EMISSION OF GAMMA RADIATION AS A FUNCTION OF TIME AFTER PHOTOFISSION OF U238

The intensity of delayed gamma rays from the photofission of U-238 was measured from 30 microsec to 8 sec after fission. The delayed gamma-ray intensity falls by a factor of about 100 between 30 and 800 microsec after fission, then remains almost constant up to the vicinity of 0.5 sec, beyond which time it decreases at a rate consistent with results obtained by Maienschein (Proc. Second Geneva Conf. P/670, 1958). The present data were normalized to the absolute data of Maienschein in the time region from 1 to 10 sec after fission. Since the apparent half-life of the decay at early times is about 80 microsec, the gamma rays associated with this period probably arise from the decay of one or more fission products formed in isomeric states. Measurements repeated with a bias energy of 510 kev showed the same initial decay characteristic, indicating that some of the gamma rays contributing to the short-lived component have energies greater than 500 kev. In an attempt to evaluate neutron backgrounds using a mercury sample, gamma rays with energies less than about 500 kev and an apparent half-life of 102 microsec were observed. These gamma rays probably resulted from the decay of a mercury reaction product not previously reported. (Author)