Comment on "Enhanced polarization and mechanisms in optically pumped hyperpolarized 3He in the presence of 4He"

Chen et al. [Phys. Rev. A 81, 033422 (2010)] claim that adding 4He to spin-exchange optical pumping (SEOP) cells confines 3He atoms to a diffusion-limited region, which effectively reduces the wall relaxation factor X. Here X is a phenomenological parameter used to characterize typical SEOP cells; it quantifies the observation that the measured linear slope of the relaxation rate with alkali-metal-atom density exceeds the spin-exchange rate. In addition they claim to demonstrate that magnetic-field gradients cause a diffusive motion of 3He. We argue that these claims and other central conclusions drawn by Chen et al. [ Phys. Rev. A 81, 033422 (2010)] are inconsistent with the physics of diffusion or should not be made from the data as presented. We also draw attention to several additional issues associated with the results, analysis, and presentation of data in Chen et al. [ Phys. Rev. A 81, 033422 (2010)] that confound clear interpretation.