S. R. Paulo

Durango apatite fission–track dating using length-based age corrections and neutron fluence measurements by natural thorium thin films and natural U-doped glasses calibrated through natural uranium thin films

Abstract In this work, we present an approach for neutron fluence measurements based on natural thorium thin films and natural uranium-doped glasses calibrated through natural uranium thin films to be used for dating with the Fission–Track Method (FTM). This neutron dosimetry approach allows the employment of FTM even when dating is carried out using low neutron thermalization facilities. Besides, it makes possible the determination of the Th/U ratio of the mineral to be dated. Durango apatite which is often employed in FTM as an age standard was analyzed. This apatite presented a fairly high Th/U ratio, 29.9±1.7. Th fissions were 18%, 12% and 10% of the total for irradiations where thermal to fast neutron flux ratios were 2.4, 4.4 and 5.2, respectively. These results show that Th fission must be considered for this apatite, when not well-thermalized irradiation facilities are used. The ratio between spontaneous and induced track length, LS/LI, close to 0.89, indicates a certain amount of rejuvenation of the age of Durango apatite. Therefore, its “apparent” age should be corrected. The application of a technique based on track-length measurements produced a corrected age of 29.7±1.1 Ma, consistent with the independent reference age of this apatite (31.4±0.5 Ma). This result represents a support for viability of the neutron dosimetry approach studied in this work for FTM.

Annealing experiments on induced fission tracks in apatite: Measurements of horizontal-confined track lengths and track densities in basal sections and randomly oriented grains

Abstract To improve kinetic models for apatite fission-track annealing, we present new experimental annealing data that complement previously published data. To determine the degree of annealing of induced tracks, surface density (ρ), and mean horizontal-confined track lengths (l), were measured, both for basal and randomly oriented faces. Our annealing data were obtained by submitting an apatite sample collected in Itambé, Bahia, Brazil, to 46 different isothermal treatments where temperature ranged from 150 to 600 °C (duration of 1, 10, 100, and 1000 h). To compare the behavior of Itambé to Durango apatite, the latter was also annealed for 1 h in 9 isothermal experiments at temperatures between 240 and 380 °C. Our results show that the l/l0 values in Durango are systematically smaller than those in Itambé sample, both in basal and random faces. The curves depicting relative track density reduction, ρ/ρ0, and relative mean confined track length reduction, l/l0, as a function of time and temperature, are similar for ρ/ρ0 > ~0.5, but different for ρ/ρ0 < ~0.5. In this interval, ρ/ρ0 can be measured but the measurement of l/l0 is very difficult because the confined tracks become undetectable. Measurements of ρ/ρ0 and l/l0 for tracks revealed in basal surfaces are systematically lower (but this difference is <3%) than those in randomly oriented ones.

Spontaneous-fission decay constant of 238U measured by nuclear track techniques without neutron irradiation

A value of the decay constant for spontaneous fission of 238U, lf, determined by particle-track detector techniques, is presented. The main source of systematic error in fission-track measurements of lf, the so called ‘neutron dosimetry’, has been avoided. Instead of calibrating the neutron source and the usual mica detector through tracks of 235U fission induced by thermal neutrons, spontaneous-fission tracks and alpha-particle tracks of 238U and 242Pu were used. A value of lf = (8.66±0.38)·10-17 y-1 has been obtained.

The Spontaneous Fission Decay Constant of 238U Using SSNTD

This work reports the results of 5 measurements of the 238U decay constant for spontaneous fission, λf carried out using solid state nuclear track detectors (SSNTD), resulting in a mean value of λf = (8.35±0.24)· 10−17 y−1. The neutron fluence of the irradiations needed for these measurements were monitored with thin films of natural uranium.

A PC compatible Brazilian software for obtaining thermal histories using apatite fission track analysis

Abstract In this work a software developed in the Instituto de Fisica Gleb Wataghin, IFGW, UNICAMP, Campinas, SP, Brazil for obtaining thermal histories using apatite fission track analysis is presented. This software works in Microsoft-Windows environment. It will be freely disposable in the web site of the Departamento de Raios Cosmicos, IFGW, UNICAMP. Thermal histories obtained through this software are compared with those deduced using Monte Trax the software compatible with Apple Macintosh developed by Gallagher.

Fission-track dating of Macusanite glasses with plateau and size correction methods

Abstract Two Macusanite pebbles (MB1 and MB2) were dated with the fission-track method. Six irradiations were carried out in different nuclear reactors: Pavia (Italy), IPEN-CNEN (Brazil) and IPEN-Lima (Peru). Measurements of the thorium and uranium induced-fission per target nucleus using natural thorium thin films and natural U-doped glasses calibrated against natural uranium thin films, together with a λ F of 8.46×10 −17 a −1 were used to determine the ages. The apparent ages were corrected using the plateau and size correction methods. Track measurements were performed by different analysts, using different counting criteria. In addition, tracks were measured on samples which had been submitted to thermal treatment as well as on samples which had not been heated. Thermal treatments were carried out to erase the fossil tracks before neutron irradiation. No significant differences have been found in individual results, using the two Macusanite pebbles and the different nuclear reactors, age correction techniques, analysts, track-counting criteria, and thermal treatments before neutron irradiation. The great majority of the results (14/17) is compatible with the Ar–Ar ages of 5.12±0.11 and 5.10±0.11 Ma , Macusanite MB1 and MB2, respectively. However, the fission-track ages are systematically less (∼8%) than the Ar–Ar ages of the two Macusanite samples studied.

Spontaneous 238U fission half-life measurements based on fission-track techniques

In the last recommendation of the International Union of Pure and Applied Chemistry (I.U.P.A.C.) on spontaneous fission half-lives for ground-state nuclides, a number of measurements of 238U based on fission-track techniques were discarded. The arguments given by the authors are not clear. In this work a more detailed discussion of these determinations is given, considering the possible systematical errors inherent in fission-track approaches.

The Th/U ratio in minerals by a fission-track technique: application to some reference samples in order to estimate the influence of Th in fission-track dating

Abstract In this work, a recent technique for the determination of the Th/U ratio in minerals and glasses was applied to relevant samples used as reference in fission-track (FT) dating. This technique, developed for estimating the influence of tracks due to the induced fission of thorium in the irradiation with neutron used in the FT dating routine, proved to be adequate for this purpose. One of the age standard used for FT dating, the Durango apatite, yielded a very high Th/U ratio (around 30). Only in case of such a high Th/U ratios did the technique used in this work yield a Th/U value with acceptable precision. For the other studied samples—glasses Macusanite, Moldavite and JAS-G1—which have Th/U ratios

Determination of the 238U spontaneous fission decay constant without neutron irradiation

We have developed a methodology for measuring the decay constant of the spontaneous fission of 238U, lf, using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by spontaneous fission of trans-uranium elements bearing a mass number close to 238 are similar to 238U 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, 242Pu, which presents a spontaneous fission half-life of 6.75.1010 y), it is possible to determine the observation efficiency of a particle-track detector for fission fragments. Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These results make it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a lf value.

Measurement of indoor 222Rn using CR-39 under a thin film geometry

Abstract In this work an assembly for indoor 222Rn measurement is presented. This assembly is made up of two acrylic plates (14 cm ×14 cm ) separated by a distance of 4 mm . To prevent radon daughters from outside entering the assembly (and produce alpha particle tracks striking the CR-39 detector placed in the central region of one of these plates), the borders of these plates were progressively closed, leaving open only a rectangular aperture of 5 mm ×4 mm . The size of this opening was determined experimentally, by exposing in two indoor environments assemblies with different apertures as follows: (i) all borders open; (ii) two borders closed; (iii) three borders closed; (iv) four borders closed but one containing a 6 cm ×4 mm opening; (v) four borders closed but one containing a 2.5 cm ×4 mm opening and (vi) four borders closed but one containing a 0.5 cm ×4 mm opening. Track density shows a noticeable decrease between assembly (i) and assembly (iii), remaining constant for smaller openings. Only 222Rn, a noble gas, should enter the assembly independently of the opening size.