Gamow-Teller strength from the 20Ne(n,p)20F reaction at En=198 MeV.

Abstract

Cross sections for the $^{20}\mathrm{Ne}$(n,p${)}^{20}$F reaction at ${\mathit{E}}_{\mathit{n}}$=198 MeV were measured at five angles using the TRIUMF charge-exchange facility and a novel high-pressure (20 atm, or 2.03 MPa) gas target. The L=0, L=1, and L\ensuremath{\ge}2 contributions to the cross sections were obtained from a multipole decomposition. Using an empirical proportionality between the cross section and Gamow-Teller (GT) strength from \ensuremath{\beta} decay, GT strength was deduced up to an excitation of 10 MeV in $^{20}\mathrm{F}$. Most of the strength is concentrated in a discrete state at ${\mathit{E}}_{\mathit{x}}$=1.0 MeV, with the extracted strength of the \ensuremath{\Delta}L=0 continuum at high excitation having a large error. These results are compared with measurements of GT strength using other probes. Comparison with the measured magnetic dipole (M1) strength in $^{20}\mathrm{Ne}$ shows that there is constructive interference between the spin part and a large orbital part of the M1 transition to the 11.26 MeV state in $^{20}\mathrm{Ne}$ (analog of the 1.0 MeV state in $^{20}\mathrm{F}$). If all the strength from the multipole decomposition is used, we find agreement of the total GT strength compared to untruncated sd shell-model calculations which use free-nucleon operators, although experimental and theoretical distributions differ in detail. The confirmation of strong orbital M1 contributions at the beginning of the sd shell together with systematics of GT quenching in heavier sd shell nuclei may provide evidence for meson exchange currents in M1 transitions.