Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session H14: Nuclear Structure: Light and Medium Mass Nuclei |
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Sponsoring Units: DNP Chair: Lee Sobotka, Washington University at St. Louis Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), St. Louis G |
Sunday, April 13, 2008 8:30AM - 8:42AM |
H14.00001: Decay of $^{10}$C excited states above the 2$p$ + 2$\alpha $ threshold and the contribution from ``democratic'' two-proton emission K.M. Mercurio, R.J. Charity, R. Shane, L.G. Sobotka, J. Elson, M. Famiano, A. Wuosmaa, A. Banu, C. Fu, L. Trache, R.E. Tribble The decay of $^{10}$C excited states to the 2$p$ +2$\alpha $ exit channel has been studied using an E/A = 10.7 MeV $^{10}$C beam inelastically scattered from a $^{9}$Be target. Levels associated with the two-proton decay to the ground state of $^{8}$Be have been observed. These include states at 5.18 and 6.54 MeV which decay by sequential two-proton emission through the long-lived intermediate state of $^{9}$B. In addition, these two states have branches, or there exist other states at almost the same energies, for which there is no long-lived intermediate state between the two proton emissions. For the 6.57 MeV state, the two protons are preferably emitted on the same side of the decaying $^{10}$C fragment. Evidence is found for a state at E$^{\ast }$= 8.4 MeV in $^{10}$C which decays through the 2.35 MeV second excited state of $^{9}$B. A large data set of kinematically complete $^{6}$Be$\to $2p + $\alpha $ events was also collected. [Preview Abstract] |
Sunday, April 13, 2008 8:42AM - 8:54AM |
H14.00002: Precision measurements of electromagnetic matrix elements as a test of Ab-Initio calculations in light nuclei E.A. McCutchan, C.J. Lister, M.P. Carpenter, R.V.F. Janssens, T.L. Khoo, T. Lauritsen, E.F. Moore, D. Seweryniak, S. Zhu Recent ab-initio shell model calculations of light nuclei have underlined the importance of 3-body forces. Gamma-ray spectroscopy of certain excited states in light nuclei can test new formulations of the interaction, as both the diagonal and off diagonal matrix elements are sensitive to it. A particularly interesting set of cases lie in the A = 10 systems, $^{10}$Be, $^{10}$B, and $^{10}$C, where the inclusion of 3-body forces is found to invert the sequence of states. This has been attributed to the important contribution of the 3-body interactions to the overall spin-orbit force. Precise ($<$10{\%}) matrix elements are needed to challenge the latest calculations, requiring a new generation of improved experiments. Lifetimes of excited levels in A = 10 nuclei, populated in the $^{6}$Li($^{6}$Li, xpxn) reaction, will be determined using high velocity DSAM lineshape measurements. Preliminary results of the experiment will be presented and discussed in terms of recent ab-initio shell model calculations. This research was supported by the DOE Office of Nuclear Physics under contract DE-AC02-06CH11357. [Preview Abstract] |
Sunday, April 13, 2008 8:54AM - 9:06AM |
H14.00003: Measurement of the ground state of $^{15}$Be A. Spyrou, T. Baumann, D. Bazin, G. Christian, S. Mosby, M. Strongman, M. Thoennessen, J. Brown, P.A. DeYoung, A. DeLine, J.E. Finck, A. Russell, N. Frank, E. Breitbach, R. Howes, W.A. Peters, A. Schiller The ground state of the neutron-unbound $^{15}$Be was measured for the first time. The experiment was performed at the National Superconducting Cyclotron Laboratory using the MoNA-Sweeper setup. The isotope of interest was produced via two-proton knockout from a $^{17}$C beam at 54 MeV/nucleon. The n-$^{14} $Be decay spectrum was reconstructed event-by-event from coincidence measurements of the $^{14}$Be fragment and the emitted neutron. The energy and position of the neutron were obtained using the Modular Neutron Array (MoNA), while for determining the same information for the $^{14}$Be fragment, a series of position and energy sensitive detectors, located at the Sweeper magnet focal plane, were used. First results from the analysis of this measurement will be presented. The present work aimed in determining the mass of the neutron-unbound $^{15}$Be and it was the first step towards the study of a possible di-neutron decay of $^{16}$Be. [Preview Abstract] |
Sunday, April 13, 2008 9:06AM - 9:18AM |
H14.00004: Isotope ratios measured in symmetric and asymmetric $^{40,48}$Ca+$^{40,48}$Ca collisions D. Henzlova, D. Brown**, B. Charity*, A. Chbihi*, D. Coupland*, R. de Souza*, J. Elson*, M. Famiano*, V. Henzl*, S. Hudan*, M. Kilburn*, J. Lee*, S. Lukyanov*, B. Lynch*, A. Rogers*, A. Sanetullaev*, L. Sobotka*, Z. Sun*, B. Tsang*, G. Verde*, M. Wallace*, M. Youngs*, G. Westfall**, A. Vander Molen** In a recent experiment performed at NSCL MSU three reaction systems with very different isospin contents were investigated at incident energy of 80A MeV -- $^{40}$Ca+$^{40}$Ca, $^{48}$Ca+$^{40}$Ca and $^{48}$Ca+$^{48}$Ca. The reactions were studied in a 4pi geometry using an MSU 4pi detector (array of 224 phoswitch scintillators) in combination with HiRA (High Resolution Array, a high granularity Si strip/CsI detector array). The former was used to determine the centrality of the collision, while the latter gave precise energy and angular information of the emitted light fragments. The measured reactions span a wide range of system isospin (N/Z=1 to 1.4) and thus serve as an important source of information on the influence of isospin of the reaction system on some of the basic properties of the dense and highly excited system formed in these collisions. Preliminary results on isotope ratios and isoscaling will be presented. This work is supported by the National Science Foundation under Grant Nos. PHY-0606007 and PHY-9977707. [Preview Abstract] |
Sunday, April 13, 2008 9:18AM - 9:30AM |
H14.00005: NP pairing in N=Z nuclei : The $^{44}$Ti($^{3}$He,p) reaction A.O. Macchiavelli, P. Fallon, R.M. Clark, M. Cromaz, I.Y. Lee, M. Wiedeking, K.E. Rehm, I. Ahmad, J. Greene, R.V.F. Janssens, M. Notani, R. Pardo, J.P. Schiffer, D. Seweryniak, X.D. Tang, A. Wuosmaa Neutron-proton pairing in N=Z nuclei is a subject of current interest in nuclear physics. Data from two-neutron transfer reactions using Ca and Ni isotopes are consistent with a picture of isovector pairing vibrations. However, it is still an open question whether the isoscalar component generates collective modes. The ($^{3}$He,p) reaction stands out as an ideal tool to study \textit{np }correlations and we started a program to study it in inverse kinematics using radioactive beams at the Argonne ATLAS facility. $^{44}$Ti (60y half-life) provides an excellent case that allows for a practical chemical separation and for better conditions to optimize the accelerator parameters. A pellet containing 100$\mu $ci of $^{44}$Ti was used in the Tandem ion source to deliver a 5 MeV/A $^{44}$Ti beam onto a $^{3}$He gas cell ($\sim $100$\mu $g/cm$^{2}$ thickness) placed in a scattering chamber in front of the FMA. A beam intensity of 10$^{6}$ /s was achieved during the four day run. Protons were detected in two Si ring detectors in coincidence with $^{46}$V recoils selected by the FMA. Details of the experiment and preliminary results will be discussed. - Supported by U.S. DOE under contracts DE-AC02-05CH11231 (LBNL) and DE-AC02-06CH11357 (ANL) [Preview Abstract] |
Sunday, April 13, 2008 9:30AM - 9:42AM |
H14.00006: Attempts to Manipulate the Decay Time of Radioactive Nuclei B. Fallin, B. Grabow, W. Tornow It has been known for 20 years that electron screening strongly changes nuclear reaction cross sections at sub-Coulomb charged-particle projectile energies. The screening energy can be increased considerably if the target atoms are implanted in a metallic host and cooled to low temperature (T$\sim $10 K). The large screening in metals derives from the Debye plasma model applied to the quasi-free metallic electrons. If ``time reversed,'' this model implies that the lifetime of radioactive nuclei placed in a metallic host can be manipulated by orders of magnitude. For $\alpha $ and $\beta ^{+}$ decay one expects a shorter half-life, while for $\beta ^{-}$ decay and EC, a longer half-life is expected. The results of prior experiments testing this theory are controversial; about half of the published data confirm an effect, while the other half observe no effect. We will report on our experimental studies using $^{64}$Cu and $^{65}$Zn nuclei produced at TUNL via the $^{63}$Cu(d,p) and $^{65}$Cu(p,n) reactions, respectively. For $^{64}$Cu, we detected the 511 keV annihilation $\gamma $ rays and for $^{65}$Zn the 1115.5 keV $\gamma $ rays using HPGe detectors. In both cases we did not observe a half-life change outside experimental uncertainties between measurements at room temperature and those with the samples cooled to T=12 K. [Preview Abstract] |
Sunday, April 13, 2008 9:42AM - 9:54AM |
H14.00007: The Role of Triaxiality in Shape-Coexistence in Light Krypton Isotopes S.M. Fischer, C.J. (Kim) Lister Shape co-existence in lead$^{1}$ and krypton$^{2-4}$ isotopes has become a cutting-edge topic in understanding the structure of heavier nuclei. Prediction of the relative binding energies of different shapes, and understanding the mixing between configurations presents a discriminating challenge to nuclear theory. In $^{72,74,76}$Kr the occurrence of two well bound shapes has been demonstrated through the observation of low-lying J$^{\pi }$ = 0$^{+}$ isomers and through radioactive beam Coulomb excitation. Roughly speaking, the shapes correspond to oblate-like and prolate-like configurations. However, the exact shapes, and the role of triaxiality has yet to be fully explored. We present new results from ``in-beam'' heavy-ion spectroscopy on $^{74}$Kr which shows that the population of the isomer is mainly through a gamma vibrational band and that considerable mixing is involved between the states built on the isomer and the gamma band. This research was supported by the DOE Office of Nuclear Physics under contract DE-AC02-06CH11357. \newline $^{1}$A. N. Andreyev \textit{et al.,} Nature \textbf{405}, 430 (2000) \newline $^{2}$E. Clement \textit{et al.}, Phys. Rev. C \textbf{75}, 054313 (2007) \newline $^{3}$E. Bouchez \textit{et al.,} Phys. Rev. Lett. \textbf{90}, 082502 (2003) \newline $^{4}$M. Bender \textit{et al.,} Phys. Rev. C \textbf{74}, 024312 (2006) [Preview Abstract] |
Sunday, April 13, 2008 9:54AM - 10:06AM |
H14.00008: Level densities of residual nuclei from the reactions $^{6}$Li on $^{58}$Fe and $^{7}$Li on $^{57}$Fe Babatunde Oginni, Steven Grimes, Alexander Voinov, Aderemi Adekola, Carl Brune, Zachary Heinen, Michael Hornish, Thomas Massey, Catalin Matei, Don Carter, John O'donnell The reactions $^{6}$Li on $^{58}$Fe and $^{7}$Li on $^{57}$Fe have been studied; these two reactions give the same compound nucleus, $^{64}$Cu. The neutron, proton and alpha spectra were measured at backward angles, and the level densities of the residual nuclei from the particle evaporation spectra have been obtained. The contribution of the breakup mechanism to the reaction cross-section was studied from $^{6}$Li on $^{197}$Au reaction. The data obtained have been compared with Hauser Feshbach model calculations performed with HF and Empire codes. Three other reactions were also studied to see how level densities change as we move away from the nuclear stability line. These are: $^{18}$O on $^{64}$Ni reaction, this gives $^{82}$Kr as compound nucleus which is on the stability line; $^{24}$Mg on $^{58}$Fe, this gives $^{82}$Sr as compound nucleus and $^{24}$Mg on $^{58}$Ni which gives $^{82}$Zr as compound nucleus; these are two and four steps away from the stability line respectively. Some results are presented. [Preview Abstract] |
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