Nuclear Track Detection最新文献

Synthetic crystals of gadolinium gallium garnets of the composition Gd₃Ga₅O₁₂, Gd_3-xCa_xGa_5-xZr_xO₁₂, and Gd₃Ga_5-2xMg_xZr_xO₁₂ were irradiated with ²⁵²Cf-fission products, ⁸⁴Kr-, ¹³²Xe- and ²³⁸U-ions in the energy range of 1.4–7.4 MeV/amu in order to study their track registration properties. Pure gadolinium gallium garnet was found to have a very high track registration threshold of ≧0.8 MeV/amu for ²³⁸U-ions. Track registration efficiency of Ca-, Zr-, and Mg-substituted garnets tends to increase with decreasing mean electron density of the crystals. No substantial anisotropy of track registration with respect to track length, track diameter and track density was observed.

A compilation of all available analyses of helium, neon and argon in chondrites, achondrites and ureilites is given. It also contains a number of so far unpublished gas analyses.

Makrofol KG solid-state track recorders (SSTRs) are used to measure the absolute fission-rate distribution in the NISUS natural-uranium shell in the three orthogonal planes. The results show that the fission-rate ratios of the outer surface to the inner surface of the shell at different positions are not constant, contrary to the prediction from the ANISN one-dimensional transport code. The minimum and maximum fission-rate ratios in the NISUS uranium shell are 7.64±0.24 and 11.58±0.27 in the horizontal midplane at θ = 30° and θ = 180°, respectively (θ = 0° is the out-of-pile direction). These ratios are compared with that of the ANISN calculation which gives the value of 10.86. The discrepancy is because of the non-uniformity of thermal neutron source distribution in the graphite cavity block.

The etching characteristics of fission tracks in sphene are shown to be significantly different before and after thermal annealing. This is due to a progressive modification of the initially anisotropic etching properties of natural sphenes owing to the accumulation of radiation damage from the α-decay of U and Th. As the level of radiation damage increases in sphene, changes occur in the colour, etching rate, track geometry, apparent etching efficiency and angular distribution of the etched tracks. Eventually the sphene behaves like an isotropic material. All these property changes are reversed by thermal annealing and are not reproduced during normal neutron irradiations. The α-damage is annealed at a lower temperature than that required for track-fading. Spontaneous (i.e. fossil) tracks in sphene are therefore not directly comparable in their etching characteristics to tracks induced in the sphene after thermal annealing. Where sphene is used for fission-track dating it is important to use laboratory techniques which do not require such a comparison of tracks before and after annealing.

Extensive measurements of cosmic-ray tracks have been made in three fragments of Barwell meteorite with a view to deduce the extent of atmospheric ablation of different crusted surfaces and in order to determine its approximate pre-atmospheric radius. Independent estimates of ablation made for several crusted surfaces from observations of fusion-crust mineralogy and thermoluminescence (TL) are in excellent accord. The observed agreement gives credence to the ablation theory and establishes usefulness of TL studies for determining atmospheric flight times. The preatmospheric ‘radius’ of Barwell is deduced to lie between 16 and 18 cm; the recovered mass amounts to 63 ± 11% of the pre-atmospheric mass. The low ablation indicates that the atmospheric and geocentric velocities of the meteorite must be less than 15 and 10 km s^-1, respectively.