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Register a new userReexamination of $^{6}$Li scattering as a Probe to Investigate the Isoscalar Giant Resonances in Nuclei
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Inelastic ${}^{6}$Li scattering at 100 MeV/u on ${}^{12}$C and ${}^{93}$Nb have been measured with the high-resolution magnetic spectrometer Grand Raiden. The magnetic-rigidity settings of the spectrometer covered excitation energies from 10 to 40 MeV and scattering angles in the range $0^circ < theta_{text{lab.}}< 2^circ$. The isoscalar giant monopole resonance was selectively excited in the present data. Measurements free of instrumental background and the very favorable resonance-to-continuum ratio of ${}^{6}$Li scattering allowed for precise determination of the $E0$ strengths in ${}^{12}$C and ${}^{93}$Nb. It was found that the monopole strength in ${}^{12}$C exhausts $52 pm 3^text{(stat.)} pm 8 ^text{(sys.)}$% of the energy-weighted sum rule (EWSR), which is considerably higher than results from previous $alpha$-scattering experiments. The monopole strength in ${}^{93}$Nb exhausts $92 pm 4^text{(stat.)} pm 10 ^text{(sys.)}$% of the EWSR, and it is consistent with measurements of nuclei with mass number of $Aapprox90$. Such comparison indicates that the isoscalar giant monopole resonance distributions in these nuclei are very similar, and no influence due to nuclear structure was observed.
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The isoscalar giant monopole, dipole, and quadrupole strength distributions have been deduced in $^{90, 92}$Zr, and $^{92}$Mo from background-free spectra of inelastic $alpha$-particle scattering at a beam energy of 385 MeV at extremely forward angles, including 0$^{circ}$. These strength distributions were extracted by a multipole-decomposition analysis based on the expected angular distributions of the respective multipoles. All these strength distributions for the three nuclei practically coincide with each other, affirming that giant resonances, being collective phenomena, are not influenced by nuclear shell structure near $Asim$90, contrary to the claim in a recent measurement.
Mid-heavy nuclei offer unique opportunities to study the collective and single-particle aspects of nuclear structure. This mass regime is a dynamic area where protons and neutrons generally occupy different orbitals, giving rise to complex structures with a wide variety of shapes, shape evolution and shape coexistence. To that end, measurements of nuclear lifetimes and electromagnetic moments ($mu$,$Q$) can be invaluable. Recent experimental activities of the NuSTRAP group in Athens have focused on $gamma$-spectroscopy studies employing the RoSPHERE array in Magurele, Romania. In recent studies [1,2], the neutron-rich $^{144-146}$Ba isotopes have exhibited octupole degrees of freedom. Similar questions exist for the lighter $^{140}$Ba isotope, which is located at the onset of octupole collectivity. In addition, understanding the structure of heavier, even-even hafnium isotopes requires more data regarding shape coexistence and shape evolution. Preliminary results on lifetimes in this area pave the way to understand dynamical phenomena prior to studying more exotic species in the future.
We searched for the bound state of the neutron-rich $Lambda$-hypernucleus ${}^{6}_{Lambda}$H, using the ${}^{6}$Li($pi^{-}, K^{+}$)X double charge-exchange reaction at a $pi^{-}$ beam momentum of 1.2 GeV/c at J-PARC. A total of $1.4 times 10^{12}$ $pi^{-}$ was driven onto a ${}^{6}$Li target of 3.5-g/$rm cm^2$ thickness. No event was observed below the bound threshold, i.e., the mass of ${}^{4}_{Lambda}$H + 2n, in the missing-mass spectrum of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction in the $2^{circ}$ < $theta_{pi K}$ < $20^{circ}$ angular range. Furthermore, no event was found up to 2.8 MeV/$c^2$ above the bound threshold. We obtained the the double-differential cross section spectra of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction in the angular range of $2^{circ}$ < $theta_{pi K}$ < $14^{circ}$. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the ${}^{6}_{Lambda}$H hypernucleus bound state. In addition, not only the bound state region, but also the $Lambda$ continuum region and part of the $Sigma^{-}$ quasi-free production region of the ${}^{6}$Li($pi^{-}, K^{+}$)X reaction, were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the $Sigma^{-}$-nucleus optical potential for $Sigma^{-}$-${}^{5}$He through spectrum shape analysis.
$^{6}$He+$t$ cluster states of exited $^{9}$Li have been measured by 32.7 MeV/nucleon $^{9}$Li beams bombarding on $^{208}$Pb target. Two resonant states are clearly observed with the excitation energies at 9.8 MeV and 12.6 MeV and spin-parity of 3/2$^{-}$ and 7/2$^{-}$ respectively. These two states are considered to be members of K$^{pi}$=1/2$^{-}$ band. The spin-parity of them are identified by the method of angular correlation analysis and verified by the continuum discretized coupled channels (CDCC) calculation, which agrees with the prediction of the generator coordinate method (GCM). A monopole matrix element about 4 fm$^{2}$ for the 3/2$^{-}$ state is extracted from the distorted wave Born approximation (DWBA) calculation. These results strongly support the feature of clustering structure of two neutron-rich clusters in the neutron-rich nucleus $^{9}$Li for the first time.
The semimicroscopic particle-hole dispersive optical model (PHDOM) is implemented to describe main properties of Isoscalar Giant Multipole Resonances (up to L=3) in medium-heavy closed-shell nuclei. The main properties are characterized by the strength distribution, transition density, partial and total probabilities of direct one-nucleon decay. Calculation results obtained for the 208Pb nucleus are compared with available experimental data.
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