Measurements of ${}^1$H$(\mathit{d}\to ,\mathit{\gamma }{)}^3$He and ${}^2$H$(\mathit{p}\to ,\mathit{\gamma }{)}^3$He at very low energies

Angular distributions of the analyzing powers ${\mathrm{iT}}_{11},$ $T_{20},$ and $T_{22}$ for ${}^1\mathrm{H}(d\to ,\gamma {)}^3\mathrm{He}$ at $40<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~110$ keV and the angular distribution of the analyzing power $A_y$ for ${}^2\mathrm{H}(p\to ,\gamma {)}^3\mathrm{He}$ at $70<~E_{\mathrm{c}.\mathrm{m}.\mathrm{}}<~210$ keV have been measured for the first time. In addition, absolute differential cross sections for proton-deuteron capture have been determined for $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=$ 75, 108, 133, and 173 keV. Thick ice or heavy ice targets and two large-volume, high-purity Ge $\gamma$-ray detectors were used. Results are in general agreement with an exact three-body calculation utilizing a realistic nucleon-nucleon potential. The vector-polarization observables are found to be especially sensitive to meson-exchange-current effects. The extracted $S\left(0\right)\mathrm{}$ value for proton-deuteron capture is $\sim 25\%$ lower than that presently used in astrophysical calculations. An expression for the thermonuclear reaction rate below 10 GK is given.