Radiation Moamiromats PERGAMON Radiation Measurements31 (1999) 651-656 POTENTIALITIES AND PRACTICAL LIMITATIONS OF ABSOLUTE NEUTRON DOSIMETRY USING THIN FILMS OF URANIUM AND THORIUM APPLIED TO THE FISSION TRACK DATING G. BIGAZZI*, S. GUEDES **, J.C. HADLER N_.**, P.J. IUNES**, S.R. PAULO***, M. ODDONE****, A.M. OSORIO A.***** AND A. Z01qlGA G.**' ****** * Istituto de Geocronologia e Geochimica Isotopica, CNR, 56100, Pisa, Italy ** Instituto de Fisica, UNICAMP, 13083-970 Campinas, SP, Brazil *** Departamento de Fisica, ICET, UFMT, 78060-900 Cuiab~t, MT, Brazil **** Dipartimento di Chimica Generale, Universit~ di Pavia, Pavia, Italy ***** Departamento de Ci~cias Ambientais, UNESP, 19060-900 Presidente Prudente, SP, Brazil ****** lnstituto Peruano de Energia Nuclear, IPEN, Av. Canada 1470, Lima 41, Peru ABSTRACT Neutron dosimetry using natural uranium and thorium thin fdms makes possible that mineral dating by the fission-track method can be accomplished, even when poor thermalized neutron facilities are employed. In this case, the contributions of the fissions of 235U, 23SU and 232Th 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 sample and the films of uranium and thorium are present) is necessary to perform the dating. However, if that ratio is unknown, it can be determined through 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. Special attention is given to the utilization of thin fdrns having very low uranium content. The problems faced suggest that it may be better to substitute these films by uranium doped standard glasses calibrated with "thicker" uranium films (thickness greater than 1.5 x 10 3 lain). KEYWORDS Neutron dosimetry; uranium thin films; thorium thin films; fission track dating. INTRODUCTION One of the most important problems that fission track dating (FTD) has faced rests on the difficulty to carry out the neutron fluence measurements utilizing the usual metallic dosimeters if low neutron thermalization facilities are employed (for instance, see Green and Hurford, 1984; Crowley, 1986; Tagami and Nishimura, 1989). This is due to the fact that these dosimeters do not respond to the contributions of 238U and 23~Fh fission induced by fast neutrons and, mainly, respond in a different way to the 235U fission induced by epithermal neutrons. The recommendation that FTD only should be performed ff facilities presenting high degree of neutron thermalization are employed constitutes an opportune way to overcome these difficulties. This was suggested by a special subcommission of the 1350-4487/99/$ - see front matter © 1999 ElsevierScience Ltd. All rights reserved. PII: S1350-4487(99)00161-4