He+ ion implantation and electron irradiation effects on cathodoluminescence of plagioclase

ORIGINAL PAPER

He+ ion implantation and electron irradiation effectson cathodoluminescence of plagioclase

Masahiro Kayama • Hirotsugu Nishido •

Shin Toyoda • Kosei Komuro • Adrian A. Finch •

Martin R. Lee • Kiyotaka Ninagawa

Received: 9 December 2012 / Accepted: 5 April 2013 / Published online: 17 April 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Cathodoluminescence (CL) spectra of unirra-

diated, He? ion-implanted and electron-irradiated plagio-

clase minerals contain the following emission bands: (1)

below 300 nm due to Pb2?, (2) at *320 and *350 nm to

Ce3?, (3) at 380–420 nm to Eu2?, Ti4? and/or Al–O-–Al/

Ti defects, (4) at 560–580 nm to Mn2? and (5) at

720–760 nm to Fe3?. During the implantation of He? ion,

much of their energy may be dissipated by partial

destruction and strain of the feldspar framework, resulting

in quenching of CL. Deconvolution of CL spectra acquired

from albite and oligoclase reveals an emission component

at 1.86 eV (666 nm) assigned to a radiation-induced defect

center associated with Na? atoms. As its intensity increases

with radiation dose, this emission component has potential

for geodosimetry and geochronometry. Electron irradiation

causes Na? migration in plagioclase, and then a consider-

able reduction in intensity of emissions assigned to impu-

rity centers, which is responsible for an alteration in the

energy state or a decrease in luminescence efficiency fol-

lowing the change of activation energy. Emission intensity

at 1.86 eV positively correlates with electron irradiation

time for unimplanted and He? ion-implanted albite and

oligoclase, but negatively for the implanted albite above

1.07 9 10-4 C/cm2. It implies that radiation halo produced

by a-particles should not be measured using CL spectros-

copy to estimate b radiation dose on albite in the high

radiation level.

Keywords Cathodoluminescence � Plagioclase � He? ion

implantation � Electron irradiation � Radiation-induced

defect center

Introduction

The formation of radiation-induced defect centers in

feldspars has been actively and extensively investigated in

order to interpret the effects of ion, proton, neutron,

electron and X-ray irradiation on their crystal structure,

chemical composition, diffusivity and optical properties,

and to explore the geoscientific and planetary scientific

applications. Thermoluminescence (TL), optical stimu-

lated luminescence (OSL) and electron spin resonance

(ESR) analyses of minerals enable estimation of the

density of various types of lattice defects produced by

natural radiations over geological timescales and can be

used for geodosimetry and geochronometry of minerals

M. Kayama (&)

Department of Earth and Planetary Systems Science, Graduate

School of Science, Hiroshima University, 1-3-1 Kagami-yama,

Higashi-Hiroshima, Hiroshima 739-8526, Japan

e-mail: [email protected]

H. Nishido

Department of Biosphere-Geosphere Science, Okayama

University of Science, 1-1 Ridaicho, Kita-ku, Okayama,

Okayama 700-0005, Japan

S. Toyoda � K. Ninagawa

Department of Applied Physics, Okayama University of Science,

1-1 Ridaicho, Kita-ku, Okayama, Okayama 700-0005, Japan

K. Komuro

Earth Evolution Sciences, University of Tsukuba,

1-1-1 Ten-nodai, Tsukuba, Ibaraki 305-8571, Japan

A. A. Finch

Department of Earth Sciences, University of St Andrews, Irvine

Building, North Street, St Andrews, Fife KY16 9AL, UK

M. R. Lee

School of Geographical and Earth Sciences, University of

Glasgow, Lilybank Gardens, Glasgow G12 8QQ, UK

123

Phys Chem Minerals (2013) 40:531–545

DOI 10.1007/s00269-013-0590-8

such as feldspar in sedimentary rocks, and volcanic ejecta

provided that the annual dose rate is also known (Wintle

and Huntly 1979; Guerin and Valldas 1980; Huntly et al.

1985; Petrov 1994). The transportation and deposition of

fluvial sediments can also be investigated using feldspar

OSL because the luminescence signals are derived from

radiative transitions of electrons trapped at lattice defects

by natural radiation after deposition and shielding from

sunlight (Shirai et al. 2008). Ion, proton and X-ray

bombardment of plagioclase crystals exposed on the sur-

faces of Moon and Mercury (i.e. space weathering) con-

tributes to the production of the lattice defects and release

of Na atoms, which is the origin of sodium ions in the

exospheres (Sprague et al. 2002; Wurz and Lammer 2003;

Lowitzer et al. 2008). As described above, there is con-

siderable scientific interest in radiation-induced defect

centers in feldspar, especially plagioclase, but their for-

mation process has not been clarified in detail. This is due

to the difficulty in characterizing radiation effect on

feldspars by ions, protons and electrons as well as a and bparticles that are seen from grain surfaces to depth of

several tens to hundreds of micrometers because con-

ventional analytical methods require the extraction of

large quantities of the mineral grains from the target

sediments.

Cathodoluminescence (CL) is the electron-stimulated

emission of photons at ultraviolet (UV) to infrared (IR)

wavelengths from a material. CL spectroscopy and

microscopy provide valuable information on the existence

and distribution of the radiation-induced defects in miner-

als with a spatial resolution of a few micrometers, and so

can elucidate the effects of radiation on the near-surface

region of mineral grains (i.e. within layers several tens to

hundreds of micrometer in thickness). According to Owen

(1988), micrometer-sized radiation halos produced by aparticles appear prominently in quartz using a CL micro-

scope, but are indistinct or invisible under a petrographic

microscope. Recently, CL measurements on ion-implanted

quartz and albite have enabled interpretation of radiation

effects, including observation of radiation damage as a CL

halo and estimation of the radiation dose (Komuro et al.

2002; Okumura et al. 2008; Krickl et al. 2008; Kayama

et al. 2011a, b; King et al. 2011). These results indicate that

the technique has the potential for estimating the a radia-

tion dose from natural radionuclides on albite. However,

almost all CL studies have focused on ion-implanted albite

rather than on other plagioclase group minerals. Also, there

have been many important studies of the radiation effects

on TL, OSL and ESR of X-ray- and c-ray-irradiated feld-

spars (Wintle and Huntly 1979; Guerin and Valldas 1980;

Huntly et al. 1985; Petrov 1994), whereas very few

investigators have reported ion implantation and electron

irradiation effects.

This study has sought to determine the impact of: (1)

He? ion implantation at 4.0 MeV, corresponding to the

energy of a-ray generated by the disintegration of 238U and232Th and (2) electron irradiation on assuming b-ray.

Samples and methods

Cathodoluminescence (CL) and Raman analyses were

carried out for single crystals of the following minerals:

albite (Ab) (Or0Ab99Or1) form a pegmatite in a Pre-

cambrian granite from Minas Gerais, Brazil; oligoclase

(Ol) (Or0Ab76An23) phenocrysts from a Late Cretaceous

granite in the Ryoke metamorphic belts from Inabu, Aichi,

Japan; andesine (Ad) (Or2Ab51An47) phenocrysts from a

Pleistocene trachyandesite in the Izu-Boin volcanic arc of

Iwo Jima, Tokyo, Japan; labradorite (La) (Or2Ab29An69)

phenocrysts from a Precambrian granitic gneiss from

Bekily, Madagascar; bytownite (By) (Or1Ab21An78)

phenocrysts from a Cenozoic weathered basalt from Chi-

huahua, Mexico; anorthite (An) (Or0Ab3An97) phenocrysts

from a Cenozoic andesite in Yoichi, Hokkaido, Japan.

Microstructures and microtextures associated with exsolu-

tion and twining were located by optical microscopy and

backscattered electron SEM imaging of the plagioclase

grains and avoid in subsequent CL work owing to their

potential impact on the results. Slices of the single crystals

(10 9 10 9 1 mm) were cut perpendicular to c-axis for

CL and Raman measurements in order to avoid a polari-

zation effects as suggested by Finch et al. (2003). The

sliced samples were polished and finished with a 1-lm

diamond abrasive. CL microscopy and spectroscopy were

preliminarily conducted on the surface of the sliced sam-

ples to select the areas with comparable CL intensity and

homogeneous distribution of the intensity before the He?

ion implantation experiments.

He? ion implantation was performed perpendicular to the

surfaces of the slices using a 3M-tandem ion accelerator

located at Takasaki Research Center of the Japan Atomic

Energy Research Institute. The ion beam had a 4.0 MeV

implantation energy which corresponds to the energy of an aparticle from 238U and 232Th disintegration. A specific dose

density was set in the range from 2.18 9 10-6 to

6.33 9 10-4 C/cm2 for the albite, bytownite and anorthite

and 5.37 9 10-6 to 5.01 9 10-4 C/cm2 for the oligoclase,

andesine and labradorite (Table 1). The implanted samples

are denoted according to their relative dose density, for

example, Ab00 for unimplanted albite and Ab10 for albite

that received the highest dose. CL spectra were acquired

from the implanted surface of the samples, which is indi-

cated by the postscript ‘‘S,’’ for example, Ol01S for oligo-

clase implanted at the lowest dose and Ol08S for oligoclase

implanted at the highest dose. The implanted samples were

532 Phys Chem Minerals (2013) 40:531–545

123

also cut perpendicular to the exposed surfaces for CL line

analysis and high-resolution CL imaging of the cross-sec-

tions. The sectional samples are denoted by the postscript

‘‘C,’’ for example, Ad00C for unimplanted andesine and

Ad10C for andesine implanted at the highest radiation dose.

The details of the He? ion implantation experiments and

sample preparation are described by Okumura et al. (2008)

and Kayama et al. (2011a).

Prolonged electron irradiation experiments were carried

out on unimplanted and He? ion-implanted plagioclase at the

highest radiation doses of 6.33 9 10-4 C/cm2 for the albite,

bytownite and anorthite and 5.01 9 10-4 C/cm2 for the

oligoclase, andesine and labradorite. The electron irradiation

was undertaken in a scanning electron microscopy-cath-

odoluminescence (SEM-CL) instrument, which comprised a

JEOL: JSM-5410 SEM and a grating monochromator

(Oxford: Mono CL2). The SEM was operated at a 15 kV

accelerating voltage and 50 nA beam current. The electron

beam was scanned over an area of 110 9 93 lm for 1 h.

Scanning electron microscopy-cathodoluminescence

(SEM-CL) was also used to obtain CL spectra with operating

conditions of 15 kV and 2.0 nA in scanning mode with a

110 9 93 lm scanning area. This beam condition was

established based on the preliminary CL spectroscopy for

the prevention of electron irradiation damage and

enhancement of the signal/noise (S/N) ratio. All CL spectra

were obtained in the range from 300 to 800 nm in 1-nm steps

and were corrected for the total instrumental response using

a calibrated standard lamp. Following Stevens-Kalceff

(2009) and Kayama et al. (2010), the corrected CL spectra in

energy units were deconvoluted into the Gaussian compo-

nents corresponding to each emission center using the peak-

fitting software (peak analyzer) implemented in OriginPro

8J SR2. High-resolution CL images were acquired using a

Gatan: MiniCL imaging system under the same condition as

CL spectral analysis by SEM-CL. More details of the

equipment construction and analytical procedures can be

found in Ikenaga et al. (2000) and Kayama et al. (2010).

Raman spectral and line analyses were performed with a

laser Raman microscope (Thermo Electron Nicolet:

Almega XR), where the Nd:YAG laser (532 nm excitation

line) were selected and controlled at 20 mW with a *1 lm

spot size. The operating conditions were set as five accu-

mulations of 10 s each in the range 200–900 cm-1 in steps

of 1 cm-1. Raman bands were calibrated by monitoring the

position of the O–Si–O bending vibration (464 cm-1) in a

high optical grade quartz standard before and after the

measurements. Further details of the instruments and ana-

lytical procedures are described by Kayama et al. (2009).

Results

He? ion-implanted plagioclase

CL microscopy and spectroscopy

Panchromatic CL images of the cross-sections of the

implanted albite (Ab01C to Ab05C, Ab09C and Ab10C) and

oligoclase (Ol05C and Ol06C) reveal a bright luminescent

band (CL halo) on dull background that extends to a depth of

*12–14 lm beneath the implanted surface (Fig. 1a).

However, cross-sections of the other albite (Ab06C, Ab07C

and Ab08C) and oligoclase (Ol01C to Ol04C, Ol07C and

Ol08C) samples have a *1-lm-wide dark line on a bright

luminescent background at *12–14 lm beneath the

implanted surface (Fig. 1b). A similar dark line was

observed in CL images of all cross-section samples of the

implanted andesine, labradorite, bytownite and anorthite,

regardless of their radiation dose (Fig. 1c–f).

Table 1 Samples of plagioclase for CL and Raman measurements

Sample no. Dose density (C/cm2) Sample no. Dose density (C/cm2)

Albite Bytownite Anorthite Oligoclase Andesine Labradorite

Ab00 By00 An00 Unimplanted Ol00 Ad00 La00 Unimplanted

Ab01 By01 An01 2.18 9 10-6 Ol01 Ad01 La01 5.37 9 10-6

Ab02 By02 An02 4.02 9 10-6 Ol02 Ad02 La02 8.95 9 10-6

Ab03 By03 An03 2.54 9 10-5 Ol03 Ad03 La03 2.69 9 10-5

Ab04 By04 An04 6.97 9 10-5 Ol04 Ad04 La04 4.48 9 10-5

Ab05 By05 An05 1.07 9 10-4 Ol05 Ad05 La05 8.71 9 10-5

Ab06 By06 An06 1.19 9 10-4 Ol06 Ad06 La06 1.63 9 10-4

Ab07 By07 An07 1.40 9 10-4 Ol07 Ad07 La07 2.61 9 10-4

Ab08 By08 An08 1.49 9 10-4 Ol08 Ad08 La08 5.01 9 10-4

Ab09 By09 An09 3.03 9 10-4

Ab10 By10 An10 6.33 9 10-4

Ab, Ol, Ad, La, By and An indicate albite, oligoclase, andesine, labradorite, bytownite and anorthite, respectively

Phys Chem Minerals (2013) 40:531–545 533

123

The CL spectrum of Ab00S has an intense emission

band with peak wavelength centered below 300 nm, and

weaker bands that peak at *380, *560 and *740 nm

(Fig. 2a). Similar emission bands occur in CL spectra of

the implanted albite, where the emission intensities at

*560 and *740 nm are lower than those of unimplanted

albite, but the intensity differences vary with radiation

dose. On the other hand, the intensity of CL emission at

*380 nm increases slightly with radiation dose. The

implanted albite also shows red CL emission at

700–750 nm, and its intensity correlates with radiation

dose. CL spectra of unimplanted and implanted oligoclase

have an intense emission band overlapped with two peaks

at *320 and *350 nm, a weak band at *420 nm and

moderate emission bands at *560, 700–750 and

*740 nm. The intensities at *420 and 700–750 nm

increase slightly with the radiation dose, but those at

*320, *350, *560 and *740 nm decrease with

increasing dose (Fig. 2b). Andesine, labradorite and

bytownite with or without implantation show doublet

Fig. 1 Panchromatic CL images of cross-sections of He? ion-

implanted a albite (Ab10C), b oligoclase (Ol08C), c andesine

(Ad08C), d labradorite (La08C), e bytownite (By10C) and f anorthite

(An10C). White lines indicate the traces of Raman line analyses for

the plagioclase, shown in Fig. 4. Scale bars are 20 lm


123

emission bands at *320 and *350 nm, intense bands at

*420 and *760 nm and relatively weak emission at

560–580 nm (Fig. 2c–e). The andesine, labradorite and

bytownite implanted at high radiation dose have lower CL

intensities in these emission bands than those samples that

received a lower dose. CL spectra of unimplanted and

implanted anorthite contain emission bands at *380,

*560 and *720 nm, which show a slight decrease or no

change in their intensity with radiation dose (Fig. 2f).

Raman spectroscopy and line analysis

Raman spectra acquired from the CL halo or dark line at

*12–14 lm depth in cross-sections of the plagioclase

samples have various peaks with a lower intensity and

broader bandwidth than those of the unirradiated sample

region at 20 lm depth (Fig. 3). Furthermore, Raman peaks

of the CL halo or dark line in each plagioclase show a

decrease in the intensity and increase in the full width at

half maximum (FWHM) and background with radiation

dose. Figure 3 also reveals that Ca-rich plagioclase tends to

have greater rate of reduction in intensity of the peaks with

He? ion implantation than Na-rich plagioclase.

The intensity of the Raman peak at *508 cm-1 was

monitored in the cross-sections of each sample implanted at

the highest radiation dose. Measurements were taken per-

pendicular to the implanted surface to 20 lm depth across

the CL halo or dark line at *12–14 lm, as illustrated by the

white line in Fig. 1. These traverses show a gradual decrease

in intensity of the peak at *508 cm-1 with implantation

depth to*11–13 lm, a sharp decrease to*12–14 lm, then

increase to 16 lm (Fig. 4). The rate of reduction in intensity

at *12–14 lm depth in Ca-rich plagioclase is appreciably

greater than for Na-rich plagioclase.

Electron-irradiated plagioclase

Panchromatic CL images of plagioclase show rectangular

areas of dull CL on a bright luminescent background; they

correspond to areas that had undergone 1 h of electron

Fig. 2 CL spectra of

a unimplanted (Ab00S) and

He? ion-implanted albite

(Ab10S), b unimplanted

(Ol00S) and He? ion-implanted

oligoclase (Ol08S),

c unimplanted (Ad00S) and He?

ion-implanted andesine

(Ad08S), d unimplanted

(La00S) and He? ion-implanted

labradorite (La08S),

e unimplanted (By00S) and He?

ion-implanted bytownite

(By10S) and f unimplanted

(An00S) and He? ion-implanted

anorthite (An10S)


123

irradiation (Fig. 5). CL spectra of the electron-irradiated

areas in unimplanted plagioclase show emission bands

centered below 300 nm in the UV peaks at *320, *350,

380–420, 560–580 and 720–760 nm in the UV-blue, yel-

low and red-IR regions, which also occur in the unirradi-

ated areas (Fig. 6). Electron irradiation reduces the

intensity of these CL emission bands relative to unirradi-

ated parts of the samples, where the rate of reduction in

intensity for An00S is appreciably lower than in the other

plagioclase minerals. Red emission at 700–750 nm is also

detectable in CL spectra of the electron-irradiated areas of

Ab00S and Ol00S, but not in Ad00S, La00S, By00S and

An00S. Figure 7 shows a change in the red emission

intensity during 600 s of electron irradiation. It reveals that

there is a large increase in intensity up to *50 s and

subsequent gradual increase to 600-s irradiation for Ab00S

and Ol00S.

Analogous emissions at *320, *350, 380–420,

560–580, 700–750 and 720–760 nm were obtained from

CL spectra of the prolonged electron-irradiated areas in the

implanted plagioclase (Ab10S, Ol08S, Ad08S, La08S,

By10S and An10S) (Fig. 8). Electron irradiation of the

implanted plagioclase, especially Na-rich samples, leads to

a sharp reduction in emission intensity, except for the red

emissions at 700–750 nm in Ol08S. With an increase in

duration up to *50 s, the red emission intensity of Ol08S

increases, but that of Ab10S decreases (Fig. 7). Subse-

quently, a gradual increase in the intensity of Ol08S and

decrease in Ab10S continues up to 600 s of electron

irradiation.

Discussion

He? ion implantation

The *12–14 lm distances of CL halo or dark line beneath

sample surfaces recognized in CL images are concordant

Fig. 3 Raman spectra obtained

from the CL halo or dark line at

*12–14 lm beneath the He?

ion-implanted surface of the

cross-section samples (‘‘Halo’’)

and unimplanted host at 20 lm

depth (‘‘Host’’) for a Ab10C,

b Ol08C, c Ad08C, d La08C,

e By10C and f An10C


123

with the maximum of the electronic energy loss of

4.0 MeV He? ions in plagioclase (Bragg and Kleeman

1905; Nogami and Hurley 1948; Faul 1954; Owen 1988;

Komuro et al. 2002; Okumura et al. 2008), but not in

complete agreement, probably due to the He? ion

implantation condition or spatial resolution of CL micros-

copy proposed by Okumura et al. (2008). Nevertheless, it is

noteworthy that He? ion implantation significantly affects

CL properties, but the impact differs between plagioclase

minerals and varies with radiation dose. He? ion implan-

tation into plagioclase leads to partial destruction of the

feldspar framework or introduction of strain, migration of

monovalent cations such as Na? to an unirradiated sample

regions and formation of lattice defects; individually or in

combination these effects result in a change of the CL

properties with the radiation dose. Outcomes of these

processes should be closely linked to chemical composi-

tion, crystal structure, diffusivity of cations, and type and

concentration of emission centers, as described in the fol-

lowing sections.

Partial destruction or strain of the framework structure

Raman spectra of these plagioclase minerals commonly

have a pronounced peak at *508 cm-1 (Fig. 3), which is

assigned to the symmetric stretching of bridging oxygens

in T–O–T linkages (ms (T–O–T)) (Matson et al. 1986). The

CL halo or dark line at *12–14 lm depth beneath the

implanted surface shows a lower intensity, broader band-

width and higher background than the unirradiated sample

region (Fig. 3). With an increase in radiation dose, the

Raman peak at *508 cm-1 also shows a decrease in the

intensity and increase in the FWHM and background

acquired from the CL halo or dark line at *12–14 lm.

These findings, therefore, reveal that He? ion implantation

partially destroys or induces strain into the feldspar

framework, and to an extent that is proportional to radia-

tion dose.

Raman spectra acquired from cross-sections of the pla-

gioclase samples and along a line normal to their implanted

surface indicate that the intensity of the peak at

Fig. 4 A plot showing the

change in intensity of the

Raman peak at 508 cm-1 from

the implanted surfaces of cross-

section samples to 20 lm depth

for a Ab10C, b Ol08C,

c Ad08C, d La08C, e By10C

and f An10C. The transects

along which Raman analyses

were acquired are indicated as

white lines in Fig. 1


123

*508 cm-1 decreases from the surface to *12–14 lm

depth (Fig. 4). The rate of reduction in intensity with depth

is greatest for anorthite followed by bytownite, labradorite,

andesine, oligoclase and albite, and so is compositionally

dependent. According to Fritz et al. (2005), the relatively

weak Al–O bonds in the plagioclase structure are broken in

preference to the stronger Si–O bonds. This suggests that

damage should scale to Al content in feldspar, and so

damage from He? ion implantation should be greater for

Ca- rather than Na-rich plagioclase. Raman line analyses of

bytownite and anorthite also show a slight decrease in the

intensity at *508 cm-1 from 6 to 12 lm regardless of the

depths with quite low electronic energy loss of He? ion,

although those of other plagioclase exhibit almost no

change in intensity up to 12 lm. This may be also due to

high sensitivity of Al-rich plagioclase to lattice damage by

He? ion implantation. Therefore, He? ion implantation

destroys the feldspar framework or induces strain, and to an

extent that depends on radiation dose and Al content,

which may be responsible for the change in CL intensity.

Fig. 5 Panchromatic CL images of a Ab00S, b Ol00S, c Ad00S, d La00S, e By00S and f An00S after electron irradiation of 110 9 93 lm areas

at 15 kV and 50 nA in scanning mode for 1 h. The electron irradiation areas are labeled


123

Cation migration

Migration of monovalent cations such Na? occurs in

feldspar due to ion implantation and electron irradiation,

where the diffusivity is closely related to radiation dose,

chemical composition and defect density (Lineweaver

1963; Jambon and Carron 1976; Petit et al. 1987; Watson

and Dohmen 2010). The extent of lattice damage caused by

ion implantation and its consequent diffusion are deter-

mined by the implantation dose (Watson and Dohmen

2010). Behrens et al. (1990) demonstrated that Na? diffu-

sivity increases with Na content in plagioclase, and Li-

neweaver (1963) found that electron irradiation transfers

alkali elements into unirradiated areas due to the formation

of the electric fields produced by displacement of oxygen.

This may be potentially caused by He? ion implantation

because the implantation leads to the formation of various

types of oxygen vacancy centers associated with Al–O-–

Al/Ti defect center and the radiation-induced defect center,

as will be described in the following section of ‘‘Defect

centers.’’ The process of alkali element migration varies

greatly between feldspar and glass, so that the diffusion

coefficient in feldspar glass is approximately one hundred

times higher than that of feldspar (Jambon and Carron

1976; Giletti and Shanahan 1997). According to Jambon

and Carron (1976), this phenomenon may be explained by

the substantially higher defect density of feldspar glass.

Since various types of lattice defects form in feldspar as the

framework is damaged during He? ion implantation

(Kayama et al. 2011a, b), structural resistance to ion

implantation, which depends on Al content in feldspar,

should link closely to Na? diffusion.

As a result, He? ion implantation into plagioclase causes

a partial destruction or strain of the framework and Na?

migration, where the ion particles may lose much of their

energy during destruction of the crystal structure or strain

than by the process during Na? migration. Chemical

analysis by wavelength dispersive X-ray spectroscopy

(WDS) reveals a slight difference in Na content of the

plagioclase between unimplanted and He? ion-implanted


unirradiated and electron-

irradiated areas in a Ab00S,

b Ol00S, c Ad00S, d La00S,

e By00S and f An00S


123

plagioclases at highest radiation dose (Table 2). As Raman

spectra of the implanted plagioclase exhibit a decrease in

intensity and an increase in FWHM and background with

radiation dose, the main impact on plagioclase of He? ion

implantation undertaken for the present study may be a

partial destruction or strain of the framework rather than

Na? migration.

Formation of defect centers

Various types of defect centers are produced in feldspar by

radiation including He? ion implantation, resulting in the

sensitizing of CL in feldspar. Finch and Klein (1999)

demonstrated that the emission intensity of UV-blue CL, as

activated by the Al–O-–Al/Ti defect center, depends on

radiation dose as a function of the defect density. Red CL

emission assigned to radiation-induced defect center is

recognized in He? ion-implanted albite, of which the

intensity increases with radiation dose and the defect

density (Kayama et al. 2011a, b). CL spectra of the present

Na-rich plagioclase reveal an increase in UV-blue and red

emission intensities with radiation dose (Fig. 2), which

may be due to the formation of radiation-induced defect

centers accompanying He? ion implantation.

Change of luminescence properties

Cathodoluminescence (CL) spectra of unimplanted and

He? ion-implanted plagioclase consist of multicausal

emission bands in the UV-blue, yellow, red and IR regions

(Fig. 2). These bands are assigned to various types of

emission centers as follows. The UV emissions, centered

below 300 nm, occur in CL spectra of unimplanted and

implanted albite and oligoclase, and may correspond to the

band at 284 nm caused by the Pb2? impurity center, as

demonstrated by Vaggelli et al. (2005). CL spectra of un-

implanted and the implanted oligoclase, andesine and

labradorite have a doublet emission band at *320 and

*350 nm, which is composed of overlapping of multiple

narrow peaks attributed to the Ce3? impurity center (Laud

et al. 1971; Gotze et al. 2000). Previous studies of feldspars

have assigned the UV-blue CL emissions at 380–420 nm to

the Eu2? impurity, Ti4? impurity centers and/or Al–O-–

Al/Ti defect (Mariano et al. 1973; Gorobets et al. 1989;

Finch and Klein 1999; Gotze et al. 2000; Lee et al. 2007;

Parsons et al. 2008; Kayama et al. 2010). The Eu2?

impurity, Ti4? impurity and Al–O-–Al/Ti defect centers

act as a dominant activator for the emission bands at

*380 nm, *405 nm and *420 nm, respectively (Kay-

ama et al. 2010, 2011a). CL spectra of unimplanted and the

implanted plagioclase show yellow emissions at

560–580 nm that are assigned to the Mn2? impurity and

red-IR emissions at 720–760 nm due to the Fe3? impurity

centers, respectively (e.g. Smith and Stenstrom 1965;

Telfer and Walker 1978; Gotze et al. 2000; Krbetschek

et al. 2002). The red emissions at *660 nm, which are

overlapped and so concealed by the yellow and the red-IR

emissions, have also been observed in CL spectra of the

implanted albite and oligoclase. The red emission intensity

increases with the radiation dose as a function of radiation-

induced defect centers, and similar patterns have been

observed in ion-implanted albite and quartz (Komuro et al.

2002; Krickl et al. 2008; Okumura et al. 2008; Kayama

et al. 2011a). The response of these CL emission intensities

to He? ion implantation seems to be significantly different

between types of emission centers, that is, the impurity and

lattice defect (Fig. 2).

Impurity center

Cathodoluminescence (CL) emission assigned to the Pb2?,

Ce3?, Eu2?, Mn2? and Fe3? impurity centers decreases in

intensity with an increase in the radiation dose (Fig. 2).

This quenching of the CL may be caused by the following

processes; ion implantation makes a change in activation

energy associated with hopping between adjacent channels

and consequently produces a reduction in the luminescence

efficiency (Curie 1963; Brooks et al. 2001). According to

Fig. 7 Plots of the change in the red emission intensity with duration

of electron irradiation up to 600 s for a Ab00S and Ab10S, and

b Ol00S and Ol08S


123

Blasse and Grabmaier (1994) and Kayama et al. (2009),

both luminescence efficiency and activation energy vary

depending on the energy state in the host structure. An

outermost electron in the impurities, related to radiative

transition, is susceptible to electronic states in the ligand

atoms, resulting in a variation of CL intensity and peak

wavelength depending on the crystal field that is a function

of the distance between impurity and the ligand (Telfer and

Walker 1978; Blasse and Grabmaier 1994). Therefore, the

partial destruction, strain or Na? migration due to He? ion

implantation may lead to a change of the energy state in the

plagioclase, resulting in a decrease in luminescence effi-

ciency of the impurity centers due to a change in the

activation energy. Alternatively, atomic bonds between the

impurities and their ligands are broken by He? ion

implantation so that the impurity centers are converted to

non-luminescent centers as the energy state is altered.

Although highly metamict zircon and glass include impu-

rity centers such as transition element and rare earth ele-

ments with sufficient contents for luminescence, there has

been less or almost no CL signal of the impurity centers

from them (e.g. Stevens-Kalceff et al. 2000; Nasdala et al.

2002). These facts suggest that the impurity centers

unlinked with their ligands (e.g. Fe3? impurity uncon-

nected with oxygens) may be non-luminescent centers even


unirradiated and electron-

irradiated areas in a Ab10S,

b Ol08S, c Ad08S, d La08S,

e By10S and f An10S

Table 2 Na content in unimplanted and He? ion-implanted

plagioclase

Na2O content

Ab00 Ol00 Ad00 La00 By00 An00

Unirradiated 11.67 10.25 6.29 5.55 4.45 0.41

Electron-irradiated 5.14 4.86 3.16 5.53 3.95 0.37

Ab10 Ol08 Ad08 La08 By10 An10

Unirradiated 11.54 9.48 6.20 5.22 4.45 0.40

Electron-irradiated 5.13 4.87 3.03 5.31 4.44 0.40

Na2O content are expressed as wt%


123

if they exist in the structural materials. Nevertheless, these

outcomes of these processes may quench CL of the pla-

gioclase activated by the impurity centers. Comparable

quenching of CL arises from Na? migration, as will be

described in the following section of ‘‘Electron

irradiation.’’

The implanted anorthite shows a slight decrease in

yellow and red-IR emission intensities with radiation dose,

in spite of being the plagioclase that is most prone to

damage. This suggests that in feldspars with high concen-

trations of emission centers, there is a greater probability

that a bond related to an emission center will be broken and

also that Na? will migrate adjacent to an emission center.

This may be responsible for the lower degree of quenching

of the CL in anorthite. However, the UV, yellow and red-

IR emission intensities of the albite decrease to variable

extents with enhanced radiation dose, regardless of rela-

tively low concentration of the impurity centers. According

to Behrens et al. (1990), the diffusivity of Na? cations

increases with Na content, and this may explain the greater

rate of reduction in these emission intensities for albite than

anorthite. Furthermore, the higher the Ca content in pla-

gioclase, the lower the probability that an impurity center

will be related to Na, so that there will be less quenching of

CL caused by Na? migration. These facts imply that Na?

migration may be a more important determinant of CL

quenching in plagioclase rather than the partial destruction

of the feldspar framework.

Defect centers

Cathodoluminescence (CL) spectra of oligoclase show an

increase in the intensity of the emission band at

*420 nm with the radiation dose (Fig. 2), which may be

due to the formation of Al–O-–Al/Ti defect centers

accompanying He? ion implantation. Finch and Klein

(1999) concluded from modeling that the intensity of the

blue CL in feldspar scales with the percentage of

Lowenstein bridges with electron holes (Al–O-–Al

defect) in the crystal structure, which is closely related to

the natural radiation dose that it has received during its

geological history. According to Petrov et al. (1989),

electron holes are trapped in the oxygen position of

feldspar by natural c-ray and X-ray irradiation. Therefore,

the Al–O-–Al/Ti defect center is formed as a conse-

quence of trapping of electron holes in Lowenstein

bridges by He? ion implantation, which may contribute to

an increase in the intensity of UV-blue emission. How-

ever, albite, andesine, labradorite, bytownite and anorthite

do not show a corresponding increase in the UV-blue

emission intensity with radiation dose (Fig. 2). According

to Petrov (1994), the ESR signal from the Al–O-–Al

defect center is almost undetectable in Ca-rich feldspar,

and this may be the explanation for the lack of an

increase in the UV-blue emission intensity in plagioclases

apart for oligoclase. With regard to the albite, electron

holes may have been already trapped in almost all

Lowenstein bridges before He? ion implantation, or the

percentage of Lowenstein bridges could have been too

low to produce a detectable variation in the intensity of

UV-blue CL.

Cathodoluminescence (CL) spectra of He? ion-implan-

ted albite and oligoclase show an increase in red emission

intensity at *700–750 nm with radiation dose (Fig. 2).

Deconvolution of CL spectra from these samples by

Gaussian fitting reveals an emission component at 1.86 eV

(666 nm) (Fig. 9), which has been recognized previously in

the deconvoluted CL spectra of He? ion-implanted albite

(Kayama et al. 2011a, b). As this emission component is

undetectable in andesine, labradorite, bytownite and anor-

thite, it is assigned to the radiation-induced defect center

associated with Na atoms. However, further studies such as

annealing experiment and ESR analysis will be necessary

for an identification of type of the radiation-induced defect

center. The intensity of the component at 1.86 eV

(666 nm) correlates with the radiation dose, of which an

increasing behavior significantly differs between the albite

and oligoclase (Fig. 10). Albite shows a linear correlation

between intensity and radiation dose up to 6.33 9 10-4 C/cm2,

whereas the oligoclase exhibits an exponential increase up

to 8.71 9 10-5 C/cm2 then a more gradual increase to

5.01 9 10-4 C/cm2. There is little difference in the gra-

dient of intensity versus dose between the albite and oli-

goclase up to 8.71 9 10-5 C/cm2. Taken together these

findings indicate that the efficiency of formation of the

radiation-induced defect center scales closely to Na content

in feldspar at the high radiation levels, but shows little

correspondence at lower doses. He? ion implantation into

oligoclase above 8.71 9 10-5 C/cm2 may change almost

the precursors into the radiation-induced defect centers

associated with Na atoms, similar to the case for red CL

in He? ion-implanted quartz (Okumura et al. 2008).

According to Kayama et al. (2011b), the intensity at

1.86 eV (666 nm) is essentially independent of the con-

centration and distribution of other emission centers, the

presence of microstructures or microtextures and crystal-

lographic orientation, which should be also recognized in

the oligoclase because of characteristic of the radiation-

induced defect center associated with Na atoms. The

radiation sensitivity obtained from CL spectral deconvo-

lution may potentially be used to estimate the natural aradiation dose that the Na-rich plagioclase has experienced.

Without a calibration of responses for mineralogical fea-

tures, this geodosimetry tool may be applied for albite in

the wide dose range, but for the oligoclase only in the low

dose range.


123

Electron irradiation

Quenching of impurity-activated CL

Dull emitting rectangles occur in panchromatic CL images

of plagioclase and correspond to the areas of the samples

that experienced prolonged electron irradiation (Fig. 5).

Therefore, in common with He? ion implantation, electron

irradiation leads to a decrease in CL intensity. Electron

irradiation has almost no impact on Raman spectra of

plagioclase, but WDS analysis reveals that there is a con-

siderable difference in Na concentrations between irradi-

ated and unirradiated areas (Table 2). These results imply

that in the case of electron irradiation, Na? migration is

more important to the quenching of CL in plagioclase than

partial destruction of the feldspar framework. Na? migra-

tion due to electron irradiation may reduce luminescence

efficiency by either a change of the activation energy or a

conversion of the emission center to a non-luminescent

center due to an alteration of its energy state, that is the

same as in the case of He? ion implantation. Additionally,

a sharp decrease in emission intensities during electron

irradiation is observed in both unimplanted and He? ion-

implanted plagioclase, implying that ion implantation has

little impact on Na? migration. The anorthite is exceptional

in showing a slight decrease in emission intensities. Its low

Na content and the low diffusivity of Na? may limit the

quenching of CL in anorthite by the prolonged electron

irradiation, which may be also expected by a slight dif-

ference in Na content between unirradiated and electron-

irradiated anorthite samples. In the case of plagioclase,

Na? migration is major cause of quenching of CL activated

by impurity centers rather than partial destruction of the

framework structure.

Formation and elimination of defect-activated CL

The intensity of the red emission at 700–750 nm in albite

and oligoclase increases with prolonged electron irradia-

tion (Figs. 6, 8), in common with He? ion implantation

(Fig. 2). The changes in red emission intensity for unim-

planted and He? ion-implanted albite and oligoclase were

monitored during electron irradiation (Fig. 7). Results

show a sharp increase in the intensity immediately after

electron irradiation up to 50 s and then gradual increase to

600 s for unimplanted albite and oligoclase, and the two

implanted samples of Ab00S to Ab05S and Ol00S to

Ol08S. The gradient gradually decreases with an increase

in electron irradiation time and reaches a saturation level at

50 s of electron irradiation, where the red emission inten-

sity as a percentage of the initial intensity is *140 % for

Ab00, *200 % for Ol00 and *180 % for Ol08 (Fig. 7).

Exceptionally, a result obtained from this CL analysis of

the implanted albite in the high radiation level (Ab06 to

Ab10) reveals that the red emission intensity decreases

sharply with electron irradiation time up to *50 s and then

decreases gradually to 600 s. These findings indicate that

the effect of electron irradiation on the red CL emission

differs significantly from the impact of He? ion

implantation.

Fig. 9 Deconvolution of CL spectra in energy units obtained from

a unimplanted (Ol00S) and b He? ion-implanted oligoclase at

5.01 9 10-4 C/cm2 (Ol08S). Data are fitted by several Gaussian

curves

Fig. 10 A plot of the intensity of the Gaussian components at

1.86 eV (666 nm) obtained using CL spectral deconvolution against

dose density of He? ion implantation for the albite (Ab00S to Ab10S)

and oligoclase (Ol00S to Ol08S)


123

The changes in red emission intensity during electron

irradiation may be closely related to the formation and

elimination of radiation-induced defect centers associated

with Na atoms in albite and oligoclase. Natural and artifi-

cial radiation, including He? ion implantation and electron

irradiation, produce not only the radiation-induced defect

center associated with Na atom and Al–O-–Al/Ti defect

center, but other types of defect centers such as another

type of oxygen vacancy, O- center associated with [Si,

M2?], Ag? and Pb2?, BOmn radical (e.g. SiO3

3-, PO32- and

NO2) and organic radical (e.g. C2H5 and CH3) (Matyash

et al. 1982; Petrov et al. 1989; Finch and Klein 1999; Gotze

et al. 2000; King et al. 2011). The oxygen vacancy con-

tributes to formation of positive charge at the oxygen

position and a consequent production of the electric fields

by displacement of oxygen. With an increase in radiation

dose, the population of oxygen vacancies and the amount

of positive charge both increase, resulting in a migration of

cations such as H?, Li? and Na? into non-radiated areas in

the structure (Lineweaver 1963; Jambon and Carron 1976;

Giletti and Shanahan 1997). According to King et al.

(2011), blue emission intensity decreases and red emission

intensity increases with radiation dose, which is caused by

a change of [AlO4/M?]0 (an emission center related to a

component at 3.3 eV) into [AlO4]- (a non-luminescence

center) plus non-bridging oxygen hole centers (NBOHC)

(emission center related to component at 1.9 eV) via

[AlO4]0 (an emission center related to a component at

3.6 eV) plus M? (free) due to ion and electron migration.

These results confirm that electron irradiation forms the

radiation-induced defect center associated with Na? atoms

and eliminates the defect center by Na? migration in the

structure due to production of large amount of the positive

charge in the oxygen position. These processes may be

responsible for an increase and decrease, respectively, in

the red emission intensity with duration of electron irra-

diation. In the case of unimplanted albite, the albite

implanted with He? ions at low doses, and the oligoclase,

formation of the radiation-induced defect center associated

with Na? cations should be a dominant process rather than

elimination of the defect center, leading to a considerable

increase in the red CL intensity up to *50 s of irradiation.

Subsequently, the formation and elimination processes may

reach equilibrium, which leads to saturation of the red

emission intensity above *50 s. The albite implanted at

high radiation doses has a sharp decrease in the emission

intensity up to *50 s and then gradual decrease to 600 s

because the elimination process predominantly occurs by

electron irradiation rather than the formation process. This

quenching of the red CL may be due to an increase in the

diffusivity of Na? cation by a partial destruction of the

feldspar framework to extent that depends on radiation

dose of He? ion implantation. Red emission intensity

correlates with electron irradiation time for unimplanted

and He? ion-implanted albite below 1.07 9 10-4 C/cm2

and for the oligoclase up to 5.01 9 10-4 C/cm2, indicating

that this emission may be used in geodosimetry to quantify

the b radiation to which the Na-rich plagioclase has been

exposed. However, there is a negative correlation between

the red emission intensity and electron irradiation time for

He? ion-implanted albite above 1.07 9 10-4 C/cm2.

Eventually, radiation halo produced by a-particles should

not be selected for the estimation of b radiation dose on

albite in the high radiation level using CL spectroscopy.

Acknowledgments We are deeply indebted to Dr. S. Nakano (Shiga

University, Shiga, Japan) for helpful suggestions on CL of feldspar.

We thank Dr. Y. Shibata (Technical center, Hiroshima University,

Hiroshima, Japan) for technical support on WDS using EPMA. The

measurement of EPMA was made using JXA-8200 at the Natural

Science Center for Basic Research and Development (N-BARD),

Hiroshima University. He? ion implantation experiments were sup-

ported by the Inter-University Program for the Joint Use of the Japan

Atomic Energy Agency (Takasaki), Grant No. 12010 to H. N.

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Documents

He+ ion implantation and electron irradiation effects on cathodoluminescence of plagioclase