Session BL: Nuclear Reactions: Heavy-Ions/Rare Isotope Beams I

Measurement of shell energies for $^{40,48}$Ca by using ($\vec{p},2p$) reaction

The binding energies and the spectroscopic factors of the proton orbits in $^{40}$Ca and $^{48}$Ca nuclei were measured by using ($\vec{p}$,2$p$) reaction at 200MeV. The aim of this experiment is to investigate the neutron-number dependence of the shell energies, which is motivated by a recent theoretical work $[1]$ on the monopole effect of the tensor force. Since the ($\vec{p}$,2$p$) reaction shows clear $j$-dependence at this energy, unambiguous $j$-assignment is expected. The experiment was performed at RCNP using a two-arm spectrometer system. The energy resolution achieved was better than 200keV FWHM. In the presentation, comparison of the present result with a previous ($\vec{d}$,$^3$He) result $[2]$ will be also given. \\[4pt] [1] T. Otsuka $et al.,$ Phys. Rev. Lett. $\textbf{95}$ (2005) 232502.\\[0pt] [2] S. M. Banks $et al.,$ Nucl. Phys. $\textbf{A437}$ (1985) 381.   

Study of spin dipole strength in $^{12}{\rm N}$ via complete polarization transfer measurements

Spin-isospin excitations in nuclei have been studied intensively in the past decades. While Gamow-Teller ($\Delta S=1,\ \Delta T=1,\ \Delta L=0$) excitaitons have been investigated exhaustively, the understanding of spin dipole (SD) ($\Delta S=1,\ \Delta T=1,\ \Delta L=1$) excitations is still rather limited with respect to the three different spin-parity states of $J^{\pi}=0^-, 1^-, 2^-$. The strength distribution of each SD state gives us fundamental information on the tensor correlations. In order to deduce the SD strength distributions in $^{12}{\rm N}$, we have measured cross sections and complete sets of polarization transfer observables for $^{12}{\rm C}(\vec{p},\vec{n})$ reaction. The measured polarization transfer observables are used to separate cross section into spin-longitudinal $ID_q$ and spin-transverse $ID_p$ polarized cross sections. These polaried cross sections enable us to separate the SD components into each $J^{\pi}$. In this talk, we will report on the obtained SD strength distributions.   

Spectroscopic Factors from the Single Neutron Pickup Reaction $^{64}$Zn($\vec{\mathrm{d}}$,t)

P.E.Garrett$^1$, G.C.Ball$^2$, J.C.Bangay$^1$, L.Bianco$^1$, G.A.Demand$^1$, T.Faestermann$^3$, P.Finlay$^1$, K.L.Green$^1$, R.Hertenberger$^4$, R.Kr\"ucken$^3$, A.A.Phillips$^1$, E.T.Rand$^1$, C.S.Sumithrarachchi$^1$, C.E.Svensson$^1$, S.Triambak$^1$, H.-F.Wirth$^4$, J.Wong$^1$, $^1$Guelph, $^2$TRIUMF, $^3$TU M\"unchen, $^4$LMU M\"unchen -- A great deal of attention has recently been paid towards high-precision superallowed $\beta$-decay ${\cal F}t$ values. With the availability of extremely high-precision $(<0.1\%)$ experimental data, precision on the individual ${\cal F}t$ values are now dominated by the $\sim1\%$ theoretical corrections$^{\mathrm{[1]}}$. This limitation is most evident in heavier superallowed nuclei (e.g. $^{62}$Ga) where the isospin-symmetry-breaking (ISB) correction calculations become more difficult due to the truncated model space. Experimental spectroscopic factors for these nuclei are important for the identification of the relevant orbitals that should be included in the model space of the calculations. Motivated by this need, the single-nucleon transfer reaction $^{64}$Zn$(\vec{\mathrm{d}}$,t)$^{63}$Zn was conducted at the Maier-Leibnitz-Laboratory (MLL) of TUM/LMU in Munich, Germany, using a 22~MeV polarized deuteron beam from the tandem Van de Graaff accelerator and the TUM/LMU Q3D magnetic spectrograph, with angular distributions from $10^{\circ}$ to $60^{\circ}$. Results from this experiment will be presented and implications for calculations of ISB corrections in the superallowed $\beta^+$ decay of $^{62}$Ga will be discussed.\newline $^{\mathrm{[1]}}$~I.S. Towner and J.C. Hardy, Phys. Rev. C $\bf{77}$, 025501 (2008).   

Determining (n,$\gamma )$ cross sections using surrogate reactions

Direct measurements of neutron-reaction cross sections on unstable nuclei are extremely challenging due to the difficulties associated with radioactive targets and neutron beams. Indirect methods, such as the surrogate reaction method, are currently the only feasible way to determine many of the cross sections for radioactive nuclei that are of interest to nuclear astrophysics, nuclear energy, and other applications. We have used the surrogate reaction method to determine (n,$\gamma )$ cross sections for $^{153,155,157}$Gd nuclei at energies up to 3 MeV through inelastic proton scattering on stable targets. The STARS/LiBerACE silicon and germanium detector arrays were used to detect $\gamma $ rays in coincidence with the scattered protons to determine $\gamma $-ray exit-channel probabilities. Techniques are being explored to extract reliable cross sections at energies for which the Weisskopf-Ewing limit of the Hauser-Feshbach theory is not applicable. This measurement will provide the first determination of the (n,$\gamma )$ cross section for $^{153}$Gd, an s-process branch-point nucleus with a half-life of 240 days. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Inelastic and Transfer Couplings in Nucleon Induced Reactions

A microscopic calculation of the optical potential for nucleon-nucleus scattering has been performed by explicitly coupling the elastic channel to all the particle-hole (p-h) excitation states in the target. These p-h states may be regarded as {\em doorway states} through which the flux flows to more complicated configurations, and to long-lived compound nucleus resonances. The random-phase approximation (RPA) and the quasi-particle RPA (QRPA) provide linear combinations of p-h states that include the residual interactions within the target, and we show results for reaction cross sections using the QRPA description of target excitations of different nuclei and coupling to all open channels. We also included couplings to relevant pick-up channels, which were found to represent a very important contribution to a more accurate and realistic description of the reaction proccess. With this procedure we observed coupling and structure effects of the studied nuclei by comparing the different coupled-channel (CC) calculations results with predictions of a well-established optical potential and with experimental data. The effect of including couplings between excited states in CC calculations was also analyzed within the RPA context and its relevance was assessed. Prepared by LLNL under Contract DE-AC52-07NA27344. LLNL-ABS-414348   

Unique double folding optical parameters for 240 MeV$^{ 6}$Li beam

Theoretical calculations with different interactions and relativistic models have shown that the location of the GMR is sensitive to the symmetry energy. To better determine the contribution from symmetry energy ISGMR measurements should be extended to unstable nuclei using inverse reactions. Most ISGMR information has come from inelastic $\alpha $ scattering but unfortunately helium does not make a good target. Chen et al.[1] have demonstrated that $^{6}$Li inelastic scattering at 40 MeV/A is a viable reaction for ISGMR studies and $^{6}$Li foil targets are well suited to these studies. Here we report results for elastic scattering of 240 MeV $^{6}$Li ions on $^{24}$Mg, $^{28}$Si, $^{40}$Ca, $^{48}$Ca, $^{58}$Ni, $^{90}$Zr, and $^{116}$Sn and inelastic scattering to low-lying states of these targets to develop a systematic optical potential that can be used in Giant Resonance studies of unstable nuclei. Density dependent double folding calculations using the M3Y effective NN interaction were used to obtain the real part of the optical potential and the transition potential. A Woods-Saxon potential was used for the imaginary part. B(EL) values obtained for low lying 2$^{+}$ and 3$^{-}$ states are mostly in agreement with the adopted values. The progress on developing a systematic potential will be presented. [1] X.Chen et al., Phys.Rev. C79,024320(2009).   

Measurement of the$^{ 235}$U(n,n')$^{235m}$U Integral Cross Section in a Pulsed Reactor

We will present the integral measurement of the neutron inelastic cross section of $^{235}$U leading to the 26-minute, E*=76.5 eV isomer state. Small samples (5-20 microgm) of isotope-enriched $^{235}$U were activated in the central cavity of the CALIBAN pulsed reactor at Valduc where a nearly pure fission neutron spectrum is produced with a typical fluence of 3x10$^{14}$ n/cm$^{2}$. After 30 minutes the samples were removed from the reactor and counted in an electrostatic-deflecting electron spectrometer that was optimized for the detection of $^{235m}$U conversion electrons. From the decay curve analysis of the data, the 26-minute $^{235m}$U component was extracted. Preliminary results will be given and compared to gamma-cascade calculations assuming complete K-mixing or with no K-mixing.   

Directional Correlation of Nuclear-Collision Probability for Aligned Beams of Deformed Nucleus

We could observe the directional correlation of the nuclear-collision probability between the direction of beam of deformed nuclei and the deformation axis for heavy-ion collisions at intermediate energies for the first time. The experiment was carried out at the HIMAC synchrotron and fragment-separator facility. Aligned nuclear beams of $^{10}$B were produced through the projectile fragmentation of 130$A$ MeV $^{11}$B primary beam on Be targets. By selecting the parallel momentum using the separator, negative or positive nuclear spin alignment of $^{10}$B relative to the beam axis were created. The ground state of $^{10}$B is considered to have a large prolate deformation of $\beta_2 \sim +0.8$. Using these aligned beams, the interaction cross sections were measured on a carbon target. The interaction probabilities were precisely obserbed as a function of longitudinal momentum of the secondary $^{10}$B beam. A clear directional correlation was observed. We will report on the details of measurements and discussions on this intriguing result.   

Shift and width of momentum distribution of projectile-like fragments produced at 290MeV/u

The shift and width of momentum distribution of projectile-like fragments (PLFs) produced at an intermediate energy were investigated experimentally. The longitudinal momentum distributions of PLFs produced from the Ar- and Kr-beams were observed at an energy of 290 MeV/u. The measurements were performed by using HIMAC facility at NIRS. Observed distributions shows an asymmetric features, which is minor than that was observed at 100 MeV/u or lower. In order to extract the reaction mechanism, observed distributions were fitted with an asymmetric Gaussian function. Based on the fitting process, the precise determination of the shift and width of momentum distribution was performed for PLFs with AF = 10 $\sim $ 40 and 20 $\sim $ 84 for Ar-beam and Kr-beam, respectively In principle, the shift and width are independent on the target (C, Al, Nb, Tb, Au). Some light PLFs produced from Kr-beam show the anomalously large momentum shift and width.   

Measurements in the Quasi-Continuum

The density of energy levels in nuclei increases rapidly as the excitation energy increases towards the particle separation energy, creating a quasi-continuum. The density of states (entropy) in a given system depends on excitations across shell gaps and the number of broken nucleon pairs. It is not possible to identify all energy levels in this quasi-continuum experimentally. Instead, average quantities such as the entropy and $\gamma$-ray strength functions are used to describe ``gross'' nuclear properties, critical in calculating nuclear reaction rates in astrophysical processes. I will discuss experimental efforts underway to measure feeding and lifetimes of quasi-continuum states using (d,p) transfer reactions. The protons and $\gamma$-rays from the reactions are detected using the STARS-LIBERACE detector array. Particle energies of detected charged particles will be used to infer the ``entrance'' excitation energy of the residual nucleus. These ``entrance'' energies are used to study feeding to discrete states and lifetimes of regions in the quasi-continuum.   

A Quadrupole Momentum Thermometer for Heavy-Ion Reactions

Various thermometers have been used to measure nuclear temperature in the context of probing the proposed nuclear liquid-gas phase transition. A recently developed thermometer derived from quadrupole fluctuations is explored. Effects on the temperature of including ground-state nuclear spins and corrections for collective motion are described. The thermometer was derived using classical Maxwell-Boltzmann statistics and an alternative Bose-Einstein correction has also been applied and tested for bosonic nuclei. Inclusion of ground-state spins in the calculation improves the agreement of the temperatures between different fragment particle types.   

Classical studies of nuclear isoscaling

The isoscaling phenomenon observed in nuclear multifragmentation experiments is expected to provide information about the nuclear equation of state. This presentation will outline a series of studies isoscaling using 1) classical molecular dynamics simulations, 2) percolation and 3) probabilistic arguments which demonstrate that systems disassembling with no more than fair sampling are expected to produce a minimum isoscaling as a general phenomenon independent of the nuclear reaction. Thus caution must be exercised when trying to extract nuclear information from the experimentally observed isoscaling parameters.