Fission tracks and thermoluminescence (TL) properties of natural sphene crystals

As a mineral with a significant U-content, the natural radiation damage in sphene is produced predominantly by the α-particles and recoiling nuclei resulting from the decay of its uranium (and thorium) impurities. The amount of damage can be estimated by measuring the fossil fission track density (FFTD) on an internal surface of a crystal. Annealing for 1h at different temperatures is found to yield an inverse correlation between FFTD and the TL ''sensitivity'' (i.e. TL output per unit γ-ray test dose). An increase in the TL sensitivity is observed at annealing temperatures above ∼ 4000C, reaching a maximum at ∼ 6000C, followed by rapid decline. At annealing temperatures beyond ∼ 7000C, the TL properties of the crystals are fully restored, and the sensitivity increases steadily, eventually tending to saturate. When preannealed samples of sphene were separately exposed to (a) thermal neutrons from a reactor to induce fission, and (b) a beam of ∼ 40 MeV α-particles from a cyclotron, the TL sensitivity against annealing temperature exhibited broadly the same features as above. (author)