Thermal donor (TD) generation in silicon at 500 degrees C was found to depend significantly on the cooling rate used after sequential annealing steps and on the nature of the ambient (air or vacuum). By performing the anneals initially under some specified cooling rate and ambient, and then changing to a new set of conditions, it was found that the: TD concentration relaxed to the value corresponding to the new conditions. These results are well explained by a self-interstitial enhancement of TD generation rate. Self-interstitials are emitted by TD clusters, and their concentration, C-i, depends on the efficiency of sinks (sample surface, bulk voids). For vacuum annealing the major sink is the sample surface. For air anneals this sink is "passivated" presumably due to oxidation of the surface and/or by surface contamination, thus leaving only voids to act as self-interstitial sinks. Fast cooling seems to partly passivate voids (presumably by a decoration mechanism), further decreasing the sink efficiency, and therefore increasing Ci and the TD generation rate. The quantitative theory of sink-controlled TD generation provides a good description of the complicated experimental kinetic curves.