The U-236(n,f) cross section has been measured in the energy range from 0.5 eV to 25 keV at the Geel Electron Linear Accelerator neutron time-of-flight facility of the Institute for Reference Materials and Measurements in Geel, Belgium. A highly enriched U-236 sample was mounted back-to-back with a B-10 sample in the center of a Frisch-gridded ionization chamber, hence realizing a 2 pi detection geometry. A U-235(n,f) cross-section control measurement was performed in the same experimental conditions. Special attention has been given to the fission resonance integral I-f and to the strongest resonance at 5.45 eV, for which a resonance analysis has been performed yielding Gamma(f) = 1.7 mu eV Both values are highly overestimated in the literature

Wagemans, Cyrillus

De Smet, Liesbeth

Vermote, Sofie

Heyse, Jan

English

Ghent University Academic Bibliography

Measurement of the 236U(n, f ) Cross Section in the NeutronEnergy Range from 0.5 eV up to 25 keVC. Wagemans,* L. De Smet, and S. VermoteUniversity of Gent, Department of Subatomic and Radiation Physics, B-9000 Gent, BelgiumJ. Heyse† and J. Van GilsEuropean Communities, Joint Research Centre, Institute for Reference Materials and MeasurementsRetieseweg 111, B-2440 Geel, BelgiumandO. SerotCEA-Cadarache, DEN0DER0SPRC0LEPh, F-13108 Saint Paul lez Durance, FranceReceived October 29, 2007Accepted January 19, 2008Abstract – The 236U(n,f) cross section has been measured in the energy range from 0.5 eV to 25 keV at theGeel Electron Linear Accelerator neutron time-of-flight facility of the Institute for Reference Materialsand Measurements in Geel, Belgium. A highly enriched 236U sample was mounted back-to-back with a 10Bsample in the center of a Frisch-gridded ionization chamber, hence realizing a 2p detection geometry. A235U(n, f) cross-section control measurement was performed in the same experimental conditions. Specialattention has been given to the fission resonance integral If and to the strongest resonance at 5.45 eV, forwhich a resonance analysis has been performed yielding Gf  1.7 meV. Both values are highly overesti-mated in the literature.I. INTRODUCTIONUranium-236 is produced by neutron capture in 235Ufuel, so neutron-induced reactions in 236U influence theneutron balance of the fuel cycle. Moreover, the 236Uatoms contribute to the long-lived radioactivity of usedfuel elements, and reprocessed uranium contains a non-negligible amount of 236U. Hence, a decent knowledgeof the 236U~n, f ! cross section is certainly desirable.The experimental database for the 236U~n, f ! reac-tion cross section in the thermal and resonance region isvery poor. Only one direct measurement with thermalneutrons is available, and the resonance data are scarceand discrepant:1. Recently, Wagemans et al.1 have performed theonly direct thermal fission cross-section measurement atthe high flux reactor of the Institute Laue-Langevin, yield-ing s~nth , f ! ~0.36 1! mb.2. In 1970, Cramer and Bergen2 made a first at-tempt to measure the 236U~n, f ! cross section from 35.2eV to 2.935 MeV in the Pommard bomb shot experiment.They used a sample containing 0.15% of 235U, but thedata were not corrected for the corresponding important235U~n, f ! contribution, so the small 236U~n, f ! reso-nances were lost in this background.3. In 1972, Theobald et al.3 performed a measure-ment at the electron linear accelerator of the Central Bu-reau for Nuclear Measurements ~CBNM !, now theInstitute for Reference Materials and Measurements~IRMM!, in Geel, Belgium, covering the energy regionfrom 5 to 415 eV. Fission neutrons were detected with a*E-mail: cyrillus.wagemans@Ugent.be†Current address: SCK•CEN, Boeretang 200, B-2400 Mol,BelgiumNUCLEAR SCIENCE AND ENGINEERING: 160, 200–206 ~2008!200liquid scintillator, and signals due to neutrons and gammarays were separated using the pulse-shape discrimina-tion technique. The sample used contained 99.69% of236U and 0.199% of 235U, which resulted in a strong235U~n, f ! background. Their sf ~E ! values are the onlydata set that has been released so far.4. Also, the measurements by Parker et al.4 at theLos Alamos Neutron Science Center spallation neutronsource suffered from a strong fission background sincethe samples used contained 0.3% of 235U and 0.9% of233U. Because the main goal of these measurements wasa study of the intermediate structure in the 236U~n, f !cross section, no sf ~E ! data set was released.5. More recently, the 236U~n, f ! cross section hasbeen measured at the n_TOF facility at CERN, Geneva,Switzerland,5 but final results have not been released sofar.In an effort to improve the database in the resonanceregion, experiments have been performed at the GeelElectron Linear Accelerator ~GELINA! neutron time-of-flight ~TOF! facility of the IRMM in Geel, Belgium,using a highly enriched 236U sample.II. EXPERIMENTAL PROCEDUREII.A. Experimental SetupThe GELINA neutron TOF facility of the IRMM isthe modernized electron linear accelerator of the CBNM,the main improvements being a reduction of the elec-tron burst width from 10 to 1 ns and an increased neu-tron flux. At present, the linear accelerator delivers apulsed and compressed electron beam that hits a rotat-ing U target, hence producing bremsstrahlung gammarays. These gamma rays create neutrons via ~g, n! and~g, f ! reactions. The neutrons are moderated in 4-cm-thick water-filled beryllium containers, resulting in abroad spectrum with energies ranging from a few milli-electron-volts up to a few mega-electron-volts. The shapeof the moderated neutron spectrum can be approxi-mated by a Maxwellian distribution at thermal energywith a 10E high-energy tail. The energy of the neutronsis determined applying the TOF method.A double Frisch-gridded ionization chamber withultrapure methane as detector gas was installed at an8.3-m-long flight path. The uranium sample ~coveredwith a very thin polyimide foil to avoid contamination ofthe chamber by uranium alpha-decay recoils! wasmounted in the center of the cathode, back-to-back witha 10B layer. A schematic view of this setup is shown inFig. 1. Two consecutive measurements were performed,one with a 236U sample and a second one with a 235Usample.Each anode signal was amplified and sent to ananalog-to-digital converter and a timing single-channelanalyzer ~TSCA!. The TSCA produces a fast logic signalTn when the anode signal crosses a certain constant frac-tion of the pulse peak. Using a time coder, the TOF isdetermined by measuring the time between the start sig-nal T0 given by the linear accelerator at each electronburst and the stop signal Tn sent by the TSCA. The TOFvalue obtained in this way was stored in a LABVIEW-based data acquisition system together with the digitizedanode pulse height for off-line analysis.II.B. SamplesThe uranium samples were prepared at the IRMMby electrolytic deposition of uranium oxide on a 20-mm-thick Al foil stretched on a metal frame. The active di-ameter was 50 mm. The isotopic enrichments and thelayer thicknesses of the 236U and 235U samples used aresummarized in Table I.For the neutron flux determination, an evapo-rated elemental 10B layer with an enrichment of 93%, aFig. 1. Schematic layout of the Frisch-gridded ionizationchamber. A, G, and C indicate anode, grid, and cathode, re-spectively. P.I. stands for polyimide foil.TABLE ISurvey of the Uranium Sample CharacteristicsIsotopic Composition ~at.%!SampleThickness~mg0cm2 ! 233U 234U 235U 236U 238U235U 110.96 0.7 0.0056 0.0408 99.0402 0.2632 0.6502236U 209.96 1.3 0.0043 99.9732 0.0225236U~n, f ! CROSS SECTION 201NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 2008thickness of ~8.05 6 0.10! mg 10B0cm2, and a diameterof 50 mm was used. The amount of 10B in the layer wasdetermined in a dedicated measurement in a neutronbeam, relative to the fission counting rate with a cali-brated 235U sample. Also, here the sample backing wasa 20-mm-thick Al foil stretched on a metal frame.II.C. MeasurementsThe 10B~n,a! 7Li reaction products ~Fig. 2! and the236U~n, f ! fragments were measured simultaneously withthe double Frisch-gridded ionization chamber. In a con-trol measurement, the 236U sample was replaced by a235U sample with the same dimensions, maintaining thesame cathode and 10B sample. This permitted us toverify if the well-known 235U~n, f ! cross section wasreproduced in our experiment. It also enabled correc-tion for the small 235U~n, f ! contribution in the mea-surement with the 236U sample ~which contains 0.0043at.% of 235U; see Table I! with 235U~n, f ! data of thesame experimental resolution. Figure 3a shows the pulse-height spectrum for 235U~n, f !; Fig. 3b shows a typical236U~n, f ! spectrum obtained in one of the runs. Be-cause of the alpha activity of the 236U sample combinedwith the 2p detection geometry, some alpha pileup waspresent, which was determined in a separate back-ground measurement.For all measurements, GELINA was operated at an800-Hz repetition frequency, the electron pulses havinga width of 1 ns. Overlapping neutrons from previousbursts were removed by a permanent cadmium overlapfilter. The time-dependent background in the TOF spec-tra was determined by putting “black resonance” filters~Rh, Au, W, Co, and Mn! in the neutron beam.III. RESULTS AND ANALYSISIII.A. Data AnalysisIn the measurements with the 236U sample, both fis-sion fragments are detected, so the fission counting rateYf is given byYf ~En !  «f @N6us6u~En ! N5us5u~En !#w~En ! YfBGR~En ! , ~1!where«f  detection geometry factorN6u  number of 236U atoms in the 236UsampleN5u  number of 235U atoms in the 236UsampleFig. 2. Pulse-height spectrum of the 10B~n,a! 7Li parti-cles. The arrow indicates the discrimination threshold.Fig. 3. ~a! 235U~n, f ! pulse-height spectrum; ~b! partial236U~n, f ! pulse-height spectrum obtained in one of the runs.202 WAGEMANS et al.NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 2008w~En !  neutron fluxs6u~En !  neutron-induced fission cross section for236Us5u~En !  neutron-induced fission cross section for235UYfBGR~En !  TOF-dependent fission background.Contributions due to the other U isotopes present in thesample are negligible.With the discriminator setting used in the 10B~n,a!7Limeasurement ~see Fig. 2!, only the alpha particles aredetected; hence, the counting rate YB is given byYB~E !  2«B NBsB~En !w~En ! YBBGR~En ! , ~2!where«B  detection geometry factorNB  number of 10B atoms in the 10B samplesB~En !  10B~n,a! 7Li reaction cross sectionYBBGR~En !  TOF-dependent background in the10B~n,a! measurement.In both cases, the time-dependent background hasbeen determined as a function of the TOF t by fitting afunction Y BGR~t ! at b c through the counting rates inthe black resonances and has been subtracted from thecounting rates Yf and YB . A typical example is given inFig. 4, which shows the background in the black reso-nances of Mn.Dividing Eqs. ~1! and ~2! yields, after correction forthe background,Yf ~En ! YfBGR~En !YB~En ! YBBGR~En !«f2«BN6us6u~En ! N5us5u~En !NBsB~En !. ~3!Since the detection geometry was 2p in both cases,«f 0«B 1; hence, the neutron-induced fission cross sec-tion of 236U can be calculated as follows:s6u~En !  2NBN6usBENDF0B-VI~En !Yf ~En ! YfBGR~En !YB~En ! YBBGR~En !N5uN6us5u~En ! , ~4!where sBENDF0B-VI~En ! is the 10B~n,a! 7Li referencecross section adopted from the ENDF0B-VI evaluateddata library.The 235U~n, f ! cross section s5u~En ! in the last termof this expression was taken from the measurements withthe 235U and 10B samples mentioned in Sec. II.C. In thisway the raw 236U~n, f ! data were corrected for the235U~n, f ! contribution due to the 235U impurities, main-taining the same energy resolution.All these operations are implemented in the AGScomputer code,6 which takes into account all sources ofuncertainty and takes care of the error propagation, pro-ducing an experimental variance-covariance matrix. Apartfrom the counting statistics, the accuracy of the data isinfluenced by ~a! an uncertainty of 1.2% on the numberof 10B atoms, ~b! an uncertainty of 0.6% on the numberof atoms in the 236U and 235U samples, ~c! the ~small!uncertainty on the 10B~n,a! 7Li reference cross section,~d! the uncertainty on the correction for the time-dependent background, and ~e! the uncertainty on thecorrection for the 235U~n, f ! contribution. Since the235U~n, f ! cross section is well known and because the236U~n, f ! and 235U~n, f ! cross sections were measuredunder the same experimental conditions, the accuracy ofthis correction is mainly determined by the 2.3% accu-racy on the 235U fraction in the 236U sample.III.B. ResultsFigure 5a shows the 236U~n, f ! cross section in theneutron energy region from 0.5 to 22 eV before correc-tion for the 235U~n, f ! contribution. The corresponding235U~n, f ! cross section obtained in the 235U measure-ment under the same conditions is shown in Fig. 5b todemonstrate the importance and the quality of thiscorrection.Figure 6 shows the 236U~n, f ! cross section in theneutron energy region from ~a! 1 to 1000 eV and ~b! 1 toFig. 4. Determination of the TOF-dependent backgroundin the 10B~n,a! measurement with a Mn ~black resonance!filter. The TOF region shown corresponds to neutron energiesbetween 18 eV and 15.5 keV.236U~n, f ! CROSS SECTION 203NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 200825 keV after correction for the 235U~n, f ! contribution.Besides the dominant resonance at 5.45 keV, four reso-nance clusters with a spacing of ;3 keV can be ob-served, as reported by Parker et al.4 Because of the shortflight path used in the present measurements, the reso-nances inside the clusters are not well resolved.IV. DISCUSSIONIV.A. Intermediate Resonance StructureThe 236U~n, f ! cross-section results shown in Fig. 6do not show the large number of resonances reported byTheobald et al.3 These resonances are probably due to asmall sensitivity for radiative capture events of the liq-uid scintillator used in their measurements and0or aninsufficient pulse-shape discrimination between gam-mas and neutrons. The present picture of fairly distantresonance clusters is a clear signature of intermediatestructure, which is not surprising given the subbarriercharacter of the low-energy neutron-induced fission of236U.IV.B. The 5.45-eV Resonance and theThermal RegionFigure 7 shows the result of a fit to the present cross-section data in the region of the 5.45-eV resonance, usingthe SAMMY R-matrix code.7 With Gg  24.5 meV andGn 2.16 meV ~as adopted in JEFF-3.1!, a fission widthGf  ~1.7 6 0.1! meV was obtained. This result is quiteFig. 5. ~a! Raw 236U~n, f ! data uncorrected for the235U~n, f ! contribution; ~b! the corresponding 235U~n, f ! crosssection.Fig. 6. The 236U~n, f ! cross section from ~a! 1 to 1000 eVand ~b! 1 to 25 keV.204 WAGEMANS et al.NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 2008comparable with the value of ~1.3 6 0.1! meV reportedby Parker et al.,4 but it is two orders of magnitude lowerthan the value Gf  290 meV adopted in JEFF-3.1. ABreit-Wigner extrapolation of the 5.45-eV resonance tailtoward 0.0253 eV using the resonance parameters ob-tained in the present work yields a thermal contributionof ~0.226 0.02! mb ~Fig. 8!, in agreement with the ex-perimental thermal cross-section value of ~0.3 6 1! mbof Wagemans et al.1 For a good analytical description ofthe experimental ~n,g! cross section, the evaluated datafiles include a bound state ~e.g., at 7.0 eV in JEFF-3.1!. If this bound state also has a fission component,this will lead to a small thermal contribution, which shouldbe added to this 0.22 mb. The lower curve in Fig. 8 is an~extrapolated! result of the present measurements, whichyield sf ~E ! values about two orders of magnitude smallerthan the corresponding JEFF-3.1 values ~the same is truefor JENDL-3.3 and ENDF0B-VII.0!.IV.C. The Fission Resonance IntegralIn Table II, the values for the fission resonanceintegralIf  0.5 eV100 keVsf ~E !dEE,denoted by If ~100! in Table II, calculated from the JEFF-3.1, JENDL-3.3, and ENDF0B-VII.0 evaluated data li-braries are reported. Since the 236U~n, f ! cross-sectiondata obtained in the present measurements only coverthe neutron energy region from 0.5 eV to 25 keV, also theintegral If ~25! beingIf  0.5 eV25 keVsf ~E !dEEhas been calculated, which is almost equal to If ~100!.The comparison in Table II clearly demonstrates that thefission resonance integrals calculated from all evaluateddata libraries are too large by two orders of magnitude.Also in Table II, the thermal neutron–induced fis-sion cross sections adopted in the evaluated data librar-ies are reported and compared with the experimental valueof Wagemans et al.1 and with the value deduced abovefrom a Breit-Wigner extrapolation of the 5.45-eV reso-nance as determined in the present work. Also here allevaluated data files report values that are too large bytwo orders of magnitude.Fig. 7. SAMMY fit to the 5.45-eV resonance data points.Fig. 8. Comparison of the JEFF-3.1 evaluation below10 eV ~intermittent line! with the present results extrapolatedto the thermal energy region ~full line! using SAMMYcalculations.TABLE IIComparison of the Thermal Fission Cross Section and theFission Resonance Integral for 236UReferencesf ~ thermal!~mb!If ~100!~b!If ~25!~b!ENDF0B-VII.0 47.1 4.460 4.452JEFF-3.1 61.3 4.336 4.334JENDL-3.3 61.3 4.363 4.359Wagemans1 0.36 1This work 0.22a 6 0.02 0.0326 0.001aBreit-Wigner extrapolation of the 5.45-eV resonance.236U~n, f ! CROSS SECTION 205NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 2008The comparisons in Table II clearly demonstrate thatall three evaluated data libraries need to be corrected,since all of them are still relying on the wrong fissioncross-section data of Theobald et al.3V. CONCLUSIONIn the present work, new experimental data for the236U~n, f ! cross section are reported, showing a clearsignature of intermediate structure. An R-matrix analy-sis has been performed of the dominant 5.45-eV reso-nance, yielding a fission width Gf  ~1.7 6 0.1! meV,much lower than the value of 290 meV adopted in JEFF-3.1. All these new data are consistent with the experi-mental value for the thermal 236U~n, f ! cross sectionobtained at the high flux reactor of the Institute Laue-Langevin.1 Both the results in the thermal and resonanceregion up to 25 keV are two orders of magnitude lowerthan those reported in the JEFF-3.1, JENDL-3.3, andENDF0B-VII.0 evaluated data libraries, which should berevised.REFERENCES1. C. WAGEMANS, O. SEROT, P. GELTENBORT, and O.ZIMMER, “Experimental Determination of the 236U~nth , f !Cross Section,” Nucl. Sci. Eng., 136, 415 ~2000!.2. J. CRAMER and D. BERGEN, LA-4420, p. 74, Los Ala-mos National Laboratory ~1970!.3. J. THEOBALD, J. WARTENA, H. WEIGMANN, and F.POORTMANS, Nucl. Phys. A, 181, 639 ~1972!.4. W. PARKER, E. LYNN, G. MORGAN, P. LISOWSKI, A.CARLSON, and N. HILL, Phys. Rev. C, 49, 672 ~1994!.5. U. ABBONDANNO et al., AIP Conf. Proc., 769, 724 ~2005!.6. C. BASTIAN, A. BORELLA, F. GUNSING, J. HEYSE, S.KOPECKY, G. NOGUERE, P. SIEGLER, and P. SCHILLE-BEECKX, “AGS, A Computer Code for Uncertainty Propaga-tion in Time-of-Flight Cross Section Data,” Proc. PHYSOR-2006, Vancouver, Canada, September 10–14, 2006, AmericanNuclear Society ~2006! ~CD-ROM!.7. N. LARSON, ORNL0TM-91790R6 ENDF-364, Oak RidgeNational Laboratory ~2003!.206 WAGEMANS et al.NUCLEAR SCIENCE AND ENGINEERING VOL. 160 OCT. 2008

Measurement of the U-236(n, f) cross section in the neutron energy range from 0.5 eV up to 25 keV

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Measurement of the U-236(n, f) cross section in the neutron energy range from 0.5 eV up to 25 keV

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