ASTM E666-2008 809 Standard Practice for Calculating Absorbed Dose From Gamma or X Radiation.pdf

1、Designation: E 666 08Standard 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 () 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 thea

3、bsorbed 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 t

4、he 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 e

5、nergy 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 ma

6、terial 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 all

8、 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:5E 170 Terminology Rel

9、ating to Radiation Measurementsand DosimetryE 380 Practice for Use of the International System of Units(SI) (The Modernized Metric System)6E 668 Practice for Application of Thermoluminescence-Dosimetry (TLD) Systems for DeterminingAbsorbed Dosein Radiation-Hardness Testing of Electronic DevicesE 124

10、9 Practice for Minimizing Dosimetry Errors in Radia-tion Hardness Testing of Silicon Electronic Devices UsingCo-60 Sources2.2 International Commission on Radiation Units andMeasurements (ICRU) Reports:ICRU Report 14Radiation Dosimetry: X Rays andGamma Rays with Maximum Photon Energies Between0.6 and

11、 60 MeV7ICRU Report 18Specification of High Activity Gamma-Ray Sources7ICRU Report 21Radiation Dosimetry: Electrons with Ini-tial Energies Between 1 and 50 MeV7ICRU Report 33Radiation Quantities and Units7ICRU Report 34The Dosimetry of Pulsed Radiation73. Terminology3.1 energy fluence spectrum(cE)th

12、e product of the par-ticle fluence spectrum (see Terminology E 170) and the particle1This practice is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.07 on Radiation Dosimetry for Radiation Effects on Materials an

13、d Devices.Current edition approved Nov. 1, 2008. Published January 2009. Originallyapproved in 1997. Last previous edition approved in 2003 as E 66603.2The boldface numbers in parentheses refer to the list of references appended tothis practice.3For calculation of absorbed dose in biological materia

14、ls 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 Information Computational Center, Oak Ridge National Labora-tory, P.O. Box

15、 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.5For referenced ASTM standards, visit the ASTM website, www.astm.org, orcont

16、act ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.6Withdrawn. The last approved version of this historical standard is referencedon www.astm.org.7Available from International Commission o

17、n 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 Conshohocken, PA 19428-2959, United States.energy. In this standard, the particles referred to are photons.The energy fluences spectru

18、m is the same as the energy fluenceper unit energy.3.2 energy fluence, cthe integral of the energy fluencespectrum over the complete range of particle energies that arepresent.3.3 mass-depth and mass-thickness, tthe product of alength traversed in a material and the mass density of thematerial. The

19、mass-depth and the mass-thickness have dimen-sions of mass per unit area.4. Significance and Use4.1 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, wher

20、eas exposure is related to the amount ofcharge produced in air per unit mass. Absorbed dose, asreferred to here, implies that the measurement is made underconditions of charged particle (electron) equilibrium (seeAppendix X1). In practice, such conditions are not rigorouslyachievable but, under some

21、 circumstances, can be approxi-mated closely.4.2 Different materials, when exposed to the same radiationfield, absorb different 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

22、another.Also, if the radiation is attenuated bya significant thickness of an absorber, the energy spectrum ofthe radiation 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 a

23、bsorbed dose rateranges discussed in this method, see ICRU Reports 14, 21, and 34.5. Calculation of Absorbed Dose5.1 The 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 absorpt

24、ion coefficient(2); and I is a normalizing factor. If all of the variables in Eq1 are expressed in SI units,I=1.Inthis case 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 t

25、he use of energyabsorption coefficients to calculate absorbed dose see thediscussion in Attix (1). The energy fluence spectrum, 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 signif

26、icant magnitude. Ifmaterial intervenes between the source and the point of dosedetermination, then the spectrum used in the 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 2F

27、or units and terminology in reports of data, E 170 and ICRUReport 33 may be used as guides.5.2 If the material in which the 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:5.2.1 From Ref 2, obtain values of

28、eniE!/r for eachcomponent, i.5.2.2 Determine the atomic fraction, fi, for each component.5.2.3 Calculate en(E)/r from the following equation:enE!/r5(ifieniE!/r (2)5.2.4 Values of en(E)/r must be determined for each valueof E for which c(E) is significant, where E is the photonenergy.5.3 The integral

29、 contained in Eq 1 is evaluated numerically.The values of en(E)/r in Ref 2 are tabulated for specificenergies. In evaluation 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 ap

30、propriate value of en(E)/r for the chosen energiesshould be determined by an acceptable interpolation procedure.The range 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 distor

31、t the shape of the spectrum. For the purpose ofselecting appropriate values of en(E)/r, the energy valueselected for each energy interval can be taken either as thatenergy at the beginning or midpoint of each energy intervalover the entire spectrum.5.4 The spectrum, c(E), is commonly given in arbitr

32、aryunits and may be normalized to some source parameter. If astandard or calibrated dosimeter is used, then the integral 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 v

33、alue of theintegral.6. Estimating the Absorbed Dose in One Material fromThat Measured in Another Material6.1 If the absorbed dose is known in one material, A, thenthe absorbed dose can be estimated in another material, B,using the method described in this section.6.1.1 The absorbed dose observed inA

34、occurs at some depthin the region of material A; similarly, it is desired to know theabsorbed dose in material B at some 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)

35、 then the energy fluencespectrum c(E) to be used in Eq 1 must be related to the knownsurface energy fluence spectrum co(E). A good approximationto the attenuated energy fluence spectrum at mass-depth t isgiven byctE! 5coE! exp enE!/rt! (3)where t is the mass-depth (in kgm2) of material betweenthe su

36、rface and the depth of interest, E is a particular energyrepresented in the spectrum, and ct(E) is the energy fluence perunit energy at mass-depth t. For a derivation of Eq 3 seeE666082Appendix X4. See also the qualifications of 6.1.3 and 6.1.4. Fora demonstration of the experimental plausibility of

37、 Eq 3, seeAppendix X5.6.1.2 Using Eq 1 and 3, the relationship between the knowndose DAand the desired dose DBcan be expressed asDADB5*0coE! expenAE!/rA#tA!#enAE!/rA#dE*0coE! exp enBE!/rB#tB#!enBE!/rB#dE(4)where enA, rA, and tAare the energy absorption coefficient,the density and the relevant mass-d

38、epth for material A, andwhere similar notation is used for material B. For furtherdetails on the derivation of Eq 4, see Appendix X6. All thevariables in Eq 4 are presumed to be known except the desiredvalue for DB. The integrals in Eq 4 must be performednumerically.6.1.3 The use of Eq 3 is based on

39、 the existence of chargedparticle equilibrium (for further discussion see 1.3). Thiscondition may be reasonably well met when the region ofinterest is at a sufficient distance from boundaries representingchanges in atomic number or material density (see AppendixX1).6.1.4 Wide Beam vs. Narrow Beam Ap

40、proximation.6.1.4.1 The use of the energy coefficient, en,inEq3isbased on the assumption that the irradiation approaches the“wide beam” as opposed to “narrow beam” condition. Thewide beam and narrow beam conditions represent limitingcases which are only approximately realized for real experi-ments.

41、In the narrow beam case, photons which are scatteredout of the narrow beam are assumed to be lost from the beam,and are assumed to 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 bereplace

42、d by photons scattering in from adjacent regions of thebeam. For the narrow beam limiting case, Eq 3 should bereplaced byctE! 5coE! exp E!/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

43、 lie between theresults of Eq 3 and Eq 5.6.1.4.2 It is possible to determine the magnitude of thechange which would have resulted had Eq 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! 5

44、expenBE!/rB#t!exp BE!/rB#t!expAE!/rA#t!exp enAE!/rA#t!(6)If, over the energy range of interest, F(E) differs from unityby a percentage which is greater than the acceptable dosimetryerror, then the application of this practice may be inappropri-ate. In that case an appropriate transport calculation i

45、s recom-mended (see 1.5).6.1.4.3 Depending on the scattering geometry, it is possiblefor the absorbed dose to be different from that calculated usingeither or en. The use of enaccounts for the buildup of dosedue to backscatter of radiation within a thick sample. For anextensive discussion of this an

46、d similar effects, see Ref 1.7. Accuracy7.1 The accuracy of this practice depends primarily on theaccuracy to which the incident energy spectrum is known. Ingeneral, even a poor estimate of a spectrum will give a betterestimate of the absorbed dose at a given location than onewould get by assuming s

47、ome sort of single“ effective photonenergy.” Although60Co and137Cs have well-defined primarygamma-ray energies, the radiation energy spectrum from mostpractical sources contains a significant Compton scatteredcomponent that could lead to significant errors if neglected (seeICRU Report 18).7.2 As sta

48、ted 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 thatrequired for equilibrium, the absorbed dose could be higher orlower than this method would predict. At depths greater thanre

49、quired for equilibrium, the accuracy of the results dependsprimarily upon the accuracy of the attenuation correctionapplied in Eq 3 and the knowledge of the incident energyspectrum.7.3 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 overalluncer