Use of the Method of Recording Fission Products in Insulating Material to Study Uranium and Thorium Distribution in Rocks and Mineralizations

The distribution of radioactivity in rocks and mineralizations can be studied by observing the path of gamma rays in the emulsion of nuclear photographic plates. However, there are several drawbacks to this method, e. g. the long exposure time for rocks of low radioactivity, the difficulty of distinguishing between uranium and thorium, and the influence of the radioactive equilibrium state of the sample on the results of the measurement. The method adopted in this study, however, which is based on recording U and Th fission fragments, can be used directly for distinguishing the distribution of U from that of Th, since their fissioning is brought about by neutrons of very different energy. Moreover, the result is independent of the radioactive equilibrium state of the sample and the time required for the measurement is always fairly short. We have compared the characteristics of a number of fission fragment detectors (nuclear plates, Makrofol, Mylar, ordinary glass and muscovite mica) and have found that the use of nuclear plates gives rise to serious complications, since the gamma radiation present in the nuclear reactor,' which is used for inducing the fission of U and Th, produces a general blackening of the plate which makes it impossible to distinguish the fission fragment tracks clearly. On the other hand, the other detectors studied, all of which are insulating materials, are capable of recording only particles of higher mass and are completely insensitive even to very large doses of gamma radiation. The tracks recorded by these detectors can be made visible in the optical microscope by suitable chemical treatment. Among the products we have examined, Makrofol would appear to be the most suitable insulating material for our purposes, and we used it for studying uranium distribution in thin sections and in polished sections of rock. This method, which is especially simple and fast, can also be used for analysis of U and Th content in rock samples and fractions of monominerals. In view of the small amount of material required for analysis and the fact that the sample need not be destroyed, as well as the speed with which the technique can be carried out, the method is very attractive. The counting of the tracks under the microscope, which is the longest operation, can be automated if the method is used for routine measurements. (author)