Bredeweg, T. A.

Chyzh, A.

Devlin, M.

Fotiades, N.

Gostic, J. M.

Haight, R. C.

Henderson, R. A.

Jandel, M.

Kwan, E.

Laptev, A.

Lee, H. Y.

Nelson, R. O.

O'Donnell, J. M.

Perdue, B. A.

Taddeucci, T. A.

Ullmann, J. L.

Wender, S. A.

Wu, C. Y.

UNT Digital Library

LLNL-PROC-625392Prompt fission gamma raysmeasured using liquidscintillatorsE. Kwan, C. Y. Wu, R. C. Haight, H. Y. Lee, T. A. Bredeweg,A. Chyzh, M. Devlin, N. Fotiades, J. M. Gostic, R. A.Henderson, M. Jandel, A. Laptev, R. O. Nelson, J. M.O'Donnell, B. A. Perdue, T. A. Taddeucci, J. L. Ullmann, S. A.WenderMarch 8, 2013Internation Conference on Nuclear Data for Science &TechnologyNew York City, NY, United StatesMarch 4, 2013 through March 8, 2013Disclaimer  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.  Prompt fission γ rays measured using liquid scintillatorsE. Kwan,1, ∗ C.Y. Wu,1 R.C. Haight,2 H.Y. Lee,2 T.A. Bredeweg,2 A. Chyzh,1 M. Devlin,2N. Fotiades,2 J.M. Gostic,1 R.A. Henderson,1 M. Jandel,2 A. Laptev,2 R.O. Nelson,2J.M. O’Donnell,2 B.A. Perdue,2 T.N. Taddeucci,2 J. L. Ullmann,2 and S.A. Wender21Lawrence Livermore National Laboratory, Livermore, CA 94551, USA2Los Alamos National Laboratory, Los Alamos, NM 87545, USA(Dated: February 28, 2013)The prompt γ-ray spectra from 235U(n,f) at incident energies of 1 to 20 MeV and for 235Cf(s.f.)were measured up to 4 MeV in coincidence with two parallel plate avalanche counters and the liquidscintillator array FIGARO. The unfolded γ-ray spectra from 1-4 MeV using the single value decom-position and iterative Bayesian techniques for the incident neutron energy regions corresponding tothe first, second and third chance fissions were found to nearly identical and similar to the distri-bution from the spontaneous fission of 252Cf. General agreement with the γ-ray distributions fromfission was also found with previous measurements.I. INTRODUCTIONNuclear energy will become increasing important in thedesire to reduce greenhouse gas emissions by 2050. In2009, is was the second largest low-carbon emitting en-ergy source producing 13.4% of the world’s electricity [1].Most of the available energy created from fission of a nu-cleus is carried off by the recoiling fragments in the formof kinetic energy of fragments, but ∼10% of the totalenergy in a core of a reactor is released in the form ofprompt, delayed and radiative capture γ-rays [2]. Sinceγ rays may be the dominate contributor of the heating inthe sub-assemblies and shielding of fast breeder reactors[3], detailed knowledge about the distributions of γ raysfrom fissile materials such as uranium and plutonium isneeded for the designing of proper shielding and coolingsystems. Information on observables from fission such asthe kinetic energy of the emitted neutron and γ-rays areimportant to nuclear energy designs and safeguards sce-narios. Accurate data on the properties of fission prod-ucts is also needed to improve our understanding of thefission process and to test the accuracy of nuclear re-action codes. In this short manuscript, we will discussour experimental efforts and progress using two unfold-ing techniques, Single Value Decomposition (SV D) anditerative Bayesian, to examine one of the least studiedquantities of fission, the prompt γ-rays. Results from ouranalysis from the spontaneous fission of 252Cf and fissionof 235U by fast neutrons from 1 to 20 MeV are presented.∗ Current address: National Superconducting Cyclotron Labora-tory, Michigan State University, East Lansing, MI 48824II. EXPERIMENTTwo experiments to measure the prompt γ-ray andneutron distributions from the spontaneous fission of252Cf and neutron-induced fission of 235U were carriedout at in Weapons Neutron Research facility at the LosAlamos National Laboratory using the FIGARO neutrondetector array [4]. The array held 17 Eljen EJ301 organicliquid scintillators, of which six were chosen for analysis.The detectors with 12.5 cm diameters by 5.0 cm depthsactive volumes were positioned ∼1 meter away from thecenter of the target position yielding an angular coverageof 42◦ to 125◦ relative to the neutron beam in the lab-oratory frame. The 252Cf and 235U targets, fabricatedat the Lawrence Livermore National Laboratory, werehoused between the cathodes and anodes of two Paral-lel Plate Avalanche Counters (PPAC ′s) [5]. The ura-nium PPAC,enriched to 99.91% in235U, consisted of tenfoils with total mass of ∼113 mg and the CaliforniumPPAC contained a single 252Cf foil with specific activ-ity of ∼2µCi. The PPAC’s, which served as fission tags,were designed to minimized the amount of structural ma-terial that would scatter neutrons. The fast timing of thePPAC’s, with ∼1 ns timing resolution for the photon-induced fission peak, and the EJ301 allowed for the en-ergies of the prompt neutrons and γ-rays from fission tobe resolved above ∼1 MeV and ∼120 keV, respectively.Neutrons and γ-rays events detected by the EJ301’s wereidentified using the pulse height vs. time of flight andthe pulse shape discrimination, c.f. FIG. 1 and Ref. [6],techniques. The separation between the slow (PSD1) andfast (PSD2) components of the scintillation light belowaround 120 keV in γ-ray energy and ∼1 MeV neutronenergy are indistinguishable, thus both techniques wereND 2013 Article . . . NUCLEAR DATA SHEETS E. Kwan et al.0 200 400 600 800 10000500100015002000PSD1PSD2’sγneutronsFIG. 1. The fast prompt (PSD1) vs. slow decay (PSD2) flu-orescence components of the scintillation light from a EJ301detector. Events identified as due to neutron and γ-rays in-teractions are labeled.used to identify the interactions due to γ-ray. The chosensettings for the PSD’s limited the measured energies to arange from 0.14 to ∼ 4 MeV. The ability to resolve eventsdue to neutron or γ ray interactions within the same de-tector is important not only because it allows us to ex-tract information about the γ-ray and neutron spectra,but it also opens up the opportunity to study γ-neutroncorrelations under the same systematics while simulta-neously reducing the amount of scattering material thatcan distort the low-energy part of the neutron spectrum.The neutrons and γ-ray distributions from fission wereanalyzed separately. Here, we will concentrate on test-ing two unfolding techniques to determine the true γ-raydistributions and the neutron spectra will be presentedelsewhere.The γ-rays spectra due to the spontaneous fission of252Cf and the neutron-induced fission at incident energiesof 1-2, 5-10 and 10-20 MeV, corresponding to first, sec-ond and third chance fission, respectively, on 235U wereexamined and compared to available data to determinewhether one could unfold spectra obtained from a liquidscintillator. Due to the nature of the absorbing materialin the EJ301, which consists mostly of carbon and hydro-gen, the photoelectric effect tends to be suppressed com-pared to the Compton scattering of photons which leadsto a continuum with no visible photo peak. The detectorresponse for FIGARO was built using GEANT4 [7] tosimulate the the electromagnetic interactions and deter-mine the incident γ-ray energies. The GEANT4 modelwas validated using 22 Na, 60Co and calibration sourcesand the SVD and iterative Bayesian unfolding techniquesin the Roounfold package [8]. Shown in FIG. 2 a is a com-parison of the simulation using GEANT for the 898 and1836 keV transitions from 88Y source with relative inten-sities of 93.7 to 99.2 [9], respectively, to the measured dis-tribution, which includes background subtraction for nat-ural activity in the room. The unfolding matrix obtainedusing GEANT4 was tested against the measured responseFIG. 2. (Color online) a) Comparison of GEANT4 simulationfor a 88Y source using known intensities in ENSDF for the898 and 1826 keV transitions to the measured spectra. b)The unfolded spectra using SVD and the iterative Bayesianmethods, open circle and filled squares, respectively, deducedfrom a measured spectrum of a 22Na calibration source.function from calibration sources. Shown in FIG. 2b fora 22Na source are the unfolded spectrum deduced fromfinding the eigenvalues/eigenvectors for a system of lin-ear equations such that measured spectrum equals thesum of detector responses times the real spectrum (SVD),unfilled circles, and the spectrum determined from itera-tively solving Bayes theorem (Bayesian unfolding), filledsquares. The deduced ratios of the intensities for the 1274keV transition relative to the 511 keV line are 199:100and 188:100 from the SVD and Bayesian unfolding, re-spectively, slightly larger than the expected intensity of181:100 [10]. The similarity between the Compton scat-tering distributions for 88Y and the expected intensitiesfor the 22Na transitions was used to validate the detec-tor efficiency predicted by our model. More details aboutthe experimental set up and results can found in Ref. [11].The unfolding routines in RooUnfold were tested using ameasured γ-ray spectrum of the spontaneous fission of252Cf in order to investigate whether it was possible tounfold the detector response of an liquid scintillator ar-ray from a continuous energy distribution. The numberof degrees of freedom used in the unfolding matrix used inthe SVD method was limited to incident energies of up to5 MeV to match the energy range of the measured spec-trum. A larger response matrix with incident energies upto 6 MeV was used for the Bayesian method. The un-folded spectra using the SVD and Bayesian methods areshown in FIG. 3a. Both spectra predict a similar down-ward slope which drops nearly three orders of magnitudefrom ∼1.5-4 MeV and a peak around 800 keV. In theSVD spectrum, this peak is broader and washes out thestructure at 1.4 MeV which was observed in the Bayesianunfolded spectrum. The Bayesian distributions is moreconsistent with what was observed by Ref. [12] using a 4πBaF2 array (open circles) except below ∼600 keV wherethe detector thresholds differ. The data were normalizedfrom ∼1.0-3.5 MeV for comparison purposes. Deviationsfrom previous results taken with a NaI [13] and NE-213detector [14] can be seen above ∼4 MeV, which is over2ND 2013 Article . . . NUCLEAR DATA SHEETS E. Kwan et al.FIG. 3. (Color online) a) The unfolded 252Cf spectra by theSVD (open triangles) and Bayesian (filled circles) methodsfrom the measured spectrum (histogram) compared to theDANCE BaF2 detectors (open circles). b ) Comparison of theunfolded Bayesian spectrum with previous results obtainedwith NaI (open stars) [13] and NE-213 (cross) [14] detectors.our PSD1 range. Further measurements are need in thisrange to determine if the γ rays distributions decrease asrapidly as what was deduced in the current work and inRef. [12] or has a slower drop in intensity as suggested byRef. [13] and Ref. [14].The γ-ray spectra from the fission of 235U were alsomeasured under similar conditions as in the 252Cf experi-ment except the PPAC was irradiated with fast neutronsup to 100 MeV. The data was examined at three en-ergy ranges corresponding to regions where first- (En =1-2 MeV), second- (5-10 MeV), and third-chance (10-20MeV) fission, see FIG. 4a-c, respectively, are known tooccur in order to determine whether the distributionschange with neutron energy. All events detected bythe six chosen liquid scintillator detectors were summedfor statistical purposes; thus any anisotropy due to theangular distributions was neglected. The distributionswere unfolded using a response matrix generated fromGEANT4 for incident γ-ray energies up to 5 MeV. Theincident spectra deduced from the two unfolding tech-niques,the iterative Bayesian and SVD are essentiallyidentical above 1.8 MeV. A comparison of the unfoldedpulse height spectra shown in FIG. 4d, indicate that thedistributions below 4 MeV are insensitive to the incidentneutron energy. The shape of the distributions betweenthermal measured by Verbinski et al [13] and 1 MeV neu-tron energies measured by Drake [15] also agree with thecurrent measurements. The disagreement below 800 keVis due to the threshold at 130 keV in the current workand due to poor resolution of the NE-213 detectors, theoscillations below 0.7 MeV observed by Verbinski can notbe seen. At ∼5 MeV neutron energy, the broad resonanceat ∼2.5 MeV observed by Drake [16] is not seen in thecurrent work.The population of different mass fragments from fissionby different materials may lead to variations in promptFIG. 4. (Color online) The measured and unfolded promptγ-ray distributions using the SVD and iterative Bayesian tech-niques, the black, blue and red curves respectively at incidentneutron energies of a)1-2 MeV, b) 5-10 MeV and c) 10-20MeV. d) Comparison of the three curves in a) to c) deducedusing the Bayesian technique and the Cf spectrum in FIG. 3.γ-ray spectrum observed. Experiments using the Crystalball array found enhancements in the γ-ray spectra at∼4-6 MeV for nuclei around mass ∼130 [17]. Verbinskiet al. found a systematic softening of the γ-ray spectrawith increasing mass number of the fissioning isotope atphoton energies above ∼3.5 MeV. Between 0.7 and ∼3.5MeV his unfolded prompt γ-ray energy spectra for 235U,239Pu and 252Cf were found to be similar. A compar-ison of the distributions from the current work for theprompt γ rays from fission of 235U and 252Cf is given inFIG 4d. The distribution from the spontaneous fissionof 252Cf has been normalized to the distribution from theneutron-induced fission 235U from 1 to 4 MeV for the pur-pose of comparison. The variations in near the end of thedistributions arise from the differences in the measuredstatistics at around 4 MeV. Except for the fine detailsin the californium distribution, both distributions havethe same monotonically decreasing trend with approxi-mately the same slope in the energy region from1to 4MeV. Future measurements with improved statistics willbe carried out using 235U and 239Pu targets to measurethe distributions past 4 MeV to determine if the soften-ing at the higher γ-ray energies observed by Verbinski etal. is a global feature for neutron-induced fission.III. CONCLUSIONTwo experiments measuring the prompt γ-ray distribu-tions from the spontaneous fission of 252Cf and neutron-induced fission of 235U were carried out at LANSCE us-ing the FIGARO neutron array. Two unfolding tech-3ND 2013 Article . . . NUCLEAR DATA SHEETS E. Kwan et al.niques, SVD and iterative Bayesian, were tested to de-termine the fission γ-ray distributions from 252Cf andfrom fast neutrons on 235U. The techniques were vali-dated using standard calibrations sources and with com-parison with published distributions on the spontaneousfission of 252Cf. The iterative Bayesian method is ableto reproduce the finer details observed in other measure-ments while the SVD approach yields a broader distribu-tion and smooths any fine details. The same monotoni-cally decreasing slope was observed for the γ-ray energiesfrom 1.0 to 4.0 MeV for 252Cf and the 235U regardless ofincident neutron energy. The success of using the mod-ern unfolding techniques to unfold the γ-ray distributionsfrom fission with a liquid scintillator array will pave theway for future studies on γ-neutron correlations needed toimprove the predictive capabilities for neutron and γ-rayemissions in fission models.ACKNOWLEDGMENTSThis work benefited from the use of the LANSCE ac-celerator facility and was performed under the auspicesof the U.S. Department of Energy by Lawrence Liv-ermore National Laboratory under Contract DE-AC52-07NA27344 and Los Alamos National Laboratory underContract DE-AC52-06NA25396.[1] The Organisation of Economic Cooperation and Develop-ment’s Nuclear Energy Agency, The Role of NuclearEnergy in a Low-carbon Energy Future, NEA no.6887 (2012).[2] Nuclear Engineering Theory and Technology ofCommercial Nuclear Power, 2nd edition, Taylor &Francis, pp 46 (1992).[3] A. Lüthi, Ph.D. Thesis, École Polytechnique Fédérale deLausanne (1998).[4] R.C. Haight et al., in Proceedings of the Eleventh In-ternational Symposium on Capture Gamma-Ray Spec-troscopy and Related Topics, edited by J. Kvasil, P. Ce-jnar, and M. Krticka (World Scientific Publishing Co.,Singapore, 2003), pp. 451459.[5] C.Y. Wu et al., LLNL-TR-461044, 2010.[6] G. F. Knoll, Radiation Detection and Measure-ment, 3rd edition, John Wiley and Sons, New York,1994.[7] S. Agostinelli et al. Nucl. Instr. and Meth. A 506,250 (2003).[8] T. Adye, ArXiv:phyics.data-an/1105.1160v1.[9] G. Mukherjee and A.A. Sonzogni, Nuclear DataSheets 105, 419 (2005).[10] R.B. Firestone, Nuclear Data Sheets 106, 106 (2005).[11] E. Kwan et al., Nucl. Instr. and Meth. A 688, 55(2012).[12] A. Chyzh et al. Phys. Rev. C, 85, 021601R (2012).[13] V.V. Verbinsk et al., Phys. Rev. C 7, 1173 (1973).[14] D. T. Ingersoll and B. W. Wehring, Nucl. Inst. andMeth. A, 147, 551 (1977).[15] D.M. Drake, Los Alamos Scientific Laboratory ReportNo. LA-5048, 1972.[16] D.M. Drake ,Los Alamos Scientific Laboratory ReportNo. LA-5134, 1973.[17] J. B. Fitzgerald et al., Z. Phys. A 355, 401 (1996).4

Prompt fission gamma rays measured using liquid scintillators

Prompt fission gamma rays measured using liquid scintillators

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