Abstract
The annealing of radiation damage in zircon by low-energy electron irradiation was explored systematically. Natural zircon samples spanning a wide range of self-irradiation damage were irradiated with the focused electron beam of an electron probe microanalyser. The effects of beam current and irradiation time were tested systematically, and the changes in zircon were measured using Raman spectroscopy. Our results confirm the damage-annealing effect of an accelerated electron beam. We demonstrate that non-thermal annealing occurs through electron-enhanced defect reactions and that the extent of the annealing is a function of both the irradiation time and the beam current. The complete annealing of radiation damage in zircon by an accelerated electron beam was not possible under the conditions of our experiments. Our results indicate that Raman band broadening in ion-irradiated zircon can possibly be explained through phonon confinement, as the estimated domain sizes of the crystalline volume amid recoil clusters decrease with increasing α dose. The results underlay the importance of doing Raman spectroscopy before electron-beam and ion-beam analysis. To avoid unwanted beam-induced annealing of damage in zircon during EPMA analysis, the electron energy transferred per volume unit of sample should be minimised, for instance by keeping the integrated charge low and/or by defocusing the electron beam.
Highlights
The electron-probe microanalyser (EPMA) is the most commonly used tool for determining quantitatively the major and minor element chemistry of geomaterials
In zircon damaged by ion irradiation, the crystalline domain sizes appear to decrease with increasing α dose, and rapidly reach very small dimensions
It is plausible to state that the confinement of phonons due to the decrease of coherent domains is the cause of observed Raman broadening in ion-irradiated zircon
Summary
The electron-probe microanalyser (EPMA) is the most commonly used tool for determining quantitatively the major and minor element chemistry of geomaterials. To generate the X-ray signals, samples are irradiated with a high-current electron beam for an extended time. The electron kinetic energy in an EPMA is typically 15–30 keV, which is considered low in materials science. Subthreshold collisions in low-energy electron microscopy samples may be energetic enough to change certain chemical or physical properties of beam-sensitive materials (Egerton et al 2004; Jercinovic and Williams 2005; Reed 2005 etc.). The specific problem with EPMA X-ray analysis in geosciences, especially of trace constituents such as rare earth elements in zircon, is that it is often essential to work with high electron-dose rates (beam currents of 100–200 nA and above) and long counting times (minutes to tens of minutes) to achieve good X-ray signal count statistics. Cathodoluminescence imaging and spectroscopy may require prolonged electron exposures
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