Potentialities and practical limitations of absolute neutron dosimetry using thin films of uranium and thorium applied to the fission track dating

Neutron dosimetry using natural uranium and thorium thin films makes possib le that mineral dating by the fission-track method can be accomplished, eve n when poor thermalized neutron facilities are employed. In this case, the contributions of the fissions of U-235, U-238 and Th-232 induced by thermal , epithermal and fast neutrons to the population of tracks produced during irradiation are quantified through the combined use of natural uranium and thorium films. If the Th/U ratio of the sample is known, only one irradiation (where the s ample and the films of uranium and thorium are present) is necessary to per form the dating. However, if that ratio is unknown, it can be determined th rough another irradiation where the mineral to be dated and both films are placed inside a cadmium box. Problems related with film manufacturing and calibration are discussed. Spe cial attention is given to the utilization of thin films having very low ur anium content. The problems faced suggest that it may be better to substitu te these films by uranium doped standard glasses calibrated with "thicker" uranium films (thickness greater than 1.5 x 10(13) mu m).