1、Designation: E 798 96 (Reapproved 2009)Standard Practice forConducting Irradiations at Accelerator-Based NeutronSources1This standard is issued under the fixed designation E 798; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、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.1. Scope1.1 This practice covers procedures for irradiations ataccelerator-based neutron sources. The discussion focuses o
3、ntwo types of sources, 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,
4、including spallation neutron sources (1).2Inter-est in spallation sources has increased recently due to theirproposed use for transmutation of fission reactor waste (2).1.2 Many of the experiments conducted using such neutronsources are intended to simulate irradiation in another neutronspectrum, fo
5、r example, that from a DT fusion reaction. Theword simulation is used here in a broad sense to imply anapproximation of the relevant neutron irradiation environment.The degree of conformity can range from poor to nearly exact.In general, the intent of these simulations is to establish thefundamental
6、 relationships between irradiation or material pa-rameters and the material response. The extrapolation of datafrom such experiments requires that the differences in neutronspectra be considered.1.3 The procedures to be considered include methods forcharacterizing the accelerator beam and target, th
7、e irradiatedsample, and the neutron flux and spectrum, as well as proce-dures for recording and reporting irradiation data.1.4 Other experimental problems, such as temperature con-trol, are not included.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement ar
8、e 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 and health practices and determine the applica-bility of regulatory limitations p
9、rior to use.2. Referenced Documents2.1 ASTM Standards:3C 859 Terminology Relating to Nuclear MaterialsE 170 Terminology Relating to Radiation Measurementsand DosimetryE 181 Test Methods for Detector Calibration and Analysisof RadionuclidesE 261 Practice for Determining Neutron Fluence, FluenceRate,
10、and Spectra by Radioactivation TechniquesE 263 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of IronE 264 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of NickelE 265 Test Method for Measuring Reaction Rates andFast-Neutron Fluences by Radioactivat
11、ion of Sulfur-32E 266 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of AluminumE 393 Test Method for Measuring Reaction Rates byAnaly-sis of Barium-140 From Fission DosimetersE 854 Test Method for Application and Analysis of SolidState Track Recorder (SSTR) Monitors for Rea
12、ctor Sur-veillance, E706(IIIB)E 910 Test Method for Application and Analysis of HeliumAccumulation Fluence Monitors for Reactor Vessel Sur-veillance, E706 (IIIC)3. Terminology3.1 Descriptions of relevant terms are found in TerminologyC 859 and Terminology E 170.4. Summary of Existing and Proposed Fa
13、cilities4.1 T(d,n) Sources:4.1.1 Neutrons are produced by the highly exoergic reactiond+t n+a. 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
14、 to1This practice is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications and is the direct responsibility of SubcommitteeE10.08 on Procedures for Neutron Radiation Damage Simulation.Current edition approved Aug. 1, 2009. Published September 2009. Originallyapproved in
15、1981. Last previous edition approved in 2003 as E 798 96 (2003).2The boldface numbers in parentheses refer to a list of references at the end ofthis practice.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of A
16、STMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.maximize the neutron yield (for a particular target configura-tion) from the resonance i
17、n the d-t cross section near 100 keV.The number of neutrons emitted as a function of angle (u)between the neutron direction and the incident deuteron beamis very nearly isotropic in the center-of-mass system. At adeuteron energy of 400 keV in the laboratory system, theneutron flux in the forward dir
18、ection 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 targetmaterial and the tritium deposition profile. Detailed calcula-tions should then be made f
19、or a specific facility.4.1.2 The flux seen at a point (r, u, z) in cylindricalcoordinates from a uniform T(d,n) source of diameter a isgiven by the following (5):fr, u, z! 5Y4pa2lnHk41 4r2z2!1/21 k22z2 J(1)where:k2= a2+ z2 r2, andY = the total source strength.For zaand r = 0 (on beam axis) this redu
20、ces to Y/4pz2,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 the beam axis. However, since theneutron energy is nearly constant, this drop
21、 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.4.1.4 Rotating Target Neutron Source (RTNS) I and II(5-7)RTNS I and II, whic
22、h formerly were operated at theLawrence Livermore National Laboratory, provided 14 MeVneutron source strengths of about 6 3 1012and 4 3 1013neutrons/s, respectively. Although these facilities have beenshut down, they were the most intense sources of 14 MeVneutrons built to date for research purposes
23、. 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 the target was a copper-zirconiumalloy (or copper with dispersed alumina)
24、 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 to maintain a constant flux inspite of tritium burn-up in the target. Sa
25、mples 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 absolute fluence could be routinelydetermined to 67 %. Calculated neutron f
26、lux 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 produced extending fromthermal energies to about 4 MeV (15 MeV for lithium) a
27、bovethe 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 cooling. A moreintense source (1016n/s) is now being designed employingliq
28、uid 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 mechanisms. The most important one for radiationdamage studies is the di
29、rect, 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. The high energy neutrons are thus preferentiallyemitted in the direction o
30、f the incident deuteron beam. How-ever, as the deuterons slow down in the target, lower energyneutrons will be produced with angular distributions that aremuch less forward peaked. Furthermore, when the residualnucleus is left in an excited state, the angular effects are alsomuch less pronounced. Th
31、ese latter two effects tend to decreasethe average neutron energy at angles other than 0 in thedirection of the beam.4.2.3 Neutrons can also be produced by compound nuclearreactions in which the entire deuteron is captured by the targetnucleus and neutrons are subsequently evaporated. Neutronsare pr
32、eferentially emitted with energies less than a few MeVand 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 theCoulomb field of the nucleus, although this effect is very smallfor l
33、ow-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).TABLE 1 Characteristics of T(d,n) and Be or Li(d,n) Neutron SourcesFacility Availability Beam TargetSourceSt
34、rength,n/sMaximum Fluxat Sample,n/cm2sExperimentalVolume forMaximumFlux, cm3RTNS I No longer available 400 keV d t 6 3 101210120.2RTNS II No longer available 400 keV d t 4 3 1013101310120.25.0Existing Be or Li(d,n) U.C. Davis CyclotronA3040 MeV d Solid Be or Li ;10131012;1.0Proposed Li(d,n) Conceptu
35、al design (9) 3040 MeV d Liquid Li 3 3 10161015101410.0600.0AThis is the only existing facility that has been well characterized and is readily available, although other facilities can be used.E 798 96 (2009)2Materials studies for which the maximum total neutron fluenceis desired are usually conduct
36、ed 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 also influence the neutron fieldduring a particular irrad
37、iation, 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, especially at low (40 MeV),as well as a small flux of charge
38、d particles.4.3.2 The procedures recommended in this work also applyto these other sources and should be used where applicable.However, the experimenter should always be aware of thepossibility of additional problems due to peculiarities ofindividual sources.NOTE 1The maximum occurs at about 40% of
39、the deuteron energy. (Data from Ref (6).)FIG. 3 Li(d,n) Spectra at 0 as a Function of Deuteron EnergyE 798 96 (2009)45. Characterization of Irradiation Environments5.1 ScopeThe methods used to define the flux, fluence,and spectra precisely in accelerator environments are signifi-cantly different fro
40、m 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 spectral gradi-ents, which may vary over short time intervals and may notscale linea
41、rly with beam current. For example, small changesin accelerator tuning can move the spatial location of theneutron source relative to the irradiated sample, therebychanging the flux and spectrum. Consequently, it is criticallyimportant to follow well established and well calibratedprocedures in orde
42、r 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 is im-portant to distinguish between T(d
43、,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 to flux measurement refer to b
44、oth 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 instabilities canbe defined according to
45、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 ordamage rates (see 5.3). However,
46、 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 from thatindicated by foils with
47、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 instability are the beam current
48、 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-depende
49、nt 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 equipmentshould be checked for beam leakage, linearity, and long-termstability. The output should then be recorded at regular timeintervals.5.2.1.2 Flux and Spectral InstabilitiesA change in thebeam energy will alter both the flux and spectrum, althoughmost accelerators have active means of keeping the beamenergy constant within relatively small preset limits. It is worth
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