  We present a theoretical and experimental investigation of homonuclear
ionizing collisions of laser-cooled metastable helium atoms, considering both
the fermionic He-3 and bosonic He-4 isotope. The theoretical description
combines quantum threshold behavior, Wigner's spin-conservation rule and
quantum statistical symmetry requirements in a single-channel model,
complementing a more complete close-coupling theory that has been reported for
collisions of metastable He-4 atoms. The model is supported with measurements
(in the absence of light fields) of ionization rates in magneto-optically
trapped samples, that contain about 3*10^8 atoms of a single isotope. The
ionization rates are determined from measurements of trap loss due to
light-assisted collisions combined with comparative measurements of the ion
production rate in the absence and presence of trapping light. Theory and
experiment show good agreement.
