We measure inelastic collisional cross sections for collisions between thorium (Th) and helium. We determine for Th 3F2-He the ratio of the momentum-transfer to Zeeman relaxation cross sections for collisions to be g~500 at 800 mK. For Th 3P0-He collisions, we observe no measureable quenching of this metastable state, even after 106 collisions. This allowed determination of the radiative lifetime of Th 3P0 to be t > 130 ms. This observed stability of the metastable 3P0 state is the first kind of measurement of this type for the actinides and fills out a growing body of evidence (from other Sr and Yb experiments) that such states are very stable. In particular, this opens up the possibility of trapping Th 3P0.

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