Amorphization in zircon: evidence for direct impact damage

X-ray diffraction has been used to characterize the amorphous phase present in a series of radiation-damaged natural zircons with radiation doses rang ing from 0.06 to 16 x 10(18) alpha-decay events g(-1). The fraction of amor phous material present in each of the samples studied has been determined, and its dependence on the radiation dose has been calibrated. Direct determ ination of the amorphous fraction confirms that amorphization in natural zi rcon occurs as a consequence of the direct impact within cascades caused by alpha-recoil nuclei. These results are not consistent with the commonly ac cepted double-overlap model of damage accumulation. The volume swelling of amorphous regions changes as a function of dose. Thu s, the density of amorphous regions depends on the degree of damage up to a certain point (i.e. 8 x 10(18) alpha-decay events g(-1)), unlike in previo us models for which a constant value independent of the radiation dose was assumed.