  We studied light assisted collisions of Tm atoms in a magneto optical trap
(MOT) for the first time, working on a weak cooling transition at 530.7 nm
$(4f^{13}(^2F^0)6s^2,J=7/2,F=4$ to $4f^{12}(^3H_6)5d_{5/2}6s^2,J=9/2,F=5)$. We
observed a strong influence from radiation trapping and light assisted
collisions on the dynamics of this trap. We carefully separated these two
contributions and measured the binary loss rate constant at different laser
powers and detuning frequencies near the cooling transition. Analyzing losses
from the MOT, we found the light assisted inelastic binary loss rate constant
to reach values of up to $\beta=10^{-9}$ cm$^3$/s and gave the upper bound on a
branching ratio $k<0.8\times 10^{-6}$ for the 530.7 nm transition.
