Gwin, R.

Ingle, R.W.

Todd, J.H.

Weaver, H.

Weston, L.W.

Weston, L. W.

Todd, J. H.

Ingle, R. W.

UNT Digital Library

rA DIRECT COMPARISON OF DIFFERENT EXPERIMENTAL TECHNIQUES FOR MEASURING // v -NEUTRON CAPTURE AND FISSION CROSS SECTIONS FOR 23<JPu* Lc"" 'R. Gwi:i, L. W. Weston, J. H. Todd, R. W. Ingle, and H. WeaverOak Ridge National LaboratoryOak Ridge, Tennessee 37830A comparison of the results of two different experimental methods of measuring theneutron absorption and fission cross sections for 235Pu is made. These measurements merenormalized at thermal energy and extend to 200 keV. The ratio of the neutron capture tofission cross section for 239Pu derived in these two experiments is shown to be in goodagreement.(239pu. fusion, absorption; measurement; comparison; cross sections)IntroductionMeasurements of the neutron absorption a, andneutron fission j f cross sections, and thereby the neu-tron capture o cross section, have been performed atORNL using two different techniques. These experimentscovered the neutron energy region from 0.02 eV to 200keV and represent a part of the experimental program on239Pu at ORNL. Two different techniques were used toserve as a guide for further experimentation at ORNLand to provide a measure of confidence in the measure-ments of cf./Cf.Several aspects of the two experiments were essen-tially the same, such as the measurement of the neutronflux; however, the performance and analysis of theseexperiments were independent as well as the techniquesfor observing fission and absorption events. The valueso"a> 3^, and o"c/o"f derived in these experiments aregiven in this paper along with the respective valuesfrom ENDF/B-IV MAT-1264.1 Also included are the valuesQJaf ^or 2 3 9P" from the evaluation of Sowerby andKonshin.2Similarities in the- Two ExperimentsBoth of the experiments described in this paperwere performed at ORNL using the Oak Ridge ElectronLinear Accelerator (ORELA) to produce the source of neu-trons in bursts from 5 to 30 nsec wide. The energy ofthe neutrons was measured by the time-of-flight tech-nique. The neutron flux was measured in each experi-ment using a parallel plate (pulse) ionization chamberfilled with B F V The prescription for the 10B(n,a)cross section used to extract the energy dependence ofthe neutron flux was that given in ENDF/B-III and wasthe same as that suggested by Scwerby and Patrick.3The energy range 0.02 el/ to 200 keV was covered in onerun in both experiments; this approach eliminates theproblems encountered in internormalizing runs obtainedunder different experimental conditions and which covera common but narrow energy interval. Normalization ofthe present data sets was performed in the thermalenergy region using values of cu and a from ENDF/B-IIIfor « 9Pu. f aDescription of the Two Experimental MethodsOne experiment (method 1) was performedflight path of about 20 m. A 3-in. diameter 233Pusample having a mass of 0.24 g/cm*2 was used. Fissionevents were recorded using pulse-shape discriminationto detect fission neutrons and a separate detector"total energy detector" (similar to a Hoxon-Rae de-tector) was used to measure the prompt gamma rays fol-lowing neutron absorption in the sample. In the otherexperiment (method 2), the -'-'9Pu (0.03 g/cm- ) was con-tained in a multi-plate pulse ionization chamber Alarge liquid scintillator was used with a 40 m flightpath to detect the prompt gamma rays resulting fromneutron absorption in the sample, and pulses from thisscintillator system in coincidence with pulses from thefission chamber were defined as fissions.Auxiliary experiments were performed to test vari-ous features of the experiments. For example, the neu-tron (pulse-shape discrimination) detector was run incoincidence with the 239Pu fission chamber in a seriesof measurements.1* Also, measurements of the neutronflux were made using 6Li glass.Presentation of the DataThe table shows the average values of 3_, ~5f, anda T^J°f derived from the two independent experiments.Also shown are the cross sections from ENDF/B-IV andthe values vc/Vf for 239Pu taken from the evaluationof Sowerby and Konshin. In comparing the data shownin the table an average difference refers to the sumover a number of energy intervals of the percent dif-ference between the values divided by the number ofintervals. In the cases shows in the table whereexplicit results for ar and o* are not given values of5"c/oy for larger intervals than those in the table areobtained using o, from ENDF/B-IV along with the appro-priate a /of to yield o" and a f for each subinterval.These latter values of 5" and oy are then averaged overthe large interval. The uncertainties shown for ?-and 0" in the table for method 2 represent the pre-acision of the experiments. For 51/o"* the uncertaintiesshown include the known uncertainties except those dueto errors in the cress sections below 0.4 eV used inthe normalization^ An examination of the table showsthat the results o /c* from the two experiments over-lap within thair uncertainties, and in fact about 70%of the results for method 1 fall within the uncertaintyshown for method 2. _ _The results of two experiments for o- and o agreewithin 0.7SJ and 0.3%, respectively, for the neutronenergy range from 0.1 to 1.0 keV, and the average dif-ference between the two results for 3> and a. are 1.9ST aand 2.92, respectively. Above 1 keV the neutron crosssections obtained in method 1 are about 4?i lower thanthose derived in method 2. This is thought to be dyeto difficulties iti the measurement of the neutron fluxfor method 1 at the time of these experiments.For the 28 intervals show,_in_the table,, the pres-ent two experimental values of ajof have an averagedifference_of about 6%. A comparison of the averagevalues of o"c/Of obtained in the present experimentsfor the intervals 0.1 to 1, 1 to 10, md 10 to 100 keVshows that they differ by 2.3, 1.5, and 2.32, respec-tively, and the results for method 2 are higher in eachMASTERnOM o4> Or>. a> av r*> asO CO «-4 +-4 CM r-4 vH ••*• m o i-4 O O oo r+ r*. <8orH*o2iscooIsICMMH5b«£3fci CisID"tnir-. asiSSoooooo+1 +1 +1 -H +4 -H +§ +4 ^  >H +tiIbooooooocoooooooooooooooooooo+1 +1 +1 +1 +1 +) +1 +i - I -H +1 +1 -H -« -H -H -H -H •« +( -H +1 -H -H -H +1 +) +1«4con«oooontOi-ii-ieMiHiHOOOOOOOOCJO+1 +i +1 +1 +1 +1 +1 +1 +: +1O * « a r > - ^ C M C I C M C 4 » l J C s l C J C ) r 4 n ^ » lOOOOOOOO -COOOOOOC-H +1 -to -to « 4< •« +1 •« •« +1 +1 +1 +1 +1 -H +1OOrtOOrHOOOOrHOOOOCOOOOOOOOOOOOO OOOOOOOi-<o OQOOOOOOOOOOOOOOH -H -H +1 +4 +1 -H +1 +1 +1 +1 +1 +1 +1 +1 +1I 1 • I i !S. O3-NOTICE-This /eport was prepared as an account of worksponsored by the United Stales Government. Neitherthe United States nor the United States EneigyRfsearch and Development Administration, nor any oftheir employees, nor any of their contractors,subcontractors, or their employees, makes anywarranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completenessor usefulness of any information, apparatus, product orprocess disclosed, or represents that its use would notinfrin.ee privately owned rights.Inttfrvai. The ENDF/B-IV values for 23iM>u, HAT 1264were based in part upon the data obtained in the pres-ent two experiments. _ _The relation of the present results for c^ /cr- for2 3 9Pu and other experimental data can be summarized bycomparing them with the results obtained in the evalua-tion of Sowerby and Konshin. From 0.1 to 40 keV theaverage difference between the present values (eitherset for "a-fOf of 2 3 2Pu and those of Sowerby andKonshin is about 7%. For the energy intervals 1 to10 keV_and 50 to 100 keV, the present average valuesof oJo. are 10% lower and J72 higher, respectively,than those obtained by Sowerby and Konshin.1.41.2t.O0.80.60.40.2 -I I • • • • I-METH0D2-ENOF/B I I I MAT 1159HISTOGRAM i < METHOD 1INTERVALS | a SOWERBY AND KONSHINCZIRR AND LINDSEYBANDL etal.V UDTTIN «tal.. . .110° 1O1 2 5 1CENERGY (KeV)l.C to 200 keV.Ratio of 9j9i forThe figure shows a plot of oc/o"f for 2 3 9Pu ob-tained in the present two experiments along with thosederived by Sowerby and Konshin. Experimental valuesmeasured by Czirr and Lindsey,5 Bandl et al.,£ andLottin et a!.7 are also shown on the figure. Valuesfrom ENDF/B-III, MAT 1159 are also shown in the figure.Note that the earlier version (III) of ENDF/B shown inthe figure follows vary closely the results of theevaluation of Sowerby and Konshin above 10 keV and bothof these data sets are systematically lower than theresults of the present two experiments. These twoabove evaluations follow the experimental valu£S ofLottin above 50 keV. Earlier measurements of oc/of byHopkins and Diven8 (not shown in the figure) yield avalue of .15 for o1 /of for 2i-Pu at about 60 keV, whichsupports the two evaluations shown in the figure. Thedata of Bandl et al. were normalized in the energyregion from 40 to 50 keV andthus provide informationon the energy dependence of °cfif only.The neutron cross sections ~af and o^ and espe-cially the ratio ov/of for a 3 5Pu obtained by these twoexperiments agree to within a few percent over theenergy range of the experiments. In addition, thesetwo results for a" fof for 239Pu have teen showi to bein good accord with the results; of other measurementsas reflected in the evaluation by Sowerby and Konshi'i.Many factors influence the choice of detectorsystems for the simultaneous measurement of neutroncapture and fission cross sections. The authors con-clude that for the pursuit of high accuracy measure-ments of aJo- on -233pu a fission chamber used inconjunction with total energy detectors represents alogical choice at ORELA. The total energy detectorsare small and can be easily moved from one flight sta-tion to another in order to optimize the experimentalconditions. Although the detection efficiency of thelarge liquid scintiliator is larger (about a factor of10 to 20 in the present case) than that of the totalenergy detector, a gain in the efficiency of the totalenergy detector system can be made by using additionaldetectors.In the present experiments the signai-to-time-dependent background ratio for the large liquid scin-tillator system is about a factor of 2 to 10 less thanthat observed for the total energy detector system.Some of the background in the large liquid scintiiiatorcan be reduced by the dividing of the tank into opti-cally separated sections and requiring that at leasttwo of these sections detect the event; however, thismakes the detector more sensitive to changes in the .capture gamma-ray cascade with neutron energy and italso decreases the efficiency of this detector. Thedesign and mode of use of the total energy detectorminimizes possible changes in its response as the cap-ture gamma cascade changes.A fission chamber is preferred for use becausethe fission detection efficiency can approach unity(~952 efficiency). As the efficiency for fissiondetection approaches unity, all the fission events areidentified and capture is measured directly.The direct measurement of the neutron capture ratesimplifies the normalization of cc and eliminates alarge part of the uncertainty in 'oJ'Zf when this ratiodeparts from that used in the normalization. In orderto achieve the large efficiency (~95:-) for fissionfragment detection, the thickness of the fissile iso-tope is limited to about 100 yg/cm2 and for 2 3 9Pu con-siderations of the alpha particle decay rate limit thetotal amount of the isotope that can be used with asingle detector system. The present fission chambercontained a total of 1.4 g of 239Pu and had an effi-ciency of about 50%. Investigations of signal-tobackground ratios with the total energy detector showthat measurements of o^/Of for 233Pu using 0.1 gquantities of the isotope are feasible.ReferencesResearch sponsored by the Energy Research andDevelopment Administration under contract with theUnion Carbide Corporation.Evaluated Nuclear Data File (ENDF/B) of theNational Neutron Cross Section Center. A detailed listof the evaluators for the particular isotope (definedwith a Mat number, Mat 1159 for *-'9Pu, ENDF/B, VersionIII) is g^yen in File 1 of the data tape. The ENDF/Bdata tape is available from National Neutron CrossSection Center. The average neutron cross sectionsfrom the ENDF/B tape used in the present work were ob-tained using the processing code SUPERTOG, ft. 0. Wrightet al., "SUPERTOG: A Program to Generate Five-GroupConstants and Pu Scattering Matrices from ENOF/8,"ORNL-TH-2679 (1969).2M. G. Sowerby and V. A. Konshin, Atomic Energy 6R. E. Band!, H. Miessner, and Frohner, jjjici. Sci.Review TO, No. 4. 453 (1972). Enfl.. 48, 329 (1972). _ _ — _3M. G. Sowerby et al. Proc. Conf. Nuclear Data for 7A. Lottin et al. Proc. Conf. Fast Critical Exper-Reactors, vol. 1, p. 161 (1970), IAEX iments and Their AnaTysis, ANL-7320, Arqonns, p. 22•*L. W. Weston and J. H. Todd, Phys. Rev. C 10, {1966)4, 1402 (1974). 8J. C. Hopkins and B. C. Diven, Nuci. Sci. Enq.5 J. B. Czirr and J. S. Lindsey, Proc. Conf. Nu- 2* 169 < 1 9 6 2 ^clear Data for Reactors, vol.1, p. 331 (1970}, IAEA.

Direct comparison of different experimental techniques for measuring neutron capture and fission cross sections for $sup 239$Pu

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Direct comparison of different experimental techniques for measuring neutron capture and fission cross sections for $sup 239$ Pu

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Direct comparison of different experimental techniques for measuring neutron capture and fission cross sections for sup239sup 239sup239Pu

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Direct comparison of different experimental techniques for measuring neutron capture and fission cross sections for $sup 239$ Pu