Helium irradiation study on zircon

Synthetic ZrSiO₄ and (mildly to strongly radiation-damaged) natural zircon samples were irradiated with 8.8 MeV ⁴He²⁺ ions (fluences in the range 1 x 10¹³ – 5 x 10¹⁶ ions/cm²). For comparison, an additional irradiation experiment was done with 30 MeV ¹⁶O⁶⁺ ions (fluence 1 x 10¹⁵ ions/cm²). The light-ion irradiation resulted in the generation of new (synthetic ZrSiO₄) or additional (mildly to strongly metamict natural samples) damage, respectively. The maximum extent of the damage is observed in a shallow depth range approximately 32–33 µm (8.8 MeV He) and ≤ 12 µm (30 MeV O) below the sample surface, respectively, i.e., near the end of the ion trajectories. These depth values, and the observed damage distribution, correspond well to defect distribution patters as predicted by Monte Carlo simulations. The irradiation damage is recognised from the notable broadening of Raman-active vibrational modes, lowered interference colours (i.e., decreased birefringence), and changes in the optical activity (i.e., luminescence emission). At very low damage levels, a broad-band yellow emission centre is generated whereas at elevated damage levels, this centre becomes re-suppressed and samples experience a general decrease of their emission intensity. Most remarkably, there is no indication of any structural recovery process in pre-damaged natural zircon as induced by the light-ion irradiation, which questions the relevance of alpha-assisted annealing of radiation damage in natural zircon.