# ASTM E722-14

Designation: E722 − 14Standard Practice forCharacterizing Neutron Fluence Spectra in Terms of anEquivalent Monoenergetic Neutron Fluence for Radiation-Hardness Testing of Electronics1This standard is issued under the fixed designation E722; the number immediately 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 (´) indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope1.1 This practice covers procedures for characterizing neu-tron fluence from a source in terms of an equivalent monoen-ergetic neutron fluence. It is applicable to neutron effectstesting, to the development of test specifications, and to thecharacterization of neutron test environments.The sources mayhave a broad neutron-energy range, or may be mono-energeticneutron sources with energies up to 20 MeV. This practice isnot applicable in cases where the predominant source ofdisplacement damage is from neutrons of energy less than 10keV. The relevant equivalence is in terms of a specified effecton certain physical properties of materials upon which thesource spectrum is incident. In order to achieve this, knowl-edge of the effects of neutrons as a function of energy on thespecific property of the material of interest is required. Sharpvariations in the effects with neutron energy may limit theusefulness of this practice in the case of mono-energeticsources.1.2 This practice is presented in a manner to be of generalapplication to a variety of materials and sources. Correlationbetween displacements (1-3)2caused by different particles(electrons, neutrons, protons, and heavy ions) is beyond thescope of this practice. In radiation-hardness testing of elec-tronic semiconductor devices, specific materials of interestinclude silicon and gallium arsenide, and the neutron sourcesgenerally are test and research reactors and californium-252irradiators.1.3 The technique involved relies on the following factors:(1) a detailed determination of the fluence spectrum of theneutron source, and (2) a knowledge of the degradation(damage) effects of neutrons as a function of energy on specificmaterial properties.1.4 The detailed determination of the neutron fluence spec-trum referred to in 1.3 need not be performed afresh for eachtest exposure, provided the exposure conditions are repeatable.When the spectrum determination is not repeated, a neutronfluence monitor shall be used for each test exposure.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard, except for MeV, keV, eV, MeV·mbarn, rad(Si)·cm2,rad(GaAs)·cm2.1.6 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 of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E170 Terminology Relating to Radiation Measurements andDosimetryE265 Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32E693 Practice for Characterizing Neutron Exposures in Ironand Low Alloy Steels in Terms of Displacements PerAtom (DPA), E 706(ID)E720 Guide for Selection and Use of Neutron Sensors forDetermining Neutron Spectra Employed in Radiation-Hardness Testing of ElectronicsE721 Guide for Determining Neutron Energy Spectra fromNeutron Sensors for Radiation-Hardness Testing of Elec-tronics1This practice 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 approved June 1, 2014. Published October 2014. Originallyapproved in 1980. Last previous edition approved in 2009 as E722 – 09ε1. DOI:10.1520/E0722-14.2The boldface numbers in parentheses refer to a list of references at the end ofthis practice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1E844 Guide for Sensor Set Design and Irradiation forReactor Surveillance, E 706 (IIC)E944 Guide for Application of Neutron Spectrum Adjust-ment Methods in Reactor Surveillance, E 706 (IIA)2.2 International Commission on Radiation Units and Mea-surements (ICRU) Reports:4ICRU Report 13 Neutron Fluence, Neutron Spectra, andKermaICRU Report 60 Fundamental Quantities and Units forIonizing RadiationICRU Report 85 Fundamental Quantities and Units forIonizing Radiation (Revised)3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 displacement damage function—(FD,mat(E)) anenergy-dependent parameter proportional to the quotient of theobservable displacement damage per target atom and theneutron fluence. Different displacement-related damage func-tions may exist, so the damage mode of interest and theobservation procedure shall be identified when the specificdamage function is defined. See, for example, Annexes A1.2.2and A2.2.2.3.1.1.1 Discussion—Observable changes in a material’sproperties attributable to the atomic displacement process areuseful indices of displacement damage in that material. Incases where the observed displacement damage is not in linearproportion to the applied fluence, the displacement damagefunction represents the quotient d(observed damage)/dΦ in thelimiting case of zero fluence. Examples of suitable represen-tations of displacement damage functions are given in theannexes. In the case of silicon, damage mode of interest is thechange in minority-carrier recombination lifetime in the bulksemiconductor material. While several procedures exist todirectly measure the minority carrier lifetime in bulk material,since this lifetime is related to the gain of a bipolar junctiontransistor (BJT), one observable damage metric is the BJT gaindegradation. For this damage mode, it has been shown that thedisplacement damage function may be successfully equatedwith the microscopic displacement kerma factor. This questionis discussed further in the annexes.3.1.2 microscopic displacement kerma factor—(κD,mat(E))the energy-dependent quotient of the displacement kerma pertarget atom and the neutron fluence. κD,mat(E) is proportional toKD,matĀ/Φ, where KD,matis the displacement kerma, Ā is themean atomic mass of the material and Φ is the neutron fluencefrom a monoenergetic source of energy E.3.1.2.1 Discussion—This quantity may be calculated fromthe microscopic neutron interaction cross sections, the kine-matic relations for each reaction and from a suitable partitionfunction which divides the total kerma into ionization anddisplacement kerma. The use of the term microscopic kermafactor in this standard is to indicate that energy times area peratom is used, instead of per unit mass, as in the term kermafactor defined in E170.3.1.3 fluence spectrum hardness parameter—(HEref,mat = Φeq,Eref,mat/Φ) this parameter is defined as the ratio of theequivalent monoenergetic neutron fluence to the total fluence,Φeq,Eref,mat/Φ.The numerical value of the hardness parameter isalso equal to the fluence of monoenergetic neutrons at thespecific energy, Eref, required to produce the same displace-ment damage in the specified material, mat, as unit fluence ofneutrons of neutron spectrum Φ(E).3.1.3.1 Discussion—For damage correlation, a convenientmethod of characterizing the shape of an incident neutronfluence spectrum Φ(E), is in terms of a fluence spectrumhardness parameter (4). The hardness parameter in a particularneutron field depends on the displacement damage functionused to compute the damage (see annexes) and is thereforedifferent for different semiconductor materials.3.1.4 equivalent monoenergetic neutron fluence—(Φeq,Eref,mat) an equivalent monoenergetic neutron fluence, Φeq,Eref,mat,characterizes an incident fluence spectrum, Φ(E), in terms ofthe fluence of monoenergetic neutrons at a specific energy Erefrequired to produce the same displacement damage in aspecified irradiated material, mat, as Φ(E).3.1.4.1 Discussion—Note that Φeq,Eref,matis equivalent toΦ(E) if, and only if, the specific device effect (for example,current gain degradation in silicon) being correlated is de-scribed by the displacement damage function used in thecalculation.3.1.5 fluence and fluence spectrum—see neutron fluence andneutron fluence spectrum.3.1.6 kerma factor—(Kmat(E)) the kerma per unit fluence ofparticles of energy E present in a specified material, mat. SeeTerminology E170 for the definition of kerma, and a formulafor calculating the kerma factor.3.1.6.1 Discussion—When a material is irradiated by aneutron field, the energy imparted to charged particles in thematerial may be described by the kerma. The kerma may bedivided into two parts, ionization kerma and displacementkerma. See 3.1.2.1 for the distinction between kerma factor andmicroscopic kerma factor. Calculations of ionization and mi-croscopic displacement kerma in silicon and gallium arsenideas a result of irradiation by neutrons with energies up to 20MeV are described in Refs 5-8 and in the annexes.3.1.7 neutron fluence and neutron fluence spectrum are usedin this standard, and are special cases of fluence and fluencespectrum as defined in E170.3.1.7.1 Discussion—In cases where the context makes clearthat neutrons are referred to, the terms fluence and fluencespectrum are sometimes used.4. Summary of Practice4.1 The equivalent monoenergetic neutron fluence,Φeq,Eref,mat, is given as follows:Φeq,Eref,mat5*0`Φ~E!FD,mat~E!dEFD,Eref,mat(1)4Available from International Commission on Radiation Units andMeasurements, 7910 Woodmont Avenue Suite 400 Bethesda, MD 20841-3095,http://www.icru.org/E722 − 142where:Φ(E) = incident neutron fluence spectrum,FD,mat(E) = neutron displacement damage function for theirradiated material (displacement damage perunit fluence) as a function of energy, andFD,Eref,mat= displacement damage reference value desig-nated for the irradiated material and for thespecified equivalent energy, Eref, as given in theannexes.The energy limits on the integral are determined in practiceby the incident neutron fluence spectrum and by the materialbeing irradiated.4.2 The neutron spectrum hardness parameter, HEref,mat,isgiven as follows:HEref,mat5*0`Φ~E!FD,mat~E!dEFD,Eref,mat*0`Φ~E!dE(2)4.3 Once the neutron fluence spectrum has been determined(for example, in accordance with Test Method E721) and theequivalent monoenergetic fluence calculated, then a monitor(such as an activation foil) can be used in subsequent irradia-tions at the same location to determine the fluence; that is, theneutron fluence is then described in terms of the equivalentmonoenergetic neutron fluence per unit monitor response,Φeq,Eref,mat/Mr. Use of a monitor foil to predict Φeq,Eref,matis validonly if the neutron spectrum remains constant.5. Significance and Use5.1 This practice is important in characterizing the radiationhardness of electronic devices irradiated by neutrons. Thischaracterization makes it feasible to predict some changes inoperational properties of irradiated semiconductor devices orelectronic systems. To facilitate uniformity of the interpretationand evaluation of results of irradiations by sources of differentfluence spectra, it is convenient to reduce the incident neutronfluence from a source to a single parameter—an equivalentmonoenergetic neutron fluence—applicable to a particularsemiconductor material.5.2 In order to determine an equivalent monoenergeticneutron fluence, it is necessary to evaluate the displacementdamage of the particular semiconductor material. Ideally, thisquantity is correlated to the degradation of a specific functionalperformance parameter (such as current gain) of the semicon-ductor device or system being tested. However, this correlationhas not been established unequivocally for all device types andperformance parameters since, in many instances, other effectsalso can be important. Ionization effects produced by theincident neutron fluence or by gamma rays in a mixed neutronfluence, short-term and long-term annealing, and other factorscan contribute to observed performance degradation (damage).Thus, caution should be exercised in making a correlationbetween calculated displacement damage and performancedegradation of a given electronic device. The types of devicesfor which this correlation is applicable, and numerical evalu-ation of displacement damage are discussed in the annexes.5.3 The concept of 1-MeV equivalent fluence is widely usedin the radiation-hardness testing community. It has merits anddisadvantages that have been debated widely (9-12). For thesereasons, specifics of a standard application of the 1-MeVequivalent fluence are presented in the annexes.6. Procedure for Calculating Φeq,Eref,mat6.1 To evaluate Eq 1 and 2, determine the energy limits Eminand Emaxto be used in place of zero and infinity in the integralsof (Eq 1) and (Eq 2) and the values of the displacement damagefunction FD,mat(E) for the irradiated material and perform theindicated integrations.6.1.1 Choose the upper limit Emaxto be at an energy abovewhich the integral damage falls to an insignificant level. ForGodiva- or TRIGA-type spectra, this limit is about 12 MeV.6.1.2 Choose the lower-energy limit Eminto be at an energybelow which the integral damage falls to an insignificant level.For silicon irradiated by Godiva-type spectra, this energy hasbeen historically chosen to be about 0.01 MeV. More highlymoderated spectra may require lower thresholds or specializedfiltering requirements such as a boron shield, or both.6.1.3 The values of the neutron displacement damage func-tion used in Eq 1 and 2 obviously depend on the material andthe equivalent energy chosen. For silicon, resonance effectscause large variations (by a factor of 20 or more) in thedisplacement damage function as a function of energy over therange from about 0.1 to 8 MeV (4, 5). Therefore, monoener-getic neutron sources with these energies may not be useful foreffects testing.Also, for a selected equivalent energy, the valueof FD,Eref,matat that specific energy may not be representativeof the displacement damage function at nearby energies. Insuch cases, a method of averaging the damage function over arange of energies around the chosen equivalent energy can beused. Such averaging is discussed in the annexes. Because theFD,mat(E) term is normalized by dividing by FD,Eref,matin Eq 1and 2, only the shape of the FD,mat(E) function versus energy is