Gamma Rays from Neutron Capture in Helium-3 and Deuteron Capture in Deuterium

The gamma rays from the ${\mathrm{He}}^3(n, \gamma ){\mathrm{He}}^4$ and the $\mathrm{D}(d, \gamma ){\mathrm{He}}^4$ reaction have been observed. The gamma detector, a 3-in.×4-in. NaI crystal, was surrounded by a plastic scintillator to eliminate the cosmic-ray background. A pileup rejection circuit was used to reduce the background from neutron induced reactions in the NaI. The cross section of the ${\mathrm{He}}^3(n, \gamma ){\mathrm{He}}^4$ reaction at 4 MeV is $\sigma =5\mathrm{}$ μb/sr at 90°. The intensity ratio, $\frac{I(90\mathrm{}°}{I\left(45\mathrm{}°\right)}\approx 2$, agrees with that expected for electric dipole radiation ($\Delta M=0\mathrm{}$). The $\mathrm{D}(d, \gamma ){\mathrm{He}}^4$ reaction at 1.35 MeV has a cross section $\sigma =2\mathrm{}\times {10}^{-33\mathrm{}}$ ${\mathrm{cm}}^2$/sr at 45°.