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 (He-4*up arrow) in a wide range of temperatures (0<T<0.5 K) and magnetic fields (0<B<100 kG), which includes all values of T and B relevant for trapped He-4*up arrow. At densities n less than or similar to 10(13) cm(-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 less than or similar to 10 mK and low magnetic fields B less than or similar to 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 He-4*up arrow.