Abstract
ABSTRACT Preferential loss of 234U from rocks into solutions relative to 238U has long been attributed to recoiling alpha-emitting nuclei. Using fission-track activation, direct evidence has been obtained for two mechanisms: (1) recoil ejection from grains and (2) release by natural etching of alpha-recoil tracks. A sensitive test of the second mechanism can be done using a 239PuO2 source to implant 235U as recoiling nuclei. These particles will leave similar tracks to the recoils from 238U decay, and the implanted nuclei are chemically the same as the 234U that results in nature. Another virtue of 235U nuclei is their fissionability, which allows them to be sensitively detected by neutron irradiation followed by etching of the induced particle tracks. The 235U nuclei are implanted in a mineral, the mineral is exposed to a solution, and both the mineral and the solution are tested for their 235U content. Preferential removal of the recoil nuclei occurs in a wide spectrum of natural minerals. Among the ten substances that have been studied, fractions of recoil nuclei removed with 24-hour treatments range from near zero in diopside to 62% for orthoclase. The existence of the effect in silicon, a semiconductor, supports the idea that recoil tracks are not an ionization-caused phenomenon.
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