ASTM E1250-15(2020)

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: E1250 − 15 (Reapproved 2020)
Standard Test Method for
Application of Ionization Chambers to Assess the Low
Energy Gamma Component of Cobalt-60 Irradiators Used in
Radiation-Hardness Testing of Silicon Electronic Devices
This standard is issued under the fixed designation E1250; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 Low energy components in the photon energy spectrum 2.1 ASTM Standards:
ofCo-60irradiatorsleadtoabsorbeddoseenhancementeffects E170Terminology Relating to Radiation Measurements and
in the radiation-hardness testing of silicon electronic devices. Dosimetry
These low energy components may lead to errors in determin- E668Practice for Application of Thermoluminescence-
ing the absorbed dose in a specific device under test. This Dosimetry (TLD) Systems for Determining Absorbed
method covers procedures for the use of a specialized ioniza- DoseinRadiation-HardnessTestingofElectronicDevices
tion chamber to determine a figure of merit for the relative E1249Practice for Minimizing Dosimetry Errors in Radia-
importance of such effects. It also gives the design and tionHardnessTestingofSiliconElectronicDevicesUsing
instructions for assembling this chamber. Co-60 Sources
1.2 This method is applicable to measurements in Co-60
3. Terminology
−6
radiation fields where the range of exposure rates is 7×10
−2 −1 −1
3.1 absorbed dose enhancement factor— ratio of the ab-
to3×10 Ckg s (approximately100R/hto100R/s).For
sorbed dose at a point in a material of interest to the
guidance in applying this method to radiation fields where the
equilibrium absorbed dose in that same material.
exposure rate is >100 R/s, see Appendix X1.
3.2 average absorbed dose—mass-weighted mean of the
NOTE1—SeeTerminologyE170fordefinitionofexposureanditsunits.
absorbed dose over a region of interest.
1.3 The values stated in SI units are to be regarded as the
3.3 average absorbed dose enhancement factor—ratio of
standard. The values given in parentheses are for information
the average absorbed dose in a region of interest to the
only.
equilibrium absorbed dose.
1.4 This standard does not purport to address all of the
3.4 dosimeter—any device used to determine the equilib-
safety concerns, if any, associated with its use. It is the
rium absorbed dose in the material and at the irradiation
responsibility of the user of this standard to establish appro-
position of interest. Examples of such devices include ther-
priate safety, health, and environmental practices and deter-
moluminescence dosimeters (TLDs), liquid chemical
mine the applicability of regulatory limitations prior to use.
dosimeters, and radiochromic dye films. (See Practice E668,
1.5 This international standard was developed in accor-
for a discussion of TLDs.)
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
3.5 equilibrium absorbed dose—absorbed dose at some
Development of International Standards, Guides and Recom-
incremental volume within the material in which the condition
mendations issued by the World Trade Organization Technical
of charged particle equilibrium (the energies, number, and
Barriers to Trade (TBT) Committee.
direction of charged particles induced by the radiation are
constant throughout the volume) exists. (See Terminology
E170.)
This method is under the jurisdiction of ASTM Committee E10 on Nuclear
Technology and Applicationsand is the direct responsibility of Subcommittee
E10.07 on Radiation Dosimetry for Radiation Effects on Materials and Devices. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2020. Published August 2015. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1988. Last previous approved in 2015 as E1250-15. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
E1250-15R20. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1250 − 15 (2020)
4. Significance and Use
4.1 AlthoughCo-60nucleionlyemitmonoenergeticgamma
rays at 1.17 and 1.33 MeV, the finite thickness of sources, and
encapsulation materials and other surrounding structures that
areinevitablypresentinirradiatorscancontributeasubstantial
amount of low-energy gamma radiation, principally by Comp-
tonscattering (1, 2). Inradiation-hardnesstestingofelectronic
devices this low-energy photon component of the gamma
spectrum can introduce significant dosimetry errors for a
device under test since the equilibrium absorbed dose as
measured by a dosimeter can be quite different from the
absorbed dose deposited in the device under test because of
absorbed dose enhancement effects (3, 4). Absorbed dose
enhancement effects refer to the deviations from equilibrium
absorbed dose caused by non-equilibrium electron transport FIG. 1 Schematic Diagram of Experimental Setup
near boundaries between dissimilar materials.
4.2 The ionization chamber technique described in this
method provides an easy means for estimating the importance
6. Procedure
of the low-energy photon component of any given irradiator
6.1 Assemble the ionization chamber, bias supply, and
type and configuration.
electrometer as shown in Fig. 1.
4.3 When there is an appreciable low-energy spectral com-
6.2 Turn on the bias supply, set the voltage to at least 60 V,
ponent present in a particular irradiator configuration, special
and ensure that there is no appreciable leakage current (I
leakage
experimental techniques should be used to ensure that dosim-
< 0.1 pA). Turn the bias supply off.
etry measurements adequately represent the absorbed dose in
6.3 Assemble the ionization chamber with the gold/
the device under test. (See Practice E1249.)
aluminum electrodes (the gold sides facing the inside of the
5. Apparatus chamber). Place the ionization chamber in the irradiation
position of interest. For directional sources, position the
5.1 Ionization Chamber,a specially fabricated parallel-plate
ionization chamber so that the direction of the main beam is
ionization chamber with interchangeable gold and aluminum
perpendicular to the electrode plates.
electrodes. A specific design is described in Appendix X2.
6.4 Turn on the bias supply and measure the ionization
5.2 Bias Supply, a battery or power supply capable of
current, I , with the gold/aluminum electrodes in place, gold
Au
delivering 60 to 100 V dc at a current up to 1 mA.
side facing inward.
5.3 Electrometer, an electrometer or picoammeter capable
6.5 Repeat steps 6.3 and 6.4 with aluminum electrodes and
of measuring currents as low as 30 pA with a resolution of at
measure the ionization current I . The ionization chamber
Al
least 0.1 pA.
location and orientation shall be the same for both measure-
5.4 Twinaxial Cable, the twinaxial cable that connects the
ments.
ionization chamber to the bias supply and electrometer is an
6.6 Calculate the ionization current ratio α as follows:
integral part of the ionization chamber (see Fig. 1).
α 5 I /I (1)
Au Al
NOTE2—TheionizationchamberdimensionsgiveninAppendixX2are
appropriate to TWC 78-2 twinaxial cable. This cable has the following
This ratio provides a figure of merit for the particular Co-60
physical dimensions (all dimensions given in inches):
irradiator configuration under investigation.
Nominal outer diameter 0.242
Conductor spacing (center to center) 0.076 NOTE3—Sincetherelationshipbetweenionizationchambercurrentand
Conductor dielectric outer diameter 0.076
exposure rate depends on such environmental factors as temperature,
Conductor diameter 0.037
atmospheric pressure, and relative humidity, it is important to make the
two measurements of I and I as nearly at the same time as possible to
Au Al
Other equivalent twinaxial cable types can be used, but the applicable
minimize the influence of environmental factors on the ratio I /I .
Au Al
dimensionsoftheionizationchamberbody,clamp,stem,andcableclamp
nut in Appendix X2 must then be adjusted.
7. Interpretation of Measurement Results
5.5 Triaxial Cable, the triaxial cable that connects the
7.1 Low values of the figure of merit, α (≈2 to 2.5) are
ionization chamber and the bias supply to the electrometer is
indicative of a relatively small low-energy photon component,
usually supplied with the electrometer, and must be of a type
and high values of α (>5) indicate a very large low-energy
that is compatible with the electrometer type used (see Fig. 1).
photon component. Appendix X3 gives a table of measured
values of α for a variety of typical Co-60 irradiator facilities
The boldface numbers in parentheses refer to the list of references appended to
and experimental arrangements.
this test method.
Available from Trompeter Electronics, 31186 La Baya Dr., Westlake Village, NOTE 4—Monoenergetic 1.25 MeV photon radiation would theoreti-
CA91362-4047. callyproduceavalueof α=1.6.Althoughthisvalueisnotattainablewith
E1250 − 15 (2020)
any realistic Co-60 irradiator configuration, it is a theoretical lower limit
on α.
7.2 Ifthemeasuredvalueof αis>2.5,steps6.1–6.5should
be repeated with the ionization chamber surrounded by a filter
can or box of 1.5 to 2.0 mm (approximately 0.063 in.) of lead
on the outside and 0.7 to 1.0 mm (approximately 0.030 in.) of
aluminumontheinside.Useofsuchafilterwillnormallygive
a significant reduction in the low-energy component of the
spectrum (see Practice E1249).
7.3 By repeating the procedure for a number of source
configurations and filter options, the experimental conditions
can be determined that minimize the low-energy photon
component of the source spectrum and thus minimize the
dosimetry errors for the device under test.
8. Application to Hardness Testing
8.1 Estimating the Absorbed Dose Enhancement Factor:
8.1.1 Although it is not possible to determine the absorbed
dose enhancement factor for a particular geometry of a device
undertestusingthismethod,thefigureofmerit, α,canbeused
toestimateanupperboundfortheabsorbeddoseenhancement
factor near an interface between any two materials (5).
8.1.2 Aspecificexampleofrelatingthefigureofmerit, α,to
theabsorbeddoseenhancementisgivenin8.1.4forthecaseof
asilicon-goldinterface.Thisexampleisofparticularinterestin
radiation-hardnesstestingofsiliconelectronicdevicesbecause
it does exist for many devices, and is a worst-case configura-
tion.
FIG. 2 Relationship for Estimating Absorbed Dose Enhancement
Factor in Silicon at a Silicon-Gold Interface From the Ionization
NOTE 5—Silicon-gold interfaces in electronic devices typically consist
Current Ratio
of relatively thin layers; however, the case considered here is an interface
between two layers each having a thickness capable of producing
absorbed dose equilibrium. This case has been used because it represents
aconfigurationthatisrelativelyeasytocalculate.Further,itgivesaworst gamma radiation; therefore, the dosimetry error for a device
case estimate of the absorbed dose enhancement factor for a silicon-gold
under test incurred by neglecting the low energy photon
interface.
component would be about 10%. On the other hand, a
8.1.3 Theabsorbeddoseenhancementfactorattheinterface
measured ionization current ratio of 7.5 would be considered a
is defined by the following:
poorfigureofmeritforanotherirradiatorconfiguration.Inthis
case, the corresponding estimated absorbed dose enhancement
F Si:Au 5 D IF /D eq (2)
~ ! ~ ! ~ !
DE Si Si
factorwouldbeabout3.0;therefore,neglectingthelowenergy
where:
spectral component would lead to a dosimetry error for a
D (IF) = absorbeddoseinsiliconimmediatelyadjacentto
device under test of as much as a factor of 1.8. For such a
Si
the silicon-gold interface, and
configuration, the use of a lead-aluminum filter box would
D (eq) = equilibrium absorbed dose in silicon.
Si minimize the dosimetry error, and, therefore should be consid-
ered (see Practice E1249).
8.1.4 The relationship between the ionization current ratio,
α, and an estimate of F (Si:Au) is shown in Fig. 2.The basis
DE 8.2 Selecting a Lead-Aluminum Filter for Spectrum Hard-
for this relationship is discussed briefly in Appendix X4.
ening:
8.2.1 Except for very soft spectra, the use of a filter box of
NOTE 6—Based on the assumptions inherent in Fig. 2 and Appendix
1.5to2.0mm(≈0.063in.)ofleadonthesourceside,followed
X4, monoenergetic 1.25 MeV photon radiation will produce a value of
F (Si:Au)=1.64. Such a low value is not attainable in any practical
by 0.7 to 1.0 mm (≈0.030 in.) of aluminum on the test object
DE
Co-60 irradiator configuration.
side, (see Practice E1249), will harden the spectrum suffi-
8.1.5 An estimated absorbed dose enhancement fa
...