Journal of Radioanalytical and Nuclear Chemistry, Vol. 253, No. 1 (2002) 73–76 Determination of the 238 U spontaneous fission decay constant without neutron irradiation S. Guedes, 1 * J. C. Hadler, 1 * P. J. Iunes, 1 A. K. Burke, 1 M. H. Kakazu, 2 J. E. S. Sarkis, 2 S. R. Paulo, 3 C. A. Tello 4 1 Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, UNICAMP, 13083-970 Campinas, SP, Brazil 2 Divisão de Caracterização Química, Instituto de Pesquisas Energéticas e Nucleares, IPEN/CNEN, 05508-900 São Paulo, SP, Bra 3 Departamento de Física, Instituto de Ciências Exatas e da Terra, Universidade Federal de Mato Grosso, UFMT, 78060-900, Cuiaba, MT, Brazil 4 Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, UNESP, 13506- 900, Rio Claro, SP, Brazil (Received April 2, 2001) We have developed a methodology for measuring the decay constant of the spontaneous fission of 238 U, λ f , using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by sponta of trans-uranium elements bearing a mass number close to 238 are similar to 238 U spontaneous-fission ones. Loading a thick source of uranium (thickness greater than the fission fragment range) with a small amount of a suitable trans-uranium element (for instance, 242 Pu, which presents a spontaneous fission half-life of 6.75 . 10 10 y), it is possible to determine the observation efficiency of a particle-track detector for fission fragme Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a λ f value. Introduction activation) (for instance, References 8 and 16, or directly by U (n, fission) reaction. 3,8,16 The determination of neutron fluences has been pointed out as the main source of systematic error in fission-track dating and λ f values obtained with this method. 1,10 In order to overcome difficulties arising from neutron-dose determination, fission-track researchers have avoided the use of a λ f value along with an absolute neutron-dose measurement in dating geological samples. Two calibration systems have been adopted: (1) use of a λ f value obtained under the same irradiation conditions and measurement procedures to be used in dating and (2) irradiation of a standard sample, whose age is well known by means of another radiometric method of dating, together with the sample to be dated. This last technique, the ζ-calibration, was first outlined by F LEISHER and H ART 5 and has been the most used procedure in dating geological mineral samples. The decay constant for spontaneous fission of 238 U, λ f , is a key parameter in the fission-track-age equation which was presented in a simplified form by F LEISCHER and P RICE : 4 T n I S I f = T T U N (1) where ρ S and ρ I are the superficial densities of spontaneous and induced-fission tracks, respectively, that can be measured on a suitable fission-track detector or on the sample to be dated, after suitable chemical etching, T is the exposure time or the sample age, n is the fluence of thermal neutrons, σ the crosssection for thermal fission of 235 U, and I = C 235 / C 238 the isotopic ratio between 235 U and 238 U in natural uranium. Many λ f measurements have been made by fission- track workers since the F LEISCHER and P RICEone, 4 the majority of these have been grouped around 7.0 . 10 –17 y –1 . However,when all techniques are considered, the λ f results have been placed around two central values: 7.0 . 10 –17 y –1 , 4,8,12–14 and 8.5 . 10 –17 y –1 , 3,6,14,15 which present a disagreement of about 20%. Fission-track results were obtained by applying Eq. (1) to the uranium samples or to artifacts whose ages were known a priori. This procedure involves neutron dosimetry. Neutron-fluence measure- ments were carried out using metal-activation monitors (or uranium-doped glasses calibratedby metal Thermal neutron irradiation yields fission tracks through 235 U induced-fission and makes it possible to determine the detector efficiency multiplied by the uranium content. Here, we have implicitly assumed the same detection efficiency for both 235 U and 238 U fission fragments. Such a hypothesis is reasonable since the mass numberand energy distributions of fission fragments are very similar in 238 U spontaneous and 235 U induced-fission processes. On the other hand, the same mass and energy distributions are observed in the spontaneous-fission-decay process of trans-uranium isotopes. 2 Therefore, trans-uranium elements could be used as fission-fragment sources for obtaining the * E-mail: guedes@ifi.unicamp.br; hadler@ifi.unicamp.br 0236–5731/2002/USD 17.00 Akadémiai Kiadó, Budapest © 2002 Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht