  We report the measurement of collision rate coefficient for collisions
between ultracold Cs atoms and low energy Cs+ ions. The experiments are
performed in a hybrid trap consisting of a magneto-optical trap (MOT) for Cs
atoms and a Paul trap for Cs+ ions. The ion-atom collisions impart kinetic
energy to the ultracold Cs atoms resulting in their escape from the shallow MOT
and, therefore, in a reduction in the number of Cs atoms in the MOT. By
monitoring, using fluorescence measurements, the Cs atom number and the MOT
loading dynamics and then fitting the data to a rate equation model, the
ion-atom collision rate is derived. The Cs-Cs+ collision rate coefficient
$9.3(\pm0.4)(\pm1.2)(\pm3.5) \times 10^{-14}$ m$^{3}$s$^{-1}$, measured for an
ion distribution with most probable collision energy of 95 meV ($\approx
k_{B}.1100$ K), is in fair agreement with theoretical calculations. As an
intermediate step, we also determine the photoionization cross section of Cs
$6P_{3/2}$ atoms at 473 nm wavelength to be $2.28 (\pm 0.33) \times 10^{-21}$
m$^{2}$.
