1、Designation: E 942 96 (Reapproved 2003)Standard Guide forSimulation of Helium Effects in Irradiated Metals1This standard is issued under the fixed designation E 942; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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
3、 the 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-d
4、ures 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
5、 mixed-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 d
6、iscussed in Practice E 521.1.3 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 p
7、rior to use.2. Referenced Documents2.1 ASTM Standards:C 859 Terminology Relating to Nuclear Materials3E 170 Terminology Relating to Radiation Measurementsand Dosimetry4E 521 Practice for Neutron Radiation Damage Simulationby Charged-Particle Irradiation4E 706 Master Matrix for Light-Water Reactor Pr
8、essureVessel Surveillance Standards, E706(0)4E 910 Test Method for Application and Analysis of HeliumAccumulation Fluence Monitors for Reactor Vessel Sur-veillance, E706(IIIC)43. Terminology3.1 Descriptions of relevant terms are found in TerminologyC 859 and Terminology E 170.4. Significance and Use
9、4.1 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-ization of the effect of helium on the properties of metals usingdirect irradiation methods may be impractical because
10、 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 andisolating major effects caused by the presence of helium. Theword simulation is used here in a broad sense to
11、 imply anapproximation of the relevant irradiation environment. Thereare many complex interactions between the helium producedduring irradiation and other irradiation effects, so care must beexercised to ensure that the effects being studied are a suitableapproximation of the real effect. By way of
12、illustration, detailsof helium introduction, especially the implantation tempera-ture, may determine the subsequent distribution of the helium(that is, dispersed atomistically, in small clusters in bubbles,etc.)5. Techniques for Introducing Helium5.1 Implantation of Helium Using Charged Particle Acc
13、el-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,rapid, and relatively inexpensive means of introducing largeconcentrations of helium into thin specimens. An ener
14、geticalpha 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 obtain a uniform concentration of h
15、elium throughthe thickness of a sample, it is necessary to vary the energy ofthe incident beam, rock the sample (12), or, more commonly, to1This guide is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.08 on Proce
16、dures for Neutron Radiation Damage Simulation.Current edition approved July 10, 2003. Published March 1996. Originallypublished as E 942 83. Last previous edition E 942 89.2The boldface numbers in parentheses refer to a list of references at the end ofthis guide.3Annual Book of ASTM Standards, Vol 1
17、2.01.4Annual Book of ASTM Standards, Vol 12.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.degrade the energy of the beam by interposing a thin sheet orwedge of material ahead of the target. The range of monoen-ergetic particles
18、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. The uniformi
19、ty 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 size required
20、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 frombeam heating.
21、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 stainlesssteel tensil
22、e 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 heliumimplantation is to si
23、mulate 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 in ways tha
24、t 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 forimplantation of
25、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 information (12-16).
26、 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 implantation.
27、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 4 to 5 A. A
28、 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 changing the
29、 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 divergence of the
30、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 thickness and for r
31、emoval 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 deposition,
32、 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 must be select
33、ed 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 material expe
34、ri-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-implanting accelerat
35、or (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 specimens to
36、 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 5 A striking
37、 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 cooling will
38、 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 aninert gas may be used if implants are not performed in vacuum.Water cooling is a more effective method of heat removal andpermits higher c
39、urrent 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 occurE 942 96 (2003)2between the bonding material and the specimen as a conse-
40、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 con-structi
41、on 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 satisfactory Far
42、adaycup. 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 passed by a
43、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 measure th
44、e 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 the specimen.
45、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 ondummy spec
46、imens 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 implant. This ca
47、n 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 iseasily done pri
48、or 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 spec
49、imen 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 deposited on the specimen by theincident alpha particles must be measured. Precautions must betaken to minimize leakage currents through the cooling waterby the use of low conductivity water, to suppress collection ofsecondary electrons emitted from the target by a negativelybiased aperture just ahead of the s
