimetry and fundamental radiation units

Summary

ASTM E1894-24 serves as an essential reference for the selection, calibration, and use of dosimetry systems in pulsed X-ray environments. By adhering to the guidance in this standard, professionals can significantly improve the accuracy and reliability of radiation measurements in diverse applications ranging from materials research to the qualification of high-reliability electronic devices. The standard also ensures alignment with international best practices and supports continued innovation and safety in radiation testing facilities.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E1894 − 24
Standard Guide for
Selecting Dosimetry Systems for Application in Pulsed
X-Ray Sources
This standard is issued under the fixed designation E1894; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 ISO/ASTM Standards:
ISO/ASTM 51261 Practice for Calibration of Routine Do-
1.1 This guide provides assistance in selecting and using
simetry Systems for Radiation Processing
dosimetry systems in flash X-ray experiments. Both dose and
ISO/ASTM 51275 Practice for Use of a Radiochromic Film
dose rate techniques are described.
Dosimetry System
1.2 Operating characteristics of flash X-ray sources are
ISO/ASTM 51310 Practice for Use of a Radiochromic
given, with emphasis on the spectrum of the photon output.
Optical Waveguide Dosimetry System
1.3 Assistance is provided to relate the measured dose to the
2.3 International Commission on Radiation Units (ICRU)
response of a device under test (DUT). The device is assumed
and Measurements Reports:
to be a semiconductor electronic part or system.
ICRU Report 14 Radiation Dosimetry: X rays and Gamma
Rays with Maximum Photon Energies Between 0.6 and 50
1.4 This international standard was developed in accor-
MeV
dance with internationally recognized principles on standard-
ICRU Report 17 Radiation Dosimetry: X rays Generated at
ization established in the Decision on Principles for the
Potentials of 5 to 150 kV
Development of International Standards, Guides and Recom-
ICRU Report 34 The Dosimetry of Pulsed Radiation
mendations issued by the World Trade Organization Technical
ICRU Report 51 Quantities and Units in Radiation Protec-
Barriers to Trade (TBT) Committee.
tion Dosimetry
2. Referenced Documents
ICRU Report 60 Fundamental Quantities and Units for
Ionizing Radiation
2.1 ASTM Standards:
ICRU Report 76 Measurement Quality Assurance for Ioniz-
E170 Terminology Relating to Radiation Measurements and
ing Radiation Dosimetry
Dosimetry
ICRU Report 77 Elastic Scattering of Electrons and Posi-
E666 Practice for Calculating Absorbed Dose From Gamma
trons
or X Radiation
ICRU Report 80 Dosimetry Systems for Use in Radiation
E668 Practice for Application of Thermoluminescence-
Processing
Dosimetry (TLD) Systems for Determining Absorbed
ICRU Report 85a Fundamental Quantities and Units for
Dose in Radiation-Hardness Testing of Electronic Devices
Ionizing Radiation
E1249 Practice for Minimizing Dosimetry Errors in Radia-
tion Hardness Testing of Silicon Electronic Devices Using
3. Terminology
Co-60 Sources
3.1 absorbed dose enhancement—increase (or decrease) in
the absorbed dose (as compared to the equilibrium absorbed
This guide is under the jurisdiction of ASTM Committee E10 on Nuclear
dose) at a location in a material of interest. This can be
Technology and Applications and is the direct responsibility of Subcommittee
E10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.
Current edition approved May 1, 2024. Published May 2024. Originally
approved in 1997. Last previous edition approved in 2018 as E1894 – 18. DOI: For referenced ISO/ASTM standards, visit the ASTM website, www.astm.org,
10.1520/E1894-24. or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Standards volume information, refer to the standard’s Document Summary page on
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM the ASTM website.
Standards volume information, refer to the standard’s Document Summary page on Available from the International Commission on Radiation Units and
the ASTM website. Measurements, 7910 Woodmont Ave., Suite 800, Bethesda, MD 20814, U.S.A.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1894 − 24
expected to occur near an interface with a material of higher or 4.2 This guide is written to assist the experimenter in
lower atomic number. selecting the needed dosimetry systems for use at pulsed X-ray
facilities. This guide also provides a brief summary on how to
3.2 converter—a target for electron beams, generally a high
use each of the dosimetry systems. Other guides (see Section 2)
atomic number material, in which bremsstrahlung X-rays are
provide more detailed information on selected dosimetry
produced by radiative energy loss of the incident electrons.
systems in radiation environments and should be consulted
3.3 dosimetry system—a system used for determining ab-
after an initial decision is made on the appropriate dosimetry
sorbed dose which consists of dosimeters, measurement instru-
system to use. There are many key parameters which describe
ments with their associated reference standards, and proce-
a flash X-ray source, such as dose, dose rate, spectrum, pulse
dures for the system’s use.
width, etc., such that typically no single dosimetry system can
measure all the parameters simultaneously. However, it is
3.4 DUT—device under test. This is the electronic compo-
nent or system being tested to determine its performance frequently the case that not all key parameters must be
measured in a given experiment.
during and/or after irradiation.
3.5 endpoint energy—endpoint energy refers to the peak
5. General Characteristics of Flash X-ray Sources
energy of the electron beam, usually in MeV, generated in a
5.1 Flash X-ray Overview—Flash X-ray sources operate
flash X-ray source and is equal to the terminal voltage of the
like a dental X-ray source but at much higher voltages and
accelerator in megavolts (MV). The word “endpoint” refers to
intensities and usually in a single, very short burst, see ICRU
the highest photon energy of the bremsstrahlung spectra, and
Report 17. A high voltage is developed across an anode-
this endpoint is equal to the maximum or peak in the electron
cathode gap (the diode) and field emission creates a pulsed
energy. For example, if the most energetic electron that strikes
electron beam traveling from the cathode to the anode. A
the converter is 10 MeV, this electron produces a range of
high-atomic-number element such as tantalum is placed on the
bremsstrahlung photon energies but the maximum energy of
anode to maximize the production of bremsstrahlung created
any photon is equal to 10 MeV, the endpoint energy. Most
when the electrons strike the anode. Graphite or aluminum is
photons have energies one tenth to one third of the maximum
usually placed downstream of the converter to stop the electron
electron energy for typical flash X-ray sources in the 1 MV to
beam completely but let the X-radiation pass through. Finally,
10 MV endpoint voltage region, respectively.
a debris shield made of Kevlar or low-density polyethylene is
3.6 endpoint voltage—Endpoint voltage refers to the peak
sometimes necessary to stop exploding converter material from
voltage across a bremsstrahlung diode in a flash X-ray source.
leaving the source. All of these components taken together
For example, a 10-MV flash X-ray source is designed to reach
form what is commonly called a bremsstrahlung diode.
a peak voltage of 10 MV across the anode-cathode gap which
5.2 Relationship Between Flash X-ray Diode Voltage and
generates the electron beam for striking a converter to produce
X-ray Energy of Bremsstrahlung—Flash X-ray sources produce
bremsstrahlung.
bremsstrahlung by generating an intense electron beam which
3.7 equilibrium absorbed dose—absorbed dose within some
then strikes a high atomic number (Z) converter such as
incremental volume in the target material in which the condi-
tantalum. The electron-solid interactions produce “braking”
tion of electron equilibrium (the energies, number, and direc-
radiation or, in German, bremsstrahlung. Fig. 1 shows the
tion of charged particles induced by the radiation are constant
typical range of photon energies produced by flash X-ray
throughout the volume) exists. For lower electron energies,
sources with different electron endpoint energies. The data in
where bremsstrahlung production is negligible, the equilibrium
Fig. 1 is generated by tallying the photon spectrum using ITS
absorbed dose is equal to the kerma. 5
with optimized tantalum/carbon bremsstralung converters (1).
If the average radiation produced is in the 20 to 100 keV
NOTE 1—For practical purposes, assuming the spatial gradient in the
X-ray field is small over the range of the maximum energy secondary
region, the source is said to be a “medium X-ray simulator.” If
electrons generated by the incident photons, the equilibrium absorbed
the average photon energy is in the 100 to 300 keV region, the
dose is the absorbed dose value that exists in a material at a distance from
term used is “hard X-ray simulator.” At the high end of the
any interface with another material greater than this range.
flash X-ray range are sources which produce an average photon
4. Significance and Use energy of around 2 MeV. Because this photon energy is in the
typical gamma-ray spectral range, the source is called a
4.1 Flash X-ray facilities provide intense bremsstrahlung
“gamma-ray simulator.”
radiation environments, usually in a single sub-microsecond
5.2.1 The average energy of the bremsstrahlung spectrum,
pulse, which often fluctuates in amplitude, shape, and spectrum
¯
E , through an optimized converter can be estimated using
photon
from shot to shot. Therefore, appropriate dosimetry must be
the following relationship (1):
fielded on every exposure to characterize the environment, see
ICRU Report 34. These intense bremsstrahlung sources have a ¯
=
E 5 k· ε where 5.1,k,18.9 (1)
photon
variety of applications which include the following:
¯
where E is the average energy of the bremsstrahlung
photon
(1) Studies of the effects of X-rays and gamma rays on
photons in keV and ε is the average energy of the electrons in
materials.
(2) Studies of the effects of radiation on electronic devices
such as transistors, diodes, and capacitors.
The boldface numbers in parentheses refer to the list of references at the end of
(3) Computer code validation studies. this standard.
E1894 − 24
FIG. 1 Range of Available Bremsstrahlung Spectra from Flash X-ray Sources with Optimized Bremsstrahlung Converters
the electron beam incident on the converter in keV. The value 6.3.1 Secondary Electrons—Both in the case of absorbed
of k depends on the converter thickness; thin targets will have dose in the DUT and absorbed dose in the dosimeter, the
values at the lower end of the range, while thick targets energy is deposited largely by secondary electrons. That is, the
optimized for higher incident energies will have values at the
incident photons interact with the material of, or surrounding,
upper end. When an optimized bremsstrahlung converter is
the DUT or the dosimeter and lose energy to Compton
used, a rule-of-thumb may be used that the average photon
electrons, photoelectrons, and Auger electrons. The energy
1 1
energy is about ⁄5 or ⁄6 of the electron endpoint energy (1). For
which is finally deposited in the material is deposited by these
a fixed converter design, the photon energy away from the
secondary particles.
optimization point is roughly proportional to the square root of
6.3.2 Transport of Photons—In some cases, it is necessary
the electron endpoint energy with the proportionality factor
to consider the transport and loss of photons as they move to
varying between about 5 and 19 depending upon the design
the region whose absorbed dose is being determined. A
point (1). This equation and Fig. 1 indicate that most of the
correction for the attenuation of an incident photon beam is an
photons have energies much less than the endpoint electron
example of such a consideration.
energy, or in voltage units, the flash X-ray voltage.
6.3.3 Transport of Electrons—Electron transport may cause
Additionally, the bremsstrahlung spectrum is very non-
energy originally imparted to electrons in one region to be
Gaussian so caution must be exercised in using the average
carried to a second region depending on the range of the
energy of the distribution for dosimetry planning.
electrons. As a result, it is necessary to consider the transport
6. Measurement Principles
and loss of electrons as they move into and out of the regions
whose absorbed dose is being determined. In particular, it is
6.1 Typically in flash X-ray irradiations, one is interested in
necessary to distinguish between equilibrium and non-
some physical change in a critical region of a device under test
equilibrium conditions for electron transport.
(DUT). The dosimetry associated with the study of such a
physical change may be broken into three parts:
6.3.3.1 Charged Particle Equilibrium—Occurs when the
(1) Determine the absorbed dose in a dosimeter.
numbers, energies, and angles of particles transported into a
(2) Using the dosimeter measurement, estimate the ab-
region of interest are approximately balanced by those trans-
sorbed dose in the region and material of interest in the DUT.
ported out of that region. (See “Equilibrium Absorbed Dose” in
(3) If required, relate the estimated absorbed dose in the
3.7.)
DUT to the physical change of interest (holes trapped, interface
6.3.3.2 Dose Enhancement—Because photoelectron produc-
states generated, photocurrent produced, etc.).
tion per atom is roughly proportional to the atomic number
6.2 This section will be concerned with the first two of the
raised to the fourth power for energies less than 100 keV (2),
above listed parts of dosimetry: (1) what is necessary to
one expects more photoelectrons to be produced in high atomic
determine a meaningful absorbed dose for the dosimeter, and
number layers than in low atomic number layers for the same
(2) what is necessary to extrapolate this measured dose to the
photon fluence and spectrum. Thus, there may be a net flow of
estimated dose in the region of interest. The final step in
energetic electrons from the high atomic number layers into the
dosimetry, associating the absorbed dose with a physical
low atomic number layers. This non-equilibrium flow of
change of interest, is outside the scope of this guide.
electrons may result in an enhancement of the dose in the low
6.3 Energy Deposition: atomic number layer. Dose enhancement problems are often
E1894 − 24
caused by high atomic number bonding layers (for example, (1) The intermediate cases, where secondary electron
gold) and metallization layers (for example, W-Si or Ta-Si). ranges are neither small nor large in comparison to the
dosimeter size, are cases where non-equilibrium energy depo-
6.4 Absorbed Dose in Dosimeter:
sition is to be expected.
6.4.1 Equilibrium Absorbed Dose in Dosimeter:
(2) An example of an intermediate case is: 100 keV
6.4.1.1 It is frequently possible to use dosimeters under
photons incident on a typical TLD.
approximate equilibrium conditions. The interpretation of the
(3) The careful treatment of dosimetry for intermediate
output of the dosimeter is straightforward only when the
cases requires the use of combined photon/electron radiation
energy deposition processes within the dosimeter are approxi-
transport calculations of the energy deposition in the dosim-
mately in equilibrium. That is, when the absorbed dose within
eters to get satisfactory measurements.
the dosimeter is an equilibrium absorbed dose.
6.5 Absorbed Dose in Device Under Test:
6.4.1.2 It is possible to treat non-equilibrium energy depo-
6.5.1 Absorbed Dose in Device Under Test—The conditions
sition within a dosimeter, but such an analysis requires electron
within a DUT during a flash X-ray irradiation are frequently far
and photon transport calculations, often in the form of com-
from equilibrium. In many cases, these experiments will
puter codes.
observe dose enhancement effects. As a result, it is frequently
6.4.2 Limiting Cases:
necessary to perform a dose-enhancement correction to esti-
6.4.2.1 There are two limiting cases for which the dosimeter
mate the absorbed dose within the region of interest. Unless the
data can be analyzed in a straightforward manner.
DUT meets the two limiting cases discussed in the previous
6.4.2.2 Limiting Case One: Short Electron Range:
section, which is rarely true, the dose should be obtained using
(1) For this case, secondary electron ranges are small in
combined photon/electron radiation transport calculations of
comparison with the size of the dosimeter.
the energy deposition in order to get satisfactory dose esti-
(2) Essentially all electrons which deposit energy within
mates. Within a system being tested, a calculation of the
the dosimeter will be produced within the dosimeter.
radiation transport through the system to the individual devices
(3) Non-equilibrium effects due to electron transport are
of interest must be performed.
negligible, but photon attenuation corrections may be neces-
sary. 6.6 Spectral Considerations:
(4) An example of this limiting case is: 20 keV photons 6.6.1 Broad Energy Range—The set of available flash X-ray
depositing energy in a typical (0.889 mm thick) thermolumi- sources spans a very wide photon energy range. Useful
nescence (TL) dosimeter (TLD). In this case, the secondary intensities may be obtained for energies as low as 10 keV and
electrons have ranges which are small in comparison with the as high as 10 MeV. Each individual flash X-ray source, of
size of the TLD. As a result, it is not necessary to perform a course, does not produce useful photons over such a wide
correction for the effect of electron transport on absorbed dose. range. Dosimetry for such flash X-ray sources can be simpli-
On the other hand, 20 keV photons may be significantly fied if the different flash spectra are categorized into three
attenuated while traveling through a TLD depending on the types: low energy, medium energy, and high energy. This
material. Thus a correction due to this effect may be necessary. categorization refers to the average energy of the photon
spectrum and not the higher electron endpoint energy (5.2).
6.4.2.3 Limiting Case Two: Large Electron Range:
6.6.2 Dosimetry for Three Energy Bands—A summary of
(1) When the maximum secondary electron range is large
the dosimetry requirements for the three flash X-ray energy
compared with the size of the dosimeter, the dosimeter must be
bands is provided in Table 1.
surrounded by an equilibrating layer. This layer must be chosen
to be of an appropriate thickness, density, and atomic number.
6.7 Absorbed Dose Interpretation:
Generally, the range of secondary electrons must be smaller
6.7.1 In 6.1, it is pointed out that the second of three steps
than the thickness of the equilibrating layer. For further
in the dosimetry process is the use of the measured absorbed
discussion of equilibrating layers, see Practice E668.
dose in the dosimeter to estimate the absorbed dose in a region
(2) Essentially all electrons which deposit energy within
of interest within the device under test.
the dosimeter originate in the equilibrating layer.
6.7.2 In the previous sections the dose in both the dosimeter
(3) Bragg-Gray cavity theory applies. That is, the dose
(6.4) and the DUT (6.5) is estimated. Although the dose can
within the dosimeter is the equilibrium dose for the equilibrat-
easily be calculated from tables or a simple photon transport
ing layer (corrected by the differences in electron stopping
code for the dosimeter, which is often in equilibrium, the dose
power of the dosimeter and the equilibrating layer and any
in the DUT, which is rarely in equilibrium, often requires a
photon attenuation through the material).
sophisticated combined photon/electron radiation transport
(4) An example of this limiting case is 1 MeV photons
code. Assuming this has been accomplished, one can use the
incident on a typical TLD surrounded by an appropriate
ratio of the calculated doses at a given fluence and spectrum as
equilibrating layer. In this case, the range of the secondary
the correction factor for the measured dose in the dosimeter to
electrons will be large in comparison to the size of the TLD.
estimate the actual dose in the DUT.
Thus the dose measured will be the equilibrium dose in the
D
DUT
TLD (with a small correction for the differences in the stopping
D 5 Calculated × D (2)
S D
DUT Dosimeter
D
Dosimeter
power for the electrons in the TLD material and the material of
the equilibrating layer).
where D is the absorbed dose in the device material, and
DUT
6.4.2.4 Intermediate Cases: D is the equilibrium absorbed dose in the dosimeter.
Dosimeter
E1894 − 24
TABLE 1 Flash X-ray Dosimetry Characteristics for Three Energy Bands
Flash X-Ray Type Absorbed Dose in Dosimeter Absorbed Dose in DUT
A
Low energy (average photon energy: 20–100 keV) Can get electron equilibrium. May need photon Depends on DUT:
B
C
transport correction.
May need an electron transport calculation. May need a
B
photon transport calculation.
Medium energy (average photon energy: 100–300 Often cannot get electron equilibrium and therefore Depends on DUT:
C,D
keV) need electron transport calculation. May need an electron transport calculation, especially
C,E
without use of beam filtration. May need a photon
B
transport calculation.
High energy (average photon energy: 300–3000 Can get electron equilibrium with proper equilibration Depends on DUT:
F,D C
keV) layer. May need an electron transport calculation. Usually no
D
photon transport calculation needed.
A
The dosimeter or region of interest is large compared to the maximum secondary electron range (6.4.2.2).
B
The dosimeter or region of interest is large compared to the photon range (6.4.2.2, Paragraph 4).
C
The dosimeter or region of interest is of comparable size to the maximum secondary electron range (6.4.2.4).
D
The dosimeter or region of interest is small compared to the photon range (6.4.2.3, Paragraph 4).
E
A filter may be used to essentially eliminate the lower energy portions of the flash X-ray spectrum. This makes the spectrum more nearly monochromatic and may simplify
dosimetry.
F
The dosimeter or region of interest is small compared to the electron range (6.4.2.3).
TABLE 3 Dose Rate Measurements
7. Dosimetry Systems Cerenkov/
Compton
Dosimeter Type PIN Diode Scintillator- PCD
Diode
7.1 Introduction—In this section a brief summary of eight
Photodetector
1 8 7 10 4 9
different types of dosimetry systems is given. The intent of this
Dose Rate 2 × 10 –10 10 –10 10 –10 2 ×
5 10
Range 10 –10
guide is to provide enough information so that users can decide
(Gy/s)
which system might be appropriate for their application. Users
Photon Energy >0.01 >0.5 >0.02 >0.01
are expected to investigate in more detail the proper use and Range (MeV)
Size Small to Large Very Large Small to
limitations of a system using references in this and other
Medium Medium
ASTM documents before undertaking any radiation measure-
Ease of Moderate Difficult Moderate Moderate
ments. The pros and cons of each system are quickly summa- calibration
Precision 10 % 10 % 4 % 10 %
rized in Tables 2 and 3. The values cited in these tables are
Active Material Si,GaAs Tungsten Plastic Diamond,
indicative of what is typically obtained under normal testing
GaAs
conditions. Many of these parameters can be improved at the
Availability Buy Buy or Make Make Buy or Make
System cost Low Medium High Medium
expenditure of considerable effort. Only the precision of each
dosimeter is discussed here because the accuracy of the
dosimeter will be determined by the quality of the calibration
and specific usage and this is beyond the scope of this
document.
7.2.1.1 Many materials are available for thermolumines-
cence dosimetry; however, the favorites for radiation hardness
7.2 Thermoluminescence Dosimeters:
testing are lithium fluoride (LiF), manganese activated calcium
7.2.1 Introduction—Thermoluminescence dosimeters
fluoride (CaF :Mn), and dysprosium activated calcium fluoride
(TLDs), see Terminology E170, are popular dosimeters be-
(CaF :Dy). Dosimeters are available as powder, chips made
cause they are small, passive, inexpensive, and can retain
from polycrystalline material, and discs consisting of very fine
accurate dose information for long periods of time between
powder uniformly dispersed throughout a polytetrafluoroeth-
irradiation and readout. The dose range of TLDs is typically
–4 3 –2 5
ylene (PTFE) matrix. A commonly used size of chip is 3.2 by
>10 to <5 × 10 Gy (>10 to <5 × 10 rad).
3.2 by 0.9 mm (0.125 by 0.125 by 0.035 in.). A commonly used
size of PTFE dosimeter is a disc, 6 mm in diameter and 0.4 mm
thick. In addition, these dosimeter materials can be made into
TABLE 2 Dose Measurements
arrays for dose mapping.
Radiochromic
Dosimeter Type TLD Calorimeter Optichromic
7.2.2 Principles—Thermoluminescent materials consist of a
Film
-4 3 5 4
crystalline insulator with added dopants which introduce stable
Dose Range 10 –5 × 10 10–10 0.4–2 × 10 0.01–5 ×
(Gy) 10
electron traps into the forbidden band gap. Ionizing radiation
Photon Energy >0.01 >0.01 >0.01 >0.01
creates electrons and holes which are trapped by stable traps in
Range (MeV)
the band gap. The density of filled traps is proportional to the
Size Small Medium Medium Small to
Large
dose absorbed by the material. Subsequent heating of the
Ease of Easy Easy Easy Moderate
material empties the electron traps, allowing electrons from
calibration
Precision 5–15 % 5 % 5 % 5 % F-centers to recombine with free holes at luminescence centers,
Active Material CaF , LiF Au, Si, Al Organic Organic
emitting light. The integrated light output is proportional to the
Availability Buy Make or Buy Buy Buy
density of filled traps, and therefore to the absorbed dose in the
System Cost Medium Low Low Low to High
TLD material.
E1894 − 24
7.2.2.1 To make an equilibrium dose measurement, the TLD MeV), the TLD can be treated using Bragg-Gray cavity theory,
must be enclosed in an equilibrium capsule of the appropriate with all photon interactions assumed to take place in the
material and thickness when it is exposed. Methods for equilibrium shield material, and the resultant secondary elec-
determining equilibrium capsule thickness are given in Practice trons depositing energy in the phosphor grains. There is a
E666. smooth transition to more moderate energies (photon energy
≈200 keV) where the photon interactions must be considered to
7.2.2.2 Readout of TLDs is accomplished by an instrument
occur primarily in the PTFE matrix. Finally, at the lowest
consisting of a heater, optical system, and photomultiplier
photon energies (photon energy ≤10 to 20 keV), all photon
detector to measure the light emitted by the TLD during a
interactions which eventually deposit energy in the phosphor
predetermined heating cycle, and an integrating picoammeter
grains must be assumed to originate in the individual phosphor
which can measure the current or the charge from the photo-
grain itself. The dose response of PTFE TLDs as a function of
multiplier. The total integrated charge from the photomultiplier
photon energy is therefore a complex function of energy which
during part or all of the heating cycle is usually related to the
is not easy to derive analytically. A modern electron/photon
absorbed dose in the dosimeter. TLD readout instruments are
transport code such as ITS (3) can be helpful in calculating this
available from a number of manufacturers.
relationship (4).
7.2.3 Applications—The choice of the TLD and capsule
7.2.5.1 Most TL phosphors exhibit fading with time; that is,
material depend on the application. For high-energy photon
the observed TL response becomes progressively smaller as the
sources (photon energy ≥200 keV) such as high voltage flash
60 137
time interval between irradiation and readout increases (5). For
X-ray generators and Co and Cs sources, and where the
most of the common TL phosphors the fading behavior is well
quantity of interest is Si or SiO equilibrium dose, a good
known. The fading of CaF :Mn TLDs has been reported to
combination of materials is a CaF :Mn TLD in an Al equilib-
obey the relationship (6):
rium shield (see Practice E1249). For these materials and
20.017
photon energies, all mass energy absorption coefficients and
TL /TL 5 t/t (3)
~ ! ~ !
o o
mass stopping powers are so close to those of silicon that the
where t is in hours and the subscript o refers to the time of
silicon equilibrium dose can be determined using Bragg-Gray
irradiation, and unsubscripted quantities refer to the time of
cavity theory. The response of CaF :Mn is nearly linear with
readout. The value of the coefficient 0.017 is only typical and
absorbed dose and the dosimeters retain dose information for a
can vary from batch to batch and must be determined experi-
long time with a small fading correction whose form is well
mentally. Use the procedures in 8.7 of Practice E668 to test for
known (7.2.5).
fading effects in the type of TLD chosen.
7.2.3.1 For radiation sources which have a significant por-
7.2.6 Sensitivity—TLDs can be used to measure dose from
tion of photons with energy below 200 keV, more detailed
100 μGy to 5 kGy (10 mrad to 500 krad). Most TL materials
calculations are necessary regardless of which material is
saturate in the range of 3 to 10 kGy (300 krad to 1 Mrad). The
chosen for the TLD. In this case the choice of TLD and capsule
dose response of calcium fluoride TLDs is independent of dose
material is flexible, and can be made on the basis of conve-
10 12
rate for dose rates less than 10 Gy/s (10 rad/s) (7). The dose
nience or cost.
response of lithium fluoride has been reported to be indepen-
9 11
7.2.3.2 The choice of whether powder, chips, or PTFE discs
dent of dose rate only up to 10 Gy/s (10 rad/s) (8).
are used is primarily one of convenience and cost. For specific
7.2.7 Calibration—TLD systems (dosimeter plus reader)
instructions on the proper procedures for using TLD systems to
must be calibrated in a standard radiation field before use; see
determine absorbed dose in radiation hardness testing of
ICRU Report 14. The most convenient sources for this are
60 137
electronic devices, see Practices E666 and E668.
calibrated Co or Cs sources. Care must be taken to expose
7.2.4 Advantages—TLDs are small, inexpensive, and re-
the TLDs in an appropriate equilibrium shield. If the calibra-
quire no instrumentation during irradiation. They are thus
tion source is calibrated in units of exposure rate, then the dose
ideally suited for measuring dose at many locations within a
absorbed by the TLD can be found by:
test object, or for measuring dose near the areas of interest in
µ /ρ
~ !
en
TLD
@
even very small (;3 mm) test objects. Their large sensitivity D 5 ~D /X!X exp 2 ~µ ⁄ρ! ρ x# (4)
S D
TLD air 0 en eqmat
eqmat
~µ /ρ!
en
air
range makes it possible to measure mGy to kGy doses with one
where:
dosimetry system.
D = is the dose absorbed by the TLD,
7.2.5 Limitations—Energy Range: TLDs are usable over the
TLD
X = is the free-in-air exposure of the source,
photon energy range from at least 10 keV to 10 MeV. (Indeed,
o
D /X = 33.68 Gy-kg/C (0.869 rad/R),
their sensitivity extends down into the ultraviolet region; air
μ /ρ = is the mass-energy absorption coefficient,
hence, TLDs must be protected from exposure to sunlight, etc., en
ρ = is the density of the equilibrator material, and
eqmat
especially for low dose applications.) The dose response as a
x = is the thickness of the equilibrator material.
function of energy can be calculated for TLDs which consist of
100 % TL phosphor by using standard equilibrium dose and 7.2.8 Reproducibility—The reproducibility of most TLD
Bragg-Gray cavity theories (Practice E666). However, care materials is about 68 % at 1σ. Better reproducibility can be
must be exercised when using PTFE disc dosimeters at low achieved with TLD chips by irradiating a number of them to a
energies. Since these dosimeters consist of fine grains of TL low dose, reading them, and selecting those which fall into a
phosphor distributed throughout a PTFE matrix, these simple tighter group. A reproducibility of 62 % is often achievable,
theories may not apply. At high energies (photon energy ≥2 and 61 % is possible by this means.
E1894 − 24
7.3 Calorimeters: be expanded in a power series as a function of temperature over
7.3.1 Introduction—Calorimetry comes closest of all the the temperature range of interest.
dosimetric techniques to providing a direct measure of dose or 7.3.2.2 Temperature sensors such as thermocouples (10) are
fluence. Apart from corrections for thermal leakage, thermal used routinely by properly attaching the thermocouple wires to
defect due to chemical or solid state reactions, and energy loss
the absorber. An example of a thermocouple attached to an
in fluence measurements, only the specific heat and the absorber is shown in Fig. 2. The 0.025 mm thick gold foil is
temperature rise of the absorber due to X-ray absorption need
thermally isolated from the surrounding environment by sup-
be known. The specific heat or the heat capacity of the absorber porting the foil on small nylon threads and by using small
is a thermodynamic property of the given material. Tempera-
diameter thermocouple wires. The thermocouple wires pass
ture rise measurements can be made with thermocouples,
through a lead shield and then they are soldered to miniature
thermopiles, resistive temperature detectors (RTDs),
connector pins with lead-free solder. The lead shield and
thermistors, etc.
lead-free solder are used to reduce the X-ray induced tempera-
7.3.2 Principles—The temperature rise of the absorbing
ture rise in the solder connection. To improve charged particle
material is related to the energy absorbed, enthalpy or dose, in
equilibrium and reduce fluorescence losses from the gold, a
the material through its specific heat. Namely,
gold backing foil is placed just behind the gold absorber. A
coupled photon/electron transport code should be used to
ΔH 5 C dT (5)
*
p
determine how far the geometry is from achieving charged
particle equilibrium for a particular spectrum. Although not
It has been assumed that the absorber remains solid at
shown in the figure, an optional fine wire can be welded to the
constant pressure and that no phase transition or other process
gold foil. When this is done, the calibration of the calorimeter
occurs which would complicate this simple relationship.
can be checked with a proton Van de Graaff provided the
7.3.2.1 The specific heat (9), that is, the heat capacity per
thermocouple is electrically isolated from the X-ray absorber.
unit mass for a typical absorber, does not vary significantly
with temperature, such as tantalum which varies from 139.7 to 7.3.2.3 Thin foil X-ray absorbers are typically designed for
–1 –1
141.6 J·kg ·K or aluminum which varies from 900.6 to measuring dose in the absorbing material. A thicker absorber
–1 –1
919.0 J·kg ·K between 20 °C and 60 °C. Hence, for many may be used as a total fluence detector for low energy spectra.
materials and over a practical range of temperature changes, a However, in this case a more sensitive temperature detector
constant specific heat can be used with accuracies better than such as a thermistor may be required and a much thicker
1 % not required. For improved accuracy, the specific heat may absorber. A typical thermistor has a negative temperature
FIG. 2 Cutaway View of a Typical Gold Foil Dose Calorimeter Designed for Use with FXR Spectra Which Have Maximum Energies Less
Than 2 MeV (courtesy of Maxwell Laboratories, Inc., San Diego, CA)
E1894 − 24
coefficient of resistivity, α, which decreases in magnitude from 7.3.5 Sensitivity—Apart from the sensitivity of the tempera-
about 0.039 to 0.036/°C. This coefficient is defined by: ture sensor and the specific heat of the absorber, the basic
sensitivity of the dose calorimeter is dependent upon the mass
Δρ /ρ 5 α ΔT (6)
~ ! ~ !
o
energy absorption coefficient of the X-ray absorber. The degree
where ρ is the original resistivity and Δρ and ΔT are the
to which the absorber deviates from this ideal cross section
o
change in resistivity and temperature. Hence, care must be
must be calculated with a code such as the ITS (3). A similar
taken to measure the temperature both before and after X-ray
comment can be made for total-fluence calorimeters except that
energy absorption. The temperature sensitivity of a thermistor
in this case the deviation from total incident energy absorption
may be closely approximated with the Steinhart-Hart equation
would be calculated.
(11):
7.3.6 Calibration—The output of a properly designed calo-
rimeter can be interpreted from the intrinsic knowledge of the
1/T 5 A1B~lnR!1C~lnR! (7)
specific heat of the absorbing material and the calibration of the
where T is in K and R is the thermistor resistance. The values
temperature sensor. Alternatively, several techniques are avail-
of A, B, and C should be measured for each thermistor and
able to confirm that the calorimeter has been properly de-
these values should be remeasured systematically to ensure that
signed. Examples of these techniques include embedded elec-
the thermistor has not been damaged.
tric heaters, proton Van de Graaff pulse heating, and flash lamp
7.3.3 Advantages—Calorimeter dose measurements can be
pulse heating. The temperature sensor element should be
made absolute, either intrinsically or by means of electrical
checked periodically for changes in response over time.
heating calibration.
7.3.6.1 Besides the basic calorimeter calibration, the cali-
7.3.3.1 The measurement of temperature rise comes closest
bration of the high gain recorder system should be checked. It
of any dosimetric technique to being a direct measurement of
is recommended that a step voltage pulse be used to check the
the energy involved in the absorbed dose.
gain of the system on each channel every day during a test
series in which the calorimeters are being used. The step pulse
7.3.3.2 Calorimeters are inherently dose rate independent
under adiabatic conditions, and become more convenient to use should be applied physically in place of the calorimeter so that
the entire signal line is included in the calibration.
as the dose rate increases because thermal leakage during dose
delivery becomes negligible.
7.3.7 Reproducibility—Apart from electrical noise and the
basic calibration of the specific heat and temperature detector,
7.3.3.3 Metal film calorimeters have no LET dependence
(neglecting minor differences in thermal defect, if any), since the reproducibility and accuracy of the dose or fluence mea-
surement are limited by the readability of the recorded signal.
ionic recombination is irrelevant to the temperature rise.
In some cases this may be dominated by electrical noise,
7.3.3.4 The conversion of absorbed dose to a temperature
thermal noise, or heat exchange. As a practical figure of merit,
rise takes place on the order of a few picoseconds. Hence, the
the reproducibility for flash X-ray measurements should be
temperature rise in the material for current flash X-rays follows
about 5 %.
the running integral of the dose rate, and calorimeters can be
designed to measure the true absorbed energy in a small region
7.4 Opti-chromic Dosimeters:
or the average dose in a larger volume after thermal equilib-
7.4.1 Introduction—Opti-chromic dosimeters (ODs) (13-18)
rium.
are a relatively new type of dosimeter that has many of the
7.3.4 Limitations—The temperature rises to be measured are
same advantages and uses of TLDs. They are relatively small
typically small and (in many cases) are only fractions of a
(approximately 3 mm diameter and 25 to 50 mm long), passive,
degree. Therefore, calorimetry performs best with relatively
inexpensive, and retain accurate dose information for long
large doses (12).
periods of time (months) between irradiation and measurement
7.3.4.1 The calorimeter absorber must be designed to allow of dose. The useful dose range of the ODs is >0.4 Gy to <20
the measurement of the temperature rise before excessive kGy (>40 rad to <2 Mrad).
thermal losses take place. This requirement must be considered
7.4.1.1 Organic solvents with a high refractive index are
in conjunction with requiring charged particle equilibrium.
used to fill a hollow fluorinated plastic tube having a low
refractive index, forming an optical waveguide. Radiochromic
7.3.4.2 The recording of the calorimeter signals are different
than most of the other signals associated with flash X-ray tests dye is dissolved in these solvents and sensitized by them. Glass
beads are located in the ends of the tube and serve as lenses for
and, consequently, the test has an additional complication when
calorimeters are used. In many cases calorimeter signals are a the waveguide. Dosimetry is performed by measuring changes
few tens of microvolts to a few millivolts and must be recorded in the optical density of the fluid.
with a few hundred hertz frequency response. In these cases 7.4.2 Principles—The dosimeters are supplied as fluori-
high-input-impedance, high-gain amplifiers in conjunction
nated polyethylene-polypropylene (FEP) tubing (50 or 25 mm
with low pass filters are often used. long, 3 mm o.d., wall thickness 0.3 mm) filled with hexahy-
droxyethl pararosaline cyanide (HPC) dissolved in a mixture
7.3.4.3 In some instances the dose from extremely high
containing triethyl phosphate, dimethyl sulfoxide, and polyvi-
fluences from lower energy spectra can be high enough to
nyl butral. The filled tubing is sealed at both ends with glass
either melt the absorber or, for slightly lower dose values,
beads (diameter 3 mm) forming a waveguide.
generate a thermomechanical shock which may break the
absorber, the temperature sensor, or perhaps, detach the tem- 7.4.2.1 The HPC is a colorless precursor of a common
perature sensor from the absorber. highly colored stable organic dye. When a liquid solution of
E1894 − 24
this compound, in these polar solvents, is irradiated with and/or non-uniform dose in the OD. These possible effects can
ionizing radiation having an energy exceeding ;4 eV, the be calculated using a modern Monte Carlo based electron/
cyanide group is split off. This results in an electron rearrange- photon transport code such as ITS (3) or MCNP (19-21), or
ment in the parent molecule which yields a blue dye. In this using a discrete ordinate code like CEPXS/ONELD (22).
solution, the resulting color change is very stable. The change
7.4.5.5 Time Dependence—There is no significant fade or
in the optical density is a monotonic function of the absorbed
time dependence in the optical density after irradiation for
dose in the solution.
doses <2 kGy. Above this dose there is a change in the optical
7.4.3 Applications—These dosimeters can be used for vir-
density of the glass beads in the ends of the dosimeter which
tually all applications where TLDs are used. They are some-
reaches a steady state value after a few days. This time
what easier to store, handle and read. They cannot be used for
dependence must be considered when using or calibrating these
very low doses nor for dosimetry over very small areas,
dosimeters at very high doses.
because of their large size in comparison to TLDs. For
7.4.6 Sensitivity—ODs can be used to measure dose from
high-energy photon sources (photon energy ≥300 keV) such as
0.4 to 20 kGy (40 rad to 2 Mrad) using a number of different
60 137
high-voltage flash X-ray generators and Co and Cs
HPC concentrations. The dose response of the OD is indepen-
sources, a good choice of additional equilibrium shield is 12 14
dent of dose rate up to 10 Gy/s (10 rad/s) (16).
plastic. For these high energies and materials the mass absorp-
7.4.7 Calibration—An approved method for using optical
tion coefficients and mass stopping powers are so close to those
waveguide dosimetry is given in Practice ISO/ASTM 51310.
of silicon that the silicon equilibrium dose can be determined
In order to make a valid equilibrium dose measurement, the
using simp
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E1894 − 18 E1894 − 24
Standard Guide for
Selecting Dosimetry Systems for Application in Pulsed
X-Ray Sources
This standard is issued under the fixed designation E1894; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide provides assistance in selecting and using dosimetry systems in flash X-ray experiments. Both dose and dose-rate
dose rate techniques are described.
1.2 Operating characteristics of flash X-ray sources are given, with emphasis on the spectrum of the photon output.
1.3 Assistance is provided to relate the measured dose to the response of a device under test (DUT). The device is assumed to be
a semiconductor electronic part or system.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E170 Terminology Relating to Radiation Measurements and Dosimetry
E666 Practice for Calculating Absorbed Dose From Gamma or X Radiation
E668 Practice for Application of Thermoluminescence-Dosimetry (TLD) Systems for Determining Absorbed Dose in
Radiation-Hardness Testing of Electronic Devices
E1249 Practice for Minimizing Dosimetry Errors in Radiation Hardness Testing of Silicon Electronic Devices Using Co-60
Sources
2.2 ISO/ASTM Standards:
ISO/ASTM 51261 Practice for Calibration of Routine Dosimetry Systems for Radiation Processing
ISO/ASTM 51275 Practice for Use of a Radiochromic Film Dosimetry System
ISO/ASTM 51310 Practice for Use of a Radiochromic Optical Waveguide Dosimetry System
2.3 International Commission on Radiation Units (ICRU) and Measurements Reports:
ICRU Report 14 Radiation Dosimetry: X rays and Gamma Rays with Maximum Photon Energies Between 0.6 and 50 MeV
ICRU Report 17 Radiation Dosimetry: X rays Generated at Potentials of 5 to 150 kV
This practiceguide is under the jurisdiction of ASTM Committee E10 on Nuclear Technology and Applications and is the direct responsibility of Subcommittee E10.07
on Radiation Dosimetry for Radiation Effects on Materials and Devices.
Current edition approved Dec. 1, 2018May 1, 2024. Published December 2018May 2024. Originally approved in 1997. Last previous edition approved in 20132018 as
E1894 – 13a.E1894 – 18. DOI: 10.1520/E1894-18.10.1520/E1894-24.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
For referenced ISO/ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from the International Commission on Radiation Units and Measurements, 7910 Woodmont Ave., Suite 800, Bethesda, MD 20814, U.S.A.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1894 − 24
ICRU Report 34 The Dosimetry of Pulsed Radiation
ICRU Report 51 Quantities and Units in Radiation Protection Dosimetry
ICRU Report 60 Fundamental Quantities and Units for Ionizing Radiation
ICRU Report 76 Measurement Quality Assurance for Ionizing Radiation Dosimetry
ICRU Report 77 Elastic Scattering of Electrons and Positrons
ICRU Report 80 Dosimetry Systems for Use in Radiation Processing
ICRU Report 85a Fundamental Quantities and Units for Ionizing Radiation
3. Terminology
3.1 absorbed dose enhancement—increase (or decrease) in the absorbed dose (as compared to the equilibrium absorbed dose) at
a pointlocation in a material of interest. This can be expected to occur near an interface with a material of higher or lower atomic
number.
3.2 converter—a target for electron beams, generally of a high atomic number material, in which bremsstrahlung X-rays are
produced by radiative energy lossesloss of the incident electrons.
3.3 dosimetry system—a system used for determining absorbed dose, consisting dose which consists of dosimeters, measurement
instruments, andinstruments with their associated reference standards, and procedures for the system’s use.
3.4 DUT—device under test. This is the electronic component or system being tested to determine its performance during orand/or
after irradiation.
3.5 endpoint energy—endpoint energy refers to the peak energy of the electron beam, usually in MeV, generated in a flash X-ray
source and is numerically equal to the maximum voltage in MV. The word endpointterminal voltage of the accelerator in megavolts
(MV). The word “endpoint” refers to the highest photon energy of the bremsstrahlung spectra, and this endpoint is equal to the
maximum or peak in the electron energy. For example, if the most energetic electron that strikes the converter is 10 MeV, this
electron produces a range of bremsstrahlung photon energies but the maximum energy of any photon is equal to 10 MeV, the
endpoint energy. Most photons have energies one-tenth to one-third one tenth to one third of the maximum electron energy for
typical flash X-ray sources in the 101 MV to 110 MV endpoint voltage region, respectively.
3.6 endpoint voltage—Endpoint voltage refers to the peak voltage across a bremsstrahlung diode in a flash X-ray source. For
example, a 10-MV flash X-ray source is designed to reach a peak voltage of 10-MV 10 MV across the anode-cathode gap which
generates the electron beam for striking a converter to produce bremsstrahlung.
3.7 equilibrium absorbed dose—absorbed dose atwithin some incremental volume withinin the target material in which the
condition of electron equilibrium (the energies, number, and direction of charged particles induced by the radiation are constant
throughout the volume) exists. For lower energies electron energies, where bremsstrahlung production is negligible, the
equilibrium absorbed dose is equal to the kerma.
NOTE 1—For practical purposes, assuming the spatial gradient in the X-ray field is small over the range of the maximum energy secondary electrons
generated by the incident photons, the equilibrium absorbed dose is the absorbed dose value that exists in a material at a distance from any interface with
another material greater than this range.
4. Significance and Use
4.1 Flash X-ray facilities provide intense bremsstrahlung radiation environments, usually in a single sub-microsecond pulse, which
often fluctuates in amplitude, shape, and spectrum from shot to shot. Therefore, appropriate dosimetry must be fielded on every
exposure to characterize the environment, see ICRU Report 34. These intense bremsstrahlung sources have a variety of
applications which include the following:
4.1.1 Studies of the effects of X-rays and gamma rays on materials.
(1) Studies of the effects of X-rays and gamma rays on materials.
(2) Studies of the effects of radiation on electronic devices such as transistors, diodes, and capacitors.
(3) Computer code validation studies.
4.1.2 Studies of the effects of radiation on electronic devices such as transistors, diodes, and capacitors.
E1894 − 24
4.1.3 Computer code validation studies.
4.2 This guide is written to assist the experimenter in selecting the needed dosimetry systems for use at pulsed X-ray facilities.
This guide also provides a brief summary of the information on how to use each of the dosimetry systems. Other guides (see
Section 2) provide more detailed information on selected dosimetry systems in radiation environments and should be consulted
after an initial decision is made on the appropriate dosimetry system to use. There are many key parameters which describe a flash
X-ray source, such as dose, dose rate, spectrum, pulse width, etc., such that typically no single dosimetry system can measure all
the parameters simultaneously. However, it is frequently the case that not all key parameters must be measured in a given
experiment.
5. General Characteristics of Flash X-ray Sources
5.1 Flash X-ray Facility Considerations—Overview—Flash X-ray sources operate like a dental X-ray source but at much higher
voltages and intensities and usually in a single, very short burst, see ICRU Report 17. A high voltage is developed across an
anode-cathode gap (the diode) and field emission creates a pulsed electron beam traveling from the cathode to the anode. A high
atomic–number high-atomic-number element such as tantalum is placed on the anode to maximize the production of
bremsstrahlung created when the electrons strike the anode. Graphite or aluminum is usually placed downstream of the converter
to stop the electron beam completely but let the X-radiation pass through. Finally, a debris shield made of Kevlar or low-density
polyethylene is sometimes necessary to stop exploding converter material from leaving the source. All of these components taken
together form what is commonly called a bremsstrahlung diode.
5.2 Relationship Between Flash X-ray Diode Voltage and X-ray Energy of Bremsstrahlung—Flash X-ray sources produce
bremsstrahlung by generating an intense electron beam which then strikes a high atomic number (Z) converter such as tantalum.
The electron-solid interactions produce “braking” radiation or, in German, bremsstrahlung. Fig. 1 shows the typical range of
photon energies produced by three different sources. flash X-ray sources with different electron endpoint energies. The data in Fig.
1 is generated by tallying the photon spectrum using ITS with optimized tantalum/carbon bremsstralung converters (1). If the
average radiation produced is in the 20–100 20 to 100 keV region, the source is said to be a medium–hard X-ray
simulator.“medium X-ray simulator.” If the average photon energy is in the 100 –to 300 keV region, the term used is “hard X-ray
simulator.” At the high end of the flash X-ray range are sources which produce an average photon energy of around 2 MeV. Because
this photon energy is in the typical gamma-ray spectral range, the source is called a gamma-ray simulator.“gamma-ray simulator.”
5.2.1 The average energy of the bremsstrahlung spectrum, E¯ , through an optimized converter can be estimated using the
photon
following relationship (1):
FIG. 1 Range of Available Bremsstrahlung Spectra from Flash X-ray Sources with Optimized Bremsstrahlung Converters
The boldface numbers in parentheses refer to the list of references at the end of this standard.
E1894 − 24
¯
E 5 k·=ε where5.1,k,18.9 (1)
photon
The average energy of the bremsstrahlung spectrum, E¯ , through an optimized converter can be estimated using the
photon
following relationship (1):
¯
E 5 k·=ε where5.1,k,18.9 (1)
photon
where E¯ is the average energy of the bremsstrahlung photons in keV and ε is the average energy of the electrons in the
photon
electron beam incident on the converter in keV. The value of k depends on the converter thickness:thickness; thin targets will
have values at the lower end of the range, while thick targets optimized for higher incident energies will have values at the
upper end. When an optimized bremsstrahlung converter is used, a rule-of-thumb may be used that the average photon energy
1 1
is about ⁄5 or ⁄6 of the electron endpoint energy (1). For a fixed converter design, the photon energy away from the optimiza-
tion point is roughly proportional to the square root of the electron endpoint energy with the proportionality factor varying
between about 5 and 19 depending upon the design point (1). This equation and Fig. 1 indicate that most of the photons have
energies much less than the endpoint electron energy, or in voltage units, the flash X-ray voltage. Additionally, the
bremsstrahlung spectrum is very non-Gaussian so caution must be exercised in using the average energy of the distribution for
dosimetry planning.
6. Measurement Principles
6.1 Typically in flash X-ray irradiations, one is interested in some physical change in a critical region of a device under test (DUT).
The dosimetry associated with the study of such a physical change may be broken into three parts:
6.1.1 Determine the absorbed dose in a dosimeter.
(1) Determine the absorbed dose in a dosimeter.
(2) Using the dosimeter measurement, estimate the absorbed dose in the region and material of interest in the DUT.
(3) If required, relate the estimated absorbed dose in the DUT to the physical change of interest (holes trapped, interface states
generated, photocurrent produced, etc.).
6.1.2 Using the dosimeter measurement, estimate the absorbed dose in the region and material of interest in the DUT.
6.1.3 If required, relate the estimated absorbed dose in the DUT to the physical change of interest (holes trapped, interface states
generated, photocurrent produced, etc.)
6.2 This section will be concerned with the first two of the above listed parts of dosimetry: (1)(1) what is necessary to determine
a meaningful absorbed dose for the dosimeter and (2)dosimeter, and (2) what is necessary to extrapolate this measured dose to the
estimated dose in the region of interest. The final step in dosimetry, associating the absorbed dose with a physical change of
interest, is outside the scope of this guide.
6.3 Energy Deposition:
6.3.1 Secondary Electrons—Both in the case of absorbed dose in the DUT and absorbed dose in the dosimeter, the energy is
deposited largely by secondary electrons. That is, the incident photons interact with the material of, or surrounding, the DUT or
the dosimeter and lose energy to Compton electrons, photoelectrons, and Auger electrons. The energy which is finally deposited
in the material is deposited by these secondary particles.
6.3.2 Transport of Photons—In some cases, it is necessary to consider the transport and loss of photons as they move to the region
whose absorbed dose is being determined. A correction for the attenuation of an incident photon beam is an example of such a
consideration.
6.3.3 Transport of Electrons—Electron transport may cause energy originally imparted to electrons in one region to be carried to
a second region depending on the range of the electrons. As a result, it is necessary to consider the transport and loss of electrons
as they move into and out of the regions whose absorbed dose is being determined. In particular, it is necessary to distinguish
between equilibrium and non-equilibrium conditions for electron transport.
6.3.3.1 Charged Particle Equilibrium—In some cases, Occurs when the numbers, energies, and angles of particles transported into
a region of interest are approximately balanced by those transported out of that region. Such cases form an important class of
limiting cases which are particularly easy to interpret. (See “Equilibrium Absorbed Dose” in 3.7.)
6.3.3.2 Dose Enhancement—Dose Enhancement—Because Because photoelectron production per atom is roughly proportional to
E1894 − 24
the atomic number raised to the fourth power for energies less than 100 keV (2), one expects more photoelectrons to be produced
in high atomic number layers than in low atomic number layers for the same photon fluence and spectrum. Thus, there may be
a net flow of energetic electrons from the high atomic number layers into the low atomic number layers. This non–equilibriumnon-
equilibrium flow of electrons may result in an enhancement of the dose in the low atomic number layer. Dose enhancement
problems are often caused by high atomic number bonding layers (for example, gold),gold) and metallization layers (for example,
W–Si or Ta–Si).W-Si or Ta-Si).
6.4 Absorbed Dose in Dosimeter:
6.4.1 Equilibrium Absorbed Dose in Dosimeter:
6.4.1.1 It is frequently possible to use dosimeters under approximate equilibrium conditions. The interpretation of the output of
the dosimeter is straightforward only when the energy deposition processes within the dosimeter are approximately in equilibrium.
That is, when the absorbed dose within the dosimeter is an equilibrium absorbed dose.
6.4.1.2 It is possible to treat non–equilibriumnon-equilibrium energy deposition within a dosimeter, but such an analysis requires
electron and photon transport calculations, often in the form of computer codes.
6.4.2 Limiting Cases:
6.4.2.1 There are two limiting cases for which the dosimeter data can be analyzed in a straightforward manner.
6.4.2.2 Limiting Case One: Short Electron Range:
(1) For this case, secondary electron ranges are small in comparison with the size of the dosimeter.
(2) Essentially all electrons which deposit energy within the dosimeter will be produced within the dosimeter.
(3) Non-equilibrium effects due to electron transport are negligible, but photon attenuation corrections may be necessary.
(4) An example of this limiting case would be is: 20 keV photons depositing energy in a typical (0.889 mm thick)
thermoluminescence (TL) dosimeter (TLD). In this case, the secondary electrons have ranges which are small in comparison with
the size of the TL dosimeter. TLD. As a result, it is not necessary to perform a correction for the effect of electron transport on
absorbed dose. On the other hand, 20 keV photons may be significantly attenuated while traveling through a TL dosimeter TLD
depending on the material. Thus a correction due to this effect may be necessary.
6.4.2.3 Limiting Case Two: Large Electron Range:
(1) When the maximum secondary electron range is large compared with the size of the dosimeter, the dosimeter must be
surrounded by an equilibrating layer. This layer must be chosen to be of an appropriate thickness, density, and atomic number.
Generally, the range of secondary electrons must be smaller than the thickness of the equilibrating layer. For further discussion of
equilibrating layers, see Practice E668.
(2) Essentially all electrons which deposit energy within the dosimeter originate in the equilibrating layer.
(3) Bragg-Gray cavity theory applies. That is, the dose within the dosimeter is the equilibrium dose for the equilibrating layer
(corrected by the differences in electron stopping power of the dosimeter and the equilibrating layer and any photon attenuation
through the material).
(4) An example of this limiting case is 1–MeV 1 MeV photons incident on a typical TL dosimeter TLD surrounded by an
appropriate equilibrating layer. In this case, the range of the secondary electrons will be large in comparison to the size of the TLD.
Thus the dose measured will be the equilibrium dose in the TLD (with a small correction for the differences in the stopping power
for the electrons in the TLD material and the material of the equilibrating layer.)layer).
6.4.2.4 Intermediate CasesCases:
(1) The intermediate cases, where secondary electron ranges are neither small nor large in comparison to the dosimeter size,
are cases where non-equilibrium energy deposition is to be expected.
(2) An example of an intermediate case is 100–keV is: 100 keV photons incident on a typical thermoluminescence
dosimeter.TLD.
(3) The careful treatment of dosimetry for intermediate cases requires the use of combined photon/electron radiation transport
calculations of the energy deposition in the dosimeters in order to get satisfactory measurements.
6.5 Absorbed Dose in Device Under Test:
6.5.1 Absorbed Dose in Device Under Test—The conditions within a DUT during a flash X-ray irradiation are frequently far from
equilibrium. In many cases, these are classic examples of experiments showing experiments will observe dose enhancement effects.
E1894 − 24
As a result, it is frequently necessary to perform a dose-enhancement correction in order to estimate the absorbed dose within the
region of interest. Unless the DUT meets the two limiting cases discussed in the previous section, which is rarely true, the dose
should be obtained using combined photon/electron radiation transport calculations of the energy deposition in order to get
satisfactory dose estimates. Within a system being tested, a calculation of the radiation transport through the system to the
individual devices of interest must be performed.
6.6 Spectral Considerations:
6.6.1 Broad Energy Range—The set of available flash X-ray sources spans a very wide photon energy range. Useful intensities
may be obtained for energies as low as 10 keV and as high as 10 MeV. Each individual flash X-ray source, of course, does not
produce useful photons over such a wide range. Dosimetry for such flash X-ray sources is somewhatcan be simplified if the
different flash spectra are categorized into three types,types: low energy, medium energy, and high energy. This categorization
refers to the average energy of the photon spectrum and not the higher electron endpoint energy (5.2).
6.6.2 Dosimetry for Three Energy Types—Bands—A summary of the dosimetry requirements for the three flash X-ray energy
bands is provided in Table 1.
6.7 Absorbed Dose Interpretation:
6.7.1 In 6.1.26.1, it is pointed out that the second of three steps in the dosimetry process is the use of the measured absorbed dose
in the dosimeter to estimate the absorbed dose in a region of interest within the device under test.
6.7.2 In the previous sections the dose in both the dosimeter (6.4) and the DUT (6.5) is estimated. Although the dose can easily
be calculated from tables or a simple photon transport code for the dosimeter, which is often in equilibrium, the dose in the DUT,
which is rarely in equilibrium, often requires a sophisticated combined photon/electron radiation transport code. Assuming this has
been accomplished, one can use the ratio of the calculated doses at a given fluence and spectrum as the correction factor for the
measured dose in the dosimeter to estimate the actual dose in the DUT.
D
DUT
D 5 Calculated ×D (2)
S D
DUT Dosimeter
D
Dosimeter
where D is the absorbed dose in the device material, and D is the equilibrium absorbed dose in the dosimeter.
DUT Dosimeter
where D is the absorbed dose in the device material, and D is the equilibrium absorbed dose in the dosimeter.
DUT Dosimeter
TABLE 1 Flash X-ray Dosimetry Characteristics for Three Energy Bands
Flash X-Ray Type Absorbed Dose in Dosimeter Absorbed Dose in DUT
A
Low energy (average photon energy, 20-100 keV Can get electron equilibrium. May need photon Depends on DUT:
B
C
transport correction.
May need an electron transport calculation. May need a
B
photon transport calculation.
A
Low energy (average photon energy: 20–100 keV) Can get electron equilibrium. May need photon Depends on DUT:
B
C
transport correction.
May need an electron transport calculation. May need a
B
photon transport calculation.
Medium energy (average photon energy, 100-300 Often cannot get electron equilibrium and therefore Depends on DUT:
C,D
keV) need electron transport calculation. May need an electron transport calculation, especially
C,E
without use of beam filtration. May need a photon
B
transport calculation.
Medium energy (average photon energy: 100–300 Often cannot get electron equilibrium and therefore Depends on DUT:
C,D
keV) need electron transport calculation. May need an electron transport calculation, especially
C,E
without use of beam filtration. May need a photon
B
transport calculation.
High energy (average photon energy, 300-3000 Can get electron equilibrium with proper equilibration Depends on DUT:
F,D C
keV) layer. May need an electron transport calculation. Usually no
D
photon transport calculation needed.
High energy (average photon energy: 300–3000 Can get electron equilibrium with proper equilibration Depends on DUT:
F,D C
keV) layer. May need an electron transport calculation. Usually no
D
photon transport calculation needed.
A
The dosimeter or region of interest is large compared to the maximum secondary electron range.range (6.4.2.2)).
B
The dosimeter or region of interest is large compared to the photon range.range (6.4.2.2, Paragraph 4)4).
C
The dosimeter or region of interest is of comparable size to the maximum secondary electron range.range (6.4.2.4)).
D
The dosimeter or region of interest is small compared to the photon range.range (6.4.2.3, Paragraph 4)4).
E
A filter may be used to essentially eliminate the lower energy portions of the flash X-ray spectrum. This makes the spectrum more nearly monochromatic and may simplify
dosimetry.
F
The dosimeter or region of interest is small compared to the electron range.range (6.4.2.3)).
E1894 − 24
7. Dosimetry Systems
7.1 Introduction—In this section a brief summary of eight different types of dosimetry systems is given. The intent of this guide
is to provide enough information so that users can decide which system might be appropriate for their application. Users are
expected to investigate in more detail the proper use and limitations of a system using references in this and other ASTM
documents before undertaking any radiation measurements. The pros and cons of each system are quickly summarized in Table
2Tables 2 and 3 and Table 3. The values cited in these Tablestables are indicative of what is typically obtained under normal testing
conditions. Many of these parameters can be improved at the expenditure of considerable effort. Only the precision of each
dosimeter is discussed here because the accuracy of the dosimeter will be determined by the quality of the calibration and specific
usage and this is beyond the scope of this document.
7.2 Thermoluminescence Dosimeters:
7.2.1 Introduction—Thermoluminescence Dosimetersdosimeters (TLDs), see Terminology E170, are popular dosimeters because
they are small, passive, inexpensive, and can retain accurate dose information for long periods of time between irradiation and
readout. The dose range of TLDs is typically
–4 3 –2 5
> 10>10 to < 5 <5 × 10 Gy (> 10(>10 to < 5 <5 × 10 rad).
7.2.1.1 Many materials are available for thermoluminescence dosimetry; however, the favorites for radiation hardness testing are
Lithium Fluoride (LiF), Manganese activated Calcium Fluoridelithium fluoride (LiF), manganese activated calcium fluoride
(CaF :Mn), and Dysprosium activated Calcium Fluoridedysprosium activated calcium fluoride (CaF :Dy). Dosimeters are
2 2
available as powder, chips made from polycrystalline material, and discs consisting of very fine powder uniformly dispersed
throughout a polytetrafluoroethylene (PTFE, Teflon) (PTFE) matrix. A commonly used size of chip is 3.2 by 3.2 by 0.9 mm (0.125
by 0.125 by 0.035 in.)in.). A commonly used size of TeflonPTFE dosimeter is a disc, 6 mm in diameter and 0.4 mm thick. In
addition, these dosimeter materials can be made into arrays for dose mapping.
7.2.2 Principles—Thermoluminescent materials consist of a crystalline insulator with added dopants which introduce stable
electron traps into the forbidden band gap. Ionizing radiation creates electrons and holes which are trapped by stable traps in the
band gap. The density of filled traps is proportional to the dose absorbed by the material. Subsequent heating of the material
empties the electron traps, allowing electrons from F-centers to recombine with free holes at luminescence centers, emitting light.
The integrated light output is proportional to the density of filled traps, and therefore to the absorbed dose in the TLD material.
7.2.2.1 To make an equilibrium dose measurement, the TLD must be enclosed in an equilibrium capsule of the appropriate
material and thickness when it is exposed. Methods for determining equilibrium capsule thickness are given in Practice E666.
7.2.2.2 Readout of TLDs is accomplished by an instrument consisting of a heater, optical system, and photomultiplier detector to
measure the light emitted by the TLD during a predetermined heating cycle, and an integrating picoammeter which can measure
the current or the charge from the photomultiplier. The total integrated charge from the photomultiplier during part or all of the
heating cycle is usually related to the absorbed dose in the dosimeter. TLD readout instruments are available from a number of
manufacturers.
7.2.3 Applications—The choice of the TLD and capsule material depend on the application. For high-energy photon sources
60 137
(photon energy ≥ 200 ≥200 keV) such as high voltage flash X-ray generators and Co Co and Cs Cs sources, and where the
TABLE 2 Dose Measurements
Radiochromic
Dosimeter Type TLD Calorimeter Optichromic
Film
-4 3 5 4
Dose Range 10 –5 × 10 10–10 0.4–2 × 10 0.01–5 ×
(Gy) 10
Photon Energy >0.01 >0.01 >0.01 >0.01
Range (MeV)
Size Small Medium Medium Small to
Large
Ease of Easy Easy Easy Moderate
calibration
Precision 5-15 % 5 % 5 % 5 %
Precision 5–15 % 5 % 5 % 5 %
Active Material CaF , LiF Au, Si, Al Organic Organic
Availability Buy Make or Buy Buy Buy
System Cost Medium Low Low Low to High
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TABLE 3 Dose-Rate Dose Rate Measurements
Cerenkov/
Compton
Dosimeter Type PIN Diode Scintillator- PCD
Diode
Photodetector
1 8 7 10 4 9
Dose-Rate 2 × 10 –10 10 –10 10 –10 2 ×
5 10
Range 10 –10
(Gy/s)
1 8 7 10 4 9
Dose Rate 2 × 10 –10 10 –10 10 –10 2 ×
5 10
Range 10 –10
(Gy/s)
Photon Energy >0.01 >0.5 >0.02 >0.01
Range (MeV)
Size Small to Large Very Large Small to
Medium Medium
Ease of Moderate Difficult Moderate Moderate
calibration
Precision 10 % 10 % 4 % 10 %
Active Material Si,GaAs Tungsten Plastic Diamond,
GaAs
Availability Buy Buy or Make Make Buy or Make
System cost Low Medium High Medium
quantity of interest is Si or SiO equilibrium dose, a good combination of materials is a CaF :Mn TLD in an Al equilibrium shield
2 2
(See(see Practice E1249). For these materials and photon energies, all mass energy absorption coefficients and mass stopping
powers are so close to those of silicon that the silicon equilibrium dose can be determined using Bragg-Gray cavity theory. The
response of CaF :Mn is nearly linear with absorbed dose and the dosimeters retain dose information for a long time with a small
fading correction,correction whose form is well known (7.2.5).
7.2.3.1 For radiation sources which have significant photon a significant portion of photons with energy below 200 keV, more
detailed calculations are necessary regardless of which material is chosen for the TLD. In this case the choice of TLD and capsule
material is flexible, and can be made on the basis of convenience or cost.
7.2.3.2 The choice of whether powder, chips, or PTFE discs are used is primarily one of convenience and cost. For specific
instructions on the proper procedures for using TLD systems to determine absorbed dose in radiation hardness testing of electronic
devices, see Practices E666 and E668.
7.2.4 Advantages—TLDs are small, inexpensive, and require no instrumentation during irradiation. They are thus ideally suited
for measuring dose at many locations within a test object, or for measuring dose near the areas of interest in even very small (;3
mm) test objects. Their large sensitivity range makes it possible to measure GymGy to kGy doses with one dosimetry system.
7.2.5 Limitations—Energy Range: TLDs are usable over the photon energy range from at least 10 keV to 10 MeV. (Indeed, their
sensitivity extends down into the ultraviolet region; hence, TLDs must be protected from exposure to sunlight, etc., especially for
low dose applications.) The dose response as a function of energy can be calculated for TLDs which consist of 100%100 % TL
phosphor by using standard equilibrium dose and Bragg-Gray cavity theories.theories (Practice E666)). However, care must be
exercised when using PTFE disc dosimeters at low energies. Since these dosimeters consist of fine grains of TL phosphor
distributed throughout a PTFE matrix, these simple theories domay not apply. At high energies (photon energy ≥ 2 ≥2 MeV), the
TLD can be treated using Bragg-Gray cavity theory, with all photon interactions assumed to take place in the equilibrium shield
material, and the resultant secondary electrons depositing energy in the phosphor grains. There is a smooth transition to more
moderate energies (photon energy ≈ 200 ≈200 keV) where the photon interactions must be considered to occur primarily in the
PTFE matrix. Finally, at the lowest photon energies (photon energy ≤ 10-20 ≤10 to 20 keV), all photon interactions which
eventually deposit energy in the phosphor grains must be assumed to originate in the individual phosphor grain itself. The dose
response of PTFE TLDs as a function of photon energy is therefore a complex function of energy which is not easy to derive
analytically. A modern electron/photon transport code such as ITS (3) can be helpful in calculating this relationship (4).
7.2.5.1 Most TLDTL phosphors exhibit fading of the TL response with time; that is, the observed TL response becomes
progressively smaller as the time interval between irradiation and readout increases (5). For most of the common TL phosphors
the fading behavior is well known. The fading of CaF :Mn TLDs has been reported to obey the relationship (6)
20.017
TL /TL 5 t/t (3)
~ ! ~ !
o o
where:
20.017
TL /TL 5 t/t (3)
~ ! ~ !
o o
t is in hours and the subscript o refers to the time of irradiation, and unsubscripted quantities refer to the time of readout. The
E1894 − 24
value of the coefficient 0.017 is only typical and can vary from batch to batch and must be determined experimentally. Use
the procedures in 8.7 of Practice E668 to test for fading effects in the type of TLD chosen.
where t is in hours and the subscript o refers to the time of irradiation, and unsubscripted quantities refer to the time of readout.
The value of the coefficient 0.017 is only typical and can vary from batch to batch and must be determined experimentally. Use
the procedures in 8.7 of Practice E668 to test for fading effects in the type of TLD chosen.
7.2.6 Sensitivity—TLDs can be used to measure dose from 100 μGy to 5 kGy (10 mrad to 500 krad). Most TL materials saturate
in the range of 3 to 10 kGy (300 krad to 1 Mrad). The dose response of calcium fluoride TLDs is independent of dose rate for dose
10 12
rates less than 10 Gy/s (10 rad/s) (7). The dose response of lithium fluoride has been reported to be independent of dose rate
9 11
only up to 10 Gy/s (10 rad/s) (8).
7.2.7 Calibration—TLD systems (dosimeter plus reader) must be calibrated in a standard radiation field before use; see ICRU
60 137
Report 14. The most convenient sources for this are calibrated Co Co or Cs Cs sources. Care must be taken to expose the TLDs
in an appropriate equilibrium shield. If the calibration source is calibrated in units of exposure rate, then the dose absorbed by the
TLD can be found by:
µ /ρ
~ !
en TLD
@
D 5 D /X X exp 2 µ ⁄ρ ρ x# (4)
S D~ ! ~ !
TLD air 0 en eqmat
eqmat
µ /ρ
~ !
en
air
where:
D = is the dose absorbed by the TLD,
TLD
X = is the free–in–air exposure of the source,
o
X = is the free-in-air exposure of the source,
o
D /X = 33.68 Gy–kg/C (0.869 rad/R),
air
D /X = 33.68 Gy-kg/C (0.869 rad/R),
air
μ /ρ = is the mass-energy absorption coefficient,
en
ρ = is the density of the equilibrator material, and
eqmat
x = is the thickness of the equilibrator material.
7.2.8 Reproducibility—The reproducibility of most TLD materials is about 68 % at 1σ. Better reproducibility can be achieved
with TLD chips by irradiating a number of them to a low dose, reading them, and selecting those which fall into a tighter group.
A reproducibility of 62 % is often achievable, and 61 % is possible by this means.
7.3 Calorimeters:
7.3.1 Introduction—Introduction. Calorimetry comes closest of all the dosimetric techniques to providing a direct measure of dose
or fluence. Apart from corrections for thermal leakage, thermal defect due to chemical or solid state reactions, and energy loss in
fluence measurements, only the specific heat and the temperature rise of the absorber due to X-ray absorption need be known. The
specific heat or the heat capacity of the absorber is a thermodynamic property of the given material. Temperature rise
measurements can be made with thermocouples, thermopiles, resistive temperature detectors (RTDs), thermistors, etc.
7.3.2 Principles—The temperature rise of the absorbing material is related to the energy absorbed, enthalpy or dose, in the material
through its specific heat. Namely,
∆H 5 C dT (5)
*
p
It has been assumed that the absorber remains solid at constant pressure and that no phase transition or other process occurs
which would complicate this simple relationship.
It has been assumed that the absorber remains solid at constant pressure and that no phase transition or other process occurs
which would complicate this simple relationship.
7.3.2.1 The specific heat (9), that is, the heat capacity per unit mass for a typical absorbers absorber, does not vary significantly
–1 –1
with temperature, such as tantalum which varies from 139.7 to 141.6 J·kg ·K , or for aluminum which varies from 900.6 to 919.0
–1 –1
J·kg ·K between 20°C and 60°C.20 °C and 60 °C. Hence, for many materials and over a practical range of temperature changes,
a constant specific heat can be used providedwith accuracies better than 1 % are not required. For improved accuracy, the specific
heat may be expanded in a power series in as a function of temperature over the temperature range of interest.
7.3.2.2 Temperature sensors such as thermocouples (10) are used routinely by properly attaching the thermocouple wires to the
absorber. An example of a thermocouple attached to an absorber is shown in Fig. 2. The 0.025-mm 0.025 mm thick gold foil is
E1894 − 24
FIG. 2 Cutaway View of a Typical Gold Foil Dose Calorimeter Designed for Use with FXR Spectra Which Have Maximum Energies Less
Than 2 MeV (courtesy of Maxwell Laboratories, Inc., San Diego, CA).CA)
thermally isolated from the surrounding environment by supporting the foil on small nylon threads and by using small diameter
thermocouple wires. The thermocouple wires pass through a lead shield and then they are soldered to miniature connector pins with
lead free lead-free solder. The lead shield and lead free lead-free solder are used to reduce the X-ray induced temperature rise in
the solder connection. In order to To improve charged particle equilibrium and reduce fluorescence losses from the gold, a gold
backing foil is placed just behind the gold absorber. A coupled photon/electron transport code should be used to determine how
far the geometry is from achieving charged particle equilibrium for a particular spectrum. Although not shown in the figure, an
optional fine wire can be welded to the gold foil. When this is done, the calibration of the calorimeter can be checked with a proton
Van de Graaff provided the thermocouple is electrically isolated from the X-ray absorber.
7.3.2.3 Thin foil X-ray absorbers are typically designed for measuring dose in the absorbing material. A thicker absorber may be
used as a total fluence detector for low energy spectra. However, in this case a more sensitive temperature detector such as a
thermistor may be required and a much thicker absorber. A typical thermistor has a negative temperature coefficient of resistivity,
α, which decreases in magnitude from about 0.039 to 0.036/°C. This coefficient is defined by:
∆ρ /ρ 5α ∆T (6)
~ ! ~ !
o
where ρ is the original resistivity and ∆ρ and ∆T are the change in resistivity and temperature. Hence, care must be taken to
o
measure the temperature both before and after X-ray energy absorption. The temperature sensitivity of a thermistor may be
closely approximated with the Steinhart-Hart equation (11),
1/T 5 A1B lnR 1C lnR (7)
~ ! ~ !
where T is in K and R is the thermistor resistance. The values of A,B, and C should be measured for each thermistor and
these values should be remeasured systematically to ensure that the thermistor has not been damaged.
where ρ is the original resistivity and ∆ρ and ∆T are the change in resistivity and temperature. Hence, care must be taken to
o
measure the temperature both before and after X-ray energy absorption. The temperature sensitivity of a thermistor may be closely
approximated with the Steinhart-Hart equation (11):
1/T 5 A1B lnR 1C lnR (7)
~ ! ~ !
where T is in K and R is the thermistor resistance. The values of A,B, and C should be measured for each thermistor and these
values should be remeasured systematically to ensure that the thermistor has not been damaged.
7.3.3 Advantages—Calorimeters Calorimeter dose measurements can be made absolute, either intrinsically or by means of
electrical-heating electrical heating calibration.
E1894 − 24
7.3.3.1 The measurement of temperature rise comes closest of any dosimetric technique to being a direct measurement of the
energy involved in the absorbed dose.
7.3.3.2 Calorimeters are inherently dose-rate-independent dose rate independent under adiabatic conditions, and become more
convenient to use as the dose-rate dose rate increases because thermal leakage during dose delivery becomes negligible.
7.3.3.3 Metal film calorimeters have no LET dependence (neglecting minor differences in thermal defect, if any), since ionic
recombination is irrelevant to the temperature rise.
7.3.3.4 The conversion of absorbed dose to a temperature rise takes place on the order of a few picoseconds. Hence, the
temperature rise in the material for current flash X-rays follows the running integral of the dose rate, and calorimeters can be
designed to measure the true absorbed energy in a small region or the average dose in a larger volume after thermal equilibrium.
7.3.4 Limitations—The temperature rises to be measured are typically small and (in many cases) are only fractions of a degree.
This limits calorimetry to Therefore, calorimetry performs best with relatively large doses (12).
7.3.4.1 The calorimeter absorber must be designed to allow the measurement of the temperature rise before excessive thermal
losses takestake place. This requirement must be considered in conjunction with requiring charged particle equilibrium.
7.3.4.2 The recording of the calorimeter signals are different than most of the other signals associated with flash X-ray tests and,
consequently, the test has an additional complication when calorimeters are used. In many cases calorimeter signals are a few tens
of microvolts to a few millivolts and must be recorded with a few hundred hertz frequency response. In these cases
high-input-impedance, high-gain amplifiers in conjunction with low pass filters are often used.
7.3.4.3 In some instances the dose from extremely high fluences from lower energy spectra can be high enough to either melt the
absorber or, for slightly lower dose values, generate a thermomechanical shock which may break the absorber, the temperature
sensor, or perhaps, detach the temperature sensor from the absorber.
7.3.5 Sensitivity—Apart from the sensitivity of the temperature sensor and the specific heat of the absorber, the basic sensitivity
of the dose calorimeter is dependent upon the mass energy absorption coefficient of the X-ray absorber. The degree to which the
absorber deviates from this ideal cross section must be calculated with a code such as the ITS (3). A similar comment can be made
for total-fluence calorimeters except that in this case the deviation from total incident energy absorption would be calculated.
7.3.6 Calibration—The output of a properly designed calorimeter can be interpreted from the intrinsic knowledge of the specific
heat of the absorbing material and the calibration of the temperature sensor. Alternatively, several techniques are available to
confirm that the calorimeter has been properly designed. Examples of these techniques include embedded electric heaters, proton
Van de Graaff pulse heating, and flash lamp pulse heating. The temperatur
...