The details of a recent experimental study of the ${}^2\mathrm{H}(p\to ,\gamma {)}^3$He and the ${}^1\mathrm{H}(d\to ,\gamma {)}^3$He reactions for beam energies below 80 keV are discussed. In this study, both polarized and unpolarized incident beams were used to measure the cross section $\sigma (\theta ,E)$, the astrophysical $S$ factor $S(\theta ,E)$, the vector analyzing power $A_y(\theta ,E)$, and the tensor analyzing power $T_{20}\left(\theta \right)$. In addition, the $\gamma$-ray linear polarization $P_{\gamma }\left(\theta \right)$ was measured for an unpolarized incident beam. The experimental details of these measurements are discussed and the data are analyzed to obtain the amplitudes and phases of the contributing transition-matrix elements. The results of our measurements are compared with recent ab initio three-body theoretical calculations. This comparison reveals the large sensitivity of the polarization observables, especially the vector analyzing power ${(A}_y)$, to the presence of meson-exchange current effects and indicates the need for further study. The tensor analyzing power data $\left[T_{20}\right(\theta )$] are used to extract a value of the asymptotic $D$- to $S$-state ratio, $\eta$, for ${}^3$He.

• Received 4 June 1997

DOI:https://doi.org/10.1103/PhysRevC.56.2565