Measurements of ¹H(d→,γ)³He and ²H(p→,γ)³He at very low energies

Angular distributions of the analyzing powers iT₁₁, T₂₀, and T₂₂ for ¹H(d→,γ)³He at 40<~E_c.m.<~110 keV and the angular distribution of the analyzing power A_y for ²H(p→,γ)³He at 70<~E_c.m.<~210 keV have been measured for the first time. In addition, absolute differential cross sections for proton-deuteron capture have been determined for E_c.m.= 75, 108, 133, and 173 keV. Thick ice or heavy ice targets and two large-volume, high-purity Ge γ-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(0) value for proton-deuteron capture is ∼25% lower than that presently used in astrophysical calculations. An expression for the thermonuclear reaction rate below 10 GK is given.