Ortho-to-para conversion in solid tritium. II. Experimental values

Nuclear-magnetic-resonance signal heights at a Larmor frequency of 30 MHz are used to monitor the ortho-${\mathrm{T}}_2$ concentration as a function of time at fixed temperature in solid ${\mathrm{T}}_2$. The ortho-${\mathrm{T}}_2$ concentration decreases with time due to ortho-to-para conversion in the solid. From our measurements, we find that the difference between the instantaneous signal height and the equilibrium signal height decays exponentially with time. The time constant of this decay has a minimum near 11.4 K indicative of a peak in the conversion rate near 11 K. The equilibrium signal height is observed to be larger than expected from the Boltzmann distribution. The conversion-rate data are interpreted in terms of a model where the conversion is due to electron spins (on atoms) capable of translational diffusion. The peak in the conversion rate is used to determine the characteristic frequency of the problem and the temperature dependence of the correlation time of the electron spins. The enhanced equilibrium signal is interpreted as arising from a preferential pumping of the J=1 ortho-${\mathrm{T}}_2$ level due to atom recombination. A model-dependent recombination coefficient for the atoms is extracted from the data, predicting an equilibrium number of atoms that is strongly temperature dependent.