Ortho-to-para conversion in solid tritium. I. Theoretical models

Reliable theoretical models developed to calculate the intrinsic ortho-to-para conversion rate in orientationally ordered solid ${\mathrm{H}}_2$ are used to estimate the conversion rate in solid ${\mathrm{T}}_2$. Since the Debye energy is larger than the rotational energy difference between the J=1 level and the ground state J=0, both the single-phonon transition probability and the transition probability due to the Raman effect must be taken into account. These additional intrinsic processes are treated here, but are found to be too weak to account for the experimentally observed rates. The single-phonon conversion rate due to unpaired electron spins in the solid is obtained for both localized and mobile spins. The conversion rate due to mobile unpaired electron spins is large enough to account for the experimental data if the atom density is of order ${10}^{18}$ atoms per ${\mathrm{cm}}^3$.