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Very high energy gamma rays have been shown to induce fission in elements as light as tin. Gamma radiation of more modest energies, in the low tens of MeV, can induce fission in traditionally fissile elements such as the actinides uranium, plutonium, and neptunium. Experiments have been conducted with much higher energy gamma rays, finding that the photofission cross section varies little within ranges in the low GeV range. Measurements have been made of the yields of photo-fission in uranium and thorium together with a search for photo-fission in other heavy elements, using continuous x-rays from a 100-Mev betatron. Fission was detected in the presence of an intense background of x-rays by a differential ionization chamber and linear amplifier, the substance investigated being coated on an electrode of one chamber. A Victoreen r-thimble, surrounded by 18-inch lead walls, was used to monitor the radiation. Curves were obtained of the number of fissions per roentgen unit for uranium and thorium. These are of similar shape, the uranium curve showing a rapid rise with increasing x-ray energy up to 18 Mev, followed by a gradual decrease as the maximum energy of the x-rays is further increased; the yield of fissions per roentgen at 100 Mev is about half that at 18 Mev. The ratio of uranium and thorium yields is very nearly two at all x-ray energies. No fissions were observed in intense 100-Mev irradiations of Bi, Pb, Tl Au, W, and Sm. Determination of cross sections from the yield curves is complicated by the continuous spectrum of the x-rays which has not been measured experimentally. A rough analysis of the data has been made in which a spectrum is assumed for which the intensity is constant in each unit energy interval and the r-meter efficiency calculated roughly from a simplified picture of the generation of secondaries in the lead walls. The resulting analysis of the yield curves shows that the cross section for photo-fission as a function of quantum energy passes through a maximum and then decreases and is extremely small above 30 Mev. The maximum cross section is of the order of 5×10−26 cm2 for uranium and half that for thorium. In the other elements studied, the cross section must be below 10−29 cm2.
Photodisintegration (also called phototransmutation) is a similar but different physical process, in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle.
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