Neutron-hole distributions inSn111,115,119as observed in the (He3,α) reaction

Abstract

The energy spectra of residual $^{111,115,119}\mathrm{Sn}$ nuclei have been studied with 38 keV resolution, up to excitation energies of the order of 16 MeV, using the ($^{3}\mathrm{He},\ensuremath{\alpha}$) reaction at 39 MeV. Complete angular distributions have been obtained for the $^{116}\mathrm{Sn}(^{3}\mathrm{He},\ensuremath{\alpha})^{115}\mathrm{Sn}$ reaction; two angles were measured for the other isotopes. Several new levels or groups are observed up to \ensuremath{\sim} 3.5 MeV in the three isotopes, owing to the momentum matching conditions. Spectroscopic factors are determined for all observed levels up to 3.71 MeV in $^{115}\mathrm{Sn}$ and for the low-lying strongly populated levels with known ${J}^{\ensuremath{\pi}}$ in $^{111,119}\mathrm{Sn}$. The fragmentation of the $l=4,5$ valence hole strengths and the excitation of levels with anomalous spins and parities in the intermediate energy region in $^{115}\mathrm{Sn}$ are discussed in terms of weak-coupling and two-step coupled channels calculations. The fine structure previously reported for the $1{g}_{\frac{9}{2}}$ hole orbital in $^{115}\mathrm{Sn}$ is clearly confirmed and enhanced in the present experiment. The fragmentation of this orbital is observed for the first time in $^{111}\mathrm{Sn}$, between 3.4 and 5.2 MeV and in $^{119}\mathrm{Sn}$, though to a lesser extent, between 3.8 and 6.5 MeV. In the fine structure region the $1{g}_{\frac{9}{2}}$ hole strength is found to decrease from 62% to 43% and 28% in $^{111}\mathrm{Sn}$, $^{115}\mathrm{Sn}$, and $^{119}\mathrm{Sn}$, respectively; theoretical calculations do not explain these experimental strengths and fragmentations. The first ${9/2}^{+}$, ${1/2}^{\ensuremath{-}}$, ${3/2}^{\ensuremath{-}}$ isobaric analog states are identified in $^{111}\mathrm{Sn}$. Previously unreported, weakly populated isobaric analog states are also found in $^{115}\mathrm{Sn}$ and $^{119}\mathrm{Sn}$. The energies and widths are measured. The spectroscopic factors and isospin splitting of these states are discussed in the framework of the Lane coupled-channels formalism for the neutron form factor.NUCLEAR REACTIONS $^{112,116,120}\mathrm{Sn}(^{3}\mathrm{He},\ensuremath{\alpha})$ $E=39$ MeV; measured $\ensuremath{\sigma}({E}_{\ensuremath{\alpha}},\ensuremath{\theta})$ $^{111,115,119}\mathrm{Sn}$ deduced levels, ${E}_{x}$, $l$, ${(J)}^{\ensuremath{\pi}}$, ${C}^{2}S$; zero-range DWBA and two-step process analysis; inner shells and isobaric analog states analysis: enriched targets, magnetic spectrometer.NUCLEAR STRUCTURE unified model calculations.