  We report a comparative investigation of trap loss rates in a magneto-optical
trap for two potassium isotopes, 39K and 40K, as a function of trap light
intensity. The isotopes present a quite similar behavior for the loss rates at
high intensities, and a sudden increase of the loss rates at low intensities is
present in both cases. While for 39K such increase can be explained assuming
that the major contribution to the losses comes from Hyperfine Changing
Collisions, a different loss mechanism must be considered for 40K, which has an
inverted ground state hyperfine structure. The experimental results of both
isotopes are well reproduced by an alternative model based on radiative escape
as the dominant loss mechanism.
