Atomistic simulation of track formation by energetic recoils in zircon

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We have performed classical molecular dynamics simulations of fission track formation in zircon. We simulated the passage of a swift heavy ion through crystalline zircon using cylindrical thermal spikes with energy deposition (dE/dx) of 2.5-12.8 keV nm(-1) and a radius of 3 nm. At a low dE/dx of 2.55 keV nm(-1), the structural damage recovered almost completely and a damage track was not produced. At higher values of dE/dx, tracks were observed and the radius of the track increased with increasing dE/dx. Our structural analysis shows amorphization in the core of the track and phase separation into Si-rich regions near the center of the track and Zr-rich regions near the periphery. These simulations establish a threshold dE/dx for fission track formation in zircon that is relevant to thermochronology and nuclear waste immobilization.2239Materials Sciences and Engineering Division, Office of Basic Energy Sciences (BES), US Department of Energy (DOE) [DE-AC05-76RL01830]DOE's Office of Biological and Environmental Research at Pacific Northwest National LaboratoryConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Materials Sciences and Engineering Division, Office of Basic Energy Sciences (BES), US Department of Energy (DOE) [DE-AC05-76RL01830

Fission track chemical etching kinetic model

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)In fission track thermochronology (FIT) latent fission tracks can be observed under the optical microscope only after chemical etching. The understanding of the etching process in the origin of this track is important for fission track annealing models. This may allow the determination of parameters related to etching kinetics independently of the models fit to the annealing data. In this work, a chemical etching kinetic model based on chemical principles and geometric track features is presented. The relation between track mean length, L (mu m), and etching time, t (s), is given by L = L(0)[1-(Kt+1)(M)](n) where L(0) (mu m), K (s(-1)), M and n are parameters. M and n are dimensionless. The L(0) can be associated with the initial mean length of the tracks. The model fits well experimental data for apatite and muscovite mica. In addition, it corroborates the assumption of one of the fission-track annealing models proposed for FFT that strength and efficiency of etching depend on the amount of defects (or displaced atoms) compounding the track. (C) 2009 Elsevier Ltd. All rights reserved.452157162Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

On the calibration of fission-track annealing models

To obtain thermal histories from fission track length data, the rates at which track lengths are reduced must be estimated. The usual method is to use empirical equations calibrated via laboratory analogues. A major concern is the extrapolation of the equations to predict temperatures on the geological time scale, which is far outside the range of the calibration data. Using objective statistical criteria, it is shown that the inclusion of geological data from age standards and deep boreholes is an important measure to ensure that model predictions of geological annealing rates are at least close to being correct. It is shown that high-temperature geological data should be the preferential target in the search for new field evidence. A discussion on the use and limitations of geological data and their consequences is presented. (c) 2007 Elsevier B.V. All rights reserved.2484167111

D-optimal design of fission-track annealing experiments

Fission tracks have been used to estimate time-temperature (t-T) paths of mineral samples of geological interest. Several datasets on annealing rate have been presented, especially for apatite. However, there are still open problems related mainly to chemical composition of apatite and crystallographic orientation of the analyzed crystal sections. Therefore, new annealing data are desirable. However, there are practical limitations (cost and mainly time) on the number of experiments that can be carried out. In this work, a method for optimizing the choice of the experimental conditions of the future experiments is presented, the so-called D-optimal criterion. The D-optimization takes advantage of previous knowledge on the experimental space and the model to be fit, to maximize the information to be obtained from new experiments. It has proved, in other instances, to be suitable for highly non-linear models as the Arrhenius-type equations currently used to fit the fission-track data. A sample example is given, showing that this procedure produces good choices, leading to reliable data fitting. (c) 2005 Elsevier B.V. All rights reserved.240488188

Molecular-dynamics simulation of threshold displacement energies in zircon

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Molecular-dynamics simulations were used to examine the displacement threshold energy (E(d)) surface for Zr, Si and O in zircon using two different interatomic potentials. For each sublattice, the simulation was repeated from different initial conditions to estimate the uncertainty in the calculated value of E(d). The displacement threshold energies vary considerably with crystallographic direction and sublattice. Based on the present simulations and previous experimental studies, this work recommends E(d) values of 75, 75 and 60 eV for Zr, Si and O, respectively. to be used in Monte Carlo simulations of irradiation damage profile in zircon. (C) 2009 Elsevier B.V. All rights reserved2672034313436Materials Sciences and Engineering Division, Office of Basic Energy Sciences (BES)US Department of Energy (DOE) [DE-AC05-76RL01830]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)US Department of Energy (DOE) [DE-AC05-76RL01830

Improved zircon fission-track annealing model based on reevaluation of annealing data

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)The thermal recovery (annealing) of mineral structure modified by the passage of fission fragments has long been studied by the etching technique. In minerals like apatite and zircon, the annealing kinetics are fairly well constrained from the hour to the million-year timescale and have been described by empirical and semi-empirical equations. On the other hand, laboratory experiments, in which ion beams interact with minerals and synthetic ceramics, have shown that there is a threshold temperature beyond which thermal recovery impedes ion-induced amorphization. In this work, it is assumed that this behavior can be extended to the annealing of fission tracks in minerals. It is proposed that there is a threshold temperature, T (0), beyond which fission tracks are erased within a time t (0), which is independent of the current state of lattice deformation. This implies that iso-annealing curves should converge to a fanning point in the Arrhenius pseudo-space (ln t vs. 1/T). Based on the proposed hypothesis, and laboratory and geological data, annealing equations are reevaluated. The geological timescale estimations of a model arising from this study are discussed through the calculation of partial annealing zone and closure temperature, and comparison with geological sample constraints found in literature. It is shown that the predictions given by this model are closer to field data on closure temperature and partial annealing zone than predictions given by previous models.40293106Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundacao de Amparo a Pesquisa de Sao Paulo [2007/08393-0]Materials Sciences and Engineering Division, Office of Basic Energy Sciences, US Department of Energy (DOE) [DE-AC05-76RL01830]UT-OR NL Governor's Chair programDepartment of Energy's Office of Biological and Environmental ResearchConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [473888/2007-6, 200016/2008-3]Fundacao de Amparo a Pesquisa de Sao Paulo [2007/08393-0]Materials Sciences and Engineering Division, Office of Basic Energy Sciences, US Department of Energy (DOE) [DE-AC05-76RL01830

Extrapolation of zircon fission-track annealing models

One of the purposes of this study is to give further constraints on the temperature range of the zircon partial annealing zone over a geological time scale using data from borehole zircon samples, which have experienced stable temperatures for similar to 1 Ma. In this way, the extrapolation problem is explicitly addressed by fitting the zircon annealing models with geological timescale data. Several empirical model formulations have been proposed to perform these calibrations and have been compared in this work. The basic form proposed for annealing models is the Arrhenius-type model. There are other annealing models, that are based on the same general formulation. These empirical model equations have been preferred due to the great number of phenomena from track formation to chemical etching that are not well understood. However, there are two other models, which try to establish a direct correlation between their parameters and the related phenomena. To compare the response of the different annealing models, thermal indexes, such as closure temperature, total annealing temperature and the partial annealing zone, have been calculated and compared with field evidence. After comparing the different models, it was concluded that the fanning curvilinear models yield the best agreement between predicted index temperatures and field evidence. (c) 2012 Elsevier Ltd. All rights reserved.50SI19219

Projected length annealing of etched Sm-152 ion tracks in apatite

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Slices of apatite (cut similar to 45 degrees apart from c-axis) were irradiated with Sm-152 ions and heated at different steps in order to investigate the thermal annealing property of tracks generated by these ions. The ions were impinged with 45 degrees and similar to 150 MeV at apatite surface. Samples were etched with diluted nitric acid. Results of annealed projected lengths are presented for isochronal 10, 100 and 1000 h thermal treatments (runs) for samples with and without pre-annealing preparation. For low annealing temperatures, a distinct behavior of these samples was observed: pre-annealed samples presented a faster annealing rate. At elevated temperatures, the behavior seems to be equal. A single activation energy model was fitted to data and the energy obtained is in agreement with literature. Finally, despite the different trend in comparison with annealing rates of confined fission tracks, extrapolation to geological timescales presents reasonable estimates, indicating small influence of surface effects and, in principle, the possibility to employ ion tracks as proxies for annealing kinetics. (C) 2012 Elsevier B.V. All rights reserved.2884852Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [141705/2008-6, 201847/2009-4

On epidote fission track dating

The use of epidote in fission track dating was abandoned since the beginning of the 1980s due to difficulties like absence of a standard etching procedure, obtainment of different closure temperatures and the percentage of the datable samples. The results become much more reproducible when restricting fission track analysis to a peculiar kind of track. We are also studying confined track length, what makes possible to obtain information about fossil track annealing. Fission tracks in epidote were successfully etched with 48% HF at 35 degrees C for 12.5 min. Dating samples by the external detector method was not possible due to problems in measuring the efficiency factor held between the number of fossil fission tracks and tracks induced in mica. Dating a sample from Brejui, RN, Brazil with the population method gave a corrected age of 510 +/- 69Ma, in agreement with published U/Th-Pb ages. From the fact that the fossil track length histogram was bimodal, we were able to infer that this sample registered a thermal episode during its history.These preliminary results indicate that epidote deserves further studies to establish whether it can be employed as a thermochronological tool. (c) 2005 Elsevier Ltd. All rights reserved