Two-deuteron photodisintegration of4HeatEγ=150–250MeV

Abstract

We have performed a measurement of the cross section for the ${}^{4}\mathrm{He}(\ensuremath{\gamma},dd)$ reaction with photon energies between 150 and 250 MeV. Both reaction particles were detected in coincidence. We obtained absolute cross sections at six angles and two energy bins (150--190 MeV and 190--250 MeV). A Legendre polynomial fit to the angular distribution shows a ${\mathrm{sin}}^{2}\ensuremath{\theta}$ distribution which suggests the isospin-forbidden $E1$ transition. However, a transition matrix element (TME) analysis using only strictly allowed transitions shows that the angular distribution can be described by just two TMEs: $E2$ s- and d-wave absorption on the D state of ${}^{4}\mathrm{He}.$ We performed an inverse direct-capture calculation in order to determine if the D state could contribute to such a high degree to the cross section. This calculation indicates that 90% of the cross section arises from absorption on the D state but fails to reproduce the observed ${\mathrm{sin}}^{2}\ensuremath{\theta}$ angular distribution.