Heavy-ion irradiation tracks in zircon

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Abstract
Heavy-ion irradiation (14 MeVu−1 Pb ions) of zircon crystals gives rise to linear latent tracks of 80 Å diameter and length 140 μm. Direct observation of the track core, by high-voltage high-resolution electron microscopy at atomic resolution, reveals a core having roughly circular cross-section, with some facetting of the core/matrix interface on {101} planes of zircon. The core diameter appears quite uniform. Conventional transmission electron microscopy (bright- and dark-field imaging) reveals an elastic strain field extending for a short distance into the zircon matrix. This appears to drop off more rapidly with distance, say 1/R2, than do dislocation strain fields (∼ 1/R). Analysis of the various contrast mechanisms yields the result that the core is essentially amorphous. Our observations confirm directly the conclusions of Fleischer, Price and Walker, based on track etching and electrical conductivity measurements, that the irradiation damage is confined to a 50–100 Å core region of atomically disordered material, with virtually no damage outside this region. Mechanisms for track production are discussed briefly, but it is concluded that the problem, which is now defined by our structural analysis, has not been fully appreciated by condensed matter physicists. In particular a damage confinement mechanism is required, which is not intuitively obvious. Some tentative suggestions along this direction are proposed.

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