THE RATIO OF CAPTURE TO FISSION IN $sup 235$U AND $sup 239$Pu.

Thin samples of uranium and plutonium have been irradiated in the pneumatic carrier of the NRU reactor, and their changes in isotopic composition measured by a mass spectrometer, to obtain information in radiative capture in {sup 235}U at thermal and epithermal neutron energies and on capture and absorption in {sup 239}Pu. The irradiation position, a vacant lattice site of the NRU reactor (natural uranium fuelling) was characterized by a well-moderated high-intensity thermal flux ({approx}2,5 x 10{sup 14}n/cm{sup 2} s) with an epithermal component having a relatively small departure from a 1/E shape and an intensity r of 0.018 (Westcott convention). Two experimental irradiations were carried out, in the first of which a sample of natural uranium was irradiated bare and a sample enriched in {sup 235}U was irradiated under cadmium; the integrated flux was 0.32 n/kb and the neutron temperature 60 deg. C. In the second experiment (0.71 n/kb, 47 deg. C) similar uranium samples were irradiated, and, in addition, samples of {sup 239}Pu 'spiked' with {sup 242}Pu were irradiated in the same capsule. The change in {sup 235}U content of the bare sample relative to the almost constant {sup 238}U gave the integrated flux and, when combined with the measured {sup 236}U production, the value of {alpha} for {sup 235}U. In the cadmium-covered sample, measurement of the {sup 236}U, when combined with subsidiary silver-activation measurements of the epithermal flux, gave a value of the resonance integral for radiative capture in {sup 235}U. The change in {sup 239}Pu and ({sup 240}Pu + {sup 241}Pu) contents relative to the comparatively constant {sup 242}Pu content gave values of {alpha} and absorption cross-section for {sup 239}Pu; corrections for epithermal effects were made by calculation, but they introduce only minor uncertainties. Corrections from the measured values for a Maxwellian neutron distribution to a neutron velocity of 2200 m/s involve uncertainties that are comparable with the experimental errors. (author)