The thorium content of mine dust collected at various locations along the drift to the SNO site has been measured with several techniques- direct counting, proton induced x-ray emission, x-ray fluorescence, and neutron activation analysis. The thorium content varies significantly, from a minimum of 4 ppm to a maximum of 24 ppm (from direct counting measurements). The average Th content of the six samples obtained by direct counting and by XRF measurements is a factor of two higher than the value of 5 ppm (for norite) that we have used in cleanliness calculations. The larger average concentration of Th in dust that enters the cavity via equipment and personnel therefore suggests that cleanliness requirements be tightened by a factor of 2. The results for Fe indicate a positive, though not complete, correlation with the Th content. Measuring the Th content in dust via the Fe component thus provides some help in correcting for fluctuations in Th content. Additional measurements are suggested. I
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