Preferential leaching and natural annealing of alpha-recoil tracks in metamict betaflte and samarskite
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Y. Eyal Department ofNuclear Engineering, Technion—Israel Institute of Technology, Haifa 32000, Israel (Received 3 March 1987; accepted 16 November 1987) 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 228 Th relative to its parent isotope 232 Th (by a factor of 3 to 6), but only slightly enhanced dissolution (by a factor of 1.2 to 2) of long-lived 234 U and 230 Th relative to 238U and 232Th, respectively. The betafite (a complex Ca-U-Ti-Nb oxide of the pyrochlore group, A 2 - m B2Xf, Fo - I ' H 2 O) and samarskite (a complex Y-Fe-U-Nb oxide with varying stoichiometry between ABOA and AB2O6) are x-ray and electron diffraction amorphous having experienced doses of 2.2-2.8 X10 17 alpha-decay events/mg (24-31 dpa) and 3.8^.4 X 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 actinidecontaining nuclear waste form glasses.
I. INTRODUCTION The enhanced leaching of alpha-recoil nuclides in crystalline and metamict phases [monazite, CePO 4 1>2; uraninite, UO2; and thorianite, ThO 2 3A; metamict betafite, (Ca,Na,U,Th) 1 _ 2 (Nb,Ta) 2 O 6 (OH,F) 0 . 1 ^thorite, ThSiO4 6 ] has been previously demonstrated. In these minerals, results show that individual alpha-recoil tracks are preserved for some time as disordered regions of higher chemical reactivity relative to the reactivity of the bulk material in crystalline and aperiodic phases. In this paper, we present new data on two additional metamict complex Nb-Ti oxides (betafite and samarskite). These samples were selected because, despite evidence for alteration (see Sec. I l l ) , their uranium and thorium contents are relatively constant throughout the specimen. Betafite,7 a radioactive mineral, is the Ti-rich member _of the pyrochlore group, Ax_2B2X6YQ_l (Fd3m, Z = 8), where A = Ca,
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