1、Designation: E798 16Standard Practice forConducting Irradiations at Accelerator-Based NeutronSources1This standard is issued under the fixed designation E798; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revisio
2、n. 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 practice covers procedures for irradiations ataccelerator-based neutron sources. The discussion focuses ontwo types of source
3、s, namely nearly monoenergetic 14-MeVneutrons from the deuterium-tritium T(d,n) interaction, andbroad spectrum neutrons from stopping deuterium beams inthick beryllium or lithium targets. However, most of therecommendations also apply to other types of accelerator-based sources, including spallation
4、 neutron sources (1).2Inter-est in spallation sources has increased recently due to theirdevelopment of high-power, high-flux sources for neutronscattering and their proposed use for transmutation of fissionreactor waste (2).1.2 Many of the experiments conducted using such neutronsources are intende
5、d to provide a simulation of irradiation inanother neutron spectrum, for example, that from a DT fusionreaction. The word simulation is used here in a broad sense toimply an approximation of the relevant neutron irradiationenvironment. The degree of conformity can range from poor tonearly exact. In
6、general, the intent of these experiments is toestablish the fundamental relationships between irradiation ormaterial parameters and the material response. The extrapola-tion of data from such experiments requires that the differencesin neutron spectra be considered.1.3 The procedures to be considere
7、d include methods forcharacterizing the accelerator beam and target, the irradiatedsample, and the neutron flux (fluence rate) and spectrum, aswell as procedures for recording and reporting irradiation data.1.4 Other experimental problems, such as temperaturecontrol, are not included.1.5 The values
8、stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.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
9、 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 andDosimetryE181 Test Methods for Detector Calibration and Analys
10、is ofRadionuclidesE261 Practice for Determining Neutron Fluence, FluenceRate, and Spectra by Radioactivation TechniquesE263 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of IronE264 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of NickelE265 Test M
11、ethod 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 DosimetersE854 Test Method for App
12、lication and Analysis of SolidState Track Recorder (SSTR) Monitors for ReactorSurveillance, E706(IIIB)E910 Test Method for Application and Analysis of HeliumAccumulation Fluence Monitors for Reactor VesselSurveillance, E706 (IIIC)3. Terminology3.1 Descriptions of relevant terms are found in Terminol
13、ogyC859 and Terminology E170.4. Summary of Existing and Proposed Facilities4.1 T(d,n) Sources:1This practice is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applicationsand is the direct responsibility of SubcommitteeE10.08 on Procedures for Neutron Radiation Damage Simulati
14、on.Current edition approved Oct. 1, 2016. Published December 2016. Originallyapproved in 1981. Last previous edition approved in 2009 as E798 96 (2009).DOI: 10.1520/E0798-16.2The boldface numbers in parentheses refer to a list of references at the end ofthis practice.3For referenced ASTM standards,
15、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.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428
16、-2959. United States14.1.1 Neutrons are produced by the highly exoergic reactiond+t n+. The total nuclear energy released is 17.589MeV, resulting in about a 14.8-MeV neutron and a 2.8-MeValpha particle at low deuterium beam energies (3). Thedeuteron energy (generally 150 to 400 keV) is chosen tomaxi
17、mize the neutron yield (for a particular target configura-tion) from the resonance in the d-t cross section near 100 keV.The number of neutrons emitted as a function of angle ()between the neutron direction and the incident deuteron beamis very nearly isotropic in the center-of-mass system. At adeut
18、eron energy of 400 keV in the laboratory system, theneutron flux in the forward direction is about 14 % greater thanin the backward direction, while the corresponding neutronenergy decreases from 15.6 to 13.8 MeV (4). In practice, theneutron field also depends on the gradual loss of the targetmateri
19、al and the tritium deposition profile. Detailed calcula-tions should then be made for a specific facility.4.1.2 The flux seen at a point (r, , z) in cylindricalcoordinates from a uniform T(d,n) source of diameter a isgiven by the following (5):r, , z! 5Y4a2lnHk414r2z2!1/21k22z2 J(1)where:k2= a2+ z2
20、r2, andY = the total source strength.For zaand r = 0 (on beam axis) this reduces to Y/4z2,as expected for a point source. The available irradiationvolume at maximum flux is usually small. For a sample placedclose to the target, the flux will decrease very rapidly withincreasing radial distance off t
21、he beam axis. However, since theneutron energy is nearly constant, this drop in flux is relativelyeasy to measure by foil activation techniques.4.1.3 Other existing sources, such as Cockroft-Walton typeaccelerators, are similar in nature although the availableneutron source strengths are much lower.
22、4.1.4 Rotating Target Neutron Source (RTNS) I and II(5-7)RTNS I and II, which formerly were operated at theLawrence Livermore National Laboratory, provided 14 MeVneutron source strengths of about 6 1012and 4 1013neutrons/s, respectively. Although these facilities have beenshut down, they were the mo
23、st intense sources of 14 MeVneutrons built to date for research purposes. They are discussedhere because of their relevance to any future neutron sources.Their characteristics are summarized in Table 1. A discussionof similar sources can be found in Ref (8). The deuteron beamenergy was 400 keV and t
24、he target was a copper-zirconiumalloy (or copper with dispersed alumina) vapor-plated withtritium-occluded titanium. The beam spot size was about 10mm in diameter. In addition to being rotated, the target alsowas rocked every few hours and the deuteron beam current wasincreased slowly in an attempt
25、to maintain a constant flux inspite of tritium burn-up in the target. Samples could be placedas close as 2.5 to 4.0 mm from the region of maximum d-tinteraction resulting in a typical flux of 1013n/cm2s over asmall sample. The neutron fields were well characterized by avariety of methods and the abs
26、olute fluence could be routinelydetermined to 67 %. Calculated neutron flux contours forRTNS-II are shown in Fig. 1.4.2 Be or Li(d,n) Sources (9):4.2.1 When a high-energy (typically 30- to 40-MeV) deu-teron beam is stopped in a beryllium (or lithium) target, acontinuous spectrum of neutrons is produ
27、ced extending fromthermal energies to about 4 MeV (15 MeV for lithium) abovethe incident deuteron energy (see Figs. 2-4). In existingfacilities, cyclotrons with deuteron beam intensities of 20 to 40A provide neutron source strengths in the range of 1013n/s,using solid beryllium targets with water co
28、oling. A moreintense source (1016n/s) is now being designed employingliquid lithium targets. In the remainder of this document theterm Be(d,n) source is meant as a generic term includingLi(d,n) sources, whether solid or liquid targets.4.2.2 Neutrons are produced by several competing nuclearreaction
29、mechanisms. The most important one for radiationdamage studies is the direct, stripping reaction since it pro-duces almost all of the high-energy neutrons. When theincident deuteron passes close to a target nucleus, the proton iscaptured and the neutron tends to continue on in a forwarddirection. Th
30、e high energy neutrons are thus preferentiallyemitted in the direction of the incident deuteron beam.However, as the deuterons slow down in the target, lowerenergy neutrons will be produced with angular distributionsthat are much less forward peaked. Furthermore, when theresidual nucleus is left in
31、an excited state, the angular effectsare also much less pronounced. These latter two effects tend todecrease the average neutron energy at angles other than 0 inthe direction of the beam.4.2.3 Neutrons can also be produced by compound nuclearreactions in which the entire deuteron is captured by the
32、targetnucleus and neutrons are subsequently evaporated. Neutronsare preferentially emitted with energies less than a few MeVTABLE 1 Characteristics of T(d,n) and Be or Li(d,n) Neutron SourcesFacility Availability Beam TargetSourceStrength,n/sMaximum Fluxat Sample,n/cm2sExperimentalVolume forMaximumF
33、lux, cm3RTNS I No longer available 400 keV d t 6 101210120.2RTNS II No longer available 400 keV d t 4 1013101310120.25.0Existing Be or Li(d,n) U.C. Davis CyclotronA3040 MeV d Solid Be or Li ;10131012;1.0Proposed Li(d,n) Conceptual design (9) 3040 MeV d Liquid Li 3 10161015101410.0600.0AThis is the o
34、nly existing facility that has been well characterized and is readily available, although other facilities can be used.E798 162and the angular distribution approaches isotropy at neutronenergies below 1 MeV. Neutrons also are produced by deuteronbreak-up, in which the deuteron simply breaks apart in
35、 theCoulomb field of the nucleus, although this effect is very smallfor low-Z materials.4.2.4 The neutron spectrum thus depends very strongly onthe angle from the incident deuteron direction, and the flux isvery sharply peaked in the forward direction (see Fig. 2).Materials studies for which the max
36、imum total neutron fluenceis desired are usually conducted close to the target and maysubtend a large range of forward angles (for example, 0 to 60).This practice primarily will be concerned with this close-geometry situation since it is the most difficult to handleproperly.4.2.5 Other factors can a
37、lso influence the neutron fieldduring a particular irradiation, especially beam and targetcharacteristics, as well as the perturbing influence of surround-ing materials. At present, these facilities have not been com-pletely characterized for routine use. In particular, someuncertainties exist, espe
38、cially at low (40 MeV),as well as a small flux of charged particles. The LAMPF is nowknown as the Los Alamos Neutron Science Center (LAN-SCE).44.3.2 Modern spallation neutron sources have also beenused for irradiation experiments. For example, the SwissSpallation Neutron Source, SINQ5, has a unique
39、SINQ TargetIrradiation Program (STIP). The STIP has been used in a seriesof materials irradiation experiments to investigate the effecthigh damage rates with high helium and hydrogen generationrates (24).4.3.3 The procedures recommended in this work also applyto these other sources and should be use
40、d where applicable.However, the experimenter should always be aware of thepossibility of additional problems due to peculiarities ofindividual sources.5. Characterization of Irradiation Environments5.1 ScopeThe methods used to define the flux, fluence,and spectra precisely in accelerator environment
41、s are signifi-cantly different from those used in reactor environments. Thereason for this difference is that, whereas reactors generallyproduce stable fields with gentle gradients, accelerators tend toproduce fields with very sharp spatial flux and spectralgradients, which may vary over short time
42、intervals and maynot scale linearly with beam current. For example, smallchanges in accelerator tuning can move the spatial location ofthe neutron source relative to the irradiated sample, therebychanging the flux and spectrum. Consequently, it is criticallyimportant to follow well established and w
43、ell calibratedprocedures in order to measure adequately the irradiationexposure parameters. Otherwise, it will be impossible tocorrectly calculate damage parameters such as DPA or tocorrelate materials effects measured at different facilities.5.2 System ParametersIn the following section, it isimpor
44、tant to distinguish between T(d,n) (14-MeV) sources andbroad spectrum9Be(d,n) sources. Whereas both types ofsources exhibit strong flux gradients, only the broad-spectrumsources exhibit significant spectral gradients. Consequently, inthe following subsections it should be understood that refer-ences
45、 to flux measurement refer to both facilities, whereasreferences to spectral measurement refer only to the9Be(d,n)sources.5.2.1 Beam CharacterizationIt is important to realize thatvirtually any change in the accelerator beam will produce somealteration of the neutron field. Two classes of instabilit
46、ies canbe defined according to whether they affect only the neutronflux or the neutron spectrum as well. Whereas the flux mayvary independently of the spectrum, spectral changes alwaysimply a change in flux. Flux changes are usually easy tomeasure and to account for in calculating total exposure ord
47、amage rates (see 5.3). However, spectral changes are muchharder to measure or to account for in subsequent calculations.For example, if the spectrum changes significantly even onceduring a long run, then activated foils with short half-lives mayindicate an average spectrum that is quite different fr
48、om thatindicated by foils with long half-lives. Furthermore, it may beimpossible to account for this difference unless great care isexercised to record the pertinent beam information, namelybeam current, beam energy, and spatial alignment.5.2.1.1 Flux InstabilitiesThe most important sources offlux i
49、nstability are the beam current and target condition. If thebeam is well collimated, stable in energy, and stable in spatialposition, then the flux should be directly proportional to thebeam current, neglecting target effects. At solid Be(d,n)sources, target effects are usually unimportant. However, atT(d,n) sources, time-dependent changes in the target are thedominant cause of flux instabilities (6). The beam currentshould be read using a Faraday cup or well-insulated targetassembly where possible. The current-sensing equipment4See http:/lansce.la
