The authors have completed a new set of total cross section measurements of 37 samples spanning the periodic table. The authors employed the same technique as in a previous measurement, with refinements intended to allow measurements on separated isotopes, and with improved systematic error control. The goal of the new measurement was 1% statistical accuracy in 1% energy bins with systematic errors less than 1%. This was achieved for all but the smallest samples, for which the statistical accuracy was as large as 2% in 1% bins

Abfalterer, W. P.

Bateman, F. B.

Dietrich, F. S.

Finlay, R. W.

Haight, R. C.

Morgan, G. L.

UNT Digital Library

roved fixpublicndesse;bM&b&mis dim-tt d.lltle:Author(s):Submitted toLos AlamosNAT IO NALL4BOR ATORYLAN M/h 4 I,quuaMEASUREMENT OF NEUTRON TOTAL CROSSSECTIONS IN SUPPORT OF THE APT PROGRAMw. P.R. C.G. L.:: ::R. W.AbfaltererHaightMorganBatemanDietrichFinlay15th International Conference on theApplication of Accelerators in Researchand Industry, November 4–7, 1998,Denton, TXksAlamsNatmmll.a~mt~, na~mativsatitisqwlo poitunityemployer, isoperatedbytheUniveraityofCaHfomiaforthebU.S. Depefimert of Ewrgyunder contraot W-7~N~. yac@@affie ofthisaflkk, tkWW~krw~tizestM tiU.S.@vemmeti ratalne a nonexclusive, myelty-fme Iicer?seto publish or reproduce the published form of this corhibdiin, or to allowothars to do so, for U.S. Govensmmt purposes. Los Alamoa Natkmal Lsborstory requests that the publisher identify this articleas work performed urritar the auspicea of the U.S. Depwtment of Energy. The Los Alamoa National Mmratory strongly supportsacademic frsedom an I a rwmeamhefa tfghtto publiih as sn institution, however, the Laboratorydoes not endorse the viewpointnf a m 11-dicatinnl-wmm Iant- its tm+nii.al rnrl-nr?tnossDISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.(s9/22/98 ‘lUE 13:U4 PAA >Ub titib 3’7U5 LANS(X 3 l@Jo(t4w“-’Measurement of Neutron Total Cross Sectionsin Support of the APT ProgramW. P. Abfalterer, R. C. Haight, G. L. Morgan, and F. B. BatemanLos Alanzos National Laboratory, Los Alatnos, NM 87S45 USAF. S, DietrichLawrence Liverrnore Natwnal Laborato~, Livermore, CA 94551 USAR. W. FinlayOhio University, Athens, OH45701 USA~~c~l~~’.d theWe have complet :d a new set of total cross section measurements of 37 samples spanning the p&b&c ‘ta e.sametechnique a! in a previous measurement (1), with refinements intended to allow measurementsimproved systemt tic error control. The goal of the new measurement was 1 % statistical accuracy in !E~$im;;:2emors less than 1 %. T%iswas achieved for all but the smallest samples,for whichthe statisticalaccubins.@~aj@jeas*%inl%lWTRODUCTIONNeutron total crxs sections are the most fundamentalquantity describing the interactions of neutrons with nuclei.If there is any interf ction at all, including elastic and all non-elastic interactions, then it is reflected in the total cross sec-tion. Yet the data b tse of total cross sections has significantuncertainties and, i]1regions, significant gaps, Therefore weundertook this exte] tsive program to measure these cross sec-tions.These measurements were supported by the AcceleratorProduction of Tritit m (APT) project as part of a program toimprove the physic:: in the modeling code (LAHET) used inthe design of the APT target and other parts of the facility.The new data, alon,~with those of (1), are being used in thedevelopment of a f:lobal optical potential from 20 to 2000MeV.The goal of the new measurements was 1 % statistical ac-curacy in 1 % eneq ;y bins with systematic errors less than 1%. This was achieved for all but the smallest samples, forwhich the statistic’ and systematic accuracy was as huge as2 % in 1 % energy bins. The data will be given to the Na-tional Nuclear Dats Center at the Brookhaven National Lab-oratory in 170ener ~y bins. However, the energy resolutionis significantly bett :r than this at low energies.We measured tlx total neutron cross section of 37 materi-als at the Los Alam M WNR spallation source. We employedthe same techniques as. in (1), with refinements intended toallow measurements on separated isotopes and other materi-als only available i]i small quantities.Samples were Ihe APT spallation target material W,medium-mass strut tural materials Ti, V, Cr, Mn, Fe, Co, andNi; the actinides TI i and depleted U; the materials for globaloptical model deveIopment F, Mg, P, S, K, Y, Mo, Irs, Au,Hg, and natural Pt; the light nuclei Li, B, and C, the sep-arated isotopes of light nuclei ‘Li, 7Li, lDB, 11B, and 13C;the medium- and heavy-mass separated isotopes 54’56Fe,andls2,1S3,1s4,E3t3w, me few-nucleon nuclei H and D.Total neutron cross scetions were determined by measur-ing the transmitted neutron beam through a known amount ofsample material in comparison to the transmitted beam with-out sample. If N. is the number of counts without a sample,and IViis the number of counts with a sample interposed be-tween neutron source and detector, then the transmission isgiven by:T = $ = e–”’”’ , (1)0where n denotes the number of atoms per unit volume andt the sample length. The total neutron cross section ~T canthen be determined as.T=-~~n~~~. (2)00I& and ROdenote the normalized sample-in and sample-outrates respectively, and Bi, I& the normalized backgroundrates.Therefore, for a successful total neutron cross sectionmeasurement the following ingredients were needed: haccurate measurement of the areal density (nl), knowledgeof the background rates and an accurate normalization ofsample-in and sample-out fluences. Thus we required a welldefined experimental geometry, stable detectors and elec-tronics, and a solid understanding of systematic effects suchas electronic dead time.TECHNIQUETotal cross sections were measured in a good-geometry(i.e., a geometry that minimizes in-scattering) transmissionexperiment with neutrons up to 600 MeV emanating at 30t)9/22/98 ‘lllE 15:SJb HAA 505 btia 37tJ5 LANSCE 3 lg4t)054degrees from the IJ$JSCE WNR TARGET 4 white neutronsource. The white neutron source was realized by bombard-ing a water-cooled t mgsten target with 800 MeV protons.The neutron ener ~y was determined by standard time-of-flight techniques. I or details on the proton beam structureand time-of-flight techrrique see (2).This experiment:s distinguished from (1) in a number ofways. Instead of on: detector, two detectors were employedwith different thickl \esses in order to increase the count rateat the high end oft le neutron spectrum, and also to have acheck on systematic: emors. Our count rate was furlier in-creased by a factor :?.5because of an increased beam repeti-tion rate (macropuk e).The sample wheel and detector set-up are shown in Fig.1. For a more detail :d description see (2). Neutrona traveledfrom the tungsten I reduction target through vacuum to theshutter exit window and then through air for the remainderof the flight path. .4 10.16-cm-thick piece of polyethyleneViewsof the a ]paratus for measuring neutrontI M eroas sectionsLooking down: trearn View of detectors atnear sample cl anger 40-m flightstationFIGURE 1. The lefl-handpicture showsthe monitor and samplewheelset-up,andthe -ight-hardpictureshowsthevetoanddetectorset-up. The second vcto counter is hidden behinddetector2.(C!I&) was used to harden the beam for all but the hydro-gen and deuterium ,:ross section measurements. This greatlyreduced the over-a 1 count rate in the detectors but had lit-tle effect on the rat :s above 100 MeV where high statisticalaccuracy is most di fficult to achieve. A 1.27-cm-thick pieceof lead was used to attenuate the gamma burst. ‘llvo setsof sweeping magn ?ts removed charged particles upstreamof the sample. A horseshoe magnet swept charged parti-cles out of the flight path immediately after the sample. Thesample was situate i in such a way as to completely shadowthe detectors. Sam?le diameters were typically 2.54 cm butspanned a range frtim2.117 to 3.810 cm.Data were taken in 25 sets referred to as “wheels”; thesewere distinguished by the samples mounted on the rotatingsample changer which was a wheel 63.5 cm in diameter witheight positions for ;amples including the “open*’sample.In order to asst:ss the beam stability we continuouslychecked the ratio o ~sample-out detector to monitor, and dis-carded data taken curing erratic beam conditions.Figure 2 shows :he transmission through the shadow barfor detectors 1 and 2 in comparison to the dead-time cor-rected and normalized open beam (sample-out) spectrum.dDETECTOR 2G IFFIGURE 2. Open beam (i.e. sample-out) and shadow bar time-of-flight spectra after dead time corrections for detectors 1 and 2.These show from right to left, the gamma flash, the fast neutronspectrum modified at the lower energies by transmission resonancesin carbon (CHZ filter), the deteetor threshold, and the time indepen-dent background.The basic philosophy and consequent approach to this ex-periment were to gain a clear understanding of the system-atic uncertainties involved, given the neutron beam and flightpath geometry parameters. Thisis one of the reasons that twoindependent detector systems were employed. The detectorswere distinguished by different bias settings, different typesof discriminators, and considerably different count rates fora given sample.Detector 1 was a 8.9 x 8.9 cm, 1.27-cm-thick piece ofBC404. Detector 1 was located at 37.70 + 0.01 m from theneutron source. Detector 2 was of the same construction, thethickness of the scintillator being 5.08 cm instead. Detector2 was located at 39.61 + 0.02 m=Veto counters just in front of the neutron detectors wereused to reject charged particles produced by neutron reac-tions on air in the flight path or other materials upstream ofeach detector. The monitor counter consisted of a circularplastic scintillator, 5,08 cm diameter and 0.159 cm thick.Good agreement between results from detectors 1 and 2gave confidence in the approach. In addition to the differ-ences in the detectors and the electronics, the individual de-u9/2z/Yx ‘lUk? 13:Ub YAA 3(J3 titib3“(U5 LANSGM J kJJtJtJti+●t..+$tectorcountrate characteristics were investigated by varyingthe thickness of almticulartypeof sample. Finally, as acheck on the long-t meltability of the system several sam-ples were remeasun :d as far apart as half a year, and againexcellent agreement was obtained.ELECTRONK:S AND DATA ACQUISITIONTheessential pri]iciples of the electronics are as follows:Figure 3 shows a silnplified time line. Logic TO’SassociatedFIGURE 3. Simplifi{d l’lme Line.with the proton bet m burst anrived at evenly spaced 1.8 pstime intervals, defining a time frame. These are used as stopson the TDC clocks. We checked for a busy condition at thebeginning of a time frame. If the system was not busy, then itwas free to start the slock with a neutron event. This arrange-ment allowed a clet n separation of dead time corrections● “analytic” dea i time - a neutron event within a frameprevents subscquent events within that frame fkom be-ing analyzed.. a correction ~& for busy frames.The complete (Jeetronics setup is discussed in (2).Neutron-energy high and low thresholds for detector 1 wereset to give useful tIata above 2.7 and 8 MeV, respectively.The detector 2 thre ;hold gave data above Em = 10 MeV. Nohigh bias was set f x detector 2. In the final data analysis,detector I was usa I above a neutron energy where the sta-tistical uncertainty was better than 1 %, typically 3-6 MeVdepending on the :arnple; Detector 2 was used above 10MeV.Data were acquired on a given sample for 20 seconds afterwhich the target w][eel was rotated to another position. Thisrapid sample cyclir g minimizes effects of drifts in the beamspatial and energy ]>rofiles.SAMPLE CHARACTERISTICSWe dealt with severid categories of samples: metallic sam-ples, encapsulated ]Iatural powder samples, encapsulated iso-topic powders, en.:apsulated isolopic solids, encapsulatedliquids, pressed po walers,solid hydrogen and fluorine com-pounds, and sintert d samples. All samples were cylindrical.A detailed description of the sample characteristics is pre-sented in in (2).As we discovered, uncertainties in the areal density of thesamples are in many instances the main contributing factorto the systematic uncertainty in the determination of the fi-nal total neutron cross sections. Areal density was thereforedetermined in as many as three ways: by physical measure-ments of mass and dimensions, by a bulk density measure-ment by water immersion coupled with a length measure-ment, and by gamma-ray attenuation.DATA ANALYSISRaw spectra were first processed by employing the so-called analytic dead-time correction. This correction is dueto the fact that low-energy neutrons have a smaller probabil-ity of being counted than high-energy neutrons because thefirst-arriving TDC start pulse within a given frame blocks thesystem from processing later events within that time frame.Reference (3) gives a detailed description of this effect andthe necessary comections for it.The remaining dead-time of the TDC system is taken careof by scaling the total number of logic To‘s, the numberof To’s while the system was alive (called To ~iWJ, and thenumber of times a conversion in progress was aborted by aveto-counter event (denoted v). The correction was accom-plished by multiplying the analytic-dead-time correction bythe factor TO/(To~~ue-v).Using charged-particle veto counters results in system-atic changes in cross sections that are typically in the 0.5% range. However, it is necessary to account for accidentalcoincidences between the veto and main detectors in order toavoid a count-rate-dependent systematic error.The time-independent background was then subtractedfrom the corrected spectra, and time-of-flight converted toenergy in 1 % bins. For the time-to-energy transformationwell-known carbon resonances were used to determine theflight path for detector 1 and detector 2.Finally, spectra were normalized to the monitor counts,and the total neutron cross section was calculated with allstatistical uncertainties properly propagated.SUMMARY OF SYSTEMATIC EFFECTSFor a detailed discussion of systematic effects see (2). Wewill only discuss here detector count rate stability , and de-tector 1 vs. detector 2 consistency.Detector count rate stability We tested the count rate stabil-ity of a given detector and the electronics associated with itby measuring the total neutron cross section of elements withdifferent sample length as in the case of oil-hardened toolsteel, Teflon, and also varying length carbon samples. Sys-tematic differences for the Teflon samples and steel sampleswere of the order of 0.5 %. In the case of the carbon samples,detector 2 showed better consistency above 100 MeV for the*-&innest carbon sample (C-short), where the detector-1 sys-tematic differences’ vere as high as 1.5 % when compared tothelonger carbon szmples. Weattribute these differences toa slightly better dett ctor 2 count-rate stability, and thereforeonly used detector;. data above 100 MeV for “7Li, lD’llB,and 13C.Figure 4 shows tlle ratio of cross sections using detector2 for the 2 cm and 3 cm carbon samples. The attenuationsof the two samples, It 300 MeV (near the cross section min-imum) are approxir latel y 2070 for the long carbon sampleand 4.7 % for the snort sample. The latter is comparable tothe attenuation of tbe thinnest sample of interest, llB, whichhas an attenuation ,}f about 4.5 % at 300 MeV. The slightoverall deviation of the ratio from unity (approximately 0.5%) is within the un :ertainty of the sample density determi-nations. There is n,J evidence for an energy dependence ofthe ratio beyond the level of about 0.5 %.1.101.08r“” ~ “:Ratio of cross sections for carbon samplesof differ mt lengths1.Ce.; 1.04 u(2 cm) / u(9 cm)Lc0 1.02 1- -1Neutron energy (MeV)FIGURE 4. The detl:ctor2 cross sectionratio for the 2 cm and 9cm carbonsamples ~he resultswerebinned in 8 % bkrs in order toget adequate statistic:.Detector 1 vs. detel:tor 2 consistency: The consistency of thetotal neutron cross ~ctions as a function of count rate as de-termined for detect x 1 and detector 2 was checked by com-paring the cross se :tion differences between detector 1 anddetector 2 by varyi]lg the length of carbon samples from 2 to15 cm. Only forth a thhnest carbon sample did we notice adiscrepancy above 100 MeV of the order of 1.5-2 %. Thiswas another reason for only using detector-2 data above 100MeV for the sampl :s mentioned in the previous paragraph.RESULTSFigure 5 depic~ a sampling across the periodic table ofthe total neutron cross sections measured in this experiment.Compound r~onar Icesarising from the interference of manynearby states can t e seen in the cross sections of the lighterelements such as Ivg, S, and P, whose analysis allows the ex-traction of level dt nsity information. The giant resonancesseen in the cross sections of the medium and heavier ele-ments are the result of potential scattering, which arises fromthe interferences between the incident wave function and thewave transmitted through the nuclear potential.,, ,mN.ulron EM%!” [M.VtFIGURE 5. Resultsfor 22 of the 37 samplesmeasured.The results for the total cross section differencedeuterium-hydrogen (d-h) have been used to test the Fad-deev description of the n+d total cross section between 10and 300 MeV (4).ACKNOWLEDGMENTSThanksto Sina and Thomas Manista from Dupont for gra-ciously providing us with high purity Teflon samples and try-ing to explain its physical and chemical properties, to PolyHi Solidur for the high purity polyethylene samples, to 3Mfor the CSFIS, and the Y-12 Development Division for theLiH samples. Special thanks to Dale.Sivils from CST-12 forperforming the chemical analysis of the octane and CaF1a,and to Rose Gray and Alex Herrera for x-raying a numberof samples. Another special thanks to Art Bridge and LloydHunt for all their tireless technical support in keeping thesample wheel turning and the experiment going, and to SteveWender for operations support.Work performed in part under USDOE contracts W-7405-ENG-48 (LLNL), W-7405-ENG-36 (LANL), and DE-FG02-93ER40756 (Ohio University).1.2.3.4.REFERENCESR. W.Fkday,W.P. Abfalterer,G. Fink, E. Montei,T. Adami,P. W. Lisowski, G. L. Morgan, snd R. C. Haight, Phys. Rev.C47, 237( 1993).W. P. Abfalterer, F. S. Ilietrich, R. C. Haight, G. L. Morgan,F. B. Bateman, and R. W, Firday, LA-UR-98-4058 ( 1998).M. S. Moore, Nucl. Instrum. Methods 169, 245(1980).W. P. Abfalterer, F. B. Bateman, F. S. Dietrich, Ch. Elsrer, R.W. Finlay, W. GIOckle, J. Golak, R.C. Haight, D. Huber, G. L.Morgan, and H. Witala, Phys. Rev. Lett., 81, 57(1998).

Measurement of Neutron Total Cross Sections in Support of the APT Program

https://digital.library.unt.edu/ark:/67531/metadc711922/m2/1/high_res_d/759868.pdf

Measurement of Neutron Total Cross Sections in Support of the APT Program

Abstract

Similar works

Full text

Available Versions

UNT Digital Library