Purification and low-temperature (15 mK-10 K) NMR of ultra pure ortho-deuterium

Pure solid ortho-deuterium, whose spin-lattice relaxation time in the liquid helium region is expected to be the order of days, is a promising candidate for nuclear-spin-polarized fusion fuels and targets for high-energy physics applications. For purification of ortho-deuterium by conversion of para-deuterium, the author presents here a new catalytic technique in which conversion takes place in the physisorbed phase on copper powder or on grafoil at temperatures between 12 and 8 K. This catalysis has yielded on ortho-D2 purity in which para-D2 concentration is reduced to 1.1 x 10-3, better by more than an order of magnitude than in any previous work. By subsequent aging of the purified samples at 4.2 K for about 9 months, o-D2 with residual p-D2 concentration of 5 ± 2 x 10-5 was attained, whose spin-lattice relaxation time at 1.2 K in a 0.3 T magnetic field is 11 hours. A theory of ortho-para conversion on clean metal surfaces due to interaction with conduction electrons is given. In the case of grafoil, the conversion rates in 2-D solid phases of physisorbed D2 by emission of one and two photons (of the D2 solid) are calculated. A temperature independent conversion rate is predicted and is consistent with the measurements in the 2-D solid regime. A sample of moderately pure o-D2, (p-D2 concentration of 0.004), chosen for acceptable heat of conversion as well as relatively short spin-lattice relaxation time even in the mK region, was cooled in the dilution refrigerator to 15 mK, a record low temperature for solid hydrogens. The special NMR probe and sample chamber designed and constructed for this purpose is described