1、Designation: E 1250 88 (Reapproved 2005)Standard Test Method forApplication of Ionization Chambers to Assess the LowEnergy Gamma Component of Cobalt-60 Irradiators Used inRadiation-Hardness Testing of Silicon Electronic Devices1This standard is issued under the fixed designation E 1250; the number i
2、mmediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1
3、Low energy components in the photon energy spectrumof Co-60 irradiators lead to absorbed dose enhancement effectsin the radiation-hardness testing of silicon electronic devices.These low energy components may lead to errors in determin-ing the absorbed dose in a specific device under test. Thismetho
4、d covers procedures for the use of a specialized ioniza-tion chamber to determine a figure of merit for the relativeimportance of such effects. It also gives the design andinstructions for assembling this chamber.1.2 This method is applicable to measurements in Co-60radiation fields where the range
5、of exposure rates is 7 3 106to 3 3 102Ckg1s1(approximately 100 R/h to 100 R/s). Forguidance in applying this method to radiation fields where theexposure rate is 100 R/s, see Appendix X1.NOTE 1See Terminology E 170 for definition of exposure and itsunits.1.3 The values stated in SI units are to be r
6、egarded as thestandard. The values given in parentheses are for informationonly.1.4 This standard does not purport to address all 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 det
7、ermine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 170 Terminology Relating to Radiation Measurementsand DosimetryE 668 Practice for Application of Thermoluminescence-Dosimetry (TLD) Systems for DeterminingAbsorbed Dosein Radiation-Hardness
8、Testing of Electronic DevicesE 1249 Practice for Minimizing Dosimetry Errors in Radia-tion Hardness Testing of Silicon Electronic Devices UsingCo-60 Sources3. Terminology3.1 absorbed dose, Dquotient of d ebydm, where d eisthe mean energy imparted by ionizing radiation to the matter ina volume elemen
9、t and dm is the mass of matter in that volumeelement.D 5 de/dm (1)3.2 absorbed dose enhancement factor ratio of the ab-sorbed dose at a point in a material of interest to theequilibrium absorbed dose in that same material.3.3 average absorbed dosemass-weighted mean of theabsorbed dose over a region
10、of interest.3.4 average absorbed dose enhancement factorratio ofthe average absorbed dose in a region of interest to theequilibrium absorbed dose.3.5 dosimeterany device used to determine the equilib-rium absorbed dose in the material and at the irradiationposition of interest. Examples of such devi
11、ces include ther-moluminescence dosimeters (TLDs), liquid chemical dosim-eters, and radiochromic dye films. (See Practice E 668, for adiscussion of TLDs.)3.6 equilibrium absorbed doseabsorbed dose at someincremental volume within the material in which the conditionof electron equilibrium (the energi
12、es, number, and direction ofcharged particles induced by the radiation are constantthroughout the volume) exists. (See Terminology E 170.)4. Significance and Use4.1 Although Co-60 nuclei only emit monoenergetic gammarays at 1.17 and 1.33 MeV, the finite thickness of sources, andencapsulation materia
13、ls and other surrounding structures thatare inevitably present in irradiators can contribute a substantialamount of low-energy gamma radiation, principally by Comp-ton scattering (1, 2).3In radiation-hardness testing of electronic1This method is under the jurisdiction of ASTM Committee E10 on Nuclea
14、rTechnology and Applications and is the direct responsibility of SubcommitteeE10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.Current edition approved Jan. 1, 2005. Published January 2005. Originallyapproved in 1988. Last previous approved in 2000 as E 1250-88(2000).2For
15、referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of referenc
16、es appended tothis test method.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.devices this low-energy photon component of the gammaspectrum can introduce significant dosimetry errors for adevice under test since the equilibrium abso
17、rbed dose asmeasured by a dosimeter can be quite different from theabsorbed dose deposited in the device under test because ofabsorbed dose enhancement effects (3, 4). Absorbed doseenhancement effects refer to the deviations from equilibriumabsorbed dose caused by non-equilibrium electron transportn
18、ear boundaries between dissimilar materials.4.2 The ionization chamber technique described in thismethod provides an easy means for estimating the importanceof the low-energy photon component of any given irradiatortype and configuration.4.3 When there is an appreciable low-energy spectral com-ponen
19、t present in a particular irradiator configuration, specialexperimental techniques should be used to ensure that dosim-etry measurements adequately represent the absorbed dose inthe device under test. (See Practice E 1249.)5. Apparatus5.1 Ionization Chamber, a specially fabricated parallel-plateioni
20、zation chamber with interchangeable gold and aluminumelectrodes. A specific design is described in Appendix X2.5.2 Bias Supply, a battery or power supply capable ofdelivering 60 to 100 V dc at a current up to 1 mA.5.3 Electrometer, an electrometer or picoammeter capableof measuring currents as low a
21、s 30 pA with a resolution of atleast 0.1 pA.5.4 Twinaxial Cable, the twinaxial cable that connects theionization chamber to the bias supply and electrometer is anintegral part of the ionization chamber (see Fig. 1).NOTE 2The ionization chamber dimensions given inAppendix X2 areappropriate to TWC 78-
22、2 twinaxial cable.4This cable has the followingphysical dimensions (all dimensions given in inches):Nominal outer diameter 0.242Conductor spacing (center to center) 0.076Conductor dielectric outer diameter 0.076Conductor diameter 0.037Other equivalent twinaxial cable types can be used, but the appli
23、cabledimensions of the ionization chamber body, clamp, stem, and cable clampnut in Appendix X2 must then be adjusted.5.5 Triaxial Cable, the triaxial cable that connects theionization chamber and the bias supply to the electrometer isusually supplied with the electrometer, and must be of a typethat
24、is compatible with the electrometer type used (see Fig. 1).6. Procedure6.1 Assemble the ionization chamber, bias supply, andelectrometer as shown in Fig. 1.6.2 Turn on the bias supply, set the voltage to at least 60 V,and ensure that there is no appreciable leakage current (Ileak-age5) indicate a ve
25、ry large low-energyphoton component. Appendix X3 gives a table of measuredvalues of a for a variety of typical Co-60 irradiator facilitiesand experimental arrangements.NOTE 4Monoenergetic 1.25 MeV photon radiation would theoreti-cally produce a value of a = 1.6.Although this value is not attainable
26、withany realistic Co-60 irradiator configuration, it is a theoretical lower limiton a.7.2 If the measured value of a is 2.5, steps 6.1-6.5 shouldbe repeated with the ionization chamber surrounded by a filtercan or box of 1.5 to 2.0 mm (approximately 0.063 in.) of leadon the outside and 0.7 to 1.0 mm
27、 (approximately 0.030 in.) ofaluminum on the inside. Use of such a filter will normally givea significant reduction in the low-energy component of thespectrum (see Practice E 1249).7.3 By repeating the procedure for a number of sourceconfigurations and filter options, the experimental conditionscan
28、be determined that minimize the low-energy photon4Available from Trompeter Electronics, 31186 La Baya Dr., Westlake Village,CA 91362-4047.FIG. 1 Schematic Diagram of Experimental SetupE 1250 88 (2005)2component of the source spectrum and thus minimize thedosimetry errors for the device under test.8.
29、 Application to Hardness Testing8.1 Estimating the Absorbed Dose Enhancement Factor:8.1.1 Although it is not possible to determine the absorbeddose enhancement factor for a particular geometry of a deviceunder test using this method, the figure of merit, a, can be usedto estimate an upper bound for
30、the absorbed dose enhancementfactor near an interface between any two materials (5).8.1.2 Aspecific example of relating the figure of merit, a,tothe absorbed dose enhancement is given in 8.1.4 for the case ofa silicon-gold interface.This example is of particular interest inradiation-hardness testing
31、 of silicon electronic devices becauseit does exist for many devices, and is a worst-case configura-tion.NOTE 5Silicon-gold interfaces in electronic devices typically consistof relatively thin layers; however, the case considered here is an interfacebetween two layers each having a thickness capable
32、 of producingabsorbed dose equilibrium. This case has been used because it representsa configuration that is relatively easy to calculate. Further, it gives a worstcase estimate of the absorbed dose enhancement factor for a silicon-goldinterface.8.1.3 The absorbed dose enhancement factor at the inte
33、rfaceis defined by the following:FDESi:Au!5DSiIF!/DSieq! (3)where:DSi(IF) = absorbed dose in silicon immediately adjacentto the silicon-gold interface, andDSi(eq) = equilibrium absorbed dose in silicon.8.1.4 The relationship between the ionization current ratio,a, and an estimate of FDE(Si:Au) is sh
34、own in Fig. 2. The basisfor this relationship is discussed briefly in Appendix X4.NOTE 6Based on the assumptions inherent in Fig. 2 and AppendixX4, monoenergetic 1.25 MeV photon radiation will produce a value ofFDE(Si:Au) = 1.64. Such a low value is not attainable in any practicalCo-60 irradiator co
35、nfiguration.8.1.5 An estimated absorbed dose enhancement factor at agold-silicon interface irradiated by a practical Co-60 sourcemay be obtained by using Fig. 2. For example, a measuredionization current ratio of 2.5 would be considered a goodfigure of merit for a given irradiator configuration. In
36、this case,Fig. 2 gives an estimate of the absorbed dose enhancementfactor of about 1.8 as compared to an estimated absorbed doseenhancement factor of 1.64 for monoenergetic 1.25 MeVgamma radiation; therefore, the dosimetry error for a deviceunder test incurred by neglecting the low energy photoncomp
37、onent would be about 10 %. On the other hand, ameasured ionization current ratio of 7.5 would be considered apoor figure of merit for another irradiator configuration. In thiscase, the corresponding estimated absorbed dose enhancementfactor would be about 3.0; therefore, neglecting the low energyspe
38、ctral component would lead to a dosimetry error for adevice under test of as much as a factor of 1.8. For such aconfiguration, the use of a lead-aluminum filter box wouldminimize the dosimetry error, and, therefore should be consid-ered (see Practice E 1249).8.2 Selecting a Lead-Aluminum Filter for
39、Spectrum Hard-ening:8.2.1 Except for very soft spectra, the use of a filter box of1.5 to 2.0 mm (0.063 in.) of lead on the source side, followedby 0.7 to 1.0 mm (0.030 in.) of aluminum on the test objectside, (see Practice E 1249), will harden the spectrum suffi-ciently to reduce a to #2.5 (see Tabl
40、e X3.1). This value of acorresponds to a dosimetry error of less than 10 %.8.2.2 A greater wall thickness of lead for the filter box thanspecified in 8.2.1 should be considered for a source configu-ration having a large fraction of low-energy photon compo-nents; that is, for a 6. For example, a wall
41、 thickness of 3.2mm (0.125 in.) of lead may be useful for the cases of the lastthree entries in Table X3.1.9. Precision and Bias9.1 The lowest ionization chamber current to which thismethod is applicable is 30 pA (corresponding to 7 3 106Ckg1s1approximately 100 R/h), which can be measured witha prec
42、ision of 0.5 pAor 61.7 %, as specified by the instrumentmanufacturer. The ratio IAu/IAlcan therefore be determined toan overall uncertainty of 62.4 % or better.9.2 This method provides a figure of merit usable forcomparing various source configurations, and for assessing therelative improvement that
43、 is achievable with a lead-aluminumFIG. 2 Relationship for Estimating Absorbed Dose EnhancementFactor in Silicon at a Silicon-Gold Interface From the IonizationCurrent RatioE 1250 88 (2005)3filter. This method gives no quantitative information aboutabsorbed dose enhancement factor other than an esti
44、mate of itsupper limit.10. Keywords10.1 absorbed dose; Co-60 irradiators; dose enhancement;ionization chamber; radiation hardness testingAPPENDIXES(Nonmandatory Information)X1. APPLICATION OF THIS METHOD TO HIGH EXPOSURE RATESX1.1 The limits of applicability of this method given in 1.2are for exposu
45、re rates less than 3 3 102Ckg1s1(100 R/s).It may be possible to apply this method to exposure rateshigher than 100 R/s if the following guidelines are applied.X1.2 The collection efficiency, f, of an ionization chamberis defined as the ratio of the ions collected at the electrodes tothe total number
46、 of ions generated in the active volume.Ideally, the collection efficiency of an ionization chambershould be 1.0. This method deals with current ratios and notwith absolute measurement of ionization current versus expo-sure rate. However, in order to achieve the precision of thismethod, a collection
47、 efficiency of $0.95 is recommended.X1.3 Relationship Between Collection Effciency and Mini-mum Bias Voltage:X1.3.1 The collection efficiency of a parallel plate ioniza-tion chamber is given by the following (6):f 5 1/1 1 m2Xd4/6V2!(X1.1)where:m = Boags constant for air = 36.7 V s1/2cm1/2esu1/2,X= e
48、xposure rate, R/s (1 R/s = esu cm3s1),d = electrode spacing, cm (0.3 cm for this method), andV = bias voltage, volts.X1.3.2 Solving Eq X1.1 for V results in the following:V 5 d23 fm2X/61 2 f!#1/2(X1.2)Use the following set of variables as follows:d = 0.3 cm,f $0.95,m = 36.7, andX= 100 R/s.The result
49、 is V $ 60 V.X1.4 This method may be applied at exposure rates higherthan 100 R/s if the bias voltage, V, is increased to valuesgreater than 60 V according to Eq X1.2. However, thecollection efficiency should be verified experimentally in eachcase by measuring the ionization current versus bias voltage upto the highest exposure rate anticipated. Then the methodshould yield valid results if no significant increase in measuredionization current occurs for a bias voltage greater than the onechosen.X2. COMPLETE FABRICATION DRAWINGS FOR THE GO
