Measurement of laser heating in spin exchange optical pumping by NMR diffusion sensitization gradients

Abstract

This paper details pulsed gradient NMR measurements of the H3e diffusion coefficient in sealed cells during spin exchange optical pumping. The potential of ultra low field magnetic resonance imgaing (MRI) and NMR for noninvasive measurement of cell pressure is demonstrated. Diffusion sensitization gradients allow measurement of the H3e diffusion coefficient from which the pressure and/or temperature of the gas can be determined during optical pumping. The pressure measurements were compared with neutron time of flight transmission measurements. Good agreement was observed between the temperature/pressure measurements and predictions based on Chapman–Enskog theory. The technique had sufficient sensitivity to observe the diffusion coefficient increasing with temperature in a sealed cell. With this method, evidence for laser heating of the H3e during optical pumping was found. The results show that NMR diffusion measurements allow noninvasive measurement of the cell temperature and/or pressure in an optical pumping setup. The method can be expanded using MRI to probe the spatial distribution of the diffusion coefficient. These techniques can be applied to the further investigation of polarization limiting effects such as laser heating.