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Photofission of Heavy Nuclei (Berman, Feldman) One of the original goals of our photofission program was to determine the total photon absorption cross sections in heavy nuclei. The conventional view was that if a very heavy nucleus, such as 238U, would absorb a photon of several hundred MeV, then sooner or later it would fission. If it boiled off some neutrons first, so much the better – its fissionability Z2/A would increase. Thus, measurement of photofission was tantamount to measurement of photoabsorption. This turned out not to be so, as we have seen [Cet02]. Thus, in order to measure the total photon absorption of 238U, say, we must measure all the hadronic particles that are emitted. And if we can measure the total photon absorption cross section in this way, we get a huge bonus – the energy and angular distribution of all the hadronic reaction products. These data, in turn, would enable us to deduce far more about the intranuclear cascade and fission processes in heavy nuclei than we now know. A thin-walled fission chamber, surrounded by plastic scintillators, could be used for such studies at lower energies (up to 225 MeV) at HIGS. A series of experiments performed at several accelerator facilities by G.Ya. Kezerashvili and his group at Novosibirsk [Kaz84, Iva92] have shown that a very large fraction of photofission events, ranging from about 5% at 50 MeV to over 15% at 6 GeV, are accompanied by a light, usually charged, particle in the forward direction. This result, if verified, would be most remarkable, since QED calculations to date (see [Iva92]) have not been able to reproduce such large cross sections for these (g ,xF) processes. When the Novosibirsk data are compared with a QED calculation of the g A ® e+e- A* process, the discrepancy is a factor of 200; this inspires us to test these results. Our existing PPADs will suffice as the fast fission-fragment detectors, and the pair spectrometer at HIGS (the latter a legacy from SAL) will measure the forward positrons and electrons emitted in coincidence with the fission fragments. The collaboration will include, in addition to GW personnel, J.C. Sanabria (now Associate Professor of Physics at the University of the Andes in Bogota), J.D.T. Arruda Neto (São Paulo), and Yu.A. Pakhotin (Novosibirsk), and the experiment will be dedicated to the memory of our former colleague and good friend, Guram Kezerashvili. |
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