Ultra-low temperature thermometry using zeeman perturbed NQR

We report the results of measurements of the magnetic field perturbed nuclear quadrupole, resonance (NQR) lines of powdered Sb immersed in liquid He-3 at low temperatures. The magnetic field was used to separate the frequencies of the allowed transitions between the I-Z = +/- 1/2 and +/- 3/2 levels that correspond to the zero field NQR absorption at 11.5289 MHz for Sb-121. The spectrum for the powder sample consists of two sharp lines with known separation. The ratios of the intensities of these two lines depends only on the difference in thermal population of the nuclear spin levels and can be used for thermometry at very low temperatures. Pulsed NQR measurements of this intensify ratio from 0.25 to 1.45 mK were in good agreement with the expected Boltzmann distributions. Temperature resolutions of 20 % could be achieved for signal integration times of similar to 1 hour.