S. Guedes

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.

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.

Further investigation of the initial fission-track length and geometry factor in apatite fission-track thermochronology

Abstract The external detector method (EDM) is a widely used technique in fission track thermochronology (FTT) in which two different minerals are concomitantly employed: spontaneous tracks are observed in apatite and induced ones in the muscovite external detector. They show intrinsic differences in detection and etching properties that should be taken into account. In this work, new geometry factor values, g, in apatite, were obtained by directly measuring the ρed/ρis ratios and independently determined [GQR]ed/is values through the measurement of projected lengths. Five mounts, two of which were large area prismatic sections and three samples composed of random-orientation pieces have been used to determine the g-values. A side effect of applying EDM is that the value of the initial confined induced fission track, L0, is not measured in routine analyses. The L0-value is an important parameter to quantify with good confidence the degree of annealing of the spontaneous fission tracks in unknown-age samples, and is essential for accurate thermal history modeling. The impact of using arbitrary L0-values on the inference of sample thermal history is investigated and discussed. The measurement of the L0-value for each sample to be dated using an extra irradiated apatite mount is proposed. This extra mount can be also used for determining the g value as an extension of the ρed/ρis ratio method. Eight apatite samples from crystalline basement, with grains at random orientation, were used to determine the g-values. The results found are statistically in agreement with the values found for apatite samples (from Durango, Mexico) measured in prismatic section and also measured at random orientation. There was no observable variation in efficiency regarding crystal orientation, showing that it is relatively safe using non-prismatic grains, especially in samples with paucity of grains, as it is the case of most basin samples. Implications for the ζ-calibration and for the calibration of the direct (spectrometer-based) fission-track dating are also discussed.

Indoor radon and radon daughters survey at Campinas-Brazil using CR-39: First results

The first results of a radon and radon daughters (RD) survey performed at Campinas-SP, Brazil, are presented. We employed a technique that, potentially, makes possible to measure the radon and RD activity in the air and to separate from this result the activity of radon, alone. In this preliminary paper only the former activity is studied.

Recalibration of U-doped standard glasses through uranium thin film for neutron-fluence measurements

This work presents a recalibration of U-doped standard glasses using natural uranium thin films to be used as most dedicated neutron monitor for fission-track dating. The recalibrated U-doped glasses were used to determine fission-track ages in apatite samples from Brazilian alkaline formations (Alto Paranaíba) and standard Durango apatite. Samples were irradiated in two nuclear reactors with different characteristics and the results were compared. For well-thermalized neutron facility, metal activation monitor was also used. The ages of Alto Paranaíba arch and Durango apatite agree with those determined by other radiometric dating methods and metal activation monitors. These results suggest that the presented recalibration is suitable to be used routinely for fission-track dating studies even in a non-well thermalized neutron facility.

Study of radon emanation from polymer-modified cementitious materials

Abstract A growing concern of the scientific community is the environmental impact caused by the increase of byproducts from several industries. A way of diminishing this problem is to employ waste material as an additional element in the formation of construction materials. The purpose of this study is to evaluate the influence of polymers in composites containing vegetable wastes with regard to the emanation of radon, seeking a correlation between the rate of emanation of this gas and the physical–mechanical properties of the material. In order to reach this aim, compressive strength, tensile strength, water absorption, porosity, permeability and radon emanation rate were determined for the composite. The results indicate that the addition of polymer tends to reduce the radon emanation rate and improves the physical–mechanical properties analyzed.

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.