Bulletin of the American Physical Society
2015 Fall Meeting of the APS Division of Nuclear Physics
Volume 60, Number 13
Wednesday–Saturday, October 28–31, 2015; Santa Fe, New Mexico
Session DF: Nuclear Reactions and Radioactive Beams |
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Chair: Tan Ahn, University of Notre Dame Room: Lamy |
Thursday, October 29, 2015 10:30AM - 10:42AM |
DF.00001: Equilibrium charge state distributions of Ni, Co, and Cu beams in molybdenum foil at 2 MeV/u Panagiotis Gastis, George Perdikakis, Daniel Robertson, Will Bauder, Michael Skulski, Phillipe Collon, Tyler Anderson, Karen Ostdiek, Ani Aprahamian, Wenting Lu, Robert Almus The charge states of heavy-ions are important for the study of nuclear reactions in inverse kinematics when electromagnetic recoil mass spectrometers are used. The passage of recoil products through a material, like the windows of gas cells or charge state boosters, results a charge state distribution (CSD) in the exit. This distribution must be known for the extraction of any cross section since only few charge-state can be transmitted through a magnetic separator separator for a given setting. The calculation of CSDs for heavy ions is challenging. Currently we rely on semi-empirical models with unknown accuracy for ion/target combinations in the Z$>$20 region. In the present study were measured the CSDs of the stable $^{60}$Ni, $^{59}$Co, and $^{63}$Cu beams while passing through a 1$\mu$m molybdenum foil. The beam energies were 1.84 MeV/u, 2.09 MeV/u, and 2.11 MeV/u for the $^{60}$Ni, $^{59}$Co, and $^{63}$Cu respectively. The results of this study mainly check the accuracy of the semi-empirical models used by the program LISE++, on calculating CSDs for ion/target combinations of Z$>$20. In addition, other empirical models on calculating mean charge states were compared and checked. [Preview Abstract] |
Thursday, October 29, 2015 10:42AM - 10:54AM |
DF.00002: Analysis of quasifission competition in fusion reactions forming heavy nuclei Kalee Hammerton, Zachary Kohley, Dave Morrissey, Aditya Wakhle, Krystin Stiefel, David Hinde, Mahananda Dasgupta, Elizabeth Williams, Cedric Simenel, Ian Carter, Kaitlin Cook, Dongyun Jeung, Duc Huy Luong, Steven McNeil, Chandani Palshetkar, Dominic Rafferty Heavy-ion fusion reactions have provided a mechanism for the production of superheavy elements allowing for the extension of both the periodic table and chart of the nuclides. However, fusion of the projectile and target, forming a compound nucleus, is hindered by orders of magnitude by the quasifission process in heavy systems. In order to fully understand this mechanism, and make accurate predictions for superheavy element production cross sections, a clear description of the interplay between the fusion-fission and quasifission reaction channels is necessary. The mass-angle distributions of fragments formed in 8 different Cr $+$ W reactions were measured at the Australia National University in order to explore the N/Z dependence of the quasifission process. Two sets of data were measured: one at a constant energy relative to the fusion barrier [Hammerton et al. Phys. Rev. 91, 041602(R) (2015)] and one at a constant compound nucleus excitation energy. The results of this analysis will provide insight into the effect of using more neutron-rich beams in superheavy element production reactions. [Preview Abstract] |
Thursday, October 29, 2015 10:54AM - 11:06AM |
DF.00003: Search for High Energy Alpha Particles in the Reactions of 7.5AMeV $^{197}$Au with $^{232}$Th S. Wuenschel, J.B. Natowitz, K. Hagel, M. Barbui, J. Gauthier, X. Cao, C. Ma, R. Wada, S. Kowalski, K. Schmidt, Z. Majka, Z. Sosin, A. Wieloch The search for alternative reaction paths for heavy element production requires a careful experimental investigation of mechanisms other than fusion, e.g., multi-nucleon transfer or very asymmetric fission of heavy transient systems. Many super heavy elements are expected to decay by unusually high energy alpha particle emission. Using $^{197}$Au projectiles incident on a $^{232}$Th target, we are pursuing survey experiments based upon the implantation of recoiling heavy reaction products in an active catcher composed of 63 fast scintillators. The 7.5 MeV/nucleon $^{197}$Au beam was pulsed at different intervals in order to be able to identify species of different half-life. These data will be discussed as will extensions of this program using an upgraded active catcher system consisting of YAP:Ce scintillators. These scintillators provide differentiation between n/$\gamma $, alphas, and heavier fragments. In addition to improving identification of alphas emitted from heavy residues, these detectors provide access to the neutron rich deep inelastic transfer region of the chart of the nuclides through an enhanced ability to identify fission decay. [Preview Abstract] |
Thursday, October 29, 2015 11:06AM - 11:18AM |
DF.00004: Energy Dependence of Fission Product Yields from 235U, 238U and 239Pu for Incident Neutron Energies Between 0.5 and 14.8 MeV M. Gooden, C. Arnold, T. Bredeweg, D. Vieira, J. Wilhelmy, A. Tonchev, M. Stoyer, M. Bhike, F. Krishichayan, W. Tornow, M. Fowler Under a joint collaboration between TUNL-LANL-LLNL, a set of absolute fission product yield measurements has been performed. The energy dependence of a number of cumulative fission product yields (FPY) have been measured using quasi-monoenergetic neutron beams for three actinide targets, $^{235}$U, $^{238}$U and $^{239}$Pu, between 0.5 and 14.8 MeV. The FPYs were measured by a combination of fission counting using specially designed dual-fission chambers and ?-ray counting. Each dual-fission chamber is a back-to-back ionization chamber encasing an activation target in the center with thin deposits of the same target isotope in each chamber. This method allows for the direct measurement of the total number of fissions in the activation target with no reference to the fission cross-section, thus reducing uncertainties. ?-ray counting of the activation target was performed on well-shielded HPGe detectors over a period of 2 months post irradiation to properly identify fission products. Reported are absolute cumulative fission product yields for incident neutron energies of 0.5, 1.37, 2.4, 3.6, 4.6, 5.5, 7.5, 8.9 and 14.8 MeV. These results are compared to previous measurements and theoretical estimates. [Preview Abstract] |
Thursday, October 29, 2015 11:18AM - 11:30AM |
DF.00005: Projectile fragmentation as a tool to observe shell effects close to the neutron drip-line Oleg Tarasov In recent experiments, production cross sections for a large number of neutron-rich nuclei produced from the fragmentation of $^{76}$Ge (130 MeV/u) and $^{82}$Se (139 MeV/u) beams were measured, including 21 new isotopes of the elements 17 $\le \quad Z \quad \le $ 26. Enhanced cross sections of several new nuclei were compared to a thermal evaporation framework, and also have been well reproduced by the LISE$^{++}$ Abrasion-Ablation model with masses derived from the full pf shell-model space with the GXPF1B5 effective interaction. It seems that the systematic trends in the production cross sections demonstrate changes in the nuclear mass surface, that can be explained with a shell model that predicts a subshell closure at $N=$34 around $Z=$20. In this contribution it will be discussed why projectile fragmentation is so sensitive to changes in the nuclear mass surface close to the neutron drip-line, and a new dBE production cross section systematics will be presented. [Preview Abstract] |
Thursday, October 29, 2015 11:30AM - 11:42AM |
DF.00006: Knockout reactions on p-shell nuclei for tests of structure and reaction models A.N. Kuchera, D. Bazin, M. Babo, T. Baumann, M. Bowry, J. Bradt, J. Brown, P.A. DeYoung, B. Elman, J.E. Finck, A. Gade, G.F. Grinyer, M.D. Jones, E. Lunderberg, T. Redpath, W.F. Rogers, K. Stiefel, M. Thoennessen, D. Weisshaar, K. Whitmore A series of knockout reactions on p-shell nuclei were studied to extract exclusive cross sections and to investigate the neutron knockout mechanism. The measured cross sections provide stringent tests of shell model and ab initio calculations while measurements of neutron+residual coincidences test the accuracy and validity of reaction models used to predict cross sections. Six different beams ranging from A=7 to 12 were produced at the NSCL totaling measurements of nine different reaction settings. The reaction settings were determined by the magnetic field of the Sweeper magnet which bends the residues into charged particle detectors. The reaction target was surrounded by the high efficiency CsI array, CAESAR, to tag gamma rays for cross section measurements of low-lying excited states. Additionally, knocked out neutrons were detected with MoNA-LISA in coincidence with the charged residuals. Preliminary results will be discussed. [Preview Abstract] |
Thursday, October 29, 2015 11:42AM - 11:54AM |
DF.00007: Measurement of the fusion excitation function for $^{19}$O + $^{12}$C at near barrier energies Varinderjit Singh, T.K. Steinbach, J. Vadas, B.B. Wiggins, S. Hudan, R.T. deSouza, L.T. Baby, V. Tripathi, S.A. Kuvin, I. Wiedenhover Fusion of neutron-rich light nuclei in the outer crust of an accreting neutron star has been proposed as responsible for triggering X-ray super-bursts. The underlying hypothesis in this proposition is that the fusion of neutron-rich nuclei is enhanced as compared to stable nuclei. To investigate this hypothesis, an experiment has been performed to measure the fusion excitation function for $^{18}$O and $^{19}$O nuclei incident on a $^{12}$C target. A beam of $^{19}$O was produced by the $^{18}$O(d,p) reaction at Florida State University and separated using the RESOLUT mass spectrometer. The resulting $^{19}$O beam bombarded a 100 $\mu$g/cm$^2$ $^{12}$C target at an intensity of 2-4 x 10$^{3}$ p/s. Evaporation residues resulting from the de-excitation of the fusion product were distinguished by measuring their energy and time-of-flight. Using silicon detectors, micro-channel plate detectors, and an ionization chamber, evaporation residues were detected in the angular range $\theta_{lab}$ $\leq$ 23$^{\circ}$ with high efficiency. Initial experimental results including measurement of the fusion cross-section to approximately the 100 mb level will be presented. The measured excitation function will be compared to theoretical predictions. [Preview Abstract] |
Thursday, October 29, 2015 11:54AM - 12:06PM |
DF.00008: Exploring clustering in alpha-conjugate nuclei using the thick target inverse kinematic technique for multiple alpha emission M. Barbui, K. Hagel, J. Gauthier, S. Wuenschel, V.Z. Goldberg, H. Zheng, G. Giuliani, G. Rapisarda, E-J. Kim, X. Liu, J.B. Natowitz, R.T. deSouza, S. Hudan, D. Fang Searching for alpha cluster states analogous to the $^{12}$C Hoyle state in heavier alpha-conjugate nuclei can provide tests of the existence of alpha condensates in nuclear matter. Such states are predicted for $^{16}$O, $^{20}$Ne, $^{24}$Mg, etc. at excitation energies slightly above the decay threshold. The Thick Target Inverse Kinematics (TTIK) technique can be successfully used to study the breakup of excited self-conjugate nuclei into many alpha particles. The reaction $^{20}$Ne$+\alpha $ at 11 and 13 AMeV was studied at Cyclotron Institute at Texas A{\&}M University. Here the TTIK method was used to study both single $\alpha$-particle emission and multiple $\alpha $-particle decays. Due to the limited statistics, only events with alpha multiplicity up to three were analyzed. The analysis of the three $\alpha $-particle emission data allowed the identification of the Hoyle state and other $^{12}$C excited states decaying into three alpha particles. The results will be shown and compared with other data available in the literature. Another experiment is planned in August 2015 to study the system $^{28}$Si $ + \alpha $ at 15 AMeV. Preliminary results will be shown. [Preview Abstract] |
Thursday, October 29, 2015 12:06PM - 12:18PM |
DF.00009: Neutron-removal reactions in the 100Sn region Giordano Cerizza, Kate Jones, Robert Grzywacz, Andrew Ayres, Travis Baugher, Daniel Bazin, Alexandra Gade, Dirk Weisshaar, Kathrin Wimmer, Jeff Tostevin Characterizing the nature of single-particle states outside of double shell closures is essential to a fundamental understanding of nuclear structure. This is especially true for those doubly magic nuclei that lie far from stability and where the shell closures influence nucleosynthetic pathways. The region around 100Sn is important due to the proximity of the N=Z=50 magic numbers, the proton drip line, and the end of the rp-process. However, owing to low production rates, there is a lack of spectroscopic information and no firm spin-parity assignment for ground states of odd-A isotopes close to 100Sn. Neutron knockout reaction experiments on a beam of 108Sn have been performed at the NSCL. By measuring gamma rays and momentum distributions of the reaction residues, the spins of the ground and first excited states of 107Sn have been established. The results are compared to eikonal-model reaction calculations. One-neutron knockout reactions from below the N=50 closed shell have been observed and estimated with the measurement of inclusive and exclusive cross sections. [Preview Abstract] |
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