High resolution study of isovector negative parity states in theO16(He3,t)F16reaction at 140 MeV/nucleon

Abstract

The isovector transitions from the ground state (g.s.) of $^{16}\mathrm{O}$ to the negative parity states in $^{16}\mathrm{F}$, i.e., the ${J}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ g.s., the 0.193 MeV, ${1}^{\ensuremath{-}}$ state, the 0.424 MeV, ${2}^{\ensuremath{-}}$ state, the 0.721 MeV, ${3}^{\ensuremath{-}}$ state, and the ${4}^{\ensuremath{-}}$ ``stretched'' state at 6.372 MeV, were studied by using a high resolution $^{16}\mathrm{O}(^{3}\mathrm{He},t)^{16}\mathrm{F}$ reaction at 140 MeV/nucleon. With the help of high energy resolution, these states were, for the first time, clearly resolved in a charge exchange reaction at an intermediate energy, which favorably excites spin-flip states. Angular distributions of the reaction cross sections were measured in the laboratory frame from ${0}^{\ifmmode^\circ\else\textdegree\fi{}}$ to ${14}^{\ifmmode^\circ\else\textdegree\fi{}}$. Parameters of phenomenological effective interactions were derived so as to reproduce these angular distributions in distorted wave Born approximation (DWBA) calculations. The angular distribution of the ${0}^{\ensuremath{-}}$ state could be reproduced well at ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}l{10}^{\ifmmode^\circ\else\textdegree\fi{}}$. The empirical values, however, are larger by a factor of 2--2.5 in the larger angle region, where the contribution of the so-called ``condensed pion field'' is expected. The high resolution also enabled the decay widths of these states to be measured.