The<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">H</mml:mi><mml:mo>(</mml:mo><mml:mi>p</mml:mi><mml:mo>→</mml:mo><mml:mo>,</mml:mo><mml:mi>γ</mml:mi><mml:mrow><mml:msup><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>He and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow/><mml:mrow><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow><mml:mi mathvariant="normal">H</mml:mi><mml:mo>(</mml:mo><mml:mi>d</mml:mi><mml:mo>→</mml:mo><mml:mo>,</mml:mo><mml:mi>γ</mml:mi><mml:mrow><mml:msup><mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>He reactions below 80 keV

The details of a recent experimental study of the ${}^{2}\mathrm{H}(p\ensuremath{\rightarrow},\ensuremath{\gamma}{)}^{3}$He and the ${}^{1}\mathrm{H}(d\ensuremath{\rightarrow},\ensuremath{\gamma}{)}^{3}$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 $\ensuremath{\sigma}(\ensuremath{\theta},E)$, the astrophysical $S$ factor $S(\ensuremath{\theta},E)$, the vector analyzing power ${A}_{y}(\ensuremath{\theta},E)$, and the tensor analyzing power ${T}_{20}(\ensuremath{\theta})$. In addition, the $\ensuremath{\gamma}$-ray linear polarization ${P}_{\ensuremath{\gamma}}(\ensuremath{\theta})$ 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 $[{T}_{20}(\ensuremath{\theta})$] are used to extract a value of the asymptotic $D$- to $S$-state ratio, $\ensuremath{\eta}$, for ${}^{3}$He.

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