Polarization and relaxation of protons in irradiated lanthanum magnesium nitrate

Abstract

Measurements have been made of proton polarization, proton relaxation, and the shape, intensity and spin-lattice relaxation rate of an electron spin resonance line from damage centres, in a lanthanum magnesium nitrate target during a bombardment with 146 Mev protons. The damage centres exhibit an inhomogeneously broadened line centred on a g value of 2 002 ± 0.002, and their energy of formation is found to be 1 0 ± 0 4 ev. At 9 5 Gc/s spin-lattice relaxation rates of (7 ± 3) × 102 sec-1 and (5 ± 2) × 102 sec-1 were measured at temperatures of 4 2 °K and 1 4 °K respectively. On the assumption that these centres are the sole cause of the observed decay in polarization, it is deduced that about half of the decay may be attributed to relaxation of the protons. The remainder of the decay may be attributed to detuning of protons, providing each damage centre occludes an average of about 700 protons. From the relaxation data, an order-of-magnitude value for the damage-centre spin fluctuation rate of 105 sec-1 at 35 2 Gc/s is derived. This value is used in a comparison of the observed damage-centre contribution to proton relaxation with that predicted by different theoretical models. The hindered-diffusion model of Khutsishvili, using his definition of a diffusion barrier, fits the observations to within a factor of 2. His value for the diffusion barrier radius is found to be the same as the radius of occlusion derived from the decay in polarization, namely 16 A. The shell-of-influence model of Schmugge and Jeffries leads to a relaxation rate that is faster than the observed one by a factor of 50. Unhindered diffusion, as analysed by de Gennes, is too efficient by a factor of about 400 to fit the observations.