Is there alpha-assisted annealing of radiation damage? A helium implantation study of radiation-damaged zircon and monazite

The stopping of high-energy light ions in a host solid is characterised by a very high ratio of electronic to nuclear stopping powers. Consequently, alpha particles (⁴He cores whose energies range between 3.9 and 8.8 MeV) penetrating into a host mineral cause tremendous amounts of lattice ionisation, whereas they are rather marginally efficient in generating atomic displacements. Radiohaloes in rock-forming minerals are therefore mostly characterised by relatively low levels of structural radiation damage [1,2]. It is well known, however, that ion-beam irradiation may not only create defects, but it may also cause structural reconstitution [3-5]. It has been suspected that the electronic excitation by the alpha particles may result in radiation-enhanced annealing of pre-existing radiation damage, as for instance the bulk metamictization or fission tracks [6] in minerals.
The present study aimed at checking the principal effect of alpha particles on the self-irradiation damage (i.e., predominantly alpha-recoil damage) in U- and Th-bearing accessory minerals. Three hypothetical possibilities had to be considered, namely, (i) the creation of additional damage, (ii) structural reconstitution of the pre-existing damage, or (iii) insignificant effects on the structural state. For this, a suite of well-characterised zircon and monazite samples, covering the entire range from well-crystallised to fully metamict, were irradiated with 8.8 MeV ⁴He²⁺ ions (which are the analogue of alpha particles generated in the ²¹²Po ---> ²⁰⁸Pb decay in the Th chain). Fluences were varied in the range 10¹² - 10¹⁷ He ions/cm². In the case of He-irradiated metamict (i.e., amorphous) samples, no indication of recrystallisation or nucleation was found. For all non-amorphous starting materials, we found that the degree of structural damage has always notably increased after the He irradiation. Consequently, alpha particles do create structural damage not only in well-crystallised but also in mildly to highly radiation-damaged zircon and monazite. In contrast, the hypothetical ability of alpha particles to assist damage annealing is not supported.
Our observations do not seem to confirm results of dual-beam ion irradiation experiments (i.e., the simultaneous irradiation of a solid with a heavy ion beam with relatively high nuclear excitation, and a second beam with high electronic excitation, such as high-energy light ions or electrons). Such experiments suggested that the simultaneous, intense electronic excitation may retard or prevent amorphisation by heavy ions [7]. The apparent contrast to our results may perhaps be explained by the consideration that self-irradiating minerals do virtually never experience genuine dual irradiation. Due to their generally low irradiation rates (averaging a few events per minute and mm³), alpha recoils and alpha particle irradiation in the very same volume area do not occur simultaneously but successively.

References:

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