High proton polarization in 1,2-propanediol at $^{3}He$ temperatures

A study is made of the dynamic proton polarization in 1,2-propanediol doped with paramagnetic Cr/sup V/ complexes. A method is described by which samples with a high concentration of Cr/sup V/ complexes can be prepared. Samples of 1,2-propanediol prepared in this way yielded a maximum proton polarization of 92% at a temperature of 0.48 K in a 2.5 T magnetic field. The polarization build-up time is found to be 2 to 3 min in a 7 cm/sup 3/ sample, with about 25 mW microwave power in the cavity. The microwave power needed to maintain the polarization in such a sample is about 4 mW. (28 refs).