  We have demonstrated that the ion current resulting from collisions between
metastable krypton atoms in a magneto-optical trap can be used to precisely
measure the trap loading rate. We measured both the ion current of the abundant
isotope Kr-83 (isotopic abundance = 11%) and the single-atom counting rate of
the rare isotope Kr-85 (isotopic abundance ~ 1x10^-11), and found the two
quantities to be proportional at a precision level of 0.9%. This work results
in a significant improvement in using the magneto-optical trap as an analytical
tool for noble-gas isotope ratio measurements, and will benefit both atomic
physics studies and applications in the earth sciences.
