Abstract
The rate of formation of extended defects by heavy energetic ions in track-forming minerals shows a pronounced nonlinear dependence on the linear ionization rate. It is suggested that the formation of an extended defectas recorded by small-angle x-ray scatteringrequires a certain threshold ionization density. As a consequence, the mean number of observable defects per ion track is related more directly to the fluctuation in ionization density than to its mean value.