Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session GE: Nuclear Scattering and Reactions |
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Sponsoring Units: DNP Chair: C.R. Brune, Ohio University Room: Gaylord Opryland Hermitage B |
Saturday, October 28, 2006 9:00AM - 9:12AM |
GE.00001: Coincidence Geometry Measurements of the $^1$S$_0$ Scattering Lengths in Neutron-Deuteron Breakup at 19 MeV A.S. Crowell, J. Deng, J.H. Esterline, C.R. Howell, M.R. Kiser, R.A. Macri, S. Tajima, W. Tornow, B.J. Crowe III, R.S. Pedroni, W. Von Witsch, H. Wita{\l}a Measurements of the $^1$S$_0$ neutron-neutron ($nn$) and neutron-proton ($np$) scattering lengths, $a_{nn}$ and $a_{np}$ respectively, using neutron-induced deuteron breakup were made at Triangle Universities Nuclear Laboratory (TUNL) at an incident neutron energy of 19.0 MeV. Six liquid scintillator detectors were configured in a coincidence geometry to measure the momenta of the two neutrons in two $nn$ and $np$ final-state-interaction (FSI) pairs while the energy of the proton was determined using a C$_{6}$D$_{12}$ active target. The scattering lengths were extracted from the experimental cross sections by comparison to rigorous three-nucleon calculations using the CD Bonn nucleon-nucleon potential for various values of $a_{nn}$ and $a_{np}$. In this talk results from the two $nn$ and $np$ FSI measurements and the analysis to obtain $a_{nn}$ will be presented. [Preview Abstract] |
Saturday, October 28, 2006 9:12AM - 9:24AM |
GE.00002: Three-body scattering with two charged particles: Application to direct nuclear reactions A.C. Fonseca, A. Deltuva The conventional treatment of direct nuclear reactions involves the use of optical potentials for elastic channels together with Coupled Channel theory or DWBA to describe inelastic or one particle transfer reactions such as $(d,p)$ or $(d,n)$ on a heavier nucleus. The application of exact Faddeev three-body theory to the understanding of these reactions has been always shadowed by the difficulty in dealing with the long range Coulomb interaction between the proton and the heavier target of atomic number Z. Given the progress achieved recently for $p-d$ elastic scattering and breakup [1] we show the results of calculations for the reactions $p(^{11}{\rm Be},^{11}\!{\rm Be})p$ and $p(^{11}{\rm Be},^{10}{\rm Be})d$ at 38.4 MeV per nucleon, taken as a three-body system made up of $p$, $n$, and $^{10}$Be as an inert core. The same is done for other $(p,d)$ reactions at similar energies. The results show that three-body calculations can provide a competing explanation of the data. Further studies are forthcoming. \newline \newline [1] A. Deltuva {\it et al.}, Phys. Rev. C {\bf {71}}, 054005 (2005). [Preview Abstract] |
Saturday, October 28, 2006 9:24AM - 9:36AM |
GE.00003: Neutron-Helium-3 Analyzing Power at 2.26, 3.14, 4.05, and 5.54 MeV J.H. Esterline, A.S. Crowell, C.R. Howell, A. Hutcheson, M.R. Kiser, R.A. Macri, S. Tajima, W. Tornow, B.J. Crowe, R.S. Pedroni, G.J. Weisel In continuation of an ongoing study of analyzing powers in the four-nucleon system, chosen for its sensitivity both to nucleon-nucleon phase shifts and possibly to new three-nucleon forces relevant to solving the three-nucleon analyzing power puzzle, we have measured analyzing powers for neutron-helium-3 scattering at Triangle Universities Nuclear Laboratory (TUNL) at neutron energies of 2.26, 3.14, 4.05, and 5.54 MeV, over a wide angular distribution for each energy. Neutrons were obtained using the source reactions T(p,n)$^{3}$He and D(d,n)$^{3}$He, for the lower energies (2.26 and 3.14 MeV) and higher energies (4.05 and 5.54 MeV), respectively; resulting neutron polarizations were between 0.3 and 0.5. Statistical uncertainties in the analyzing power were found in preliminary analysis to be less than 0.03 at the cross section minima, corresponding to values of analyzing powers in excess of 0.6. The data are compared to rigorous calculations based on the Yakubovsky equations, with which they are in marked disagreement, and existing proton-triton data corrected for the Coulomb barrier. [Preview Abstract] |
Saturday, October 28, 2006 9:36AM - 9:48AM |
GE.00004: The 6He and 6Li interaction with 12C at energies of 20 – 50 MeV/nucleon Olexandr Momotyuk, Kirby Kemper, Nicholas Keeley, Krzysztof Rusek The elastic scattering data for $^{6}$He+$^{12}$C [1] when plotted on top of similar $^{6}$Li+$^{12}$C data [2] shows that the absorption of $^{6}$He is weaker than $^{6}$Li, a surprising result since $^{6}$He has a much lower binding energy (0.98 MeV) than does $^{6}$Li (1.47 MeV). In order to understand the origin of this surprising result the elastic scattering cross sections for $^{6}$He and $^{6}$Li by $^{12}$C in the energy range 20 – 50 MeV/nucleon were analyzed using coupled reaction channels (CRC) calculations that employed optical potentials of Woods-Saxon type, double-folded (DF) and cluster-folded (CF) potentials. The results of these calculations and possible reasons for the weaker absorption of $^{6}$He relative to $^{6}$Li will be presented. \newline [1] V. Lapoux et al Phys. Rev. C 66, 034608 (2002) \newline [2] A. Nadasen et al Phys. Rev. C 37, 132 (1988) A. Nadasen et al ibid C 37, 674 (1993) [Preview Abstract] |
Saturday, October 28, 2006 9:48AM - 10:00AM |
GE.00005: ABSTRACT WITHDRAWN |
Saturday, October 28, 2006 10:00AM - 10:12AM |
GE.00006: Final state interactions in two-proton interferometry and decay Carlos Bertulani There is an intriguing possibility, that a diproton (2He) correlation may play an important role in the mechanism of 2p emission from nuclear states. Correlations of 1S0 proton pairs produced in (d,2He) reactions have also been used to test the Bell and Wigner inequalties against the predictions of quantum mechanics. Finally, two-particle correlations are widely used in relativistic heavy-ion physics as a tool for extracting information about the spatial and temporal extent of the system at freeze-out. We have perfomed calculations for the effect of final state interactions of the correlated pair depending on initial conditions and on the properties of the interaction. [Preview Abstract] |
Saturday, October 28, 2006 10:12AM - 10:24AM |
GE.00007: Investigation of surrogate reactions near A=100: $^{102,104}Ru(\alpha,\alpha')$ for $^{101,103}Ru(n,\gamma)$ J.A. Church, L.A. Bernstein, J.T. Burke, F. Dietrich, J. Escher, C. Forssen, E.B. Norman, H.-C. Ai, L. Phair, R. Clark, P.A. Fallon, D. Lee, I.Y. Lee, A.O. Macchiavelli, P. McMahan, S. Sinha, M. Stephens, E. R.-Vietez, M. Wiedeking For two-step, neutron-induced reactions proceeding through an equilibrated intermediate state, an alternate, ``surrogate reaction'' technique \footnote{J.D. Cramer and H.C. Britt, Nucl. Sci. Eng., {\bf{41}}, 177 (1970).} is applicable. Measured decay probabilities for the intermediate nucleus formed via a light-ion reaction are combined with optical-model calculations for the formation of the same intermediate nucleus via the n- induced reaction, and result in the overall (n, $\gamma$/n/2n) cross sections. $^{102,104}Ru(\alpha,\alpha')$ were studied separately as surrogate reactions for $^{101,103}Ru (n,\gamma)$. The test, $^{101}Ru(n,\gamma)$, has been previously measured directly (EXFOR). The unknown, $^{103}Ru (n,\gamma)$, is a branch in the s-process. Energies of scattered $\alpha$ particles were detected in double-sided silicon detectors (STARS) over scattering angles of 42-60 degrees. Ge clover detectors (LiBerACE) were used to count $\gamma$-rays in coincidence with $\alpha$ particles scattered at energies corresponding to 0-3 MeV equivalent neutron energy in the desired (n,$\gamma$) reaction. Work performed under the auspices of the U.S. DOE by the Univ. of CA, LLNL contract No. W-7405-Eng-4, and DOE grants DE-FG02-91ER-40609 and DE-FG03- 03NA00081, LDRD-04-ERD-057. [Preview Abstract] |
Saturday, October 28, 2006 10:24AM - 10:36AM |
GE.00008: Towards improved optical potentials for composite particle scattering Helmut Leeb, Wolfgang Dungel, Roman Kogler Quantitative evaluations of almost all nuclear reaction cross sections depend strongly on the quality of the optical potentials. This is particularly true for nuclear reactions involving composite projectiles or ejectiles, e.g. deuterons, alpha-particles etc. Despite their importance for various applications (embrittlement of materials, nuclear astrophysics) the current status of microscopic approaches is not fully satisfactory with regard to their predictive power. In this contribution we present a study of a microscopic approach to the alpha-nucleus optical potential, which is based on a consistent treatment of the composite nature of the collision partners. In addition, we also consider contributions due to the breakup in the case of loosely bound projectiles. [Preview Abstract] |
Saturday, October 28, 2006 10:36AM - 10:48AM |
GE.00009: Covariance analysis for the fission program at LANSCE F. Tovesson, T.S. Hill, K.M. Hanson, P. Talou, T. Kawano, R.C. Haight, L. Bonneau An experimental program at Los Alamos Neutron Scattering Center (LANSCE) has been developed to precisely measure differential fission cross sections over 10 decades in incident neutron energy for a range of actinides relevant to advanced nuclear reactor designs and transmutation concepts. As the need for uncertainty quantification (UQ) and covariance matrix evaluations significantly increased in the past few years, the detailed assessment and reporting of experimental uncertainties has become crucial. We will report on the analysis of the sensitivity vectors and covariance matrices for some of the fission data taken at LANSCE and provide examples of the impact experimental covariance data has in the evaluation process. [Preview Abstract] |
Saturday, October 28, 2006 10:48AM - 11:00AM |
GE.00010: Using the 152Sm(p,3n)150Eu* Reaction as a Means to a Clean 150Gd target B. Barquest, J. Cerny III, C. Jewett, D. Bleuel, M.A. McMahan, L. Ahle, L. Bernstein, J. Burke As part of an ongoing program to measure direct neutron cross sections on radioactive targets, we have been investigating the feasibility of making a clean 150Gd target - a long-lived alpha-emitter of interest for stockpile stewardship - by making isomeric 150Eu, which decays to 150Gd with an 89{\%} branching ratio. In the first part of this study, the 152Sm(p,3n) reaction was studied at several energies ranging from 19-27 MeV, using an array of five clover Ge detectors to look at the relative population of the ground state ($\tau _{1/2}$ = 36.9 years) and the isomer ($\tau _{1/2}$ = 12 hour) of 150Eu. In the second experiment, we bombarded a thicker 152Sm target at 25 MeV and measured the delayed gamma rays from the ground state and isomer to get an absolute production yield. Preliminary results from these studies will be presented including a tentative excitation function. [Preview Abstract] |
Saturday, October 28, 2006 11:00AM - 11:12AM |
GE.00011: Time-variability of alpha from realistic models of Oklo reactors Chris Gould, Eduard Sharapov, Steve Lamoreaux We reanalyze Oklo $^{149}$Sm data using realistic models of the natural nuclear reactors. Disagreements among recent Oklo determinations of the time evolution of $\alpha$, the electromagnetic fine structure constant, are shown to be due to different reactor models, which led to different neutron spectra used in the calculations. We use known Oklo reactor epithermal spectral indices as criteria for selecting realistic reactor models. Two Oklo reactors, RZ2 and RZ10, were modeled with MCNP. The resulting neutron spectra were used to calculate the change in the $^{149}$Sm effective neutron capture cross section as a function of a possible shift in the energy of the 97.3-meV resonance. We independently deduce ancient $^{149}$Sm effective cross sections, and use these values to set limits on the time-variation of $\alpha$. Our study resolves a contradictory situation with previous Oklo $\alpha$-results. Our suggested $2 \sigma$ bound on a possible time variation of $\alpha$ over two billion years is stringent: $ -0.11 \le \frac{\Delta \alpha}{\alpha} \le 0.24$, in units of $10^{-7}$, but model dependent in that it assumes only $\alpha$ has varied over time. [Preview Abstract] |
Saturday, October 28, 2006 11:12AM - 11:24AM |
GE.00012: Scattering Kernel for Phase II Solid Methane Yunchang Shin, Christopher Lavelle, William Michael Snow, Chen-Yu Liu, David Baxter Methane is one of the few substances that possess free rotor groups even in the solid phase at low temperature. These rotational degrees of freedom allow for efficient energy loss for cold neutrons and $\textrm{CH}_{4}$ is therefore a good choice for a low energy neutron moderator. In addition to its use as a cold neutron moderator at LENS, solid methane is planned to be used as a pre moderator for an ultracold neutron (UCN) source at the NC State PULSAR reactor and can also be used as a premoderator for solid $\textrm{O}_{2}$ UCN source at LENS. We have developed a simple model for the double differential cross section for solid methane for incident neutron energies from 0.1 meV to 1000 meV and temperatures from 4 K to 20.4 K. In this temperature range the solid methane exists in a phase (called phase II) with partial orientation of the free rotors. We adapted this analytical model due to Ozaki. $\it{et~al.}$, which describes free and hindered rotor excitations in phase II valid for neutron energies below 6.5 meV, and convoluted it with a phonon model good for energies between 6.5 and 1000 meV. The parameters of the model are consistent with neutron and Raman measurements of the excitations of phase II $\textrm{CH}_{4}$. We present the total cross section and MCNP simulation from the scattering kernel and compare to the measurement of LENS at IUCF. [Preview Abstract] |
Saturday, October 28, 2006 11:24AM - 11:36AM |
GE.00013: Solid Oxygen Ultra-cold Neutron Source for Fundamental Physics Experiments Yunchang Shin, Chen-Yu Liu Ultra-cold neutrons (UCN) are neutrons with energy of a few hundred neV. This energy is so low that UCN experience total external reflection from material surfaces. High precise measurements of fundamental physics such as neutron lifetime measurement or neutron EDM are possible with UCN. However a deficiency of intensity restricts us from achieving meaningful measurements. A UCN source with higher intensity is necessary to perform these precise measurements. Solid oxygen may be an attractive choice as a UCN source with this demand. Theoretical calculations predicted the possible advantages of solid oxygen. However, it has been experimentally shown that the UCN production rate from solid oxygen highly depends on the crystal condition, especially at low temperatures. We are testing crystal growths of solid oxygen over a wide range of temperature. Preliminary results will be presented. [Preview Abstract] |
Saturday, October 28, 2006 11:36AM - 11:48AM |
GE.00014: Level density and $\gamma $-strength function for A=56-60 nuclei A.V. Voinov, C.R. Brune, S.M. Grimes, M.J. Hornish, T.N. Massey The knowledge of the compound nuclear cross sections is important in various fields of nuclear science and applications. The level density and $\gamma $-strength function are the main quantities needed for such calculations and corresponding experimental data are in high demand. Traditionally, the nuclear level density below the particle separation energy is determined from neutron resonance spacing and low-lying discrete levels by means of interpolation function based on Fermi-gas model. But the question of how reliable these level densities are for reaction rate calculations remains open. Differential cross sections with $^{3}$He and deuteron beams on A=56-60 nuclei have been measured at Edwards Laboratory of Ohio University. Level density parameters are obtained and compared to available systematics. The results on the $\gamma $-strength function for $^{56,57}$Fe nuclei obtained from ($^{3}$He,$\alpha \gamma )$ and ($^{3}$He,$^{3}$He'$\gamma )$ reactions measured at Oslo Cyclotron Laboratory will be presented. The data show a large enhancement relative to existing models in the low-energy region ($<$3 MeV). [Preview Abstract] |
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