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Photofission of Heavy Nuclei (Cetina, Heimberg, Feldman, Berman) The g5 experiment ran for about four days in April, 1998. This was the commissioning experiment for the Photon Tagger, and we obtained data over its full energy range for three different incident electron energies – 0.9, 1.7, and 4.0 GeV, thus yielding photofission data from 0.2 to 3.9 GeV. The tagged photons were incident on thin fission-foil targets positioned, each with its own parallel-plate avalanche detector for the fission fragments, in a common reaction chamber filled with isobutane at a pressure of 15 torr. In spite of the adverse running conditions at this early date and the short running time made available to us, we were able to obtain data of high quality. These data were the first physics results from Hall B to be published [Cet00]. We found that the photofission cross section for 237Np is substantially larger than that for 238U, showing that the fission probability of the latter could not be 100%, as had been commonly believed prior to this experiment. However, the near-equality of the photofission cross section per nucleon for 237Np and the photon absorption cross section per nucleon for lead, together with the asymptotic behavior of the photofission cross sections as a function of fissionability Z2/A, leads one to the conclusion that the fission probability for 237Np is indeed nearly 100%. This in turn enables us to determine the degree of shadowing of photons by heavy nuclei in the GeV energy region; the onset of the shadowing effect is seen to occur below 1 GeV, and its magnitude is approximately 20% by 4 GeV. In order to understand these data better, we have teamed up with Dr. I. Pshenichnov (INR, Moscow) to calculate photofission probabilities and total cross sections with a cascade-evaporation-fission model of intermediate-energy photonuclear reactions. It includes multiparticle production in photoreactions on intranuclear nucleons, pre-equilibrium emission, and the statistical decay of excited residual nuclei via competition of evaporation, fission, and multifragmentation processes. This calculation [Psh03] predicts that even for the heaviest actinides the fission process is not solely responsible for the entire photoabsorption cross section, because certain residual nuclei that are created by deep photospallation at GeV photon energies have relatively low fission probabilities. Thus, the total photoabsorption cross section is at least 10% larger than the measured photofission cross sections, which calls into question the concept of a "Universal Curve" for the photoabsorption cross section per nucleon for all nuclei.
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