JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

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Cathodoluminescence of Albite Activated by
Alpha-particle Induced Luminescence Centers
H. Nishido a) , M. Kayama a) , S. Toyoda b) , K. Komuro c) and K. Ninagawa b)
a) Research Institute of Natural Sciences, Okayama University of Science, b) Department of Applied
Physics, Okayama University of Science, c) Earth Evolution Sciences, University of Tsukuba
Cathodoluminescence (CL) is an optical phenomenon
that is based on the generation of visible radiation through
sample excitation by high-energy electrons.
Radiation-damage halos caused by alpha-particles are
recognized in mineral grains of quartz and biotite with
radioactive uranium- and thorium-bearing mineral
inclusions. However, the halos in feldspar minerals, most
common mineral in the Earth's crust, have not been studied
from the point of CL characterization, where
radiation-damages are not observable with a polarized
microscope. They can be detected only with a CL
microscope and are called CL halos. Until now the
formation of such halo has not been investigated in detail.
In this study, CL of He + ion implanted albite in feldspar
group have been measured to clarify the halo for
geodosimetry and geochronology.
Single crystals of albite (Ab 98~100Or 2~0) from Minas
Gerais, Brazil (Ab1), Niigata, Japan (Ab2) and Shiga, Japan
(Ab3) were selected for CL measurements. He + ion
implantation (dose density: 1.23 × 10−4 ~ 7.38 × 10−4 C/cm 2 )
on the samples was performed using a 3 MV-tandem ion
accelerator at the Takasaki Advanced Radiation Research
Institute of the Japan Atomic Energy Agency. The energy
was set at 4 MeV corresponding to the energy of
238
alpha-particles from U. A scanning electron
microscopy-cathodoluminescence (SEM-CL), which is
comprised of SEM (JEOL: JSM-5410) combined with a
grating monochromator (OXFORD: Mono CL2), was used
to measure CL spectra ranging from 300 to 800 nm at 15 kV
acceleration voltage and a beam current of 1.0 nA. All CL
spectra were corrected for total instrumental response, which
was determined using a calibrated standard lamp.
CL imaging of Ab1, Ab2 and Ab3 shows CL halo on the
surface of He + ion implanted sample. Approximately
15 μm width of CL halo in the section is consistent with
theoretical range of alpha-particles from disintegration of
238
U in albite. Their CL spectra in the halos consist of four
emission bands at around 400, 580, 660 and 730 nm. The
emissions at 400, 580 and 730 nm can be assigned to Ti 4+ ,
Mn 2+ and Fe 3+ impurity centers, respectively. The
intensities of these CL emissions decrease with an increase
in radiation dose of He + implantation. Raman spectroscopy
on these albite in halo areas reveals pronounced peaks at 480,
520 and 780 cm -1 of which full width at half maximum
(FWHM) increase with an increase in radiation dose.
It implies that He + implantation causes a breakage of the
framework, suggesting a reduction of CL emission centers
assigned to Ti 4+ , Mn 2+ and Fe 3+ impurity centers. In the
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case of an emission band at around 660 nm, however, the
higher the radiation dose of He + implantation, the higher the
intensity of CL emission at 660 nm becomes. These results
suggest that CL emission at around 660 nm might be
assigned to radiation induced defect center formed by He +
ion implantation.
In general, a CL spectral peak fitted by one Gaussian
curve in energy units can be assigned to one specific type of
emission center. These spectra of implanted albite can be
deconvoluted by Gaussian curves in energy units, resulting
in four components centered at 3.05, 2.10, 1.86 and 1.56 eV,
where a number of the Gaussian component can be
determined by chi-square test for CL spectral data fitted
with the smallest margin of error. An integral intensity of
the Gaussian component at 1.86 eV positively correlates
with radiation dose of He + implantation in the halo area of
individual albite. It suggest that the component at 1.86 eV
(660 nm) might be assigned to radiation-induced defect
center formed by He + ion implantation (Fig. 1). CL line
analysis along the halo section reveals that a change in the
CL intensity along depth direction substantially corresponds
to the Bragg’s curve, implying energy loss process of
specific ionization along the track of a changed particle.
Therefore, the CL emission of albite related to
radiation-induced defect center can be used to quantitatively
evaluate the radiation dose of alpha-particles induced on
feldspar minerals as an indicator applied for a geodosimeter.
Fig. 1 Deconvolution of CL spectrum of Ab3
(Ab 98~100Or 2~0 from Shiga, Japan) by Gaussian
curves.