Cathodoluminescence (CL) is an effective method for detection of various types of emission centers (lattice defects and trace elements), and CL observation of several minerals has been applied in many geoscientific fields, e.g., observation of growth structure, interpretation of diagenesis and studies of sediments, evaluation of metamorphic and metasomatic processes in several minerals (e.g., Marshall, 1988; Götze
The CL emission component related to radiation-induced damage during metamictization in zircon has not been identified as a specified emission peak separated from the spectra overlapped with multiple emission peaks. In this study, we carried out 4.0 MeV He+ ion-implantation experiments for undoped synthetic zircon 4.0 MeV is similar to the energy of α particles emitted from 238U and 232Th. The CL data obtained from the implanted zircon have been analyzed using a spectral deconvolution method to clarify a relationship between the CL emission component and metamictization by comparing with those obtained from natural zircon.
CL analysis was carried out for unimplanted and He+-ion implanted samples of synthetic zircon (termed SZ) with colorless and transparent euhedral crystals of ~0.2 mm size. This SZ was made from pure crystal powders of ZrO2 and Li2SiO3 (99.9 wt. % in purity) with a molar ratio of 15.77 mol % : 8.46 mol % by a fusion method using a Li-Mo Flux powders (Wako Pure Chemical Industries, Ltd) under atmosphere based on the procedure reported in Hanchar
He+ ion implantation was conducted perpendicular to the (100) surface using a 3M-tandem ion accelerator at Takasaki Research Center of the Japan Atomic Energy Research Institute. The ion-beam was set to achieve 4.0 MeV at the time of implantation, corresponding to the energy of α particles from 238U and 232Th decays. Specific dose density was set in the range from 2.23×10–5 C/cm2 to 2.14×10–3 C/cm2. In this case, the dose of 2.5×10–5 C/cm2 corresponds to the exposed dose estimated from the radiation of uranium of 1000 ppm in natural zircon for 1.0×106 y. Detailed information on the He+ ion implantation experiment and sample preparation has been reported in Okumura
Color CL images of the samples were obtained by a cold-cathode microscope (Luminoscope: Nuclide ELM–3R), which is composed of an optical microscope, an electron gun, and a cooled charge–coupled device (CCD) camera. The instrument was operated at an accelerating voltage of 7.5 kV and a beam current of 0.5 mA with a 0.5 s exposure.
A scanning electron microscopy-cathodoluminescence (SEM–CL) measurement was carried out using a JEOL: JSM–5410 SEM combined with a grating monochromator (Oxford: Mono CL2). All CL spectra obtained in the range from 300 nm to 800 nm with 1 nm steps were corrected for the total instrumental response, which was determined using a calibrated standard lamp. The operating conditions were set as follows: an accelerating voltage of 15 kV and a beam current of 0.1 nA in scanning mode with a 220×185 μm scanning area.
Color CL images of unimplanted SZ shows a bright blue CL emission with homogeneous features by an examination using Luminoscope, without any growth zoning (oscillatory and/or sector zoning) commonly found in magmatic zircon. CL spectrum of the SZ obtained with an SEM–CL is characterized by enhanced emission bands centered at ~310 nm and ~380 nm in a UV-blue region (
CL spectrum of synthetic zircon (SZ) after total instrumental correction operated at 0.1 nA in a scanning mode.
In general, CL spectra of natural minerals are composed of multiple emission peaks derived from a variety of impurity and defect centers. Therefore, a spectral deconvolution should be required for the characterization of constituent emission components in zircon CL. Recently, Tsuchiya
The peak positions in the range of 300 nm to 400 nm as reported by Cesbron
Deconvolution of the CL spectra in energy units obtained from SZ by using a Gaussian curve fitting. Measured spectrum shown by a black solid line; deconvoluted components by broken lines of blue one in the blue region and orange one in the yellow region; sum of the components by the red dotted line.
A faint signal of yellow emission is detected as an emission component centered at 2.15 eV (
Color CL images of unimplanted and an example of He+ ion-implanted (2.14×10–3 C/cm2) samples are shown in
Color CL images of unimplanted (top) and He+ ion-implanted (bottom) SZ samples at 2.14×10–3 C/cm2.
CL spectra of unimplanted and He+ ion implanted (2.23×10–5 C/cm2 to 2.14×10–3 C/cm2) samples are indicated in
CL spectra of unimplanted and He+ ion-implanted SZ samples at 2.23×10–5 C/cm2 to 2.14×10–3 C/cm2.
In a yellow region, CL intensity shows an increase with increasing in radiation dose up to 2.39×10–4 C/cm2 (
CL spectra of the samples implanted at 2.23×10–5 C/cm2 and 2.14×10–3 C/cm2 were converted into energy units, which were decomposed using a Gaussian curve to clarify the emission components for the yellow CL related to the radiation-induced defects (
Spectral deconvolution of the CL spectra in energy units obtained from implanted samples at (a) He+ ion-implanted at 2.23×10–5 C/cm2 and (b) at 2.14×10–3 C/cm2 by using a Gaussian curve fitting. Measured spectrum shown by a black solid line; deconvoluted components by broken lines of blue one in the blue region and orange one in the yellow region; sum of the components by the red dotted line.
Spectral deconvolution of the CL spectra in energy units from Malawi zircon by using a Gaussian curve fitting. Measured spectrum shown by a black solid line; deconvoluted components by broken lines of green one for REE activations and orange one in the yellow region; sum of the components by the red dotted line.
Furthermore, Götze
A plot of integral CL intensities of emission components at (a) 1.96 eV, (b) 2.16 eV and (c) 3.26 eV against radiation dose (C/cm2) for unimplanted and He+ ion-implanted samples at 2.23×10–5 C/cm2 to 2.14×10–3 C/cm2.