Designation:E521–96(Reapproved2003)
Standard Practice for
Neutron Radiation Damage Simulation by Charged-Particle Irradiation1
This standard is issued under thefixed designation E521;the number immediately following the designation indicates the year of original adoption or,in the ca of revision,the year of last revision.A number in parenthes indicates the year of last reapproval.A superscript epsilon(e)indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This practice is intended to provide the nuclear rearch community with recommended procedures for the simulation of neutron radiation damage by charged-particle irradiation.It recognizes the diversity of energetic-ion producing devices,the complexities in experimental procedures,and the difficulties in correlating the experimental results with tho produced by reactor neutron irradiation. Such results may be ud to estimate density changes and the changes in microstructure that would be caud by neutron irradiation.The information can also be uful in elucidating fundamental mechanisms of radiation damage in reactor materials.
1.Scope
1.1This practice provides guidance on performing charged-particle irradiations of metals and alloys.It is generally confined to studies of microstructural and microchemical changes carried out with ions of low-penetrating power that come to rest in the specimen.Density changes can be measured directly and changes in other properties can be inferred.This information can be ud to estimate similar changes that would result from neutron irradiation.More generally,this informa-tion is of value in deducing the fundamental mechanisms of radiation damage for a wide range of materials and irradiation conditions.
1.2The word simulation is ud here in a broad n to imply an approximation of the relevant neutron irradiation environment.The degree of conformity can range from poor to nearly exact.The intent is to produce a correspondence between one or more aspects of the neutron and charged particle irradiations such that fundamental relationships are established between irradiation or material parameters and the material respon.
1.3The practice appears as follows:
Section
Apparatus4
Specimen Preparation5-10
Irradiation Techniques(including Helium Injection)11–12
Damage Calculations13
Postirradiation Examination14-16
Reporting of Results17
Correlation and Interpretation18-22
1.4This standard does not purport to address all of the safety concerns,if any,associated with its u.It is the responsibility of the ur of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to u.
2.Referenced Documents
2.1ASTM Standards:
C859Terminology Relating to Nuclear Materials2
E798Practice for Conducting Irradiations at Accelerator-Bad Neutron Sources3
E821Practice for Measurement of Mechanical Properties During Charged-Particle Irradiation2
E910Test Method for Application and Analysis of Helium Accumulation Fluence Monitors for Reactor Vesl Sur-veillance,E706(IIIC)2
E942Guide for Simulation of Helium Effects in Irradiated Metals2
3.Terminology
3.1Definitions of Terms Specific to This Standard:
3.1.1Descriptions of relevant terms are found in Terminol-ogy C859and Terminology E170.
3.2Definitions:
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3.2.1damage energy,n—that portion of the energy lost by an ion moving through a solid that is transferred as kinetic energy to atoms of the medium;strictly speaking,the energy
1This practice is under the jurisdiction of ASTM Committee E10on Nuclear Technology and Applications and is the direct responsibility of Subcommittee E10.08on Procedures for Neutron Radiation Damage Simulation.
Current edition approved Jan.10,1996.Published March1996.Originally published as E521–76.Last previous edition E521–89.
2Annual Book of ASTM Standards,V ol12.01.
3Annual Book of ASTM Standards,V ol12.02. 1
Copyright©ASTM International,100Barr Harbor Drive,PO Box C700,West Conshohocken,PA19428-2959,United States.
transfer in a single encounter must exceed the energy required to displace an atom from its lattice cite.
3.2.2displacement,n—the process of dislodging an atom from its normal site in the lattice.
3.2.3path length,n—the total length of path measured along the actual path of the particle.
3.2.4penetration depth,n—a projection of the range along the normal to the entry face of the target.
3.2.5projected range,n—the projection of the range along the direction of the incidence ion prior to entering the target.
3.2.6range,n—the distance from the point of entry at the surface of the target to the point at which the particle comes to rest.
3.2.7stopping power(or stopping cross ction),n—the energy lost per unit path length due to a particular process; usually expresd in differential form as−d E/d x.
3.2.8straggling,n—the statisticalfluctuation due to atomic or electronic scattering of some quantity such as particle range or particle energy at a given depth.
3.3Symbols:Symbols:
A1,Z1—the atomic weight and the number of the bombard-ing ion.
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A2,Z2—the atomic weight and number of the atoms of the medium undergoing irradiation.
depa—damage energy per atom;a unit of radiation expo-sure.It can be expresd as the product of s¯de and thefluence. dpa—displacements per atom;a unit of radiation exposure giving the mean number of times an atom is displaced from its lattice site.It can be expresd as the product of s¯d and the fluence.
heavy ion—ud here to denote an ion of mass>4.
light ion—an arbitrary designation ud here for conve-nience to denote an ion of mass#4.
T d—an effective value of the energy required to displace an atom from its lattice site.
s d(E)—an energy-dependent displacement cross ction;s¯d denotes a spectrum-averaged value.Usual unit is barns.山楂作用
s de(E)—an energy-dependent damage energy cross ction; s¯de denotes a spectrum-averaged value.Usual unit is barns-eV or barns-keV.
4.Significance and U
4.1A characteristic advantage of charged-particle irradia-tion experiments is preci,individual,control over most of the important irradiation conditions such as do,do rate, temperature,and quantity of gas prent.Additional at-tributes are the lack of induced radioactivation of specimens and,in general,a substantial compression of irradiation time, from years to hours,to achieve comparable damage as mea-sured in displacements per atom(dpa).An important applica-tion of such experiments is the investigation of radiation effects in not-yet-existing environments,such as fusion reactors. 4.2The primary shortcoming of ion bombardments stems from the damage rate,or temperature dependences of the microstructural evolutionary process in complex alloys,or both.It cannot be assumed that the time scale for damage evolution can be comparably compresd for all process by increasing the displacement rate,even with a correspon
ding shift in irradiation temperature.In addition,the confinement of damage production to a thin layer just(often;1µm)below the irradiated surface can prent substantial complications.It must be emphasized,therefore,that the experiments and this practice are intended for rearch purpos and not for the certification or the qualification of equipment.
4.3This practice relates to the generation of irradiation-induced changes in the microstructure of metals and alloys using charged particles.The investigation of mechanical be-havior using charged particles is covered in Practice E821.
5.Apparatus
5.1Accelerator—The major item is the accelerator,which in size and complexity dwarfs any associated equipment. Therefore,it is most likely that irradiations will be performed at a limited number of sites where accelerators are available(a 1-MeV electron microscope may also be considered an accel-erator).
5.2Fixtures for holding specimens during irradiation are generally custom-made as are devices to measure and control particle energy,particleflux,and specimen temperature.Deci-sions regarding apparatus are therefore left to individual workers with the request that accurate data on the performa
nce of their equipment be reported with their results.
6.Composition of Specimen
6.1An elemental analysis of stock from which specimens are fabricated should be known.The manufacturer’s heat number and analysis are usually sufficient in the ca of commercally produced metals.Additional analysis should be performed after other steps in the experimental procedure if there is cau to believe that the composition of the specimen may have been altered.It is desirable that uncertainties in the analys be stated and that an atomic basis be reported in addition to a weight basis.
7.Preirradiation Heat Treatment of Specimen
7.1Temperature and time of heat treatments should be well controlled and reported.This applies to intermediate anneals during fabrication,especially if a metal specimen is to be irradiated in the cold-worked condition,and it also applies to operations where specimens are bonded to metal holders by diffusion or by brazing.The cooling rate between annealing steps and between thefinal annealing temperature and room temperature should also be controlled and reported.
7.2The environment of the specimen during heat treatment should be reported.This includes description of container, measure of vacuum,prence of gas(flowing or steady),and the prence of impurity absorbers such as metal sponge.Any discoloration of specimens following an anneal should be reported.
7.3High-temperature annealing of metals and alloys from Groups IV,V,and VI frequently results in changes,both positive and negative,in their interstitial impurity content. Since the impurity content may have a significant influence on void formation,an analysis of the specimen or of a companion piece prior to irradiation should be performed.Other situations, such as lective vaporization of alloy constituents during annealing,would also require afinal
analysis.
7.4The need for care with regard to alterations in compo-sition is magnified by the nature of the spec
imens.They are usually very thin with a high expod surface-to-volume ratio. Information is obtained from regions who distance from the surface may be small relative to atomic diffusion distances.
8.Plastic Deformation of Specimen
8.1When plastic deformation is a variable in radiation damage,care must be taken in the geometrical measurements ud to compute the degree of deformation.The variations in dimensions of the larger piece from which specimens are cut should be measured and reported to such a precision that a standard deviation in the degree of plastic deformation can be assigned to the specimens.A measuring device more accurate and preci than the common hand micrometer will probably be necessary due to the thinness of specimens commonly irradiated.
8.2The term cold-worked should not stand alone as a description of state of deformation.Every effort should be made to characterize completely the deformation.The param-eters which should be stated are:(1)deformation process(for example,simple tension or compression,swaging,rolling, rolling with applied tension);(2)total extent of deformation, expresd in terms of the principal orthogonal natural strain components(e1,e2,e3)or the geometric shape changes that will allow the reader to compute th
e strains;(3)procedure ud to reach the total strain level(for example,number of rolling pass and reductions in each);(4)strain rate;and(5)defor-mation temperature,including an estimate of temperature changes caud by adiabatic work.
又白又大8.2.1Many commonly ud deformation process(for example,rolling and swaging)tend to be nonhomogeneous.In such cas the strain for each pass can be best stated by the dimensions in the principal working directions before and after each pass.The strain rate can then be specified sufficiently by stating the deformation time of each pass.
9.Preirradiation Metallography of Specimen
9.1A general examination by light microscopy and transmission-electron microscopy should be performed on the specimen in the condition in which it will be irradiated.In some cas,this means that the examination should be done on specimens that were mounted for irradiation and then un-mounted without being irradiated.The microstructure should be described in terms of grain size,phas,precipitates, dislocations,and inclusions.
9.2A ction of a reprentative specimen cut parallel to the particle beam should be examined by light microscopy.Atten-tion should be devoted to the microstructure within a distance from the incide
nt surface equal to the range of the particle,as well as to theflatness of the surface.
10.Surface Condition of Specimen
10.1The surface of the specimen should be clean andflat. Details of its preparation should be reported.Electropolishing of metallic specimens is a convenient way of achieving the objectives in a single operation.The possibility that hydrogen is absorbed by the specimen during electropolishing should be investigated by analys of polished and nonpolished speci-mens.Deviations in the surface form the perfect-planar condi-tion should not exceed,in dimension perpendicular to the plane,10%of the expected particle range in the specimen.
10.2The specimen may be irradiated in a mechanically polished condition provided damage produced by polishing does not extend into the region of postirradiation examination.
11.Dimension of Specimen Parallel to Particle Beam 11.1Specimens without support should be thick enough to resist deformation during handling.If a disk having a diameter of3mm is ud,its thickness should be greater than0.1mm.
11.2Supported specimens may be considerably thinner than unsupported specimens.The minimum t
hickness should be at least fourfold greater than the distance below any surface from which significant amounts of radiation-produced defects could escape.This distance can sometimes be obrved as a void-free zone near the free surface of an irradiated specimen.
12.Helium
12.1Injection:
12.1.1Alpha-particle irradiation is frequently ud to inject helium into specimens to simulate the production of helium during neutron irradiations where helium is produced by transmutation reactions.Helium injection may be completed before particle irradiation begins.It may also proceed incre-mentally during interruptions in the particle irradiation or it may proceed simultaneously with particle irradiation.The last ca is the most desirable as it gives the clost simulation to neutron irradiation.Some techniques for introducing helium are t forth in Guide E942.
12.1.2The influence of implantation temperature on helium distribution(that is,disperd atomistically,in small clusters, in bubbles,etc.)is known to be important.The conquences of the choice of injection temperature on the simulation should be evaluated and reported.
12.2Analysis and Distribution:
12.2.1Analysis of the concentration of helium injected into the specimens should be performed by mass spectrometry. Using this technique,the helium content is determined by vaporizing a helium-containing specimen under vacuum,add-ing a known quantity of3He,and measuring the4He/3He ratio. This information,along with the specimen weight,will give the average helium content in the specimen.The low-level2He addition is obtained by successive expansion through cali-brated volumes.The mass spectrometer is repeatedly calibrated for mass fractionation during each ries of runs by analyzing known mixtures of3He and4He.Other methods of measure-ment,such as the nondestructive a-a scattering technique,may be employed,but their results should be correlated with mass spectrometric results to ensure accuracy.Refer to Test Method E910and Guide E942for additional details.
12.2.2In many experiments,attempts are made to achieve uniformity of helium content within the damage region by varying the incident energy of the alpha-particle beam and by avoidingfluence variations on the specimen surface.The success of the attempts should be measured by analyzing parate ctions of the specimen for helium.It may
be
necessary to u veral companion specimens for this pur-po.Variation of helium concentration through the thickness of the specimen as well as variations across the specimen can also be nondestructively measured with the a-a scattering technique.
12.3Alpha-Particle Damage—Alpha-particle irradiation produces some displacement damage in the specimen.This damage,which changes as the specimen is heated for irradia-tion by other particles,may influence the radiation effects subquently produced.Therefore,in tho cas where helium injection precedes the particle irradiation,a specimen should be brought to the irradiation temperature in the same manner as if it were going to be irradiated and then examined by transmission-electron microscopy at ambient temperature to characterize the microstructure.
13.Irradiation Procedure
13.1Quality of Vacuum—Contamination of the specimen surface by oxidation or deposition of foreign matter and diffusion of impurities into the specimen must be avoided.A vacuum of133µPa(10–6torr)or smaller should be maintained during irradiation for most nonreactive metals.High-temperature irradiation of metals from Groups IV,V,or VI should be done in a vacuum of1.33µPa(101
8torr)or smaller. Oil-diffusion pumps should be cold-trapped to restrict the passage of hydrocarbons into the target chamber and beam tube.The visual appearance of the specimen after irradiation and the vacuum maintained during irradiation should be reported.
13.2Specimen Temperature:
13.2.1The temperature of the specimen should not be allowed to vary by more than610°C.It should be controlled, measured,and recorded continuously during irradiation.Infra-red nsors offer a direct method of measuring actual tempera-ture of the specimen surface.If thermocouples are ud,they should be placed directly on the specimen to avoid temperature gradients and interfaces between the thermocouple and the specimen,which will produce a difference between the ther-mocouple reading and the actual temperature of the specimen volume being irradiated.A thermocouple should not be ex-pod to the particle beam becau spurious signals may be generated.
13.2.2Beam heating should be as small as practical relative to nonbeam heating to minimize temperaturefluctuations of the specimen due tofluctuations in beamflux and energy.If a direct measurement of specimen temperature during irradiation cannot be made,then the specimen temperature should be calculated.Details of the calculation should be fully reported.
13.3Choice of Particle—Since the accelerated particles usually come to rest within the specimen,the possibility of significant alterations in specimen composition exists with concomitant effects on radiation damage.If metallic ions are ud,they should be of the major constituents of the specimen. Electron irradiation pos no problems in this regard.
13.4Choice of Particle Energy:
13.4.1Three criteria should be considered in the choice of particle energy:
(1)The range of the particle should be large enough to ensure that the region to be examined posss a preirradiation microstructure that is unperturbed by its proximity to the surface.
(2)The point defect concentration during irradiation in the obrved volume should not differ substantially from that expected of irradiated volumes located far from free surfaces.
(3)The energy deposition gradient parallel to the beam across the volume chon for obrvation should be small over a distance that is large compared to typical diffusion distances of defects at the temperature of interest.
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The best measure of surface influence is the obrvation of denuded zones for the microstructural fe
ature of interest.The width of denuded zones for voids can be significantly larger or smaller than tho obrved for dislocations.The volume of the specimen to be examined should lie well beyond the denuded zone becau steep concentration gradients of point defects may exist on the boundary of such zones.Gradients in the deposited energy can be reduced by rocking the specimen (varying the angle between the beam and the specimen surface),but local time-dependentflux variations will exist.
13.4.2The nominal energy of the accelerated particle should be verified periodically by calibration experiments. The experiments should be reported and an uncertainty assigned to the energy.
13.5Purity of Beam:
13.5.1The u of a bending magnet is an effective way of lecting a particular ion for transit through the beam tube to the specimen.However,it is possible that the lected ions will interact with foreign atoms in the beam tube,causing foreign atoms to strike the specimen also and altering the charge and energy on the lected ion.
13.5.2A good vacuum in the beam tube will eliminate the significance of the effects,and therefore this vacuum should be monitored during irradiation.A discoloration of the speci-men surface could in
dicate a problem in this regard even though a satisfactory vacuum exists in the vicinity of the specimen.
13.6Flux:
13.6.1The particleflux on the specimen should be recorded continuously during irradiation and integrated with time to give thefluence.This is particularly important since most accelera-tors do not produce a constantflux.Flux andfluence should be reported as particles/m2·s and particles/m2.For the ca where the particle comes to rest within the specimen,the specimen holder asmbly should be designed as a Faraday cup.Theflux measured this way should be checked with a true Faraday cup that can be moved in and out of the beam.If the particles are transmitted through the specimen,a Faraday cup can be positioned on the exit side forflux measurement.Variations in flux during the irradiation should be reported.
13.6.2It is desirable that theflux be the same everywhere on the specimen surface.The actualflux variation in a plane parallel to the specimen surface should be measured and considered when interpreting results of postirradiation exami-nation.A beam profile monitor is recommended for this purpo.It is possible to mitigate the effects of a spatially nonhomogeneous beam by moving the beam over the
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of the specimen during irradiation.A defocud beam should be ud;the maximum translation should be less than the beam half-width.
13.6.3Rastering (periodic scanning)of a focud beam over the specimen will subject the specimen to periodic local flux variations.It is recommended that a rastered beam be avoided for the simulation of a constant neutron flux,although it may be appropriate for the simulation of a puld neutron flux.Radiation-induced defect structures that evolve under such puld conditions can differ substantially from tho that evolve in a constant flux.It should be noted that puld operation is an inherent characteristic of many accelerators.14.Damage Calculations
14.1Scope —This ction covers methods and problems of determining displacement rates for ions and electrons in the energy ranges most likely to be employed in simulations of fission and fusion reactor radiation effects.The are 0.1to 70MeV for ions and 0.2to 10MeV for electrons,although not all energies within the ranges are treated with equal precision.To provide the basis for subquent descriptions of neutron-charged particle correlations,the calculation of displacement rates in neutron irradiations is also treated.
14.2Energy Dissipation by Neutrons and Charged Particles —See Appendix X1.
14.3Particle Ranges —Ions suffer negligible deflections in encounters with electrons;hence,if electron loss dominate,differences between range,projected range,and path length will be small.Furthermore,energy dissipation in this ca is by a large number of low-energy-exchange events,so range straggling will be small and,at a given depth (except near end of range),energy straggling will be small.The conditions apply to light ions for energies down to the tens of keV range,but only at much higher energies for heavy ions such as nickel.14.3.1Light Ions :
14.3.1.1Stopping powers of light ions are easiest to calcu-late in the range of veral MeV to veral tens of MeV ,but the calculations cannot be done accurately from first prin-ciples.At lower energies,
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heavy reliance must be placed on the few experimental measurements of stopping powers.Several tabulations of stopping powers and the path lengths deduced from them exist (1-5).4
14.3.1.2Although the work by Janni (4)appears to be the most comprehensive one for protons,experimental range data (6)have been produced that are in disagreement with his tables for 1-MeV protons incident on steel.In view of the better agreement of the tables of Williamson et al (2)with the data,it was recommended (7)that the latter tables be ud for the path length of protons in iron and nickel and their alloys.Ranges can be obtained from the path length values by subtracting a correction for multiple scattering as given by Janni,but this correction is only −2.2%at 0.1MeV ,decreas-ing to −0.8%at 5MeV for protons incident on iron.Ranges for iron should be valid also for steels and nickel-ba alloys to within the accuracy of the tables (veral percent).The
referenced tables should be consulted for data on proton ranges in other metals (the distinction between path length and range is generally ignored)and for deuteron and alpha ranges (5).Range estimates can conveniently be made for deuterons and alphas in terms of tho for protons for energies at which the stopping power is primarily electronic by employing the following equations:
R a ~E !>R p ~E /4!(1)R d ~E !>2R p ~E /2!
(2)
The approximations agree with tabulated values to within better than 5%for alpha energies >8MeV and deuteron energies >2MeV ,the accuracy increasing with increasing energy.
14.3.2Heavy Ions :
教师节英语怎么说14.3.2.1Heavy ions suffer increasing range straggling as the energy is decread—the spread in range is a large fraction of the mean range at 1MeV .This corresponds to an increasing fraction of energy lost as kinetic energy imparted to atoms (nuclear stopping)as oppod to excitation and ionization of electrons (electronic stopping).
14.3.2.2Ranges of heavy ions in the low MeV range cannot be calculated with high accuracy.A mi-empirical tabulation of ranges by Northcliffe and Schilling is available (1),and a more recent tabulation of range distributions and stopping powers is contained in a ries of books edited by Ziegler and co-workers (5).Note that the ranges in Ref (1)(actually path lengths)have been corrected for nuclear stopping,whereas their tabulated stopping powers are for electronic stopping only.
14.3.2.3Ranges are generally tabulated as areal densities,for example,mg/cm 2;as such they are inv
ariant to changes in mass density.In particular,they apply to material containing voids.The linear range is obtained by dividing the areal density by the mass density—the latter must of cour be the actual density,including a correction for void volume if prent.An increa in range straggling and energy straggling is caud by the production of voids during an irradiation (8).
14.3.2.4Ranges can be computed with a code developed by Johnson and Gibbons (9).It is included as a sub-routine in the E-DEP-1Code (e 13.5.3).It permits evaluations of projected ranges and range straggling as well.14.3.3Electrons :
14.3.3.1Electrons are subject to many large-angle scatter-ing events;hence range straggling is vere.In radiation damage studies,however,the primary concern is with the passage of electrons through relatively thin targets in which the fractional energy loss is small.This loss can be estimated for many purpos using the following general prescription.The principal loss mechanisms are ionization and radiation.If x is the projected range and N and Z are the atomic density and atomic number of the target,respectively:
d E /d x |ion a NZ (3)d E /d x |rad a NZ 2E
(4)
for E >1MeV .Hence,given values for some reference material,energy dissipation for any other material can be
4
The boldface numbers in parenthes refer to the list of references appended to this
practice.
