Static Spin Temperature Experiments and the Approach to Thermal Equilibrium in the Rotating Reference Frame

Nuclear magnetic resonance experiments were performed which further verify Redfield's assumption that, for large rf fields $H_1\left(\nu \right)$, a nuclear spin system is properly described by a spin temperature in a reference frame rotating about the Zeeman field with frequency $\nu$. As with similar experiments recently performed by Slichter and Holton, the interpretation of our results does not require any assumption about the spin-lattice relaxation mechanism. The measurements were made on the ${\mathrm{Na}}^{23}$ spins in NaCl and the ${\mathrm{F}}^{19}$ spins in Ca${\mathrm{F}}_2$. In both materials the agreement between experiment and theory was very good. From the sodium chloride results it is concluded that quadrupole interactions, caused by strains and imperfections, produce an unobservably small increase in the local field $H_l$, in the crystals used.