Nuclear Alignment at Low Temperatures

The population distribution of nuclei aligned at low temperatures is derived by means of an expansion of the density matrix in powers of $\frac{A}{\mathrm{kT}}$ with an arbitrary external field. It is shown how this is related to the angular distribution of emitted $\gamma$ radiation. The influence of interionic exchange and dipole coupling is examined to order ${\left(\frac{A}{\mathrm{kT}}\right)}^3$ and is found to be generally small and cannot account for some experimental results. The case of finite crystal field interaction is considered as it applies to the angular distribution.