RadiativenpCapture and Polarization Effects

For many years a 10% discrepancy has existed between theory and experiment for the total radiative thermal $\mathrm{np}$ capture cross section. This process is usually considered to be dominated by a ${}_{}{}^1{}_{}{}^{}S\to d$ transition. Breit and Rustgi have shown that a significant amount of ${}_{}{}^3{}_{}{}^{}S\to d$ transition would be detectable in experiments with polarized $n$ and $p$. We show that the ${}_{}{}^3{}_{}{}^{}S\to d$ transition can be described with a general transition current containing two parameters. On the assumption that the amplitude is large enough to explain the discrepancy, we give angular distributions for the $\gamma$-ray emission from polarized $n$ on polarized $p$. On the other hand, we calculate theoretical values for the parameters which indicate that the ${}_{}{}^3{}_{}{}^{}S\to d$ transition is probably not large enough to explain the discrepancy, and we discuss the correctness of the theory.