1、Designation: E 666 03Standard Practice forCalculating Absorbed Dose From Gamma or X Radiation1This standard is issued under the fixed designation E 666; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A n
2、umber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This practice presents a technique for calculating the
3、absorbed dose in a material from knowledge of the radiationfield, the composition of the material, (1-5)2,3and a relatedmeasurement. The procedure is applicable for X and gammaradiation provided the energy of the photons fall within therange from 0.01 to 20 MeV.1.2 A method is given for calculating
4、the absorbed dose ina material from the knowledge of the absorbed dose in anothermaterial exposed to the same radiation field. The procedure isrestricted to homogeneous materials composed of the elementsfor which absorption coefficients have been tabulated (2). Italso requires some knowledge of the
5、energy spectrum of theradiation field produced by the source under consideration.Generally, the accuracy of this method is limited by theaccuracy to which the energy spectrum of the radiation field isknown.1.3 The results of this practice are only valid if chargedparticle equilibrium exists in the m
6、aterial and at the depth ofinterest. Thus, this practice is not applicable for determiningabsorbed dose in the immediate vicinity of boundaries betweenmaterials of widely differing atomic numbers. For more infor-mation on this topic, see Practice E 1249.1.4 Energy transport computer codes4exist that
7、 are formu-lated to calculate absorbed dose in materials more preciselythan this method. To use these codes, more effort, time, andexpense are required. If the situation warrants, such calcula-tions should be used rather than the method described here.1.5 This standard does not purport to address al
8、l of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:E 170 Terminology Rel
9、ating to Radiation Measurementsand Dosimetry5E 380 Practice for Use of the International System of Units(SI) (the Modernized Metric System)6E 665 Practice for Determining Absorbed Dose VersusDepth in Materials Exposed to the X-Ray Output of FlashX-Ray Machines5E 668 Practice for Application of Therm
10、oluminescence-Dosimetry (TLD) Systems for DeterminingAbsorbed Dosein Radiation-Hardness Testing of Electronic Devices5E 1249 Practice for Minimizing Dosimetry Errors in Radia-tion Hardness Testing of Silicon Electronic Devices UsingCo-60 Sources52.2 International Commission on Radiation Units andMea
11、surements (ICRU) Reports:ICRU Report 14Radiation Dosimetry: X Rays andGamma Rays with Maximum Photon Energies Between0.6 and 60 MeV7ICRU Report 18Specification of High Activity Gamma-Ray Sources7ICRU Report 21Radiation Dosimetry: Electrons with Ini-tial Energies Between 1 and 50 MeV7ICRU Report 33Ra
12、diation Quantities and Units7ICRU Report 34The Dosimetry of Pulsed Radiation73. Significance and Use3.1 The absorbed dose is a more meaningful parameter thanexposure for use in relating the effects of radiation on materi-als. It expresses the energy absorbed by the irradiated materialper unit mass,
13、whereas exposure is related to the amount ofcharge produced in air per unit mass. Absorbed dose, asreferred to here, implies that the measurement is made under1This practice is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of Subc
14、ommitteeE10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.Current edition approved July 10, 2003. Published August 2003. Originallyapproved in 1997. Last previous edition approved in 1997 as E 66697.2The boldface numbers in parentheses refer to the list of references appen
15、ded tothis practice.3For calculation of absorbed dose in biological materials such as tissue or bone,etc., ICRU Report 14 provides more information and procedures for a more accuratecalculation than this practice.4Information on and packages of computer codes can be obtained from TheRadiation Safety
16、 Information Computational Center, Oak Ridge National Labora-tory, P.O. Box 2008, Oak Ridge, TN 37831-6362. This information center collects,organizes, evaluates, and disseminates shielding information related to radiationfrom reactors, weapons, and accelerators and to radiation occurring in space.5
17、Annual Book of ASTM Standards, Vol 12.02.6Annual Book of ASTM Standards, Vol 14.02.7Available from International Commission on Radiation Units and Measure-ments (ICRU), 7910 Woodmont Ave., Suite 800, Bethesda, MD 20814.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohoc
18、ken, PA 19428-2959, United States.conditions of charged particle (electron) equilibrium (seeAppendix X1). In practice, such conditions are not rigorouslyachievable but, under some circumstances, can be approxi-mated closely.3.2 Different materials, when exposed to the same radiationfield, absorb dif
19、ferent amounts of energy. Using the techniquesof this standard, charged particle equilibrium must exist inorder to relate the absorbed dose in one material to theabsorbed dose in another.Also, if the radiation is attenuated bya significant thickness of an absorber, the energy spectrum ofthe radiatio
20、n will be changed, and it will be necessary tocorrect for this.NOTE 1For comprehensive discussions of various dosimetry methodsapplicable to the radiation types and energies and absorbed dose rateranges discussed in this method, see ICRU Reports 14, 21, and 34.4. Calculation of Absorbed Dose4.1 The
21、absorbed dose, D, at a point may be expressed as:D 5 I*0cE!enE!/rdE (1)where c(E) is the energy fluence per unit energy at the pointof interest; en(E)/r is the mass energy absorption coefficient(2); and I is a normalizing factor. If all of the variables in Eq1 are expressed in SI units,I=1.Inthis ca
22、se the units for Dare Gy (J kg1), of c(E), are m2,ofen/r are m2kg1, and ofE are J. For an alternative use of the normalizing factor I, seeAppendix X2. For further information on the use of energyabsorption coefficients to calculate absorbed dose see thediscussion in Attix (1). The energy fluence spe
23、ctrum, c(E), isthat which is incident at the point where the dose is to bedetermined. In practice, the limits of integration are the limitsof energy over which c(E) is of a significant magnitude. Ifmaterial intervenes between the source and the point of dosedetermination, then the spectrum used in t
24、he calculation mustbe the output spectrum of the source modified by the absorbingeffects of the intervening material. The values of en(E)/r arefound in the tables of Ref 2.NOTE 2For units and terminology in reports of data, E 170 and ICRUReport 33 may be used as guides.4.2 If the material in which t
25、he absorbed dose is to becalculated is a homogeneous combination of materials notlisted in the tables of Ref 2,en(E)/r is determined as follows:4.2.1 From Ref 2, obtain values of eniE!/r for eachcomponent, i.4.2.2 Determine the atomic fraction, fi, for each component.4.2.3 Calculate en(E)/r from the
26、 following equation:enE!/r5(ifieniE!/r (2)4.2.4 Values of en(E)/r must be determined for each valueof E for which c(E) is significant, where E is the photonenergy.4.3 The integral contained in Eq 1 is evaluated numerically.The values of en(E)/r in Ref 2 are tabulated for specificenergies. In evaluat
27、ion of the integral referred to in actualpractice, it is often desirable to choose energy intervals thatwould not correspond to the tabulated values in Ref 2. In suchcases, the appropriate value of en(E)/r for the chosen energiesshould be determined by an acceptable interpolation procedure.The range
28、 of energy over the total photon spectrum is dividedinto energy intervals or bins. The width of these bins issomewhat flexible but should be chosen small enough so as notto distort the shape of the spectrum. For the purpose ofselecting appropriate values of en(E)/r, the energy valueselected for each
29、 energy interval can be taken either as thatenergy at the beginning or midpoint of each energy intervalover the entire spectrum.4.4 The spectrum, c(E), is commonly given in arbitraryunits and may be normalized to some source parameter. If astandard or calibrated dosimeter is used, then the integral
30、in Eq1 must be calculated for the material from which this dosimeteris constructed. The value of I is then given by the observeddose, D, measured by the dosimeter divided by the value of theintegral.5. Estimating the Absorbed Dose in One Material fromThat Measured in Another Material5.1 If the absor
31、bed dose is known in one material, A, thenthe absorbed dose can be estimated in another material, B,using the method described in this section.5.1.1 The absorbed dose observed inAoccurs at some depthin the region of material A; similarly, it is desired to know theabsorbed dose in material B at some
32、depth in the region ofmaterial B. If it is presumed that we know the surface energyfluence spectrum co(E) (the energy fluence spectrum incidenton the surface of materials A and B) then the energy fluencespectrum c(E) to be used in Eq 1 must be related to the knownsurface energy fluence spectrum co(E
33、). A good approximationto the attenuated energy fluence spectrum at mass-depth t isgiven byctE! 5coE!eenE!/rt(3)where t is the mass-thickness (in kgm2) of material betweenthe surface and the depth of interest, E is a particular energyrepresented in the spectrum, and ct(E) is the energy fluence perun
34、it energy at mass-depth t. For a derivation of Eq 3 seeAppendix X4. See also the qualifications of 5.1.3 and 5.1.4. Fora demonstration of the experimental plausibility of Eq 3, seeAppendix X5.5.1.2 Using Eq 1 and 3, the relationship between the knowndose DAand the desired dose DBcan be expressed asD
35、ADB5*0coE!eenAE!/rA#tA#enAE!/rA#dE*0coE!eenBE!/rB#tB#enBE!/rB#dE(4)where enA, rA, and tAare the energy absorption coefficient,the density and the relevant mass-thickness for materialA, andwhere similar notation is used for material B. For furtherdetails on the derivation of Eq 4, see Appendix X6. Al
36、l thevariables in Eq 4 are presumed to be known except the desiredvalue for DB. The integrals in Eq 4 must be performednumerically.5.1.3 The use of Eq 3 is based on the existence of chargedparticle equilibrium (for further discussion see 1.3). Thiscondition may be reasonably well met when the region
37、 ofE666032interest is at a sufficient distance from boundaries representingchanges in atomic number or material density (see AppendixX1).5.1.4 Wide Beam vs. Narrow Beam Approximation.5.1.4.1 The use of the energy coefficient, en,inEq3isbased on the assumption that the irradiation approaches the“wide
38、 beam” as opposed to “narrow beam” condition. Thewide beam and narrow beam conditions represent limitingcases which are only approximately realized for real experi-ments. In the narrow beam case, photons which are scatteredout of the narrow beam are assumed to be lost from the beam,and are assumed t
39、o have no further importance to the experi-ment. In the broad beam case, photons which are scattered outof a given small region of the broad beam are presumed to bereplaced by photons scattering in from adjacent regions of thebeam. For the narrow beam limiting case, Eq 3 should bereplaced byctE! 5co
40、E!eE!/rt(5)where is the photon attenuation coefficient. Values of(E)/r are found in the tables of Ref 2. For most practicalproblems the results of photon attenuation lie between theresults of Eq 3 and Eq 5.5.1.4.2 It is possible to determine the magnitude of thechange which would have resulted had E
41、q 1 and Eq 5 beenused rather than using Eq 1 and Eq 3 in order to develop Eq 4.The resulting change in the ratio DA/DBcalculated by Eq 4 isrelated to the factorFE! 5eenBE!/rB#teBE!/rB#teAE!/rA#teenAE!/rA#t(6)If, over the energy range of interest, F(E) differs from unityby a percentage which is great
42、er than the acceptable dosimetryerror, then the application of this practice may be inappropri-ate. In that case an appropriate transport calculation is recom-mended (see 1.5).5.1.4.3 Depending of the scattering geometry, it is possiblefor the absorbed dose to be higher than calculated either by or
43、en. This buildup of dose is often due to backscatter ofradiation within a thick sample. For an extensive discussion ofthis and similar effect, see Ref 1.6. Accuracy6.1 The accuracy of this practice depends primarily on theaccuracy to which the incident energy spectrum is known. Ingeneral, even a poo
44、r estimate of a spectrum will give a betterestimate of the absorbed dose at a given location than onewould get by assuming some sort of single“ effective photonenergy.” Although60Co and137Cs have well-defined primarygamma-ray energies, the radiation energy spectrum from mostpractical sources contain
45、s a significant Compton scatteredcomponent that could lead to significant errors if neglected (seeICRU Report 18).6.2 As stated in 1.3, the results of this practice are not validunless charged particle equilibrium conditions exist in thematerial at the depth of application. For depths less than that
46、required for equilibrium, the absorbed dose could be higher orlower than this method would predict. At depths greater thanrequired for equilibrium, the accuracy of the results dependsprimarily upon the accuracy of the attenuation correctionapplied in Eq 3 and the knowledge of the incident energyspec
47、trum.6.3 The procedures used in this method neglect the possiblenonlocality of energy deposition by secondary electrons but docorrect for production of bremsstrahlung by secondary elec-trons. For the energy range specified in this practice, theseconsiderations contribute about 5 % or less to the ove
48、ralluncertainty.7. Keywords7.1 calculation of absorbed dose; charged particle equilib-rium; radiation dosimetryAPPENDIXES(Nonmandatory Information)X1. CHARGED PARTICLE EQUILIBRIUM THICKNESSX1.1 Whenever a material is irradiated with X or gammarays, there is initially an increase in energy absorption
49、 as theradiation penetrates the material. After some finite thickness,the radiation energy absorption reaches a maximum and thendecreases. The thickness necessary to reach the maximumenergy deposition is commonly called the “charged particleequilibrium” thickness and is a function of the radiation energyand the mass energy absorption coefficient of the materialbeing irradiated.X1.2 Fig. X1.1 is a typical plot of energy deposition as afunction of depth in a material. The absorbed dose of 0.85 isuse
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