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ASTM E721-16

Designation E721 − 16Standard Guide forDetermining Neutron Energy Spectra from Neutron Sensorsfor Radiation-Hardness Testing of Electronics1This standard is issued under the fixed designation E721; 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 guide covers procedures for determining theenergy-differential fluence spectra of neutrons used inradiation-hardness testing of electronic semiconductor devices.The types of neutron sources specifically covered by this guideare fission or degraded energy fission sources used in either asteady-state or pulse mode.1.2 This guide provides guidance and criteria that can beapplied during the process of choosing the spectrum adjust-ment methodology that is best suited to the available data andrelevant for the environment being investigated.1.3 This guide is to be used in conjunction with Guide E720to characterize neutron spectra and is used in conjunction withPractice E722 to characterize damage-related parameters nor-mally associated with radiation-hardness testing of electronic-semiconductor devices.NOTE 1Although Guide E720 only discusses activation foil sensors,any energy-dependent neutron-responding sensor for which a responsefunction is known may be used 1.2NOTE 2For terminology used in this guide, see Terminology E170.1.4 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.5 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 Standards3E170 Terminology Relating to Radiation Measurements andDosimetryE261 Practice for Determining Neutron Fluence, FluenceRate, and Spectra by Radioactivation TechniquesE262 Test Method for Determining Thermal Neutron Reac-tion Rates and Thermal Neutron Fluence Rates by Radio-activation TechniquesE263 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of IronE264 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of NickelE265 Test Method for Measuring Reaction Rates and Fast-Neutron Fluences by Radioactivation of Sulfur-32E266 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of AluminumE393 Test Method for Measuring Reaction Rates by Analy-sis of Barium-140 From Fission DosimetersE523 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of CopperE526 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of TitaniumE704 Test Method for Measuring Reaction Rates by Radio-activation of Uranium-238E705 Test Method for Measuring Reaction Rates by Radio-activation of Neptunium-237E720 Guide for Selection and Use of Neutron Sensors forDetermining Neutron Spectra Employed in Radiation-Hardness Testing of ElectronicsE722 Practice for Characterizing Neutron Fluence Spectra inTerms of an Equivalent Monoenergetic Neutron Fluencefor Radiation-Hardness Testing of Electronics1This guide 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 Dec. 1, 2016. Published December 2016. Originallyapproved in 1980. Last previous edition approved in 2011 as E721 – 11. DOI10.1520/E0721-16.2The boldface numbers in parentheses refer to the list of references at the end ofthis guide.3For 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 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 IICE944 Guide for Application of Neutron Spectrum Adjust-ment Methods in Reactor Surveillance, E 706 IIAE1018 Guide for Application of ASTM Evaluated CrossSection Data File, Matrix E706 IIBE1297 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of NiobiumE1855 Test Method for Use of 2N2222A Silicon BipolarTransistors as Neutron Spectrum Sensors and Displace-ment Damage Monitors3. Terminology3.1 Definitions The following list defines some of thespecial terms used in this guide3.1.1 effectthe characteristic which changes in the sensorwhen it is subjected to the neutron irradiation. The effect maybe the reactions in an activation foil.3.1.2 responsethe magnitude of the effect. It can be themeasured value or that calculated by integrating the responsefunction over the neutron fluence spectrum. The response is anintegral parameter. Mathematically, the response, R5iRi,where Riis the response in each differential energy region at Eiof width ∆Ei.3.1.3 response functionthe set of values of Riin eachdifferential energy region divided by the neutron fluence in thatdifferential energy region, that is, the set fi Ri/ΦEi∆Ei.3.1.4 sensoran object or material sensitive to neutronsthe response of which is used to help define the neutronenvironment. A sensor may be an activation foil.3.1.5 spectrum adjustmentthe process of changing theshape and magnitude of the neutron energy spectrum so thatquantities integrated over the spectrum agree more closely withtheir measured values. Other physical constraints on thespectrum may be applied.3.1.6 trial functiona neutron spectrum which, when inte-grated over sensor response functions, yields calculated re-sponses that can be compared to the corresponding measuredresponses.3.1.7 prior spectruman estimate of the neutron spectrumobtained by transport calculation or otherwise and used asinput to a least-squares adjustment.3.2 Abbreviations3.2.1 DUTdevice under test.3.2.2 ENDFevaluated nuclear data file.3.2.3 NNDCNational Nuclear Data Center atBrookhaven National Laboratory.3.2.4 RSICCRadiation Safety Information ComputationCenter at Oak Ridge National Laboratory.3.2.5 TREEtransient radiation effects on electronics.4. Significance and Use4.1 It is important to know the energy spectrum of theparticular neutron source employed in radiation-hardness test-ing of electronic devices in order to relate radiation effects withdevice performance degradation.4.2 This guide describes the factors which must be consid-ered when the spectrum adjustment methodology is chosen andimplemented. Although the selection of sensors foils and thedetermination of responses activities is discussed in GuideE720, the experiment should not be divorced from the analysis.In fact, it is advantageous for the analyst conducting thespectrum determination to be closely involved with the designof the experiment to ensure that the data obtained will providethe most accurate spectrum possible. These data include thefollowing 1 measured responses such as the activities offoils exposed in the environment and their uncertainties, 2response functions such as reaction cross sections along withappropriate correlations and uncertainties, 3 the geometryand materials in the test environment, and 4 a trial function orprior spectrum and its uncertainties obtained from a transportcalculation or from previous experience.5. Spectrum Determination With Neutron Sensors5.1 Experiment Design5.1.1 The primary objective of the spectrum characteriza-tion experiment should be the acquisition of a set of responsevalues activities from effects reactions with well-characterized response functions cross sections with re-sponses which adequately define as a set the fluence values atenergies to which the device to be tested is sensitive. Forsilicon devices in fission-driven environments the significantneutron energy range is usually from 10 keV to 15 MeV. Listsof suitable reactions along with approximate sensitivity rangesare included in Guide E720. Sensor set design is also discussedin Guide E844. The foil set may include the use of responseswith sensitivities outside the energy ranges needed for the DUTto aid in interpolation to other regions of the spectrum. Forexample, knowledge of the spectrum below 10 keV helps in thedetermination of the spectrum above that energy.5.1.2 An example of the difficulty encountered in ensuringresponse coverage over the energy range of interest is thefollowing If fission foils cannot be used in an experimentbecause of licensing problems, cost, or radiological handlingdifficulties especially with235U,237Np or239Pu, a large gapmay be left in the foil set response between 100 keV and 2MeVa region important for silicon and gallium arsenidedamage see Figs.A1.1 andA2.3 of Practice E722. In this casetwo options are available. First, seek other sensors to fill thegap such as silicon devices sensitive to displacement effectssee Test Method E1855,93Nbn,n93mNb see Test MethodE1297or103Rhn,n103mRh. Second, devote the necessaryresources to determine a trial function that is close to the realspectrum. In the latter case it may be necessary to carry outtransport calculations to generate a prior spectrum whichincorporates the use of uncertainty and covariance information.5.1.3 Other considerations that affect the process of plan-ning an experiment are the following5.1.3.1 Are the fluence levels low and of long duration sothat only long half-life reactions are useful This circumstancecan severely reduce the response coverage of the foil set.5.1.3.2 Are high gamma-ray backgrounds present which canaffect the sensors or affect the devices to be tested5.1.3.3 Can the sensors be placed so as to ensure equalexposure This may require mounting the sensors on a rotatingE721 − 162fixture in steady-state irradiations or performing multipleirradiations with monitor foils to normalize the fluence be-tween runs.5.1.3.4 Do the DUT or the spectrum sensors perturb theneutron spectrum5.1.3.5 Are response functions available that account forself-shielding for all sensors using n,γ or non-threshold n,freactions, unless the material is available in a dilute form ofcertified composition5.1.3.6 Can the fluence and spectrum seen in the DUT testlater be directly scaled to that determined in the spectrumcharacterization experiment by monitors placed with thetested device5.1.3.7 Can the spectrum shape and intensity be character-ized by integral parameters that permit simple intercomparisonof device responses in different environments Silicon is asemiconductor material whose displacement damage functionis well established. This makes spectrum parameterization fordamage predictions feasible for silicon.5.1.3.8 What region of the spectrum contributes to theresponse of the DUT In other words, is the spectrum welldetermined in all energy regions that affect device perfor-mance5.1.3.9 How is the counting system set up for the determi-nation of the activities For example, are there enough countersavailable to handle up to 25 reactions from a single exposureThis may require as many as six counters. Or can theavailable system only handle a few reactions before theactivities have decayed below detectable limits5.1.4 Once the experimental opportunities and constraintshave been addressed and the experiment designed to gather themost useful data, a spectrum adjustment methodology must bechosen.5.2 Spectrum Adjustment Methodology5.2.1 After the basic measured responses, responsefunctions, and trial or prior spectrum information have beenassembled, apply a suitable spectrum adjustment procedure toreach a solution that satisfies the criteria of the chosenprocedure. It must also meet other constraints such as, thefluence spectrum must be positive and defined for all energies.The solution is the energy-dependent spectrum function, ΦE,which approximately satisfies the series of Fredholm equationsof the first kind represented by Eq 1 as followsRj5 *0σjEΦE dE 1 j n 1whereRj measured response of sensor j,σjE neutron response function at energy E for sensor j,ΦE incident neutron fluence versus energy, andn number of sensors which yield n equations.NOTE 3Guides E720 and E844 provide general guidance on obtaininga suitable set of responses activities when foil monitors are used.Practice E261 and Test Method E262 provide more information on thedata analysis that generally is part of an experiment with activationmonitors. Specific instructions for some individual monitors can be foundin Test Methods E263 iron, E264 nickel, E265 sulfur-32, E266aluminum, E393 barium-140 from fission foils, E523 copper, E526titanium, E704 uranium-238, E705 neptunium-237, E1297 nio-bium.5.2.2 One important characteristic of the set of equationsEq 1 is that with a finite number of sensors, n, which yield nequations, there is no unique solution. Exact solutions toequations Eq 1 may be readily found, but are not generallyconsidered useful. When the least squares adjustment methodis used, equations Eq 1 are supplemented by the constraintthat the solution spectrum must be approximately equal to theprior spectrum. This additional constraint guarantees that theset of equation is overdetermined and that a unique leastsquares solution does exist. The tolerances of the approxima-tions are dependent on the specified variances and covariancesof the prior spectrum, the response functions, and the measuredresponses. When other adjustment methods are used it must beassumed that the range of physically reasonable solutions canbe limited to an acceptable degree.5.2.3 Neutron spectra generated from sensor response datamay be obtained with several types of spectrum adjustmentcodes. One type is linear least squares minimization used bycodes such as STAY’SL 2 or the logarithmic least squaresminimization as used by LSL-M2 3. When the spectrumadjustments are small, these methods yield almost identicalresults. Another type is the iterative method, an example ofwhich is SAND II 4. If used properly and with sufficient,high-quality data, this method will usually yield nearly thesame values as the least squares methods 610 to 15 for theprimary integral parameters discussed in E722.NOTE 4Another class of codes often referred to as Maximum Entropy5 has also been used for this type of analysis.5.2.4 Appendix X1 and Appendix X2 discuss in some detailthe implementation and the advantages and disadvantages ofthe two approaches as represented by LSL-M2 and SAND-

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