Radiation-induced defect centers: Luminescence and optical absorption study of helium-irradiated diamond and zircon

The impact of radioactivity can generate optically active defect centers in minerals. These defects may first affect light absorption, i.e., they may cause radio-coloration or -de-coloration. Second, radio-induced defects may enhance or suppress luminescence emissions of their host minerals. The action of such centers is, for instance, seen in spotted diamond specimens showing green or brown radio-coloration (the latter is typically associated with yellowish-green photoluminescence). Another example is the yellow broad-band luminescence emission of zircon, which is commonly observed in cathodo- and photoluminescence spectra of this mineral. So study whether, and how, natural alpha radiation generates and affects such centers, flat polished diamond and zircon samples were irradiated in a tandem accelerator facility with 8.8 MeV He²⁺ ions, which are the analog of alpha particles generated in the ²¹²Po α-decay (Th decay chain).
Helium ions were found to penetrate 29 μm into diamond and 32 μm into zircon, respectively, which corresponds very well to ranges predicted by Monte Carlo simulations using the SRIM code. Notable pale green coloration of diamond was observed to start at 10¹⁴ to 10¹⁵ He/cm². Spots irradiated with 10¹⁷ He/cm² appeared dark green and were found to show initial amorphization. Green colors transformed to orange-brown through heat-treatment at about 550 °C, which is mainly due to the disappearance of the ∼16,000 cm^-1 GR1 band. The latter process was found to be associated with the appearance of intense green UV-induced photoluminescence. Associated observations include strong volume expansion due to the accumulated radiation-damage, which may result in notable up-doming of radiohaloes. First studies of He-implanted zircon indicated a similar luminescence behavior, with an irradiation-induced broad-band yellow emission at 575 nm. This emission band decreases in intensity in samples that were affected by natural radiation damage prior to the He irradiation experiment.
The observed depth profiles of the luminescence emission intensity in the two minerals correspond to the calculated defect distribution profiles but not to the ionization distribution profiles. This suggests that ionization alone is insufficient to create optically active centers. Additional carbon- (diamond) and oxygen-irradiation experiments (zircon) were done to generate similar structural damage with an elemental species which is already present in the respective host mineral. Observations on these samples are largely similar to those on He-irradiated samples. Consequently, optically active centers are related to structural point defects that are created by atomic knock-ons whereas our observations did not yield independent evidence which suggests that He ions themselves might be optically active species.