Bulletin of the American Physical Society
2008 Annual Meeting of the Division of Nuclear Physics
Volume 53, Number 12
Thursday–Sunday, October 23–26, 2008; Oakland, California
Session LE: Nuclear Structure: Light Nuclei |
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Chair: Alexandra Gade, Michigan State University Room: Simmons Ballroom 1 |
Sunday, October 26, 2008 8:30AM - 8:42AM |
LE.00001: Two-Body Photodisintegration of $^4$He into p+t Rakhsha Nasseripour, Barry Berman The two-body photodisintegration of $^4$He into a proton and a triton has been studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid $^4$He target. This is the first measurement of the photodisintegration of $^4$He above 0.4 GeV. The differential cross sections for the $\gamma$$^4$He$\rightarrow$ pt reaction have been measured as a function of photon-beam energy and proton-scattering angle, and are compared with the latest model calculations by Laget [1]. At 0.6-0.8 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. At the same time, the strength of the one-body mechanisms is overestimated at higher energies and at small proton-scattering angles. These results reinforce the conclusion of our previous study of the three-body breakup of $^3$He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV [2]. \newline [1] J.-M. Laget, private communication (2008) \newline [2] S. Niccolai {\it et al.}, Phys. Rev. C {\bf 70}, 064003 (2004) [Preview Abstract] |
Sunday, October 26, 2008 8:42AM - 8:54AM |
LE.00002: Improved DSAM measurements in $^{10}$Be as a test of Ab-Initio calculations E.A. McCutchan, C.J. Lister, M.P. Carpenter, R.V.F. Janssens, T.L. Khoo, T. Lauritsen, D. Seweryniak, I. Stefanescu, S. Zhu Recent ab-initio calculations in light nuclei have emphasized the significance of 3-body forces. In the A = 10 systems, the inclusion of 3-body forces inverts the sequence of states, which has been attributed to the important contribution of the 3-body interaction to the overall spin orbit force. To challenge these latest calculations, a new generation of improved DSAM measurements is necessary to precisely ($<$5{\%}) determine matrix elements between excited states. Lifetimes of excited levels in $^{10}$Be, populated in the $^{7}$Li($^{7}$Li,$\alpha )$ reaction, are determined using high velocity DSAM measurements. The recoiling $^{10}$Be were detected at zero degrees using the Argonne Fragment Mass Analyzer (FMA) and $\gamma $ rays measured with Gammasphere, the combination allowing for the collection of very clean $\gamma $-ray spectra and the elimination of cascade feeding. Preliminary results of the experiment will be presented and discussed in terms of recent ab-initio calculations. This research is supported by the DOE Office of Nuclear Physics under contract DE-AC02-06CH11357. [Preview Abstract] |
Sunday, October 26, 2008 8:54AM - 9:06AM |
LE.00003: $^{25}$Al levels observed in the $^{28}$Si(p,$\alpha )^{25}$Al reaction S.T. Pittman, D.W. Bardayan, J.C. Blackmon, R.L. Kozub, M.S. Smith The level structure of $^{25}$Al has been studied at the ORNL Holifield Radioactive Ion Beam Facility (HRIBF) by measuring the angular and energy distributions of alpha particles from the $^{28}$Si(p,$\alpha )^{25}$Al reaction. Proton beams ($\sim $10 nA) at laboratory energies of 40- and 42-MeV, respectively, were generated by the 25 MV tandem accelerator and bombarded a natural silicon target (50 $\mu $g/cm$^{2})$. Alpha particles were detected and identified in the Silicon Detector Array (SIDAR) in the ``telescope'' configuration [1]. Angular distributions were extracted for strongly populated states, and distorted-wave Born approximation (DWBA) calculations were performed using the code DWUCK4 to determine spin and parity. Results of this experiment, including angular distributions of alpha particles and spin and parity assignments for $^{25}$Al excited states, will be discussed. [1] D.W. Bardayan \textit{et al}., Phys. Rev. C \textbf{65}, 032801(R) (2002). *This work was supported by research grants from the Department of Energy. [Preview Abstract] |
Sunday, October 26, 2008 9:06AM - 9:18AM |
LE.00004: Narrowing of the neutron sd-pf shell gap in $^{29}$Na Aaron Hurst The wave-function composition for the low-lying states in $^{29}$Na was explored by measuring their electromagnetic properties using the Coulomb-excitation technique. A beam of $^{29}$Na$^{5+}$ ions, postaccelerated to 70~MeV, bombarded a $^{110}$Pd target with a rate of up to 600 particles per second, in the first physics experiment using the {\small ISAC-II} facility at {\small TRIUMF}. Six segmented clover detectors of the {\small TIGRESS} $\gamma$-ray spectrometer were used to detect deexcitation $\gamma$ rays in coincidence with scattered or recoiling charged particles in the segmented silicon detector, {\small BAMBINO}. A preliminary reduced transition matrix element $| \langle \frac{5}{2}^{+}_{1} ||E2|| \frac{3}{2}^{+}_{\rm gs} \rangle | = 0.229(20)$~$e$b was derived for $^{29}$Na from the measured $\gamma$-ray yields for both projectile and target. This first-time measured value is consistent with the most recent Monte Carlo shell-model calculation (MCSM) of Utsuno {\it et al.}, predicted to be 0.232~$e$b, indicating an approximately equal admixture of both $sd$ and $pf$ components in the wave function, and also providing evidence for the narrowing of the $sd$-$pf$ shell gap from $\sim 6$~MeV for stable nuclei to $\sim 3$~MeV for $^{29}$Na. [Preview Abstract] |
Sunday, October 26, 2008 9:18AM - 9:30AM |
LE.00005: Z=14 and Z=16 shell closures Mathis Wiedeking, P. Fallon, A.O. Macchiavelli It has been known for a long time that the interaction between valence neutrons and protons plays a pervasive role in the evolution of nuclear structure with changing neutron and proton number. With experimental data far from stability becoming available we present a systematic investigation of experimental excitation energies and electromagnetic properties in nuclei along the N=10, 11, and 12 isotones. We have searched available experimental data for signatures of shell closures far from stability. These data are compared to N$_{n}$-N$_{p}$ models and are found to be consistent with the proposed proton shell closures at Z=14 [1] and Z=16 [2]. [1] P.D. Cottle, Phys. Rev. C 76, 027301 (2007) [2] Z. Dlouhy et al., Nucl. Phys. A722, 36 (2003) [Preview Abstract] |
Sunday, October 26, 2008 9:30AM - 9:42AM |
LE.00006: A microscopic hyper-spherical model: application to $^6$He Ivan Brida, Filomena Nunes We have developed a microscopic cluster model of light two neutron halo nuclei that incorporates the few-body asymptotics in full extent. The wavefunction of the system consists of a core and two valence neutrons. The core is given in terms of correlated Gaussians. The three-body dynamics between the core and valence neutrons are taken into account by means of the hyper-spherical functions containing an exponentially decaying hyper-radial part. The center of mass motion is removed by construction. In this talk, we present the first results of our model applied to $^6$He. The central Minnesota N-N interaction with a spin-orbit addition is used to bind the system. Basic structural observables, such as binding relative to $^4$He, radii and one-body densities are in agreement with other microscopic calculations employing similar N-N interactions. The microscopic description of the core allows us to test the efficiency of Pauli projection techniques employed in the few-body models. We demonstrate that proper antisymmetrization is crucial to bind $^6$He against three-body break-up. We also present overlap functions between $^4$He and $^6$He with the aim of future reaction calculations. [Preview Abstract] |
Sunday, October 26, 2008 9:42AM - 9:54AM |
LE.00007: Precision Test of the Isobaric Multiplet Mass Equation in the A = 32, T = 2 Quintet R. Ferrer, A.A. Kwiatkowski, G. Bollen, C.M. Campbell, C.M. Folden III, D. Lincoln, D.J. Morrissey, G.K. Pang, A. Prinke, J. Savory, S. Schwarz Masses of the radionuclides $^{32,33}$Si and $^{34}$P and of the stable nuclide $^{32}$S have been measured with the Low Energy Beam and Ion Trap (LEBIT) Penning trap mass spectrometer. Relative mass uncertainties of 3 x 10$^{-8}$ and better have been achieved. The measured mass value of $^{32}$Si differs from the literature value [1,2] by four standard deviations. The precise mass determination of $^{32}$Si and $^{32}$S have been employed to test the isobaric multiplet mass equation for the A = 32, T= 2 isospin quintet. The experimental results indicate a significant deviation from the quadratic form. This work has been supported by Michigan State University, the NSF under contract number PHY- 0606007, and the DOE under the contract DE-FG02-00ER41144. References: 1. G. Audi, A.H. Wapstra, and C. Thibault, Nucl. Phys. A729 (2003) 337 2. A. Paul, S. R\"{o}ttger, A. Zimbal, and U. Keyser, Hyperfine Interact. 132 (2001) 189 [Preview Abstract] |
Sunday, October 26, 2008 9:54AM - 10:06AM |
LE.00008: Next generation $\Lambda$-hypernuclear spectroscopy via the (e, e$^\prime$K$^+$) reaction at Jefferson Lab Tomofumi Maruta Spectroscopic study via the (e, e$^\prime$K$^+$) reaction is a very important technique to investigate $\Lambda$-hypernuclear structure; the reaction favorably excites spin-filp states and on light nuclei, compared to the meson-induced reactions, produces mirror hypernuclei. So far, it is the only technique that allows absolute mass determination with accuracies of a $\approx$ 100 keV or better. Two previous experiments that we performed, JLab E89-009 and E01-011, established the experimental technique, and the latter obtained hypernuclear mass spectra up to A $\approx$ 30 with, for reaction spectroscopy, unprecedented energy resolution as good as 400 keV (FWHM). Our next experiment, E05-115, will investigate $\Lambda$-hypernuclei in wide mass region up to $^{52}_{\Lambda}$V. A newly constructed electron spectrometer (HES) and splitter magnet will increase the yield by almost one order of magnitude while preserving the achieved energy resoluton. Together with the existing Kaon Spectrometer (HKS), they are scheduled for installation in Jefferson Lab's Hall C in 2009. The planned experimental program will explore $\Lambda$-hypernuclei beyond the $p$-shell for the first time in (e, e$^\prime$K$^+$) reaction spectroscopy. This presentation will give a preparation status report and outline of the experimental program and technique of the next experiment. [Preview Abstract] |
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