Bulletin of the American Physical Society
2007 Annual Meeting of the Division of Nuclear Physics
Volume 52, Number 10
Wednesday–Saturday, October 10–13, 2007; Newport News, Virginia
Session BG: Nuclear Structure I |
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Chair: William Walters, University of Maryland Room: Newport News Marriott at City Center Pearl Salon III |
Thursday, October 11, 2007 2:00PM - 2:12PM |
BG.00001: Ab Initio Calculations of Electroweak Transitions in A=8 Nuclei Muslema Pervin, Steven Pieper, Robert Wiringa The variational Monte Carlo (VMC) and Green's function Monte Carlo (GFMC) techniques are powerful tools for calculating properties of light nuclei. These methods in combination with the Argonne $v_{18}$ two-nucleon and Illinois-2 three-nucleon potentials, have recently been applied to calculate electroweak matrix elements in A$=$6, 7 nuclei with some success. We are now applying the GFMC method to additional off-diagonal matrix elements for nuclei with A$=$ 8. The electroweak transitions for A$=$8 are more challenging because many involve transitions from a big component of an initial state to a small component of a final state. In addition some of the physical states have significant isospin mixing. Of particular interest are the weak decay processes $^8$Li($\beta^-)^8$Be$^*$ and $^8$B($\beta^+)^8$Be$^*$. The many-body calculations were performed on the parallel computers of the Laboratory Computing Resource Center, Argonne National Laboratory. This work is supported by the U. S. Department of Energy, Office of Nuclear Physics, under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, October 11, 2007 2:12PM - 2:24PM |
BG.00002: Faddeev calculations for $_{\Lambda}^7 $He and $_{\Lambda}^9$Be: bound states and low-lying resonances Igor Filikhin, Vladimir Suslov, Branislav Vlahovic $_{\Lambda}^7$He and $_{\Lambda}^9$Be hypernuclei are considered in the three-body cluster models $_{\Lambda}^5$He+n+n [1] and $\alpha +\alpha +\Lambda$ [2], respectively. Configuration space Faddeev calculations are performed for the hyperon binding energy. For $_{\Lambda}^7$He we obtained the binding energy 5.35 MeV which is in agreement with previous theoretical predictions and preliminary experimental value (5.4 MeV) [3]. We have found that the binding energy of $_{\Lambda}^9$Be depends strongly on the $\alpha \Lambda$ potential used. A variant of the method of analytical continuation in coupling constant [4] is applied to calculate the energies of low-lying levels of the $_{\Lambda}^7 $He and $_{\Lambda}^9$Be. The second bound state of $_{\Lambda}^7$He with total angular momentum J=3/2$^+$(5/2$^+)$ is found. The bound states of $_\Lambda ^7 $He can be classified as an analog of the $^6$He ground band. For $_{\Lambda}^9$Be concluded that the ground band of this nucleus cannot be explained by similar classification related to the core nucleus $^8$Be. We have obtained a new 2$_2^+ $ resonance state that is close to the $\alpha +\alpha +\Lambda $ threshold. Overall, calculated resonance energies differ from previous theoretical predictions. This work is supported by the DoD W911NF-05-1-0502 and NASA NAG3-804 grants. 1. I. Filikhin, et al. J. Phys. G31 389 2005 2. I. Filikhin, et al. J. Phys. G30 513 2004 3. O. Hashimoto, HYP2006 Mainz, October 11-14, 2006 4. V. I. Kukulin, et al. Theory of Resonances (Kluwer Academic, Dordrecht, 1989) [Preview Abstract] |
Thursday, October 11, 2007 2:24PM - 2:36PM |
BG.00003: Study of the $\beta$-decay of $^{11}$Li at ISAC/TRIUMF Caleb Mattoon, F. Sarazin, C. Andreoiu, A. Andreyev, R.A.E. Austin, G.C. Ball, R.S. Chakrawarthy, D. Cross, E.S. Cunningham, J. Daoud, P.E. Garrett, G.F. Grinyer, G. Hackman, D. Melconian, C. Morton, C. Pearson, J. Ressler, J. Schwartzenberg, M.B. Smith, C.E. Svensson The $\beta$-decay of $^{11}$Li was investigated using the 8$\pi$ $\beta$-decay spectrometer, an array of 20 Compton-suppressed HPGe detectors and 20 plastic scintillators for $\beta$-particle detection. Doppler-broadened line shapes appear in the experimental spectrum, resulting from the decay of excited states of $^{10}$Be populated by $\beta$-delayed neutron emission. Using a Monte Carlo simulation, we have simulated the $\beta$-delayed neutron decay and subsequent $\gamma$-ray emission, in order to explore the excited states in $^{10}$Be and the neutron-emitting states in $^{11}$Be. Overall agreement is observed with the two most recent studies [F. Sarazin et al, PRC70 (2004) and Y.Hirayama et al, PLB611 (2005)]. Analysis however also suggests the existence of small contributions from previously unobserved neutron branches. [Preview Abstract] |
Thursday, October 11, 2007 2:36PM - 2:48PM |
BG.00004: Study of the $^3$He($^{14}$O,$\alpha)^{13}$O reaction Scott Marley, N.J. Goodman, J.C. Lighthall, A.H. Wuosmaa, C.L. Jiang, M. Notani, R.C. Pardo, K.E. Rehm, J.P. Schiffer, X.D. Tang, L. Jisonna We have studied the structure of $^{13}$O through the $^3$He($^{14}$O,$\alpha)^{13}$O reaction. Recent work examining the resonance elastic scattering of p + $^{12}$N has provided information on the excited states of $^{13}$O up to 4 MeV (B.B. Skorodumov et al., Phys. Rev. C 75, 024607 (2007)). Little is known about the excited states of this isotope at excitation energies above 6.02 MeV. Due to a low separation energy (S$_p$ = 1.516 MeV) the most likely populated states are 1p and 2p unbound. States in $^{13}$O above the alpha-particle decay threshold may also have astrophysical significance, and may influence the rate of the $^9$C($\alpha,p)^{12}$N reaction in dense, low metallicity stars. The ATLAS in-flight radioactive beam facility at Argonne National Laboratory produced an $^{14}$O beam of 10$^5$ particles per second at 148 MeV. The beam bombarded a cryogenic $^3$He target cell. Three annular segmented Si detectors detected light reaction products between $\theta_{lab}$=8 and 48 degrees. Heavy beam-like fragments were identified using a four segment Si E-$\Delta$E telescope covering laboratory angles between 1.4 and 7 degrees. Preliminary results will be discussed. Work supported by the U. S. Department of Energy, Office of Nuclear Physics under contracts DE-FG02-04ER41320 (WMU) and DE-AC02-06CH11357 (ANL). [Preview Abstract] |
Thursday, October 11, 2007 2:48PM - 3:00PM |
BG.00005: Study of low-lying resonant states in $^{16}$F using an $^{15}$O radioactive ion beam Dongwon Lee, Kari Perajarvi, James Powell, Jim O'Neil, Dennis Moltz, Vladilen Goldberg, Joseph Cerny Among the A=16, T=1 isobaric triad, many states in $^{16}$O and $^{16}$N have been well established, but less has been reported about $^{16}$F. Experimental studies with stable beams have established spin-parity values for the first four low-lying states of $^{16}$F, but only upper limits or rough estimates of their level widths have been reported. The spins and parities of the low-lying states have been found to be 0$^{-}$, 1$^{-}$, 2$^{-}$, and 3$^{-}$ in ascending order in energy, and are believed to have $^{15}$O core-single proton configurations of 1p$_{1/2}^{-1}$ 2s$_{1/2}$ for the 0$^{-}$, 1$^{-}$, and 1p$_{1/2}^{-1}$ 1d$_{5/2}$ for the 2$^{-}$, 3$^{-}$. A recently developed $^{15}$O (T$_{1/2 }$= 122 sec.) radioactive ion beam from the BEARS (Berkeley Experiments with Accelerated Radioactive Species) facility was used to study the structure of $^{16}$F using $^{15}$O+p elastic scattering and the Thick Target Inverse Kinematics method on a polyethylene target. The level widths of the first four states in $^{16}$F were determined using R-matrix analysis, and our results show that the 0$^{-}$ state has a level width of 22.8 $\pm $ 14.4 keV, and that the broad 1$^{-}$ state has a width of 103 $\pm $ 12 keV. The level width of the 2$^{-}$ state is found to be 4.0 $\pm $ 2.5 keV which is much narrower than the compiled value, while 15.1 $\pm $ 6.7 keV for the 3$^{-}$ state is in good agreement with previous studies. [Preview Abstract] |
Thursday, October 11, 2007 3:00PM - 3:12PM |
BG.00006: Coupled SU(3) models of rotational states in nuclei and quasi-dynamical symmetry Gabriela Thiamova, David John Rowe This work reports a first step towards the development of a model of low-lying nuclear collective states based on the progression from weak to strong coupling of a combination of systems in multiple SU(3) irreps. The motivation for such a model comes partly from the persistence of rotational structure observed experimentally and in many calculations. This work considers the spectra obtainable by coupling two SU(3) irreps via a qadr-qadr interaction. For a particular value of this interaction, the two irreps combine to form strongly-coupled irreps while for zero interaction the results are mixtures of many such strongly-coupled irreps. A notable result is the persistence of the rotor character of the low-energy states for a wide range of the interaction strength. Also notable is the fact that, for very weak interaction strengths, the energy levels of the yrast band resemble those of a vibrational sequence while the B(E2) transition strengths are close to those of an axially symmetric rotor, as observed in many nuclei. An application to shape coexistence in $^{16}$O is considered to show that the model gives an indication of which \textit{np-nh} states are likely to contribute to the low-energy states of nuclei. [Preview Abstract] |
Thursday, October 11, 2007 3:12PM - 3:24PM |
BG.00007: Unbound states of the neutron-rich oxygen isotopes C.R. Hoffman, S.L. Tabor, M. Thoennessen, T. Baumann, D. Bazin, A. Gade, W.A. Peters, A. Schiller, J. Brown, P.A. Deyoung, R. Howes, N. Frank, B. Luther, H. Scheit, J. Hinnefeld The energies of the ground state decay of $^{25}$O and excited states in $^{24}$O were measured for the first time. From these energies the size of the $N = 16$ shell gap may be deduced. Due to the lack of observation of an excited state in $^{24}$O using $\gamma$-ray spectroscopy, along with the known unbound nature of $^{25}$O, techniques involving neutron spectroscopy had to be applied. $^{25}$O ($^{24}$O$^{*}$) was populated via proton (proton-neutron) removal from a $^{26}$F beam. Complete 4-vector reconstruction revealed resonant structures in the decay spectrum for n-$^{22}$O, n-$^{23}$O and n-$^{24}$O coincidence events. From the n-$^{24}$O and n-$^{23}$O decay spectrum the energies of the ground state of $^{25}$O and first excited state of $^{24}$O were measured. Using n-$^{22}$O coincident events of neutron multiplicity $\geq 2$, clear correlation between a low and high energy resonance was observed. With the known placement of the low energy resonance as the first excited state in $^{23}$O, the high energy resonance has been attributed to an excited state in $^{24}$O. [Preview Abstract] |
Thursday, October 11, 2007 3:24PM - 3:36PM |
BG.00008: Structure of $^{21}$Mg excited states studied in single neutron knockout C. Aa. Diget, P. Adrich, D. Bazin, M.D. Bowen, B.A. Brown, C.M. Campbell, J.M. Cook, A. Gade, T. Glasmacher, S. McDaniel, A. Obertelli, K. Siwek, J.R. Terry, D. Weisshaar, K. Hosier, D. McGlinchery, L.A. Riley Previously, the $^{21}$Mg excited-state properties have been studied using the three-neutron transfer reaction $^{24}$Mg($^{3}$He,$^{6}$He)$^{21}$Mg directed by comparison to the $^{21}$F mirror nucleus. When comparing the experimental results to USD shell-model calculations, however, significant discrepancies are found. To resolve these discrepancies, we investigated the nucleus in a one-neutron knockout reaction $^{9}$Be($^{22}$Mg,$^{21}$Mg)X and studied the subsequent $\gamma$ decay using in-beam $\gamma$-ray spectroscopy. From the $\gamma$-decay measurement, energies of the excited states were determined. Furthermore, properties of the states were identified through the decay branching ratios to lower lying states, the spectroscopic factors for the $^{22}$Mg ground state with respect to the individual $^{21}$Mg states, and the longitudinal momentum distribution of the residual nucleus which is sensitive to the orbital angular momentum of the knocked-out neutron. [Preview Abstract] |
Thursday, October 11, 2007 3:36PM - 3:48PM |
BG.00009: Coulomb Excitation of Radioactive $^{21}$Na with TIGRESS and BAMBINO Michael Schumaker The first experiment to use modules of the TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer (TIGRESS) with an accelerated radioactive ion beam has been performed successfully, using the ISAC facility at TRIUMF. The low-energy structures of the five-particle nuclei ${}^{21}$Na and ${}^{21}$Ne were examined, in order to test model predictions of these deformed nuclei. For $^{21}$Na, the $\pm$86\% uncertainty on the previously accepted B(E2) value, resulting from the dominance of M1 decay in these nuclei, has made comparison difficult. Beam particles were accelerated to 1.7~MeV/A, and Coulomb excited in a $^{nat}$Ti target. Recoiling projectile and target ions were detected by the BAMBINO segmented silicon detector, while $\gamma$-ray yields were determined using two TIGRESS detectors perpendicular to the beam axis. For $^{21}$Na and $^{21}$Ne, Coulomb excitation from the 3/2$^{+}$ ground state to the 5/2$^{+}$ state was observed. Strongly enhanced B(E2) values were determined, which will be compared with the simplest quadrupole rotor model, and shell model predictions. [Preview Abstract] |
Thursday, October 11, 2007 3:48PM - 4:00PM |
BG.00010: Ground state properties of pf shell even-even nuclei in a mean-field model with a modified separable monopole interaction. Jirina Stone, William Walters The non-relativistic Hartree-Fock mean field model with modified separable monopole interaction has been used to calculate ground state properties of even-even 20$<$Z$<$32 nuclei, including binding energies, mean-square radii, charge density distributions, neutron skins and geometrical shapes. Parameters of the separable interaction are adjusted to known properties of finite nuclei, including nuclei away from closed shells, as well as to expected features of symmetric and asymmetric nuclear matter and the mass-radius relationship in cold, non-rotational neutron stars. The results are compared to the outcome of the same model with more traditional Skyrme-type interactions SLy4 and SkI3. The similarities and differences of the models are discussed and attributed to the treatment of isospin degrees of freedom. Calculated bulk properties, such as binding energies and shape parameters, are also compared to the Finite-Range-Liquid-Droplet (FRLD) model predictions. [Preview Abstract] |
Thursday, October 11, 2007 4:00PM - 4:12PM |
BG.00011: Shell structure near $^{42}$Si: one proton knockout from $^{44}$S L.A. Riley, T.R. Baugher, K.E. Kosier, P.D. Cottle, K.W. Kemper, P. Adrich, D. Bazin, J.M. Cook, C. AA. Diget, A. Gade, D.A. Garland, T. Glasmacher, A. Ratkiewicz, K.P. Siwek, D. Weisshaar The $d_{5/2}$ proton strength in $^{43}$P was studied via the one-proton knockout reaction $^9$Be($^{44}$S,$^{43}$P)X. Gamma rays were measured in coincidence with the knockout products. We extracted spectroscopic factors of states populated in the reaction from measured parallel momentum distributions. An expanded level scheme will be presented and the size of the $Z=14$ proton subshell closure will be discussed. [Preview Abstract] |
Thursday, October 11, 2007 4:12PM - 4:24PM |
BG.00012: Determination of effective charges in the lower $pf$ shell J.M. Cook, P. Adrich, D. Bazin, M.D. Bowen, B.A. Brown, C.M. Campbell, A. Gade, T. Glasmacher, S. McDaniel, A. Obertelli, K. Siwek, J.R. Terry, D. Weisshaar Recently, the use of the standard effective charges, $e_p=1.5$ and $e_n=0.5$, has been called into question, and the noncanonical values of $e_p=1.15$ and $e_n=0.8$ have been suggested for the upper $fp$ shell\footnote{R.~du~Rietz \textit{et~al.}, Phys. Rev. Lett. \textbf{93}, 222501 (2004)}. $^{50}$Ca is ideally situated for determining the $e_n$ effective charge due to its closed $\pi(sd)$ shell. The results of the measurement of the \mbox{B(E2; 0$_{1}^{+}\rightarrow2_{1}^{+}$)} transition rate in $^{50}$Ca, which is proportional to $e_n^2$, via intermediate-energy Coulomb excitation at the National Superconducting Cyclotron Laboratory will be reported. In this experiment, $\gamma$ rays were observed using the high-efficiency NaI APEX Array in conjunction with particle identification by the S800 Spectrograph. [Preview Abstract] |
Thursday, October 11, 2007 4:24PM - 4:36PM |
BG.00013: Level densities of residual nuclei from the reaction Li6 on Fe58. Babatunde Oginni, Steven Grimes, Alexander Voinov, Aderemi Adekola, Carl Brune, Don Carter, Zack Heinen, Micheal Hornish, Tom Massey, Catalin Matei, John O'donnell We investigated the level densities of residual nuclei from compound reactions using a Li6 beam. The (Li6,p) and (Li6,$\alpha )$ reactions have been studied at 15MeV for a Fe58 target. Proton and alpha spectra were measured at angles 23.5, 37.7, 68, 98, 142.5 and 157.5 over a range of excitation energies in the residual nuclei. The contribution of the breakup reaction to the residual nuclei was studied from Li6 on Au197 reaction. The evaporated spectra have been compared with theoretical models based on Hauser-Feshbach and Empire codes. We present some of the results obtained. [Preview Abstract] |
Thursday, October 11, 2007 4:36PM - 4:48PM |
BG.00014: GFMC Calculations of Isospin-Mixing in $^8$Be Robert B. Wiringa, Steven C. Pieper, Muslema Pervin $^8$Be has two 2$^+$ states at 16.6 and 16.9 MeV excitation that are strongly isospin-mixed. We have performed microscopic Green's function Monte Carlo calculations of the isospin-mixing matrix elements between the T=0 and 1 states using the realistic Argonne v$_{18}$ + Illinois-2 Hamiltonian, which includes strong CSB components and a full electromagnetic interaction. We obtain 80\% of the empirically-determined matrix element, with 2/5 of our result coming from the terms beyond Coulomb, confirming an earlier variational study. We have also calculated the mixing between the nearby T=0,1 pairs of 1$^+$ and 3$^+$ states. Finally, we have examined the mixing of the T=1 2$^+$ states with the first T=0 excited 2$^+$ state at 3.0 MeV, which is the final state for weak decays from either $^8$Li or $^8$B. We find this state, which is an important laboratory for testing various aspects of weak interactions, to have extremely small T=1 contamination. [Preview Abstract] |
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