Session B7: Nuclear Structure and Reactions

Population of low-spin levels in $^{75,77}$Ge in neutron-capture reactions

The structure of nuclei near $^{76}$Ge has been of interest owing to the possibility of neutrinoless double beta decay. To further characterize the low-energy, low-spin structure for $^{75}$Ge and $^{77}$Ge, we have studied the gamma rays following neutron capture on highly enriched $^{74}$Ge and $^{76}$Ge, respectively, in experiments performed at the cold-neutron beam line at the NIST Center for Neutron Research. The new spectra will be shown and insight into the level structures presented.   

Measurements of Dipole Excitations in $^{48}$Ca Between E$_{\gamma}$ = 9.5 and 15.3 MeV

$^{48}$Ca photodisintegration cross sections were measured using $\gamma$-ray beams of 34 different energies between 9.5 and 15.3 MeV generated with the TUNL High-Intensity $\gamma$-ray Source (HI$\gamma$S). The cross sections were measured with a 2.7 g $^{48}$Ca target enriched to 92\% using an array of $^{3}$He proportional-counters embedded in thermalizing polyethylene. The absolute cross sections for the $^{48}$Ca($\gamma$,n) reaction were obtained by normalization to the well known $^{2}$H($\gamma$,n) reaction cross sections. Single neutron emission is the dominant channel in this energy range and its detection provides a measurement of dipole excitation strength such as the previously identified M1 strength at E$_{\gamma}$ = 10.22 MeV,\footnote{W.Steffen \textit{et al.} Nucl. Phys. A \textbf{404}, 413 (1983).} for which a new value will be presented. Structures on the low-energy tail of the E1 GDR were also observed and will be presented.   

Transient field $g$ factor measurement on radioactive $^{100}$Pd(2$^+_1$) via $\alpha$ transfer

$^{100}$Pd has four proton holes in the $g_{9/2}$ and four neutrons in the $d_{5/2}$ orbitals around the $N$ = $Z$ =50 shell closures and is therefore a suitable candidate for studying single particle effects in the nuclear wave function. The $\alpha$ transfer from a carbon target to an energetic beam of $^{96}$Ru close to the Coulomb barrier has been used to populate the 2$^+_1$ state in radioactive $^{100}$Pd via the reaction $^{12}$C($^{96}$Ru,$^8$Be)$^{100}$Pd. $^{96}$Ru beams of 343 MeV were provided by the Yale WNSL accelerator. The two $\alpha$ particles from the breakup of $^8$Be and the carbon ions which Coulomb excited the Ru projectiles were detected in a Si detector in coincidence with the $\gamma$ rays recorded in four Ge Clover detectors. Angular correlations and precessions have been measured via the transient field technique. Preliminary data yield the first measurement of the $g$ factors of the 2$^+_1$ state in $^{100}$Pd and of the 4$^+_1$ state in $^{96}$Ru.   

Properties of 130Te for Neutrinoless Double Beta Decay

We have started a set of measurements aimed at determining the nuclear structures relevant to the decay from 130Te to 130Xe. So far the neutron occupation has been determined for 130Te at the Yale tandem accelerator, and similar measurements are planned for 130Xe. The proton BCS pairing is known to be broken in 130Xe (W.P.Alford et al, Nuclear Physics A323, pp329, 1979) who have seen strong population of excited 0+ states in the pair-adding 130Te(3He,n)132Xe reaction. We have carried out similar studies for pair-removal for neutrons, and within the experimental uncertainties of a few percent find no corresponding breaking in the neutron BCS state in the 130Te(p,t) reaction.   

Feeding of isomers of stable 70$<$Z$<$80 isotopes in neutron induced reactions

The ($n,n^{\prime}\gamma$) reactions on stable Au, Ir and Lu isotopes were used to determine the feeding of the 11/2$^{-}$ isomers in $^{191}$Ir, $^{193}$Ir and $^{197}$Au and, more recently, of the 1$^{-}$ isomer in $^{176}$Lu, an odd-odd nucleus. The data were taken in four separate experiments, one for each isotope, using the GEANIE spectrometer comprised of 26 Ge detectors. The pulsed neutron source of the LANSCE/WNR facility provided neutrons with incident energies from 1 to 300~MeV, determined using the time-of-flight technique. The experimental results for $^{191}$Ir, $^{193}$Ir and $^{197}$Au were compared with theoretical predictions from the GNASH reaction model, a step that is essential to take into account the weak $\gamma$ rays feeding the isomers and not observed experimentally. GNASH reaction model calculations in $^{176}$Lu are in progress. Results from all four experiments will be discussed. This work was performed under the auspices of the U.S. DOE in part by the University of California, Los Alamos National Laboratory (LANL) under Contract W-7405-ENG-36, and in part by Los Alamos National Security, LLC, LANL under Contract DE-AC52-06NA25396, and in part by the University of California, Lawrence Livermore National Laboratory (LLNL) under Contract W-7405-ENG-48 and in part by Livermore National Security, LLC, LLNL under Contract DE-AC52-07NA27344.   

Analyzing powers for the $^{12}C(^7\vec{Li}, t/d/p)$ reactions

The first, second, and third rank tensor analyzing powers $^TT_{10}$, $^TT_{20}$, $^TT_{30}$, and $T_{20}$, as well as cross section angular distributions for $^{12}C(^7\vec{Li}, t/d/p)$ at 34 MeV were measured. An earlier work\footnote{E.E.~Bartosz, {\it etal} Phys. Lett {\bf B 488} (2000) 138} showed that the ratio $\frac{3\,^TT_{30}}{^TT_{10}}$ for the elastic, inelastic, and single nucleon reactions could distinguish peripheral, in plane reaction processes, from other processes. The present work was undertaken to determine if the $(^7\vec{Li}, d/p)$ transfer reactions could be described as a direct process. High statistics were obtained because the analyzing powers $^TT_{30}$ are small. Large values for $^TT_{10}$ were found for the $(^7\vec{Li}, d/p)$ reaction indicating that they do not proceed primarily through the compound nucleus process. Results of FRDWBA calculations will be compared to experiment to determine spectroscopic amplitudes and the nature of the transfer reactions. Results indicate that all reactions are direct to some degree and there is evidence for the direct transfer of a $^{4,5}\mathrm{He}$ cluster, but not a $^6\mathrm{He}$ cluster.   

Impact Parameter Characterization, Reaction Plane Determination, and Flow Analysis of 35 MeV/u $^{70}$Zn+$^{70}$Zn, $^{64}$Zn+$^{64}$Zn, $^{64}$Ni+$^{64}$Ni

The NIMROD-ISiS array was recently used for the collection of 35 MeV/u reaction systems consisting of $^{70}$Zn+$^{70}$Zn, $^{64}$Zn+$^{64}$Zn, $^{64}$Ni+$^{64}$Ni. The primary motivation for obtaining this data set was to examine the flow of isotopically identified fragments. In order to complete the flow analysis, an estimation of the impact parameter and reaction plane was completed. The impact parameter characterization was obtained through the binning of experimental distributions of global variables. The AMD (Antisymmetrized Molecular Dynamics) model coupled with Gemini was utilized to investigate the accuracy of the impact parameter characterization. In particular, combining the transverse energy of Z=1-2 fragments and the neutron multiplicity, both with equal weighting, provided reasonable impact parameter determination. Similarly, a comparison of different reaction plane determination methods will be shown demonstrating the accuracy of each calculation. Lastly, initial collective flow measurements of isotopically resolved light fragments will be presented from both experimental data and theoretical simulations.   

Probing densities of hot nuclei

Study of density dependence of symmetry energy requires quantitative information about the excitation energy dependence of nuclear density. In the present work, kinetic energy spectra of fragments arising from the reactions $^{78,86}$Kr+$^{58,64}$Ni, ($E_{lab}$= 35 MeV/A) have been analyzed to get information about the nuclear density. A reconstructed quasi-projectile was chosen as the fragmenting source with appropriate conditions. In order to investigate the excitation energy dependence of the nuclear density, kinetic energy spectra of fragments, gated on different excitation energy of the fragmenting source, were analyzed within the framework of a binary break up. In order to account for the variation in Coulomb repulsion seen by a fragment in different break up events, an average Coulomb repulsion was calculated by varying the charge of the residual nucleus. In the analysis, the radius constant was one of the free parameters. The density profile obtained from the values of radius constant at various excitation energies was found to be in a reasonable agreement with that reported in literature based on similar analysis.   

Dynamic Fragmentation of Ca Projectiles in Intermediate-Energy Heavy-Ion Collisions

Charged products released in reactions of $^{48}$Ca+$^{112,124}$Sn and $^{40}$Ca+$^{112}$Sn at 45 AMeV were measured using the CHIMERA 4$\pi $ multi-detector array of the LNS-INFN in Catania, Italy. Emission patterns of these products were characterized with the aid of Wilczy\~nski-type dissipation plots and invariant velocity plots. Additionally the angular distribution of the binary (Z$_{1}$,Z$_{2}\ge $3) breakup of the primary projectile-like fragment was found to exhibit a distinct forward-backward asymmetry, which appears to be inconsistent with a two-step reaction model: a dissipative reaction stage followed by statistical decay of primary products.