Analysis sensitivities achievable with the thermal ionization, mass spectrometric technique for trace quantities of uranium and plutonium are improved by replacement of the commonly employed direct evaporation, single-filament source with a diffusion controlled source. The sample on the single filament surface is coated with a high work function, refractory metal using a R.F. sputtering technique. The arrival of sample atoms at the ionizing surface is controlled by diffusion through the applied coaling. Nickel, tungsten and rhenium coatings were evaluated. Only the rhenium coats were found to be thermally stable, adherent, and visibly defect free. Coated and uncoated uranium and plutonium samples were mass spectrornetrically analyzed. The coated samples were at least four times more efficient in sample utilization than the uncoated samples. Analytical sensitivities of at least 1 X grams plutonium on the filament were observed with comparable sensitivities estimated by extrapolation for uranium.
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