1、Designation: E1250 15Standard 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 E1250; the number immediately following
2、 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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 Low energy components
3、 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. Thismethod covers procedures f
4、or 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 of exposure rates is
5、7 106to3102Ckg1s1(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 E170 for definition of exposure and its units.1.3 The values stated in SI units are to be regarded as thestandard. The
6、 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 determine the applica-bility o
7、f regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E170 Terminology Relating to Radiation Measurements andDosimetryE668 Practice for Application of Thermoluminescence-Dosimetry (TLD) Systems for Determining AbsorbedDose in Radiation-Hardness Testing of Electronic Device
8、sE1249 Practice for Minimizing Dosimetry Errors in Radia-tion Hardness Testing of Silicon Electronic Devices UsingCo-60 Sources3. Terminology3.1 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.3.2
9、 average absorbed dosemass-weighted mean of theabsorbed dose over a region of interest.3.3 average absorbed dose enhancement factorratio ofthe average absorbed dose in a region of interest to theequilibrium absorbed dose.3.4 dosimeterany device used to determine the equilib-rium absorbed dose in the
10、 material and at the irradiationposition of interest. Examples of such devices include ther-moluminescence dosimeters (TLDs), liquid chemicaldosimeters, and radiochromic dye films. (See Practice E668,for a discussion of TLDs.)3.5 equilibrium absorbed doseabsorbed dose at someincremental volume withi
11、n the material in which the conditionof charged particle equilibrium (the energies, number, anddirection of charged particles induced by the radiation areconstant throughout the volume) exists. (See TerminologyE170.)4. Significance and Use4.1 Although Co-60 nuclei only emit monoenergetic gammarays a
12、t 1.17 and 1.33 MeV, the finite thickness of sources, andencapsulation materials 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 el
13、ectronicdevices this low-energy photon component of the gamma1This method is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applicationsand is the direct responsibility of SubcommitteeE10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices.Current edition
14、approved June 1, 2015. Published August 2015. Originallyapproved in 1988. Last previous approved in 2010 as E1250-10. DOI: 10.1520/E1250-15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volum
15、e information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to the list of references appended tothis test method.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1spectrum c
16、an introduce significant dosimetry errors for adevice under test since the equilibrium absorbed 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
17、 the deviations from equilibriumabsorbed dose caused by non-equilibrium electron transportnear 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 irra
18、diatortype and configuration.4.3 When there is an appreciable low-energy spectral com-ponent 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 Pra
19、ctice E1249.)5. Apparatus5.1 Ionization Chamber, a specially fabricated parallel-plateionization 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
20、 mA.5.3 Electrometer, an electrometer or picoammeter capableof measuring currents as low as 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
21、. 1).NOTE 2The ionization chamber dimensions given in Appendix X2 areappropriate to TWC 78-2 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 0.0
22、76Conductor diameter 0.037Other equivalent twinaxial cable types can be used, but the applicabledimensions 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 supp
23、ly to the electrometer isusually supplied with the electrometer, and must be of a typethat 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 le
24、ast 60 V,and ensure that there is no appreciable leakage current (Ileakage5) indicate a very large low-energyphoton component. Appendix X3 gives a table of measuredvalues of for a variety of typical Co-60 irradiator facilitiesand experimental arrangements.NOTE 4Monoenergetic 1.25 MeV photon radiatio
25、n would theoreti-cally produce a value of = 1.6.Although this value is not attainable withany realistic Co-60 irradiator configuration, it is a theoretical lower limiton .7.2 If the measured value of is 2.5, steps 6.1 6.5 shouldbe repeated with the ionization chamber surrounded by a filtercan or box
26、 of 1.5 to 2.0 mm (approximately 0.063 in.) of leadon the outside and 0.7 to 1.0 mm (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 E1249).4Available from Trompeter Electronics
27、, 31186 La Baya Dr., Westlake Village,CA 91362-4047.FIG. 1 Schematic Diagram of Experimental SetupE1250 1527.3 By repeating the procedure for a number of sourceconfigurations and filter options, the experimental conditionscan be determined that minimize the low-energy photoncomponent of the source s
28、pectrum and thus minimize thedosimetry errors for the device under test.8. 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
29、method, the figure of merit, , can be usedto estimate an upper bound for the absorbed dose enhancementfactor near an interface between any two materials (5).8.1.2 Aspecific example of relating the figure of merit, ,tothe absorbed dose enhancement is given in 8.1.4 for the case ofa silicon-gold inter
30、face.This example is of particular interest inradiation-hardness testing 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 considere
31、d here is an interfacebetween two layers each having a thickness capable 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 sili
32、con-goldinterface.8.1.3 The absorbed dose enhancement factor at the interfaceis defined by the following:FDESi:Au! 5 DSiIF!/DSieq! (2)where:DSi(IF) = absorbed dose in silicon immediately adjacent tothe silicon-gold interface, andDSi(eq) = equilibrium absorbed dose in silicon.8.1.4 The relationship b
33、etween the ionization current ratio, and an estimate of FDE(Si:Au) is shown 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
34、. Such a low value is not attainable in any practicalCo-60 irradiator configuration.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 co
35、nsidered a goodfigure of merit for a given irradiator configuration. In 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
36、 for a deviceunder test incurred by neglecting the low energy photoncomponent 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 enhan
37、cementfactor would be about 3.0; therefore, neglecting the low energyspectral 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 con
38、sid-ered (see Practice E1249).8.2 Selecting a Lead-Aluminum Filter for 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 E1249)
39、, will harden the spectrum suffi-ciently to reduce to 2.5 (see Table X3.1). This value of corresponds 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 lo
40、w-energy photon compo-nents; that is, for 6. For example, a wall 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 7 106
41、Ckg1s1approximately 100 R/h), which can be measuredwith a precision of 0.5 pA or 61.7 %, as specified by theinstrument manufacturer. The ratio IAu/IAlcan therefore bedetermined to an overall uncertainty of 62.4 % or better.FIG. 2 Relationship for Estimating Absorbed Dose EnhancementFactor in Silicon
42、 at a Silicon-Gold Interface From the IonizationCurrent RatioE1250 1539.2 This method provides a figure of merit usable forcomparing various source configurations, and for assessing therelative improvement that is achievable with a lead-aluminumfilter. This method gives no quantitative information a
43、boutabsorbed dose enhancement factor other than an estimate 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
44、applicability of this method given in 1.2are for exposure rates less than 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 chamber isdefined as the ratio of th
45、e ions collected at the electrodes to thetotal number of ions generated in the active volume. Ideally,the collection efficiency of an ionization chamber should be1.0. This method deals with current ratios and not withabsolute measurement of ionization current versus exposurerate. However, in order t
46、o achieve the precision of this method,a collection 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 1/11m2Xd4/6V2! (X1.1)where:m = Boa
47、gs constant for air = 36.7 V s1/2cm1/2esu1/2,X= exposure 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 =
48、0.3 cm,f 0.95,m = 36.7, andX= 100 R/s.The result 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 mea
49、suring 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 GOLD- AND ALUMINUM-WALLEDIONIZATION CHAMBERX2.1 This design is for a parallel plate ionization chamberwith an active volume of 40-mm diameter and 3-mm thickness.The sensitivity of this ionization chamber with aluminumplates is a
