Bredeweg, T. A.

Chyzh, A.

Couture, A.

Gostic, J.

Haight, R. C.

Henderson, R.

Jandel, M.

Kwan, E.

Lee, H. Y.

O'Donnell, J.

Ullmann, J.

Wu, C. Y.

UNT Digital Library

LLNL-PROC-505354Neutron Capture and FissionMeasurements on Actinides atDANCEA. Chyzh, C. Y. Wu, E. Kwan, R. Henderson, J. Gostic,J. Ullmann, J. O'Donnell, T. A. Bredeweg, M. Jandel, A.Couture, R. C. Haight, H. Y. LeeOctober 13, 2011CGS-14Guelph, CanadaAugust 28, 2011 through September 2, 2011Disclaimer  This document was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor Lawrence Livermore National Security, LLC, nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or Lawrence Livermore National Security, LLC. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or Lawrence Livermore National Security, LLC, and shall not be used for advertising or product endorsement purposes.  October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs141Neutron Capture and Fission Measurements on Actinides atDANCEA. Chyzh∗, C. Y. Wu, E. Kwan, R. A. Henderson, J. M. GosticLawrence Livermore National Laboratory,Livermore, CA 94550, USA∗E-mail: chyzh1@llanl.govwww.llnl.govJ. L. Ullmann, T. A. Bredeweg, M. Jandel, A. Couture,J. M. O’Donnell, R. C. Haight, and H. Y. LeeLos Alamos National Laboratory,Los Alamos, NM 87545, USAwww.lanl.govThe prompt γ-ray energy and multiplicity distributions in the spontaneous fis-sion of 252Cf have been measured using a highly granular 4π γ-ray calorimeter.Corrections were made for both energy and multiplicity distributions accord-ing to the detector response, which is simulated numerically using a modelvalidated with the γ-ray calibration sources. A comparison of the total γ-rayenergy distribution was made between the measurement and a simulation byrandom sampling of the corrected γ-ray energy and multiplicity distributionsthrough the detector response. A reasonable agreement is achieved betweenthe measurement and simulation, indicating weak correlations between γ-rayenergy and multiplicity. Moreover, the increasing agreement with increasingmultiplicity manifests the stochastic aspect of the prompt γ decay in sponta-neous fission. This calorimeter was designed for the study of neutron capturereactions and an example is given, where the238Pu(n,γ) measurement was car-ried out in the laboratory environment for the first time.Keywords: Neutron capture, spontaneous fission, DANCE, Cf-252, Pu-238,gamma-ray energy, multiplicity.1. IntroductionUnderstanding the fission process is important for the design of nuclearreactor and the safety of its operation. The prompt energy released in fis-sion is derived from three components; namely, fission fragments and theirprompt neutrons as well as γ decays. Significant amount of work has beenOctober 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs142carried out for the measurement and modeling of the mass and kinetic en-ergy distributions of fission fragments1 as well as the energy distributionand the average multiplicity of emitted neutrons.2,3 However, there is onlylimited measurement and modeling for the γ-ray decay.With the advance of modern highly granular 4π γ-ray detector ar-rays such as the Detector for Advanced Neutron Capture Experiments(DANCE)4 at Los Alamos Neutron Science Center (LANSCE), the pre-cision measurement of the prompt γ-ray multiplicity as well as energy dis-tribution in fission becomes feasible. The unique features of DANCE suchas the nearly γ-ray energy independence of the multiplicity response, makeit possible, for the first time, to unfold the γ-ray multiplicity distribution infission. This enables one to quantify the correlation between γ-ray energyand multiplicity and advance our understanding of the prompt γ-ray decayprocess in fission.Such a study has been carried out for the spontaneous fission of 252Cf.The prompt γ rays have been measured by the DANCE array in coinci-dence with the detection of fission fragments by a parallel-plate avalanchecounter (PPAC). The prompt γ-ray energy and multiplicity distributionswere derived in the off-line analysis and their unfoldings were performedusing both Bayesian5 and Singular Value Decomposition (SVD)6 methodsaccording to the detector response. The study of correlation between γ-rayenergy and multiplicity was made by investigating the total γ-ray energy be-tween the measurement and simulation by random sampling their unfoldeddistributions through the detector response. Details of this measurementand results are presented below.2. ExperimentA 252Cf source with a strength ∼0.15 µCi was prepared by stippling thematerial on a 3 µm thick titanium foil, which was then covered by the 1.4µm thick aluminized mylar before being inserted into the PPAC, to serveas a cathode. Two anodes, made of the same thickness aluminized mylar,were placed on both sides of the cathode at a distance of 3 mm. The PPACwas operated with the isobutane gas pressure ∼4.00 torr stabilized by afeedback loop of constant gas flow. It has efficiency ∼82% for the detectionof fission fragments with a bias of 375 V applied on anodes and provides afast and clean fission trigger for the DANCE array to record the coincidentγ rays from fission fragments with a time window ∼ 30 ns. Details of thedesign and fabrication for this PPAC is described in Ref.7DANCE is a 4π γ-ray calorimeter and consists of 160 BaF2 crystalsOctober 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs143with equal solid-angle coverage. It is designed to study the neutron-capturereactions on small quantities of radioactive and rare stable nuclei. Theneutron-capture event is recognized by the measured total γ-ray energy andthe summed photopeak is equivalent to the reaction Q value. Besides its useas a calorimeter for the study of the neutron-capture reaction, DANCE canbe used for the precision measurement of the γ-ray energy and multiplicitydistributions as well as the total γ-ray distribution in fission as long as themeasurement is accompanied by a charged-particle detector, such as thePPAC mentioned above, for the fission-fragment detection.The experiment was fielded at LANSCE over a period of five days.A total of ∼3×106 fission events were selected for the current study byplacing a time window ∼10 ns on the coincident time spectrum betweenDANCE and PPAC, where a time resolution ∼2 ns is achieved. The pulseheight and timing information for both DANCE and PPAC were derivedfrom the recorded waveforms by the 500 MS/s, 300 MHz bandwidth Acqirisdigitizers. The coincident γ-ray energy and multiplicity distributions weredetermined from the DANCE array for those fission events. To avoid thesumming effect on the γ-ray energy distribution, the spectrum, shown inFig. 1, was obtained by requiring the γ ray detected by the BaF2 crystalwithout any adjacent crystals being triggered. The sensitivity of DANCEfor the γ-ray spectroscopy study is clearly demonstrated, where the strengthof γ ray falling monotonously as a function of energy over a range of fivedecades until tapering off ∼8 MeV is observed.The γ-ray energy and multiplicity response for the DANCE array hasbeen studied extensively.8,9 A physical model of DANCE together with thesurrounding material was created to study the response numerically andthe validation was carried out by the comparison between the measurementand simulation using GEANT410 with three γ-ray calibration sources, 22Na,60Co, and 88Y. Many unique features of DANCE are identified, such as thedetection efficiency, the peak-to-total ratio, and the multiplicity distributionare all nearly independent of γ-ray energy ranging from 150 keV up to 10MeV. For instance, the detection efficiency varies between 84 to 88%, thepeak-to-total is nearly constant ∼ 55%, and the average multiplicity differsno more than 6%.11 Note that the values of γ-ray energy and multiplicitywere derived by lumping together adjacent crystals if triggered. Introducingthe physical model of PPAC into the simulation reduces the efficiency by4% and 1% for the γ-ray energy of 200 keV and 1 MeV, respectively.The neutron capture measurement for 238Pu was carried out in Nov -Dec, 2010, using the DANCE array. The isotopic enriched target (99.35%)October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs144Fig. 1. Prompt γ-rays of the 252Cf spontaneous fission. (a): measured (black crosses),unfolded (red circles). (b): Verbinski (black triangles), unfolded (red circles).was prepared at LLNL with the new electroplating cell capable of depositingthe isotope simultaneously on both sides of the 3-µm thick Ti backing foil.A total mass of 395 µg with an activity of 6.8 mCi was deposited onto theOctober 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs145area of 7 mm in diameter. The target was covered by 1.4 µm double-sidealuminized mylar before inserted into a vacuum-tight container. The totalbeam-on-target time was about 14 days.3. Results and discussionAs was mentioned earlier, the least known quantity for the prompt energyreleased in the fission process is the γ decay. The current work yields aprecision measurement of the prompt γ-ray energy and multiplicity distri-butions for the spontaneous fission of 252Cf, which allows to quantify thecorrelations between γ-ray energy and multiplicity. However, before such astudy can be carried out, the unfolding procedure is needed to correct thosemeasured distributions according to the detector response.The relationship between the measured distribution Mobs and the ac-tual one Mphys can be expressed as a matrix equation:Mobs = R ·Mphys (1)where R is the response matrix for a given detector system. In principle,one can derive the actual distribution from the measured one by invertingthe response matrix, but in reality, this technique fails to produce sensiblesolutions in most cases. An extensive discussion on this subject and thetechniques developed to achieve better solutions can be found in Ref.12 Inour current work, the unfolding of measured γ-ray energy and multiplicitydistributions was carried out using both the Bayesian and SVD methodsimplemented in the standalone software package.13 The results derived fromboth methods are essentially identical. More on the unfolding of measuredenergy and multiplicity by DANCE can be found in Ref.14The response matrix of DANCE for both γ-ray energy and multiplicitywas simulated numerically using GEANT4 according to the model vali-dated by the γ-ray calibration sources discussed earlier. For the energyresponse, the simulation is restricted by the same experimental condition,which requires the γ-ray detected by the BaF2 crystal without any adjacentcrystals being triggered. This reduces the average detection efficiency from86% to 65% but the peak-to-total ratio remains the same ∼55%. The un-folded γ-ray energy distribution together with the measured one is shown inFig. 1(a) and the comparison with the previous measurement15(b) is shownin Fig. 1. The agreement is reasonable in general until ∼4 MeV where thecurrent data drops steeper toward higher γ-ray energy.For the multiplicity response, the matrix was created by the simulationsweighted by the unfolded γ-energy distribution to minimize the residual ef-October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs146fect of the nearly energy independence. The unfolded γ-ray multiplicitydistribution together with the measured one is shown in Fig. 2(a) and thecomparison with the semi-empirical formulated distribution16 is shown inFig. 2(b). The agreement is good in general even though the current datashifts toward higher γ-ray multiplicity. Note that it is the first time thatthe actual γ-ray multiplicity in the spontaneous fission is derived experi-mentally.To quantify the correlations between the γ-ray energy and multiplicityfor the prompt γ decay in spontaneous fission, a comparison of the to-tal γ-ray energy was made between the measurement and a Monte Carlosimulation by random sampling of their unfolded distributions through thedetector response of DANCE. The simulated total γ-ray energy distributionwas obtained with one million samples selected according to the multiplic-ity distribution. A matching number of γ rays with the chosen multiplicitywas selected according the energy distribution before going through thedetector response. Comparison between the measurement and simulationis shown in Fig. 3(a), where the agreement is reasonable, indicating weakcorrelations between the γ-ray energy and multiplicity.More detailed study of their correlations was made by comparisons withthe partition on the measured multiplicity, which shown in Fig. 3(b),(c),(d).The agreement in reproducing the average energy and variance is increasingwith increasing multiplicity, which manifests the stochastic aspect of theprompt γ decay in spontaneous fission of 252Cf. Obviously, the stochasticaspect can not account for all correlations without considering the detailsof nuclear structure of fission fragments. Nevertheless, this is an importantfinding to advance the modeling of the prompt γ decay in fission.The relative yield of neutron capture cross section for 238Pu as a func-tion of neutron incident energy is shown in Fig. 4. This spectrum wasobtained by setting gates on the total energy Etot = 5.7 – 6.5 MeV closeto 5.646-MeV, the Q-value of 238Pu(n, γ), and the multiplicity, M = 3 –4. This is to minimize the contribution from the non-negligible amount ofdaughter nucleus, 234U, and fission. Background subtraction was made forthe residual contribution of 234U and the contribution from neutrons elas-tically scattered off the container Kapton windows, the backing foil, and238Pu itself. A complementary experiment equipped with a fission counterwill be fielded to measure the fission contribution quantitatively to completethe final analysis of the neutron capture cross section for 238Pu.October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs147Fig. 2. Prompt γ-ray multiplicity of the 252Cf spontaneous fission. (a): measured (blackcircles), unfolded (red squares). (b): Brunson (blue triangles), unfolded (red squares).4. SummaryThe prompt γ-ray energy and multiplicity distributions in spontaneous fis-sion of 252Cf were measured using the DANCE array. Both distributionsOctober 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs148Fig. 3. Total γ-ray energy of the 252Cf spontaneous fission: measured (black), simu-lated (red). (a): M = 1 –16. (b): M = 1 – 5. (c): M = 6 – 8. (d): M = 9 – 16.were unfolded using Bayesian and SVD methods and consistent results wereobtained. A comparison of the total γ-ray energy distribution was madebetween the measurement and simulation by random sampling of both un-folded distributions through the detector response. A reasonable agreementbetween the measurement and simulation implies weak correlations betweenthe γ-ray energy and multiplicity. The stochastic aspect is manifested bythe increasing agreement with increasing multiplicity, which provides im-portant physics for modeling the prompt γ decay in the fission process. Thismeasurement will be extended to study the correlations between the γ-rayenergy and multiplicity for the prompt γ decay in the neutron-inducedfission.An example is given for the neutron capture measurement using theDANCE array, where the relative cross section for the 238Pu(n,γ) reactionwas derived. This reaction was measured for the first time in the laboratoryenvironment. To complete the final determination of cross section, we needto subtract the fission contribution, which will be measured in an additionalexperiment equipped with a fission counter.October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs149Fig. 4. The relative yield of the 238Pu(n, γ) cross section.We thank Prof. D. Gogny for many fruitful discussions during the courseof this work. This work performed under the auspices of the US Departmentof Energy by Lawrence Livermore National Security, LLC under contract DE-AC52-07NA27344 and by Los Alamos National Security, LLC under contractDE-AC52-06NA25396.References1. F. Gonnenwein, The Nuclear Fission Process, editor C. Wagemans, (CRCPress, 1991), p. 287 and references therein.2. D.G. Madland and J.R. Nix, Nucl. Sci. Eng. 81, 213 (1982).3. H.-H. Knitter, U. Brosa, and C. Budtz-Jorgensen, The Nuclear Fission Pro-cess, editor C. Wagemans, (CRC Press, 1991), p. 497 and references therein.4. M. Heil et al., Nucl. Instrum. Methods in Phys. Res. A 459, 229 (2001).5. G. D’Agostini, Nucl. Instrum. Mthods in Phys. Res. A 362, 487 (1995).6. A. Höcker, V. Kartvelishvili, Nucl. Instrum. Methods in Phys. Res. A 362,487 (1995).7. C.Y. Wu et al., LLNL-TR-462118, Lawrence Livermore National Laboratory(2010).8. R. Reifarth et al., LA-UR-03-5560, Los Alamos National Laboratory (2003).October 12, 2011 17:36 WSPC - Proceedings Trim Size: 9in x 6in cgs14109. M. Jandel et al., Nucl. Instrum. Methods in Phys. Res. B 261, 1117 (2007).10. S. Agostinelli et al., Nucl. Instrum. Methods in Phys. Res. A 506, 550 (2003).11. A. Chyzh et. al., LLNL-TR-452298, Lawrence Livermore National Labora-tory (2010).12. G. Cowan, Statistical Data Analysis, (Clarendon Press, Oxford, 1998).13. T.J. Adye, http://hepunx.rl.ac.uk/∼adye/software/unfold/RooUnfold.html, RooUnfold 1.1.0.14. A. Chyzh et al., LLNL-TR-460216, Lawrence Livermore National Laboratory(2010).15. V.V. Verbinski, H. Weber, R.E. Sund, Phys. Rev. C 7, 1173 (1973).16. G.S. Brunson Jr, LA-940B-T, Los Alamos National Laboratory (1982).

Neutron Capture and Fission Measurements on Actinides at DANCE

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Neutron Capture and Fission Measurements on Actinides at DANCE

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