Criteria for the occurrence of fission-fragment tracks in crystalline substances

The tracks left in minerals by fission fragments arising from the spontaneous decay of uranium impurities serve as an important method for geological dating. If it is assumed that fission tracks in crystalline substances have a structure similar to that of an amorphous phase, then four criteria concerning whether such substances will be useful for dating can be made. These are the existence of amorphization following heavy-ion impact, the identification of an etchant, a high enough crystallization temperature, and the position of the substance on a scale of amorphizability. The criteria are illustrated by applying them to apatite, cobalt titanate, and columbite. Amorphization occurred with all three following bombardment with 10 to 35 keV Kr⁺ ions. Amorphous apatite was more successfully etched with aqueous KOH than with the commonly used HNO₃, while amorphous columbite could be etched with an HF-HNO₃ mixture. The crystallization temperatures, which lay in the interval 810 to 905 K for experiments of short duration, were judged to be sufficiently high to insure track retention at the temperature of the surface of the earth over a geological time scale. The positions on a scale of amorphizability were established by noting the bombardment doses at which there was sufficient amorphization to cause the loss of one-half of an inert-gas marker when specimens were heated or etched. Considering all four criteria, it is concluded that apatite and columbite but not cobalt titanate should be useful for dating, with a similar conclusion holding for other substances which are structurally or chemically similar.This paper is dedicated to Professor C.A. Winkler of McGill University on the occasion of his sixty-fifth birthday by one of his former students (R.K.).