sphere or might have some other aspherical concave shape however a truncated conical shape is relatively easy to fabricate and has been found to provide a considerable reduction in the reversion of particulate contamination from exhaust line 25 the exact nature of the gaseous flow which results in this reduction in particulate contamination has not been fully analyzed however it is believed that two mechanisms may be involved 1 a decoupling of the gas within exhaust line 25 from the pattern of swirling gases which occurs in bell jar 7 during back filling reducing the generation of eddy currents in exhaust line 25 and 2 a tendency to trap a large percentage of any mobile particles in exhaust line 25 especially in the region between the lower surface of baffle 33 and the adjacent portions of sleeve 29 the second of these two mechanisms suggests that the convex shape of baffle 33 as viewed from the side facing into exhaust line 25 also contributes to the performance in reducing particle contamination such a convex shape produces between baffle 33 and the adjacent inner surface of sleeve 29 an annular region having a progressively converging shape which is well suited to trapping a majority of particles moving in.