Kaeppeler, F.

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November 1970Institut für Angewandte KernphysikProjekt Schneller BrüterA Measurement of the Fission Cr.oss Section235of U at 440 and 530 keV Neutron EnergyF. KäppelerKFK 1313November 1970KERNFORSCHUNGS ZENTRUM KARLSRUHEKFKInstitut für Angewandte KernphysikProjekt Schneller BrüterA Measurement of the Fission Cross Sectionof 235U at 440 and 530 keV Neutron Energy*F. Käppeler* Paper presented at the EANDC Symposiumon Neutron Standards and Flux Normalization,Argonne, October 21 - 23, 1970Gesellschaft für Kernforschung mbH., Karlsruhe1313ABSTRACTThe fission cross section of 235U has been measured at twoneutron energies using agas scintillation chamber for registratingthe fission events and a telescope-like proton recoil detector fordetermining the neutron flux. The neutrons were produced via the7Li(p,n)7Be reaction at the Karlsruhe 3 MV pulsed Van de Graaffaccelerator. The time of flight method served for background dis-crimination. At 440 ~ 25 keV neutron energy the result was0f = 1.17 barns + 3.5 % and at 530 + 30 keV 0f = 1.17 barns + 3.5 %.ZUSAMMENFASSUNGDer Spaltquerschnitt von 235U wurde für 2 Neutronenenergiengemessen. Die-Spaltereignisse wurden in einem Gasszintillations-zähler nachgewiesen und der Neutronenfluß mit einem teleskop-ähn-lichen Protonenrückstoßdetektor bestimmt. Als Neutronenquelle wurdeam gepulsten 3 MV Van de Graaff-Beschleuniger in Karlsruhe die7Li(p,n)7Be-Reaktion verwendet. Mit Hilfe der Flugzeitmethode wurdegegen Untergrund diskriminiert. Das Ergebnis war für 440 : 25 keV0f = 1.17 barns .~ 3.5 %und für 530 + 30 keV 0f = 1.17 barns ~ 3.5 %.A MEASUREMENT OF THE FISSION CROSSSECTION OF 235U AT 440 AND 530 keVNEUTRON ENERGYF. KÄPPELERGESELLSCHAFT FüR KERNFORSCHUNG,INSTITUT FüR ANGEWANDTE KERNPHYSIKKARLSRUHE, FED.REP. OF GERMANYI. INTRODUCTIONThe discrepancies in the existing values of the fission crosssection of 235U in the neutron energy region between 0.1 and 1 MeVlet it appear worthwhile to try a measurement with a novel method,the accuracy of which .is expected to be ultimately + 2.5 %or better.At the present stage, scattering corrections and statistics are notas small as they could be, and the uncertainties are still 3 to 4 %.The detectors which were used in this experiment were describedelsewhere 1,2 in detail. Therefore only the specific features of theexperimental technique are given here.- 2 -11. EXPERIMENTAL ARRANGEMENTFission DetectorThe fission detector was agas scintillation chamber filled witha mixture of 85 %Argon and 15 %Nitrogen at atmospheric pressure. Itis shown in Fig. 1 together with a pulse height distribution of thefission fragments. The chamber is viewed by two photomultipliers sothat geometry effects are minimized. The efficiency of the detector(the fraction of the pulse-height spectrum above the electronic thresh-old) was determined to be 0.99 ~ 0.5 %. Not shown in Fig. 1 is a trans-port chamber into which the fissile sample could be withdrawn. Thus thetime-correlated background could be measured with the sample out of thechamber. It was found that the time-dependent background amounted to(4 ~ 0.5) % of the counting rate. The time-independent background dueto alpha particles or fission events induced by room-scattered neutronswas about 2.5 %.Proton Recoil DetectorThe proton recoil detector was the same as the one reported atthe Helsinki conference 2. The surface of the solid state detector wasmetallized with 25 pg/cm2 of Gold and the thickness of the insensitiveregion, measured with a collimated beam of alpha particles impingingunder different angles, was about 100 pg/cm2 silicon under experimentalconditions. This corresponds to an energy loss of 34 keV for 440 keVprotons. In Fig. 2 the pulse height distributions of the spectra withcovered and uncovered solid state detector are plotted for the twoenergies.SamplesThe samples were fabricated by CBNM/Euratom in Geel. The fissilelayer consists of uranium acetate electrosprayed 3 onto a 0.9 mm thicksheet of stainless steel, 70 mm in diameter and with a polish to better- 3 -than 1 pm. The thickness of the layer was 92 ~g/cm2 and the diameter40 mm. From the specific activity and low-geometry alpha counting thetotal mass of uranium was determined to be 1.161 mg U :t 0.8 %. Only asmall correction for the isotopic composition had to be made, becausethe abundance of 235U was 99.5 + 0.01 %. An estimation followingWhite 4 shows that an amount Of-0.6 %of the fission fragments areabsorbed in the fissile layer itself. The absorption correction thenbecomes 0.994 :t 0.3 %.The radiator for the proton recoil detector consists of glyceroltristearate (C17H35COO)3C3H5' evaporated 5 onto a similar backing of0.9 mm stainless steel as the fissile layer. By careful weighing witha vacuum balance the total mass of the radiator material was determinedto be 1.619 mg + 0.5 %. The diameter of the layer was 40 mm and thethicknes$ 128 p;/cm2•ElectronicsFig. 3 shows a block diagram of the electronics. From the fissionevents pulse height spectra and time-of-flight spectra were recordedsimultaneously. A coincidence requirement between the outputs of thetwo photomultipliers provides diserimination against multiplier noise.Two speetra of eaeh type were taken beeause both multiehannel ana~yzerswere routed by the same sampIe ehanger automatie as the proton reeoildeteetor.The electronics of the proton recoil detector is the same as des-2eribed in Ref. exeept for a new timing unit. This unit improved thetime resolution to 12nsee independent of the pulse height.Neutron SourceNeutrons were produeed with the Karlsruhe 3 MV pulsed Van de Graaffaeeelerator via the 7Li(p,n)7Be reaction. The pulse width was 1 nseeand the repetition rate 106 -1- • The neutron energy was determined withsee- 4 -a Li-glass detector at a distance of 3.02 m and with a time resolutionnsec 7 .<2 ---- • Normally the L~-targets were 40 to 60 keV thick. Ta-back-mings for the 7Li-targets and a shielding of 2 cm Pb reduced the y-flashof the accelerator.111. CORRECTIONS AND UNCERTAINTIESBesides the corrections mentioned above there are those due toscattered neutrons. For the determination of theinfluence of scatteredneutrons on the measured fission cross section four different detectorarrangements were used.In the first arrangement (geometry I) the detectors were placedback-to-back behind a 450 mm long collimator of paraffin that was coatedby cadmium sheet. The scattering correction for this geometry mustaccount for neutrons scattered in the backings of the fissile layer andof the radiators, and in the bronze windows between them. With the ca1-cu1ated correction factor, K1 = 0.910 : 2.5 %, the fission cross sectionat 440 + 26 keV was found to be Gf = 1.16 barn + 4.5 %.Another measurement at the same energy was made with the detectorslocated in symmetrie positions with respect to the proton beam axis,without co1limator (geometry 11). In this geometry, the correction dueto scattered neutrons from the backings becomes smaller (K2 = 0.982 +0.5 %), but there is additional scattering from the detector walls. Byadding an identical blind fission chamber to the experiment, it waspossible to measure the inf1uence of neutrons scattered from the walls.It was found to require an additional correction of K3 = 0.92 : 1.2 %.The va1ue determined with this geometry (after application of K2 and K3)was Gf = 1.18 barns + 4 %.A third possibility is to determine the fission cross section froma measurement with the back-to-back arrangement without co11imator usingthe correction K3 for the scattered neutrons from the detector walls.However, one has to account for the fact that for this arrangement the- 5 -proton recoil detector is situated closer to the fission chamber thanin geometry 11. The necessary correction is estimated to beK4 = 1.5 K3 = 0.88 ! 3 %. The resulting fission cross section valuewas 0f = 1.16 barns ! 6 %, again at 440 ! 25 keV.The uncertainties, which are given for these three values of 0fcontain all uncertainties listed in Table I, including the statisticaluncertainties.IV. RESULTS AND DISCUSSIONFission cross sections of 235U have been measured relative to thehydrogen (n,p) cross section at two energies. The result is440 + 25 keV530 + 30 keV0f = 1.17 barns + 3.5 %0f = 1.17 barns + 3.5 %.Fig. 4 shows a comparison between these two values and the existingmeasurements. Agreement is found within the quoted uncertainties bet-4ween these values and the data of White , and Szabo, Marquette, Fortet Lerqy 6. The fission cross sections measured by Allen and Ferguson 7,Diven 8 and Smirenkin 9 are somewhat higher while the data given byPönitz 10 and - to some extent - those given by Gorlov 11 are lower byabout 10 %.It is planned to increase the accuracy of this experiment in thefuture by using thinner backings, more samples and longer data acqui-sition times. The aim is to establish a data set in the energy rangebetween 0.3'and 1.2 MeV with an uncertainty of ~2.5 %.- 6 -ACKNOWLEDGMENTI wish to thank F. Fröhner for his help and contribution tothis work and A. Ernst and R. Bandl for helpful discussions. I amalso indebted to all people in Geel who prepared and analyzed thesampIes in a very careful way as weIl as to the Van de Graaff teamfor their good service.- 7 -REFERENCES1) E. Pfletschinger and F. Käppeler, A Measurement of the FissionCross Section of 239pu and 233U Relative to 235U, Nuc.~.Eng.40 : 375 - 382 (1970)2) F. Käppeler and F.H. Fröhner, Absolute Neutron Flux Determination,to be published in the Proceedings of the Second InternationalConference on Nuclear Data for Reactors in Helsinki, June 1970,by the IAEA, Vienna.3) V. Verdingh and K.F. Lauer, Preparation by Electro-Spraying ofThin Uranium, Plutonium and Boron samples for Neutron CrossSection Measurements in 41r -Geometry, Nucl.lnstr.Meth. 21 :161 - 166 (1963)4) P.H. White, Measurements of the 235U Neutron Fission Cross Sectionin the Energy Range 0.04 MeV to 14 MeV, J.~.Energy 19 :325 - 342 (1965)5) H.L. Eschbach, priv. comm.6) I. Szabo et al., Mesure Absolue de la Section Efficace de Fissionde 235U et de 239pu entre 0.025 et 1 MeV, to be published in theProceedings of the Second International Conference on Nuclear Datafor Reactors in Helsinki, June 1970, by the IAEA, Vienna7) W.D. Allen and A.T.G. Ferguson, The Fission Cross Sections of233U, 235U, 238U and 239pu for Neutrons in the Energy Range 0.030MeV to 3.0 MeV, Proc.Phys.~. 70A : 573 - 585 (1957)8) B.C. Diven, Fission Cross Sections of 235U for Fast Neutrons,Phys.!!!. 105 : 1350 - 1353 (1957)9) G.N. Smirenkin, V.G. Nesterov and I.I. Bondarenko, 233U, 235Uand 239pu Fission Cross Sections for 0.3 - 2.5 MeV Neutrons,Atomnaya Energiya 13 : 366 - 368 (1962); engl. ~.i.Atomic Energy13 : 974 - 976 (1963)10) W.P. Pönitz, Measurement of the 235U Fission Cross Section in thekeV Energy Range, Conf. on Neutron Cross Sect. and Tech., Vol.I :503 - 511, Washington, D.C. (1968), D.T. Goldman, Ed.- 8 -11) G.V. Gorlov et al., The Fission Cross Sections ot 233U and 235Utor Neutrons Having Energies between 3 and 800 keV, ~.~.Energy12 A : 79 - 82 (1960)TABLE ISource of Uncertainty UncertaintyNumber of Hydrogen Atoms 0.5 %Number of Uranium Atoms 0.8 %Efficiency of the Proton Recoil Detector 1.5 %Efficiency of the Fission Chamber 0.5 %Absorption in the Fissile Layer 0.3 %Isotopic Composition of 235U 0.01 %Correction for (n,y)-Background 0.5 %Correction for Equal Flux 0.3 %(For Back-to-Back Position only)Fig. 1Fig. 2Fig. 3Fig. 4FIGURE CAPTIONSThe gas scintillation chamber (Top view) and a pulse heightdistribution of the fission fragments obtained under ex-perimental conditions.Pulse height distributions from the proton recoil detector.Electronic equipmentComparison of the present work (full circles) with othermeasur ements.BACKING(0.9 mm STAINLESS STEEL)FISSILE LAYERPM TUBE(VALVO 56 UVP)QUARTZ WINDOWn •NEUTRON WINDOW(0.1 mm BRONZE)SCINTILLATION CHAMBER(0.4 mm BRONZE)THICKNESS OF 92 119FISSILE LAYER cm2 ••• •(f)I- •....z:::> ~ •>- ••0:::: •« • •0::: ••I- ••ä:i•0::: •« •••.....J •W • •z •z« •:r:u0:::: ••wCL(f) • •I-Z ••:::>0 •u ••• •••••••r I I I I ••I ICHANNEL NUMBERFIG.lRADIATOR 119128 -.-THICKNESS cm2 0000MAX. NEUTRON 460 KeVENERGY 0 00NEUTRON 50 KeV 0ENERGY SPREAD00o SPECTRUM+ BACKGROUND--e-- BACKGROUNDSUBTRACTED700•oo 0o 000 0o00o 0o00o 0o560 KeV128~cm2o+00NEUTRON 60 KeVENERGY SPREADMAX. NEUTRONENERGYRADIATORTHICKNESS-IUJZZ«IU0::UJCLu'J......z::::>ouFIG.2STOP FROMACCELERATORTIMINGUNITBUFFERMEMORYGATEGENERATORSPECTROSCOPYAMPLIFIERSTOP FROMACCELERATORLINEARGATE sSTRETCHER512 CHANNELANALYZER1024 CHANNELANALYZERON LINECOMPUTERTWO PULSE HEIGHTSPECTRATWO TIME-OF-FLIGHTSPECTRATWO 64x64 CHANNEL MATRICES(PULSE HEIGHT versus TIMEOF FLIGHT)FIG.31.011.5I/lc:Ci 1.4.0..:::>I.!'l('I')N..-'U 1.31.21.1v ALLEN & FERGUSON0 SMIRENKIN! KFK A WHITE---- DAVEY 0 SZABO+ GORLOV0 E9 POENITZ,17 X DIVEN0"'i00 v---- -v----- ) 0 ,. v Xo --_-- 0 0 v OXo --__---.....-. ---.- -,.. -,... T- ---U U V-U - - ...._ 0 X--- -- 0 o 0---.---1--__ ...-0-+ -'-----_...--+~~ +e~ E90.9300FIG.4400 500NEUTRON ENERGYJ KeV600..700

A Measurement of the Fission Cross Section of ²³⁵U at 440 and 530 keV Neutron Energy

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A Measurement of the Fission Cross Section of ²³⁵U at 440 and 530 keV Neutron Energy

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