1、Designation: E 798 96 (Reapproved 2003)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 (e) 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
3、ontwo 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, f
5、or 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 thefundamenta
6、l 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, t
7、he 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 This standard does not purport to address all of thesafety concerns, if any, associated with
8、 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 prior to use.2. Referenced Documents2.1 ASTM Standards:C 859 Terminology Relating to Nuclear Materials3E 170 Terminology Rel
9、ating to Radiation Measurementsand Dosimetry4E 181 Test Methods for Detector Calibration and Analysisof Radionuclides4E 261 Practice for Determining Neutron Fluence Rate, Flu-ence, and Spectra by Radioactivation Techniques4E 263 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation
10、 of Iron4E 264 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of Nickel4E 265 Test Method for Measuring Reaction Rates andFast-Neutron Fluences by Radioactivation of Sulfur-324E 266 Test Method for Measuring Fast-Neutron ReactionRates by Radioactivation of Aluminum4E 393 Tes
11、t Method for Measuring Reaction Rates by Analy-sis of Barium-140 from Fission Dosimeters4E 854 Test Method for Application and Analysis of SolidState Track Recorder (SSTR) Monitors for Reactor Sur-veillance, E 706 (IIIB)4E 910 Specification for Application and Analysis of HeliumAccumulation Fluence
12、Monitors for Reactor Vessel Sur-veillance, E 706 (IIIC)43. Terminology3.1 Descriptions of relevant terms are found in TerminologyC 859 and Terminology E 170.4. Summary of Existing and Proposed Facilities4.1 T(d,n) Sources:4.1.1 Neutrons are produced by the highly exoergic reactiond+t n+a. The total
13、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 to1This practice is under the jurisdiction of ASTM Committee E10 on NuclearTechnology and Applications a
14、nd is the direct responsibility of SubcommitteeE10.08 on Procedures for Neutron Radiation Damage Simulation.Current edition approved Jan. 10, 1996. Published March 1996. Originallypublished as E 798 81. Last previous edition E 798 89.2The boldface numbers in parentheses refer to a list of references
15、 at the end ofthis practice.3Annual Book of ASTM Standards, Vol 12.01.4Annual Book of ASTM Standards, Vol 12.02.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.maximize the neutron yield (for a particular target configura-tion) from
16、the resonance in 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
17、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 targetmaterial and the tritium deposition profile. Detailed calcula-tions should
18、 then be made for 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
19、axis) this reduces 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 cons
20、tant, 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.4.1.4 Rotating Target Neutron Source (RTNS) I and II(5-7)RTNS
21、 I and II, which 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 re
22、search 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 the target was a copper-zirconiumalloy (or copper with dis
23、persed 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 to maintain a constant flux inspite of tritium burn-up in
24、 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 absolute fluence could be routinelydetermined to 67 %. Calcu
25、lated 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 produced extending fromthermal energies to about 4 MeV (15 MeV
26、 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 cooling. A moreintense source (1016n/s) is now being design
27、ed 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 mechanisms. The most important one for radiationdamage st
28、udies 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. The high energy neutrons are thus preferentiallyemitted in
29、the direction of 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
30、 pronounced. These 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.
31、 Neutronsare preferentially 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
32、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).TABLE 1 Characteristics of T(d,n) and Be or Li(d,n) Neutron SourcesFacility Availability Beam
33、 TargetSourceStrength,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 L
34、i(d,n) Conceptual 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 (2003)2Materials studies for which the maximum total neutron fluenceis desired are
35、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 also influence the neutron fieldduring a p
36、articular 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, especially at low (40 MeV),as well as a small
37、 flux of charged 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 a
38、t about 40% of the deuteron energy. (Data from Ref (6).)FIG. 3 Li(d,n) Spectra at 0 as a Function of Deuteron EnergyE 798 96 (2003)45. Characterization of Irradiation Environments5.1 ScopeThe methods used to define the flux, fluence,and spectra precisely in accelerator environments are signifi-cantl
39、y 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 spectral gradi-ents, which may vary over short time intervals and may
40、 notscale linearly 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 calibratedpro
41、cedures 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 is im-portant to distingu
42、ish 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 to flux measure
43、ment 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 instabilities canbe define
44、d 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 ordamage rates (see
45、 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 from thatindicated
46、 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 instability are t
47、he 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) source
48、s, 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 equipmentshould be checked for beam leakage, linearity, and long-termstability. The output
49、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 worthmentioning that beam stability is often linked to beam currentsince beam control systems may use slits or aperatures whichin turn limit the transmission through the machine. Hence,attempts to maximize the beam current may allow a widerrange of particle trajectories, resulting in a lar
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