Bulletin of the American Physical Society
2006 Division of Nuclear Physics Annual Meeting
Wednesday–Saturday, October 25–28, 2006; Nashville, Tennessee
Session BC: Nuclear Structure I |
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Sponsoring Units: DNP Chair: Volker Werner, Yale University Room: Gaylord Opryland Tennessee B |
Thursday, October 26, 2006 2:00PM - 2:12PM |
BC.00001: A Direct Measurement of the Neutron-Neutron Scattering Length: Progress and Status S.L. Stephenson, B.E. Crawford, C.R. Howell, W. Tornow, G.E. Mitchell, W.I. Furman, A.R. Krylov, E.V. Lychagin, A. Yu. Muzichka, G.V. Nekhaev, E.I. Sharapov, V.N. Shvetsov, A.V. Strelkov, Yu. I. Chernukhin, Ya. Z. Kandiev, B.G. Levakov, A.E. Lyzhin A direct measurement of the neutron-neutron scattering length $(a_{nn})$ provides a unique contribution to understanding charge symmetry breaking in the nuclear force. A brief status report on such an experiment at the YAGUAR reactor facility will be given. [Preview Abstract] |
Thursday, October 26, 2006 2:12PM - 2:24PM |
BC.00002: Variational microscopic treatment of halo nuclei Ivan Brida, Filomena Nunes, Kalman Varga We present first results of a variational microscopic model for two neutron halo nuclei. The wavefunction of the system consists of two parts: a core and the valence neutrons. The core is given in terms of correlated Gaussians. The three-body asymptotics and dynamics between the core and valence neutrons are taken into account by means of hyperspherical functions employed to describe the motion of the valence particles. To avoid the spurious motion of the center of mass, Jacobi coordinates are used for the entire system. The wavefunction is properly antisymmetrized. Both - core's and valence - pieces of wavefunction contain nonlinear parameters involved in energy minimization procedure. Results for $^6$He are presented and compared with the three-body model. [Preview Abstract] |
Thursday, October 26, 2006 2:24PM - 2:36PM |
BC.00003: Ab Initio Calculations of Electroweak Matrix Elements. Muslema Pervin, Robert Wiringa, Steven Pieper , Kenneth Nollett 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 $v_{18}$ (AV18) two-nucleon and Illinois-2 (IL2) three-nucleon potentials, reproduce the energies of many bound and narrow states in nuclei up to A$=12$. They have also been applied to calculate properties beyond the structures of nuclei, for example, radiative capture cross sections and electroweak matrix elements. We are now exploring some nuclear transitions, particularly electromagnetic transition strengths and nuclear beta decay rates, with the GFMC technique using the AV18 $+$ IL2 potential for nuclei with A$=$ 6 to 8. A few of the transitions have previously been calculated using the more approximate VMC technique but with an older potential. However, GFMC wave functions are better approximations to the true wave functions, hence GFMC results for these transitions should be more reliable. [Preview Abstract] |
Thursday, October 26, 2006 2:36PM - 2:48PM |
BC.00004: Towards a Laser Spectroscopic Determination of the $^8$He Nuclear Charge Radius P. Mueller, K. Bailey, R.J. Holt, R.V.F. Janssens, Z.-T. Lu, T.P. O'Connor, J.P. Schiffer, I. Sulai, M.-G. Saint Laurent, J.-Ch. Thomas, A.C.C. Villari, O. Naviliat-Cuncic, X. Flechard, S.-M. Hu, G.W.F. Drake, M. Paul $^8$He (t$_{1/2}$ = 119 ms) has the highest neutron to proton ratio of all bound nuclei. Precision measurements of its nuclear structure shed light on nuclear forces in neutron rich matter that, for example, play a critical role in neutron stars. Our experiment to measure the $^8$He nuclear charge radius is based on our previous work of high-resolution laser spectroscopy of helium atoms cooled and confined in a magneto-optical trap. This technique enabled us to accurately measure the atomic isotope shift between $^6$He and $^4$He and thereby to determine the $^6$He rms nuclear charge radius to be 2.054(14) fm. We are currently well on the way to improve the overall trapping efficiency and signal-to-noise ration of our system to compensate for the shorter lifetime and lower production rates of $^8$He as compared to $^6$He. The $^8$He measurement is planned to be carried out at the GANIL cyclotron facility in Caen, France in late 2006. [Preview Abstract] |
Thursday, October 26, 2006 2:48PM - 3:00PM |
BC.00005: Large Study of Short-Range Correlations in $^{12}$C(e,e$^{\prime}$pn) Ramesh Subedi Experiment E01-015 at JLab Hall A has been investigating electron induced two nucleon emission from carbon with the goal of being sensitive to and studying short-range correlations. The experiment took data from January through April 2005 at a kinematic coverage of Q$^2$ = 2 (GeV/c)$^2$, $x_{B}$ = 1.2 and the missing momentum from $^{12}$C(e,e$^{\prime}$p) reaction ranging 250 to 650 MeV/c. Two high resolution spectrometers were used for e' and p detection in the reaction $^{12}$C(e,e$^{\prime}$pn). The recoiling neutrons were detected by a neutron detector which covered an 81 msr solid angle and consisted of 4 layers of neutron counters, each layer being 10 cm thick. We used a time-of-flight method to reconstruct the neutron momentum. Short-range correlations can be emulated by various two-body effects, so we chose an anti-parallel kinematics with high Q$^2$ at $x_{B}>$1 to minimize meson exchange currents, isobar currents and final-state interactions. Preliminary analysis results from $^{12}$C(e,e$^{\prime}$pn) reaction will be presented. [Preview Abstract] |
Thursday, October 26, 2006 3:00PM - 3:12PM |
BC.00006: Microscopic Coupled Cluster approach to Neutron Rich Nuclei Gaute Hagen, David Dean, Morten Hjorth-Jensen, Thomas Papenbrock Building nuclei from the ground up starting with the bare nucleon-nucleon interaction is a major challenge in nuclear structure today. We present results starting with the fully microscopic Coupled Cluster approach. Coupled Cluster theory builds correlation on a given reference state through an exponential correlation operator acting on the reference state. We have implemented Coupled Cluster with two- and three-body forces and present ab-initio calculations of O16 starting with a two- and three-body low-momentum interaction. Further, we have extended the Coupled Cluster technique to the complex energy plane using a complex single-particle basis where bound, resonant and continuum states are treated on equal footing. This allows for an ab-initio description of nuclei far from the valley of stability, and results of the various ground-states of the Helium isotopes, starting with a realistic nucleon-nucleon interaction are presented. Within this formalism we are able to reproduce basic properties of these nuclei, such as He5 unbound and He6 bound in its ground state. [Preview Abstract] |
Thursday, October 26, 2006 3:12PM - 3:24PM |
BC.00007: Study of $^{18}$Ne Structure by $^{14}$O+Alpha Elastic Resonance Reaction Changbo Fu, V.Z. Goldberg, G.V. Rogachev, G.G. Chubarian, G. Tabacaru, M. McCleskey, Y. Zhai, T. Al-Abdullah, L. Trache, A. Banu, R.E. Tribble Data on the $\alpha $ cluster structure in N$\ne $Z nuclei are very scarce, however a recent work [1] showed unusual features of $\alpha $ cluster states in these nuclei. It was also shown in [1] that a comparison study of mirror $\alpha $ cluster states in T=1 nuclei can result in understanding of some details of nuclear structure resulting in $\alpha $ cluster configuration. Besides this, the astrophysical impact of the knowledge of the $^{14}$O+ $\alpha $ interaction is well known. We report the use of an $^{14}$O beam at the Texas A{\&}M University [2], produced by MARS facilities [3], to study resonances in the $^{14}$O+ $\alpha $ interaction. The $\alpha -^{14}$O resonance interaction was studied using the Thick Target Inverse Kinematics (TTIK) method. The time of flight method, providing for the possibility of the identification of low energy particles, was used to identify reaction products. Nine states with large $\alpha $ cluster reduced widths were found in $^{18}$Ne in the excitation energy region 8-16 MeV. The alpha structure in $^{18}$Ne and in $^{18}$O is discussed using results obtained in on $^{14}$C+$\alpha $ interaction. \newline References [1] V. Z. Goldberg et al., Phys. Rev. C 69, 024602 (2004). [2] V. Z. Goldberg et al., Phys. Rev. C 69, 031302 (2004) [3] R. E. Tribble et al., Nucl. Phys. \textbf{A701}, 278c (2002) [Preview Abstract] |
Thursday, October 26, 2006 3:24PM - 3:36PM |
BC.00008: Production of light neutron-rich nuclei in fusion-evaporation reactions M. Wiedeking, P. Fallon, A.O. Macchiavelli, L.W. Phair, D.L. Bleuel, R.M. Clark, M. Cromaz, M-A. Deleplanque, J.D. Gibelin, I-Y. Lee, L.G. Moretto, E. Rodriguez-Vieitez, D. Ward, L.A. Bernstein, J.T. Burke, B.F. Lyles I will discuss our work to extend the experimental data on light neutron-rich nuclei and present new results on $^{18}$N (Z=7), which is sufficiently far from stability to exhibit modified shell structure, yet still within the reach of stable beam facilities. $^{18}$N was produced using the $^{9}$Be($^{11}$B,2p)$^{18}$N fusion reaction at LBNL's 88-Inch Cyclotron and studied using STARS-LIBERACE, a large area segmented silicon $\Delta $E-E detector telescope and six HPGe ``Compton Suppressed'' Clover detectors. Previous information on the excited states of $^{18}$N came from $^{18}$C beta-decay and charge-exchange reactions only. A key aspect of the current measurement was to use the 2-proton evaporation reaction channel. The large Q-value and the chosen beam energy (50 MeV) suppressed the evaporation of additional neutrons, with the result that a 2p ``gate'' uniquely selected the $^{18}$N products. [Preview Abstract] |
Thursday, October 26, 2006 3:36PM - 3:48PM |
BC.00009: First excited state of doubly-magic $^{24}$O N. Frank, A. Schiller, T. Baumann, J. Brown, P. DeYoung, J. Hinnefeld, R. Howes, J.-L. Lecouey, B. Luther, W.A. Peters, M. Thoennessen Neutron separation energy systematics indicate the formation of a new magic number $N=16$ close to the dripline. The energy of the first 2$^{+}$ state may indicate or invalidate the existence of a shell closure. The search for excited states in $^{23.24}$O using in beam $\gamma$ ray spectroscopy has yielded no results, which could indicate that the 2$^{+}$ state is neutron unbound. In order to unambiguously identify $^{24}$O as a doubly magic nucleus, we therefore have resorted to neutron decay spectroscopy. Experimentally, the two-proton-knockout reaction of a 86 MeV/u $^{26}$Ne beam on a Be target at the fast- fragmentation radioactive beam facility of the National Superconducting Cyclotron Laboratory was investigated and $\sim 500$ neutron-$^{23}$O coincidences were recorded using the Sweeper/MoNA setup. From these events, a decay-energy spectrum was reconstructed which combined with the neutron separation energy of $^{24}$O yields an excitation energy of the first excited state of $^{24}$O in the order of 3.6 MeV, in agreement with new shell-model calculations. [Preview Abstract] |
Thursday, October 26, 2006 3:48PM - 4:00PM |
BC.00010: Compressional-mode Giant Resonances in $^{24}$Mg. P.V. Madhusudhana Rao, T. Li, B.K. Nayak, U. Garg, M. Itoh, M. Yosoi, M. Uchida, H. Takeda, Y. Yasuda, H. Sakaguchi, H. Fujimura, K. Hara, M. Fujiwara, T. Kawabata, H. Akimune, M.N. Harakeh A precise measurement of the incompressibility of nuclear matter (K$_{nm})$ is required for understanding the properties of nuclei, neutron stars and supernova explosions. This can be achieved through the measurements of compressional-mode giant resonances--the isoscalar giant monopole resonance (ISGMR) and the isoscalar giant dipole resonance (ISGDR). ~We have performed measurements on GMR and ISGMR in $^{24}$Mg using forward-angle inelastic scattering of 400 MeV $\alpha $-particles. Differential angular distributions were measured over an angular range of 0-10 degree and an excitation energy range of 10-50 MeV. Strength distributions for both the compressional-mode giant resonances were obtained by employing the multi-pole decomposition procedure. The experimental results and their implications on the K$_{nm}$ will be discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:00PM - 4:12PM |
BC.00011: Studying the Transition to the Island of Inversion E. Rodriguez-Vieitez, P. Fallon, R.M. Clark, M. Cromaz, M.A. Deleplanque, I.Y. Lee, A.O. Macchiavelli, F.S. Stephens, M. Wiedeking, S.G. Prussin, D. Bazin, C.M. Campbell, J.M. Cook, D.-C. Dinca, A. Gade, T. Glasmacher, W.F. Mueller, K. Yoneda The existence of deformed (2p2h) intruder ground states in A$\sim $30 N$\sim $20 nuclei (``island of inversion'') signals a modification of conventional shell structure in neutron-rich nuclei. While intruder ground states have been identified in e.g. $^{30}$Ne and $^{31}$Na, questions remain as to where the normal-to-deformed transition occurs and the nature of their collectivity: data on excited states will help answer these questions. An experiment was conducted at MSU to study N$\sim $20 Ne and Na nuclei. A 140 MeV/A $^{48}$Ca primary beam produced secondary-beam ``cocktails'' ($^{29}$Na/$^{30}$Mg/$^{32}$Al, $^{32}$Mg/$^{33}$Al/$^{35}$Si) which underwent secondary reactions to produce Ne and Na; $\gamma $-ray decays were detected by the segmented Ge array, SeGA, and $\gamma -\gamma $ coincidences were critical to establish a correct level scheme in e.g. $^{28}$Ne. The data provide information on the transition to the island of inversion and a test of recent shell-model calculations. [Preview Abstract] |
Thursday, October 26, 2006 4:12PM - 4:24PM |
BC.00012: $^{32}{\rm S}(p,\gamma)$ and its relevance to calibrating a $^{33}{\rm Ar}$ beta-delayed proton spectrum Smarajit Triambak, Alejandro Garcia, Dan Melconian, Meghan Mella, Owen Biesel Bounds on scalar contributions to the weak-interaction have been obtained from a measurement of the $^{32}{\rm Ar}$ $e^+ - \nu$ correlation which is determined from the shape of the beta-delayed proton spectrum.\footnote{E.~G.~Adelberger {\it et al}, Phys. Rev. Lett, {\bf 83}, 1299 (1999)} The proton-energy calibration plays an important role and is obtained using delayed proton groups from $^{33}{\rm Ar}$ corresponding to resonances in $^{32}{\rm S}(p,\gamma)$. In order to check for potential systematic effects and to improve the precision of the above experiment we have performed a high precision measurement of excitation energies of the relevant states corresponding to $^{32}{\rm S}(p,\gamma)$ resonances. We also measured the relative gamma branches and width of the second $J^\pi = 3/2^+$ excited state in $^{33}{\rm Cl}$. The relative gamma branches are in significant disagreement with a previous measurement\footnote{M.~M.~Aleonard {\it et al}, Nucl. Phys. {\bf A 257}, 490 (1976)}. Using our value for the width we are able to resolve an apparent discrepancy. [Preview Abstract] |
Thursday, October 26, 2006 4:24PM - 4:36PM |
BC.00013: Large Scale Shell Model Studies of M1 Strength in Argon and Calcium Isotopes A.F. Lisetskiy, E. Caurier, K. Langanke, G. Martinez-Pinedo, P. von Neumann-Cosel, F. Nowacki, A. Richter We have calculated the M1 strength distributions in the $^{36-40}$Ar and $^{40}$Ca isotopes within large-scale shell model studies which consider valence nucleons in the sd and pf shells. While the M1 strength in $^{36}$Ar is well reproduced within the sd shell, the experimentally observed fragmentation of the M1 strength in $^{38}$Ar and $^{40}$Ca requires n-particle n-hole excitations with $n\ge4$ from the $sd$ to the $pf$ shell. The mechanism of M1 strength fragmentation and the role of different n-particle n-hole cross-shell excitations are discussed. [Preview Abstract] |
Thursday, October 26, 2006 4:36PM - 4:48PM |
BC.00014: Binding-Energy Systematics of 0$^{+}$, 2$^{+}$, 3$^{-}$, and 4$^{-}$, T=0 States of Even-Even Self-Conjugate Nuclides from $^{16}$O to $^{40}$Ca Friedrich Everling Binding energies of self-conjugate even-even nuclides are plotted as --\textit{B}$^{*}$+(9.5 MeV)$^ {.}$ \textit{A}\textrm{ versus mass number } \textit{A}, \textrm{ where }\textit{B}$^{*} $ is the binding energy of ground states and levels. A diagram from \textit{A}=0 to 76 mainly for ground states shows a subshell systematics. In a diagram from \textit{A} =16 to 40, established and hypothetical 0$^{+}$ levels are shown; 24 states of supposed 1d$_{5/2}$, 2s$_{1/2}$, and 1d$_ {3/2}$ subshell occupations are connected by almost linear trends. Surprisingly, early insufficient measurements at E$_{x} =0.65$ MeV in $^{20}$Ne and 0.5 (and 0.43) MeV in $^{32}$S fit the trends. A diagram for the 0$^{+}$, 2$^{+}$, 4$^{+}$, and 6$^ {+}$ band from $^{16}$O to $^{28}$Si suggests the 0$^{+}$ head in $^{20}$Ne to be at 0.65 MeV. A systematics of 2$^{+}$ states supports both levels. A plot of 3$^{-}$ and 4$^{-}$ states contains two pairs of nearly parallel and linear 3-point trends. Two odd 2s$_{1/2}$ and 1f$_ {7/2}$ nucleons couple to 3$^{-}$ and (not completely established) to 4$^{-}$ in trends $\approx$1.6 and $\approx$1.7 MeV above. Below each of the two pairs of trends, the 0$^{+}$ trends are expected to be also nearly linear, which they are with these complementary $^{20}$Ne and $^{32}$S levels. A table suggests a total of 18 important experimental investigations. [Preview Abstract] |
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BC.00015: Binding-Energy Systematics of 0$^{+}$, \textit{T}=1 and 1$^{+}$, 2$^{+}$ and 3$^{+}$, \textit{T}=0 States of Odd-Odd Self-Conjugate Nuclides from $^{14}$N to $^{38}$K Friedrich Everling Binding energies of self-conjugate odd-odd nuclides are plotted as --\textit{B}$^{*}$+(9.5 MeV)$\cdot$ \textit{A} \textrm{ versus mass number }\textit{A}.\textrm{ Four diagrams show all }0$^{+}$, \textit{T}=1 and 1$^{+}$, 2$^{+}$, and 3$^{+} $, \textit{T}=0 states and those without \textit{J}$^{\pi}$. The connection of states by almost linear trends leads to approximate parallelograms or to single trends if supplemented by complementary levels and reference points. Including 8 unrearranged states according to the phenomenon of 6 MeV steps, 99 states are involved with supposedly relatively clean subshell configurations. Of these, 65 have the corresponding spin, while for 15 known levels the spin has not yet been determined. Four states are complementary and 15 are just reference points, most of them probably forbidden. Two additional complementary levels are isobaric analogs at \textit {A}=14. Trends with different spins are combined in pairs to show parallelisms associated with a spin-flip. The energy gain upon addition of four 1d$_{5/2}$ nucleons is almost constant, independent of the occupation of the 1d$_{5/2}$ and 1d$_{3/2}$ subshells, but not of the 2s$_{1/2}$ subshell. A table lists 40 levels for which experimental clarification is needed. The nuclide $^{22}$Na has 9 reference points, probably mainly forbidden states caused by the abnormal ground state of the $^ {20}$Ne core. [Preview Abstract] |
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