The photofission yield-mass curve of natural uranium for 48-Mev x-rays was determined radiochemically. Isolation of selected peak and trough fission products at various maximum x-ray energies, 7 Mev, 10 Mev, 16 Mev, 21 Mev, 48 Mev, 100 Mev, and 300 Mev showed that the peak-to-trough ratio decreased from 300 to 4 as the maximum x-ray energy increased from 7 Mev to 300 Mev. The observed photofission yield curves may be quantitatively interpreted as the superposition of two components, a low-energy asymmetric (doublehumped) curve and a high-energy single-humped curve. The average cross section for symmetric fission for photons in the energy range 16 Mev to 300 Mev is about 7 millibarns.
I. INTRODUCTIONM ANY investigations of high-energy fission, induced by particles and x-rays, have been reported. 1-12 One striking feature of these studies is the decrease in the peak-to-trough yield ratio of the yieldmass curve as the energy of the bombarding particle increases, resulting in the one hump yield-mass curve as the energy of the particle enters the hundred-Mev range. A systematic investigation of the photofission of a heavy nuclide at various energies has not been reported.The purpose of this work was essentially threefold; (1) to determine the yield-mass curve of natural uranium for 48-Mev maximum x-rays by the comparison method; 4,13,14 (2) to study the changes in the photoyield curve of uranium as a function of increasing maximum x-ray energy from 7 Mev to 300 Mev; and (3) to study the change in the neutron-to-proton ratio of primary fission fragments with increasing x-ray energy. On the assumption that the observed fission-yield curves are composites of two idealized curves, 14 one *
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