Thermalization of magnetically trapped metastable helium

We have observed thermalization by elastic collisions of magnetically trapped metastable helium atoms. Our method directly samples the reconstruction of a thermal energy distribution after the application of a RF knife. The relaxation time of our sample toward equilibrium gives an elastic collision rate constant of $\alpha \sim 5\times {10}^{-9}\hspace{0.5em}{\mathrm{cm}}^3/\mathrm{s}$ at a temperature of 1 mK. This value is close to the unitarity limit.