Inelastic decay processes in a gas of spin-polarized triplet helium

We calculate the rates of elastic and inelastic collisions in a gas of spin-polarized triplet helium ${(}^4$ ${\mathrm{He}}^{\mathrm{*}}$↑) in a wide range of temperatures (0TBT and B relevant for trapped ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{\mathrm{*}}{}_{}{}^{}$↑. At densities n≲${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$, where three-body recombination is not important, the main inelastic processes are spin relaxation and relaxation-induced ionization, both induced by the spin-dipole interaction in pair collisions. For ultralow temperatures T≲10 mK and low magnetic fields B≲100 G, the leading decay channel is relaxation-induced ionization. If either T or B is higher, the gas decays through spin relaxation. The ratio of elastic to inelastic rate is found to be large even in the nanokelvin regime, which is promising for evaporative cooling of ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{\mathrm{*}}{}_{}{}^{}$↑. © 1996 The American Physical Society.