A users' guide to fission track dating calibration

Abstract

Misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations. Problems of registration geometry, inhomogeneous uranium distribution, accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods, and impose strict limitations upon even their routine application. Complexities of unique, absolute thermal neutron dosimetry are illustrated by the calibration and use of the NBS pre-irradiated glass SRM 962. The 20% disparity in measured λ f decay constant values cannot be isolated from neutron fluence (φ) measurement and calibration. Frequently a calibration ratio (φ/λ f) is evaluated against an age standard and then split into its component parts. To illustrate this interdependence of λ f and φ, zircon fission track ages, in agreement with independent K Ar ages, are obtained by calculating the same track count data with each of the preferred values of λ f (λ f = 7.03 × 10 −17yr −1and8.46 × 10 −17yr −1) together with appropriate, selected neutron dosimetry schemes. An alternative approach is presented, formally relating unknown ages of samples to known ages of standards, either by direct comparison of standard and sample track densities, or by the repeated calibration of a glass against age standards. Practical recommendations are given for the reporting and critical assessment of fission track data.