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{He}*↑) in a wide range of temperatures (0T0.5 K) and magnetic fields (0B100 kG), which includes all values of T and B relevant for trapped⁴He*↑. At densities n{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{}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 ⁴He*{↑}. {} 1996 The American Physical Society.