Abstract
A small, 14 inch diameter by 12 inch in length, fast neutron fission counter probe employing highly depleted uranium as the fission material has been developed. The isotopic analysis of the uranium, obtained by the 3-stage mass spectrometer at this Laboratory, is as follows: U238 (essentially 100%), U236 (0.03 ± 0.02 ppm), U235 (0.90 ± 0.20 ppm), and U234 (0.04±0.02 ppm). The uranium was plated out of solution to a yield of approximately 1.72 mg/cm2. Detailed measurements of the fast neutron flux in the Flexible Plastic Reactor (105–106 ncm−2 at KAPL, obtained with the U238 fission counter, have shown it to be insensitive to both thermal neutrons and high energy (>5.3 MeV) gamma rays. The photofission of the U3O8 was checked in the critical assembly by making a fast neutron scan with red phosphorus (12″ × 110″) pellets). Since the (n,p) threshold reaction in phosphorus (≈ 1 MeV) is approximately the same as the (n,f) reaction in U238, the two scans should yield, as is found to be the case, identical results provided there is no appreciable influence of photofission in the U3O8.
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