1、Designation: E942 96 (Reapproved 2011)Standard Guide forSimulation of Helium Effects in Irradiated Metals1This standard is issued under the fixed designation E942; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last re
2、vision. 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 This guide provides advice for conducting experimentsto investigate the effects of helium on the properties of metalswhere th
3、e technique for introducing the helium differs in someway from the actual mechanism of introduction of helium inservice. Simulation techniques considered for introducing he-lium shall include charged particle implantation, exposure toa-emitting radioisotopes, and tritium decay techniques. Proce-dure
4、s for the analysis of helium content and helium distributionwithin the specimen are also recommended.1.2 Two other methods for introducing helium into irradi-ated materials are not covered in this guide. They are theenhancement of helium production in nickel-bearing alloys byspectral tailoring in mi
5、xed-spectrum fission reactors, andisotopic tailoring in both fast and mixed-spectrum fissionreactors. These techniques are described in Refs (1-5).2Dualion beam techniques (6) for simultaneously implanting heliumand generating displacement damage are also not includedhere. This latter method is disc
6、ussed in Practice E521.1.3 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.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 sta
7、ndard 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:3C859 Terminology Relating to Nuclear MaterialsE170 Terminology Relating to Radiation Measurements andDosimetryE521 Practice f
8、or Neutron Radiation Damage Simulationby Charged-Particle IrradiationE706 Master Matrix for Light-Water Reactor Pressure Ves-sel Surveillance Standards, E 706(0)E910 Test Method for Application and Analysis of HeliumAccumulation Fluence Monitors for Reactor Vessel Sur-veillance, E706 (IIIC)3. Termin
9、ology3.1 Descriptions of relevant terms are found in TerminologyC859 and Terminology E170.4. Significance and Use4.1 Helium is introduced into metals as a consequence ofnuclear reactions, such as (n, a), or by the injection of heliuminto metals from the plasma in fusion reactors. The character-izati
10、on of the effect of helium on the properties of metals usingdirect irradiation methods may be impractical because of thetime required to perform the irradiation or the lack of aradiation facility, as in the case of the fusion reactor. Simula-tion techniques can accelerate the research by identifying
11、 andisolating major effects caused by the presence of helium. Theword simulation is used here in a broad sense to imply anapproximation of the relevant irradiation environment. Thereare many complex interactions between the helium producedduring irradiation and other irradiation effects, so care mus
12、t beexercised to ensure that the effects being studied are a suitableapproximation of the real effect. By way of illustration, detailsof helium introduction, especially the implantation tempera-ture, may determine the subsequent distribution of the helium(that is, dispersed atomistically, in small c
13、lusters in bubbles,etc.)5. Techniques for Introducing Helium5.1 Implantation of Helium Using Charged Particle Accel-erators:5.1.1 Summary of MethodCharged particle acceleratorsare designed to deliver well defined, intense beams of monoen-ergetic particles on a target. They thus provide a convenient,
14、rapid, and relatively inexpensive means of introducing largeconcentrations of helium into thin specimens. An energetic1This guide is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.08 on Procedures for Neutron Rad
15、iation Damage Simulation.Current edition approved June 15, 2011. Published July 2011. Originallyapproved in 1983. Last previous edition approved in 2003 as E942 96(2003). DOI:10.1520/E0942-96R11.2The boldface numbers in parentheses refer to a list of references at the end ofthis guide.3For reference
16、d 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 standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Con
17、shohocken, PA 19428-2959, United States.alpha particle impinging on a target loses energy by exciting orionizing the target atoms, or both, and by inelastic collisionswith the target atom nuclei. Particle ranges for a variety ofmaterials can be obtained from tabulated range tables (7-11).5.1.1.1 To
18、obtain a uniform concentration of helium throughthe thickness of a sample, it is necessary to vary the energy ofthe incident beam, rock the sample (12), or, more commonly, todegrade the energy of the beam by interposing a thin sheet orwedge of material ahead of the target. The range of monoen-ergeti
19、c particles is described by a Gaussian distribution aroundthe mean range. This range straggling provides a means ofimplanting uniform concentrations through the thickness of aspecimen by superimposing the Gaussian profiles that resultfrom beam energy degradation of different thicknesses ofmaterial.
20、The uniformity of the implant depends on the numberof superpositions. Charged particle beams have dimensions ofthe order of a few millimetres so that some means of translatingthe specimen in the beam or of rastering the beam across thespecimen must be employed to uniformly implant specimens ofthe si
21、ze required for tensile or creep tests. The rate of heliumdeposition is usually limited by the heat removal rate from thespecimens and the limits on temperature rise for a givenexperiment. Care must be exercised that phase transformationsor annealing of microstructural components do not result fromb
22、eam heating.5.1.2 LimitationsOne of the major limitations of thetechnique is that the thickness of a specimen that can beimplanted with helium is limited to the range of the mostenergetic alpha particle beam available (or twice the range ifthe specimen is implanted from both sides). Thus a stainless
23、steel tensile specimen is limited to 1.2 mm thickness using a70-MeV beam to implant the specimen from both sides. Thislimiting thickness is greater for light elements such as alumi-num and less for heavier elements such as molybdenum.5.1.2.1 One of the primary reasons for interest in heliumimplantat
24、ion is to simulate the effects resulting from theproduction of helium by transmutation reactions in nuclearreactors. It should be appreciated that the property changes inirradiated metals result from complex interactions between thehelium atoms and the radiation damage produced during theirradiation
25、 in ways that are not fully understood. Energeticalpha particles do produce atomic displacements, but in amanner atypical of most neutron irradiations. The displacementrate is generally higher than that in fast reactor, but the ratio ofhelium atoms to displaced atoms is some 103times greater forimpl
26、antation of stainless steel with a 50-MeV alpha beam.5.1.3 ApparatusApparatus for helium implantation isusually custom designed and built at each research center andtherefore much variety exists in the approach to solving eachproblem. The general literature should be consulted for de-tailed informat
27、ion (12-16). Paragraphs 5.1.3-5.1.3.4 providecomments on the major components of the helium implantationapparatus.5.1.3.1 AcceleratorCyclotrons or other accelerators areused for helium implantation experiments because they arewell suited to accelerate light ions to the high potentialsrequired for im
28、plantation. Typical Cyclotron operating charac-teristics are 20 to 80 MeV with a beam current of 20 A at thesource. It should be noted, however, that the usable beamcurrent delivered to the specimen is limited by the ability toremove heat from the specimens which restricts beam currentsto a limit of
29、 4 to 5 A. A beam-rastering system is the mostpractical method for moving the beam across the samplesurface to uniformly implant helium over large areas of thespecimen.5.1.3.2 Beam Energy DegraderThe most efficient proce-dure for implanting helium with an accelerator, because of thetime involved in
30、changing the energy, is to operate theaccelerator at the maximum energy and to control the depth ofthe helium implant by degrading the beam energy. Thisprocedure offers the additional advantages that range stragglingincreases with energy, thus producing a broader depth profile,and the angular diverg
31、ence of the beam increases as a conse-quence of the electronic energy loss process, thus increasingthe spot size and reducing the localized beam heating. Thebeam energy degrader requires that a known thickness ofmaterial be placed in front of the beam with provisions forremotely changing the thickne
32、ss and for removal of heat fromthe beam energy degrader. Acceptable methods include arotating stepped or wedged wheel, a movable wedge, or a stackof foils. Beam degrader materials can be beryllium, aluminum,or graphite. The wedge or rotating tapered wheel designsprovide a continuous change in energy
33、 deposition, so as toprovide a uniform distribution of helium in the specimen butintroduce the additional complexity of moving parts andcooling of thick sections of material. The stacked foil designsare simpler, can be cooled adequately by an air jet, and havewell calibrated thickness. The design mu
34、st be selected on thebasis of experiment purpose and facility flexibility. Concentra-tions of helium uniform to within 65 % can be achieved bysuperposition of the depth profiles produced by 25-m incre-ments in the thickness of aluminum beam degrader foils.Uniformity of 610 % is recommended for all m
35、aterial experi-ments. Distributing helium over more limited depth ranges (as,for example, when it is only required to spread helium aboutthe peak region of heavy ion damage, in specimens that will beexamined by transmission electron microscopy) can be done bycycling the energy of the helium-implanti
36、ng accelerator (15) inplace of degrader techniques.5.1.3.3 Specimen HolderThe essential features of thespecimen holder are provisions for accurately placing thespecimen in the beam and for cooling the specimens. Addi-tional features may include systems for handling and irradiat-ing large numbers of
37、specimens to improve the efficiency of thefacility and to avoid handling the specimens until the radioac-tivity induced during the implantation has had an opportunityto decay. Some method of specimen cooling is essential sincea degraded, singly charged beam of average energy of 20 MeVand current of
38、5 A striking a 1-cm2nickel target, 0.025 cmthick, deposits 100 W of heat into a mass of 0.22 g. Assumingonly radiative heat loss to the surroundings, the resulting rise intemperature would occur at an initial rate of about 1300 Ks1and would reach a value of about 2000 K. Techniques used forspecimen
39、cooling will depend on whether the implantation isperformed in air or in vacuum and on the physical character-istics of the specimen. Conductive cooling with either air or anE942 96 (2011)2inert gas may be used if implants are not performed in vacuum.Water cooling is a more effective method of heat
40、removal andpermits higher current densities to be used on thick tensilespecimens. The specimens may be bonded to a cooled supportblock or may be in direct contact with the coolant. Care mustbe exercised to ensure that metallurgical reactions do not occurbetween the bonding material and the specimen
41、as a conse-quence of the beam heating, and that hot spots do not developas a consequence of debonding from thermal expansion of thespecimen. Silver conductive paint has been used successfullyas a bonding agent where the temperature rise is minimal.Aluminum is recommended in preference to copper for
42、con-struction of the target holder because of the high levels ofradioactivity induced in copper.5.1.3.4 Faraday Cup and Charge Integration SystemAFaraday cup should be used to measure the beam currentdelivered to the target. A600 mm long by 50 mm diameteraluminum tube closed on one end makes a satis
43、factory Faradaycup. An electron suppressor aperture insulated from the Fara-day cup and positively charged is necessary to collect theelectrons emitted from the degrader foils so as to give accuratebeam current readings. Beam current density and beam profilecan be determined by reading the current p
44、assed by a series ofapertures of calibrated size that can be placed in the beam. Thetarget holder assembly must be insulated from its surroundings,and deionized (low conductivity) water must be used forcooling purposes to permit an integration of current deliveredto the target and thereby accurately
45、 measure the total heliumimplanted independent of fluctuations in the beam current. Anegatively biased aperture must be placed between the targetholder and the degrader foils to suppress secondary electronsemitted from the target that would give erroneously high valuesof total charge deposited on th
46、e specimen.5.1.4 ProcedurePrior to the actual implantation of heliumin a specimen, certain standardization and calibration proce-dures should be performed.The temperature rise to be expectedfrom beam heating and the intended specimen cooling modemust be measured. Such measurements can be performed o
47、ndummy specimens using a thermocouple embedded in thesample behind the beam spot or with an infrared pyrometercapable of reading the surface temperature of an area the sizeof the beam spot. The thickness of the beam energy degradermust be accurately measured to determine the depth of thehelium impla
48、nt. This can be determined from a measurementof the mean energy of the emergent particles from the degraderusing a detector placed directly in the beam line behind thedegrader.5.1.4.1 The uniformity of the flux must be determined forthe implant conditions and for each degrader thickness. This iseasi
49、ly done prior to implantation using a small-diameteraperture that can be moved into the centerline of the particlebeam to compare the flux on the axis to the average flux on thespecimen. The Faraday cup is placed behind this small apertureto measure the current, and the ratio of peak current density onthe specimen to the average current density can then bedetermined for each degrader thickness since the ratio of thearea of small aperture to the total implant area is known. Analternative is the use of a commercially available beam profilemonitor.5.1.4.2 The total charge
