Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session CB: Mini-symposium on Stewardship Science I |
Hide Abstracts |
Sponsoring Units: DNP Chair: Cornelius Beausang, University of Richmond Room: Gaylord Opryland Tennessee A |
Friday, October 27, 2006 9:00AM - 9:36AM |
CB.00001: Nuclear Science for Stockpile Stewardship Invited Speaker: Stockpile Stewardship is based on our ability to make predicitons in complex systems for which a number of physics desciplines are required: material properties at high temperature and pressure, high energy density physics and nuclear physics. This talk will provide an overview of the role of nuclear science, both physics and radio-chemisty, with repect to both short term and long term needs of stockpile stewardship. [Preview Abstract] |
Friday, October 27, 2006 9:36AM - 9:48AM |
CB.00002: Theoretical Description of the Fission Process Witold Nazarewicz Spontaneous fission is one the oldest decay modes known, but is still not fully understood. On the one hand, various nuclear structure models have been applied to fission barriers, lifetimes, and mass/charge distributions, and they provide a good overall description of the phenomenon and, in many cases, detailed predictions. On the other hand, the full-fledged, non-adiabatic description of fission, based on effective nucleon-nucleon interactions, still does not exist. The aim of our project on ``Theoretical Description of the Fission Process,'' supported by NNSA (www.phys.utk.edu/witek/fission/fission.html), is to attack the problem of spontaneous fission using modern theoretical methods and state-of-the-art computational tools. During the first stage of the project, we have studied static fission barriers of the even-even actinide and transactinide nuclei within the self-consistent Density Functional Theory. The computations are carried out applying a code that makes it possible to break all self-consistent symmetries of the nuclear mean field, including axial symmetry and reflection symmetry. Particular attention has been paid to symmetry-breaking effects along the fission path. [Preview Abstract] |
Friday, October 27, 2006 9:48AM - 10:00AM |
CB.00003: Compound-nuclear reaction cross sections via the Surrogate method: considering the underlying assumptions Jutta Escher, Frank Dietrich The Surrogate Nuclear Reactions approach makes it possible to determine compound-nuclear reaction cross sections indirectly. The method has been employed to determine (n,f) cross sections for various actinides, including unstable species [1-4]; other, primarily neutron- induced, reactions are being considered also [5,6]. The extraction of the sought-after cross sections typically relies on approximations to the full Surrogate formalism [7]. This presentation will identify and critically examine the most significant assumptions underlying the experimental work carried out so far. Calculations that test the validity of the approximations employed will be presented. [1] J.D. Cramer and H.C. Britt, Nucl. Sci. and Eng. 41, 177(1970); H.C. Britt and J.B. Wilhelmy, ibid. 72, 222(1979) [2] M. Petit et al, Nucl. Phys. A735, 345(2004) [3] C. Plettner et al, Phys. Rev. C 71, 051602(2005); J. Burke et al, Phys. Rev. C. 73, 054604(2006) [4] W. Younes and H.C. Britt, Phys. Rev. C 67, 024610(2003); 68, 034610(2003) [5] L.A. Bernstein et al, AIP Conf. Proc. 769, 890(2005) [6] J. Escher et al, Nucl. Phys. A758, 43c(2005) [7] J. Escher and F.S. Dietrich, submitted (2006) [Preview Abstract] |
Friday, October 27, 2006 10:00AM - 10:12AM |
CB.00004: Testing the Absolute Surrogate Technique for Astrophysics D.L. Bleuel, R.M. Clark, P. Fallon, J.D. Gibelin, I-Y. Lee, A.O. Macchiavelli, M.A. McMahan, L. Phair, E. Rodriguez-Vieitez, M. Wiedeking, L.A. Bernstein, J.T. Burke, R.D. Hoffman, B.F. Lyles, E.B. Norman Neutron-induced reaction cross-sections on unstable nuclei are difficult to impossible to measure. However many of these reactions are of central importance to the understanding of stellar nucleosynthesis and the interpretation of radiochemical data from nuclear tests. In this talk we will present results from an experiment designed to ``benchmark'' the use of the absolute probability surrogate method to determine (n,xn$\gamma$) cross sections. Furthermore, $^{157}$Gd($^{3}$He,$\alpha$xn$\gamma$) probabilities will be compared to $^{155}$Gd(n,xn$\gamma$) cross sections calculated using the STAPRE reaction model. The $^{157}$Gd($^{3}$He,$^{3}$He/$\alpha$) reaction was performed at the 88-inch Cyclotron at Lawrence Berkeley National Laboratory, as a surrogate for $^{155}$Gd(n,$\gamma$), $^{154}$Gd(n,2n$\gamma$), and $^{154}$Gd(n,n$^{\prime}\gamma$) reactions. In conjunction with a future-planned $^{155}$Gd($^{3}$He,$\alpha$) experiment, the $^{153}$Gd(n,$\gamma$) cross section, an important s-process branch point reaction, will be determined through use of the ratio method. [Preview Abstract] |
Friday, October 27, 2006 10:12AM - 10:24AM |
CB.00005: Benchmarking the Surrogate Ratio Method Using $^{234}$U($\alpha$,$\alpha'f$)/$^{236}$U($\alpha $,$\alpha'$f) S.R. Lesher, C.W. Beausang, J.T. Burke, L.A. Bernstein, J.A. Church, F.S. Dietrich, J. Escher, B.F. Lyles, K.J. Moody, E.B. Norman, H. Ai, R.M. Clark, M.A. Deleplanque, P. Fallon, I.Y. Lee, A.O. Macchiavelli, M.A. McMahan, L. Phair, E. Rodriguez-Vieitez The Surrogate Ratio Method [1] is expected to demonstrate less sensitivity to differences in spin between the neutron-induced and surrogate reactions than the absolute surrogate method. The first ratio method experiment showed that the $^{237}$U/$^{239}$U fission probability ratio was independent of whether the nuclei were formed using neutron-capture or the (d,p) reaction over a wide range of equivalent neutron energy [1]. However, this result had significant ($>$20{\%}) uncertainty. In order to benchmark the ratio method with greater precision a new experiment was performed at the 88-Inch Cyclotron at LBNL using the Silicon Telescope Array for Reaction Studies (STARS) where the ratio of the $^{234}$U($\alpha$,$\alpha'$f) and $^{236}$U($\alpha$,$\alpha'$f) probabilities were compared to the known $^{233}$U(n,f)/$^{235}$U(n,f) cross section ratio. This talk will discuss the surrogate ratio method and the preliminary results of our measurement. This work was sponsored by UC-LLNL under Contract No. W-7405-Eng-48 and Grant Nos. DE-FG-05NA25929, DE-FG52-06NA26206, and DE-FG02-05ER41379. \newline [1] C. Plettner, \textit{et al.}, Phys. Rev. C 71, (2005) 051602(R) [Preview Abstract] |
Friday, October 27, 2006 10:24AM - 10:36AM |
CB.00006: The Effect of the J-pi Population Mismatch on the Surrogate Method Bethany Lyles, Lee Bernstein, Jason Burke, Larry Phair, Julien Gibelin, Darren Bleuel, Mathis Wiedeking, Roderick Clark, Augusto Macchiavelli, Peggy McMahan, Cornelius Beausang, Shelly Lesher, Cybele Jewett, Eric Norman The surrogate method is an indirect means for determining neutron-induced reaction cross sections on unstable nuclei. This is accomplished by measuring the relevant decay probabilities of the composite nucleus of interest produced via a light-ion induced surrogate reaction using a stable target and beam. To properly characterize the surrogate method, the effect of differences in angular momentum populations between the surrogate light-ion and desired neutron-induced reactions must be determined. To this end, the following experiment was performed at the 88'' Cyclotron at Lawrence Berkeley National Laboratory: 235U(3He,af) and 238U(3He,af) as surrogates for 233U(n,f) and 236U(n,f), respectively. The extracted cross sections were compared to known values and the implication on the J-pi population mismatch will be discussed. [Preview Abstract] |
Friday, October 27, 2006 10:36AM - 10:48AM |
CB.00007: Applying the Surrogate Technique to Stockpile Stewardship J.T. Burke, L.A. Bernstein, J. Escher, L. Ahle, J.A. Church, F.S. Dietrich, K.J. Moody, E.B. Norman, L. Phair, P. Fallon, R.M. Clark, M.A. Deleplanque, D.L. Bleuel, I.Y. Lee, A.O. Macchiavelli, M.A. McMahan, M. Wiedeking, E. Rodriguez-Vieitez, F.S. Stephens, C. Plettner, H. Ai, C. Beausang, B. Cridder Recent work has focused on developing the absolute probability and external ratio surrogate techniques for use in determining neutron-induced cross sections on unstable nuclei. In this talk we will present a new ``internal'' ratio method where a ratio of different exit channel probabilities for the same compound nucleus can be used to determine the cross section for an unknown exit channel when a cross for a different exit channel (e.g. determining (n,$\gamma )$ when (n,f)) is known. This technique can be used in combination with an ``external'' ratio method measurement to ``bootstrap'' several surrogate (n,x) cross section from a single surrogate measurement. We will present results from two experiments using STARS+LIBERACE at the 88-Inch cyclotron at LBNL to determine the $^{237}$U(n,$\gamma )$ and $^{237}$U(n,2n) cross sections using this ``internal'' ratio method. This work was sponsored by UC-LLNL under Contract No. W-7405-Eng-48 and Grant Nos. DE-FG-05NA25929, DE-FG52-06NA26206, and DE-FG02-05ER41379. [Preview Abstract] |
Friday, October 27, 2006 10:48AM - 11:00AM |
CB.00008: Pre-equilibrium nucleon emission and it's effect on the surrogate measurement of $^{237}$U(n, f) H. Ai, C.W. Beausang, L. Ahle, L.A. Bernstein, J.T. Burke, J.A. Church, K.J. Moody, E.B. Norman, W. Younes, D.L. Bleuel, R.M. Clark, P. Fallon, I.Y. Lee, A.O. Macchiavelli, M.A. McMahan, L.W. Phair, E. Rodriguez-Vieitez, S. Sinha, M. Wiedeking The surrogate ratio technique of using surrogate measurements to estimate the cross section ratio of two different but similar reactions was found to be reliable by recent works. To further confirm the validity of the surrogate method, we performed an experment at LBL to extract the $^{237}$U(n, f) reaction cross section from the surrogate $^{238}$U($\alpha$, $\alpha$'f) reaction directly, without the use of ratios. Interestingly, the directly estimated $^{237}$U(n, f) reaction cross section was lower than that obtained by the ratio method for equivalent neutron energy above one neutron separation energy. A possible explaination is that a direct reaction has occured where a single nucleon in the composite system is excited with enough energy to escape, leaving a nucleus with much less excitation energy deduced from energy of the scattered $\alpha$ beam. To check this we searched for scattered $\alpha$'s in coincidence with other charged particles, and evidence of such events will be presented. This work was supported in part by U.S. DOE Grant No. DE-FG02-91ER-40609, DE-FG02-05ER41379, DE-FG52-06NA26206, and Contract No. W-7405-Eng-48, DE-AC03-76SF0098. [Preview Abstract] |
Friday, October 27, 2006 11:00AM - 11:12AM |
CB.00009: Surrogate Reaction Measurements of Fission $\gamma $-ray Spectra with Liberace and STARS. C.C. Jewett, S.R. Lesher, J.T. Burke, C.W. Beausang, L.A. Bernstein, H. Ai, J.A. Church, R.M. Clark, M.A. Deleplanque, F.S. Dietrich, J. Escher, P. Fallon, I.Y. Lee, B.F. Lyles, A.O. Macchiavelli, M.A. McMahan, K.J. Moody, E.B. Norman, L. Phair, E. Rodriguez-Vieitez Very few high-resolution measurements have been made of the prompt $\gamma $-ray spectrum following non-spontaneous fission. However, this information is useful for stockpile stewardship and can also provide a wealth of information about the fission process. Recent experiments using STARS+LIBERACE at the 88-Inch cyclotron designed to determine surrogate neutron-induced cross sections in the Uranium isotopic chain have also produced a wide range of triple-coincident particle-fission-$\gamma $ data which can be used to determine fission $\gamma $-ray spectra for $^{234-238}$U and $^{235,238}$Np compound nuclei. In this talk we will present the spectra from these different systems and compare $\gamma $-ray spectra from different ``chance'' fission channels over a wide range of isotopes and energies. This work was sponsored by UC-LLNL under Contract No. W-7405-Eng-48 and Grant Nos. DE-FG-05NA25929, DE-FG52-06NA26206, and DE-FG02-05ER41379. [Preview Abstract] |
Friday, October 27, 2006 11:12AM - 11:24AM |
CB.00010: ABSTRACT WITHDRAWN |
Friday, October 27, 2006 11:24AM - 11:36AM |
CB.00011: Simultaneous measurement of (n,$\gamma$) and (n,fission) cross sections with the DANCE array T.A. Bredeweg, M. Jandel, M.M. Fowler, E.M. Bond, J.M. O'Donnell, R. Reifarth, R.S. Rundberg, J.L. Ullmann, D.J. Vieira, J.B. Wilhelmy, J.M. Wouters, R.A. Macri, C.Y. Wu, J.A. Becker We have recently begun a program of high precision measurements of the key production and destruction reactions of important radiochemical diagnostic isotopes, including several isotopes of uranium, plutonium and americium. The Detector for Advanced Neutron Capture Experiments (DANCE), a 4$\pi$ BaF$_2$ array located at the Los Alamos Neutron Science Center, will be used to measure the neutron capture cross sections for most of the isotopes of interest. Since neutron capture measurements on many of the actinides are complicated by the presence of $\gamma$-rays arising from low-energy neutron-induced fission, we are currently using a dual parallel-plate avalanche counter with the target material electro-deposited directly on the center cathode foil. This design provides a high efficiency for detecting fission fragments and allows loading of pre-assembled target/detector assemblies into the neutron beam line at DANCE. An outline of the current experimental program will be presented as well as results from measurements on $^{235}$U and $^{252}$Cf that utilized the fission-tag detector. [Preview Abstract] |
Friday, October 27, 2006 11:36AM - 11:48AM |
CB.00012: Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on Uranium Using a Pulsed and Monoenergetic Beam at TUNL A. Hutcheson, A.S. Crowell, J.H. Esterline, B. Fallin, C.R. Howell, M. Kiser, A.P. Tonchev, W. Tornow, J.H. Kelley, C.T. Angell, M. Boswell, H.J. Karwowski, R.S. Pedroni, G.J. Weisel, J.A. Becker, D. Dashdorj, R.A. Macri, R.O. Nelson Precision measurements have been performed on $^{235,238}$U targets at Triangle Universities Nuclear Laboratory using a pulsed and monoenergetic neutron beam. The excitation function of (n,2n) reaction has been studied with incident neutron energies between 5 and 15 MeV and beam flux of 10$^{4}$ n s$^{-1}$ cm$^{-2}$ at target position. Multiple (n,n') and (n,2n) partial cross sections have been measured using clover and planar HPGE detectors in this energy range, and experimental results will be compared with the Hauser-Feshbach model. A more in-depth explanation of the experimental techniques and analysis will be presented. [Preview Abstract] |
Friday, October 27, 2006 11:48AM - 12:00PM |
CB.00013: Measurement of the $^{235m}$U Production Cross Section Using a Critical Assembly* Robert Macri, Nicolas Authier, John Becker, Gilbert Belier, Evelyn Bond, Todd Bredeweg, S. Glover, Vincent Meot, Robert Rundberg, David Vieira, Jerry Wilhelmy Measurements of the creation and destruction cross sections for actinide nuclei constitute an important experimental effort in support of Stockpile Stewardship. In this talk I will give a progress report on the effort to measure the production cross section of the $^{235m}$U isomer integrated over a fission neutron spectrum. This ongoing experiment is fielded at CEA in Valduc, France, taking advantage of the CALIBAN critical assembly. This effort is performed in collaboration with LANL, LLNL, Bruyeres le Chatel, and Valduc staff. This experiment utilizes a technique to measure internal conversion electrons from the $^{235m}$U isomer with the French BIII detector (Bruyeres le Chatel), and involves a substantial chemistry effort (LANL) to prepare targets for irradiation and counting, as well as to remove fission fragments after irradiation. Experimental techniques will be discussed and preliminary data presented. *Work performed under the auspices of the U.S. Department of Energy by Los Alamos National Laboratory (W-7405-ENG-36) and Lawrence Livermore National Laboratory (W-7405-ENG-48), and CEA-DAM under CEA-DAM NNSA-DOE agreement. [Preview Abstract] |
Friday, October 27, 2006 12:00PM - 12:12PM |
CB.00014: Measurement of the $^{241}$Am(n,2n) Reaction Cross Section with the Activation Technique A. Tonchev, A. Crowell, B. Fallin, C. Howell, A. Hutcheson, W. Tornow, J. Kelley, C. Angell, H. Karwowski, R. Pedroni, J. Becker, D. Dashdorj, R. Macri, J. Wilhelmy, E. Bond, J. Fitzpatrick, A. Slemmons, D. Vieira High-precision measurements of the $^{241}$Am(n,2n)$^{240}$Am reaction have been performed with neutron energies from 8.8 to 14.0 MeV. The monoenergetic neutron beams were produced via the $^{2}$H(d,n)$^{3}$He reaction using the 10 MV Tandem accelerator at TUNL. The radioactive targets consisted of 1mg highly-enriched $^{241}$Am sandwiched between four different thin monitor foils. They were irradiated with a neutron flux of 3x10$^{7}$ n cm$^{-2}$ s$^{-1}$. After each irradiation the induced activity in the targets and monitors was measured off-line with 60\% HPGe detectors. Our preliminary neutron induced cross sections will be compared with recent literature results and statistical model calculations using the GNASH and EMPIRE codes. [Preview Abstract] |
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