Pauli blocking of collisions in a quantum degenerate atomic Fermi gas

We have produced an interacting quantum degenerate Fermi gas of atoms compo sed of two spin states of magnetically trapped K-40. The relative Fermi ene rgies are adjusted by controlling the population in each spin state. Thermo dynamic measurements reveal a resulting imbalance in the mean energy per pa rticle between the two species, which is a factor of 1.4 at our lowest temp erature. This imbalance of energy comes from a suppression of collisions be tween atoms in the gas due to the Pauli exclusion principle. Through measur ements of the thermal relaxation rate we have directly observed this Pauli blocking as a factor of 2 reduction in the effective collision cross sectio n in the quantum degenerate regime.