Response of cathodoluminescence of alkali feldspar to He+ ion implantation and electron irradiation

Cathodoluminescence (CL) of minerals such as quartz and zircon has been extensively studied to be used as an indicator for geodosimetry and geochronometry. There are, however, very few investigations on CL of other rock-forming minerals such as feldspars, regardless of their great scientific interest. This study has sought to clarify the effect of He+ ion implantation and electron irradiation on luminescent emissions by acquiring CL spectra from various types of feldspars including anorthoclase, amazonite and adularia. CL intensities of UV and blue emissions, assigned to Pb²⁺ and Ti⁴⁺ impurity centers respectively, decrease with an increase in radiation dose of He⁺ ion implantation and electron irradiation time. This may be due to decrease in the luminescence efficiencies by a change of the activation energy or a conversion of the emission center to a non-luminescent center due to an alteration of the energy state. Also, CL spectroscopy of the alkali feldspar revealed an in-crease in the blue and yellow emission intensity assigned to Al-O^?-Al/Ti defect and radiation-induced defect centers with the radiation dose and the electron irradiation time. Taken together these results indicate that CL signal should be used for estimation of the α and β radiation doses from natural radionuclides that alkali feldspars have experienced.     

Thermal effects on cathodoluminescence in forsterite

Cathodoluminescence (CL) spectral analysis has been conducted for luminescent forsterite (olivine) of terrestrial and meteoritic origins. Two emission bands at 3.15 and 2.99 eV in blue region can be assigned to structural defect centres and two emission bands at 1.91 and 1.74 eV in red region to impurity centres of Mn²⁺ and Cr³⁺, respectively. These emissions reduce their intensities at higher temperature, suggesting a temperature quenching phenomenon. The activation energy in the quenching process was estimated by a least-square fitting of the Arrhenius plots using integrated intensity of each component as follows; blue emissions at 3.15 eV: 0.08–0.10 eV and at 2.99 eV: 0.09–0.11 eV, red emissions at 1.91 eV: ～0.01 eV and at 1.74 eV: ～0.02 eV. The quenching process can be construed by the non-radiative transition by assuming the Mott-Seitz model. The values of activation energies for blue emissions caused by structural defects correspond to the vibration energy of Si-O stretching mode in the lattice, and the values for red emissions caused by Mn and Cr impurity centres to Mg-O vibration energy. It implies that the temperature quenching energy might be transferred as a phonon to the specific lattice vibration.