Preferential leaching and natural annealing of alpha-recoil tracks in metamict betaflte and samarskite

Abstract

Leaching experiments on naturally occurring, metamict betafite and samarskite minerals in a bicarbonate-carbonate solution show strongly enhanced release to the solution of short-lived 228Th relative to its parent isotope 232Th (by a factor of 3 to 6), but only slightly enhanced dissolution (by a factor of 1.2 to 2) of long-lived 234U and 230Th relative to 238U and 232Th, respectively. The betafite (a complex Ca–U–Ti–Nb oxide of the pyrochlore group, A2−mB2X6Y0−1.H2O) and samarskite (a complex Y–Fe–U–Nb oxide with varying stoichiometry between AB O4 and AB2O6) are x-ray and electron diffraction amorphous having experienced doses of 2.2–2.8 × 1017 alpha-decay events/mg (24–31 dpa) and 3.8–4.4 × 1017 alpha-decay events/mg (39–45 dpa), respectively. The isotopic fractionation is attributed to the radiation damage created by the alpha-decay events. Individual alpha-recoil tracks are preserved for some time as disordered regions of higher chemical reactivity in already fully damaged, aperiodic areas that result from the alpha-decay events. The annealing time of the alpha-recoil track within the aperiodic atomic array of the metamict state is calculated to be 29 300 ± 8100 years for samarskite and 42 700 ± 13 700 years for the betafite. These data plus data on an earlier studied betafite sample (annealing time = 2000 ± 1300 years) give an average annealing time of 25 000 ± 21 000 years. The variation in calculated annealing time is due, in part, to post-metamict alteration of the sample, particularly for the betafite. The wider range of values for the betafite is attributed to its greater degree of alteration. These results demonstrate that recoil nuclei of alpha-decay events may be selectively leached from damaged, aperiodic phases, but that these tracks are subject to low-temperature annealing in short periods of time relative to the age of the samples (∼ 500 m.y.). The same phenomena are expected in actinide-containing nuclear waste form glasses.