Luminescence from natural fluorite crystals

Nucl. Tracks Radial. Mews., Vol.20, No. 3, pp. 475485, ht. J. Radiat. Appl. Instrum., Part D Printed in Great Britain

LUMINESCENCE

1992

0735-245X/92

$5.00 + Ml

Pergamon Press Ltd

FROM NATURAL

FLUORITE CRYSTALS

T. CALDERON,* M.-R. KHANLAItY,? H. M. &NDELLt and P. D. TOWNSEND? *Departamento de Quimica Agricola-Geologia-Geoquimica, Universidad Autonoma de Madrid, Cantoblanco, Madrid, Spain, and tSchoo1 of Mathematical and Physical Sciences, University of Sussex, Brighton BNl 9QH, U.K. (Received

13 November

1991; accepted

15 February 1992)

Abstract-Luminescence spectra are reported for a range of natural fluorite crystals of different colours which contain rare earth impurities. Whilst photoluminescence and optical absorption spectra reveal different impurities such as Ce, Eu and Sm, the thermoluminescence spectra are dominated by signals from Dy ions with only traces of emission from other impurities. The reasons for the apparent differences between photoluminescence, optical absorption and thermoluminescence analyses are discussed. Glow peak temperatures are similar in each sample, suggesting that the basic trapping sites are closely related; however, fading studies indicate that this is only partially correct. In all cases the thermoluminescence recombination sites involve rare earth ions. There is the possibility that the trapping and luminescence sites are not independent but part of large defect complexes.

1. INTRODUCTION

NATURALfluorite (CaF,) is a common mineral which exhibits a large variety of colours and hues. Pure calcium fluoride forms as a body centred cubic crystal lattice and the mineral frequently contains rare earth (RE) ions. Historically it was thought that these impurities primarily enter the lattice substitutionally as trivalent ions on Ca *+ lattice sites. The necessary charge compensation may be achieved by close association with fluorine interstitial ions, resulting in a dipolar complex of (RE3+-F-). This arrangement may then show tetragonal symmetry, C4v (Kitts and Crawford, 1974). Alternative compensators include O*- substitutional ions (Bill et al., 1967; Bill and Galas, 1978). Other possibilities are for RE3+ and RE*+ ions which are non-locally compensated and these possess cubic (Oh) symmetry. However there are now numerous data which indicate that the defect sites may involve many more lattice ions. The more complex variants are typified by a point defect notation such as a 4: 3:2 for a RE in CaF, which implies four interstitials, three vacancies and two impurity interstitials in a single package (e.g. AgulloLopez et al., 1988; Hayes and Stoneham, 1985). Previous work related to optical defect properties in CaF, includes optical absorption data (Merz and Persham, 1967; Kaplyanskii et al., 1963; Manthey, 1973; Bill and Galas, 1978), electron paramagnetic resonance (EPR) (Weber and Bierig, 1964; Nakata et al., 1976), photoluminescence (Calderon et al., 1990) and thermoluminescence (TL) (Ratnam and Bose, 1966; Sunta 1977, 1979; Bangert et al., 1982) and these have revealed a variety these crystals particularly related

ties. From a size consideration it has been suggested by Sunta (1979) that rare earth impurities, particularly cerium, can dominate the TL response of this material to ionizing radiation. Cerium is the largest of the RE ions so may emphasize strain distortions which result in effective charge trapping. However, in the light of more recent data on more complex RE defect clusters, the size factors are less obvious. Cerium was noted to be of particular relevance even when defects were formed by ion implantation or modified by prolonged thermal treatments (Bangert et al., 1982). In all cases this suggests an important role for cerium as a recombination centre, even when other RE ions coexist in the sample. In this paper we discuss TL production above room temperature in “natural” as well as “pure” synthetic fluorite crystals. The role of RE impurities in this mechanism will be apparent. Optical absorption, PL, and TL data of fluorite samples are presented and discussed. From the earlier experiments it was apparent that changes occur during room temperature storage, or thermal treatments and thus a secondary objective of this study has been to seek changes of emission spectra caused by such treatments after irradiation. The TL curves of laboratory irradiated samples fade during storage and anomalous fading has been observed in one sample type for the low temperature glow peaks ( < 9O’C). Discussion of a possible mechanism which allows loss of TL signal is also made.

2. EXPERIMENTAL

of defect states in

to RE impurities. The high sensitivity of TL in CaF, has enabled some characterization of glow peaks with specific impuri-

Samples

of natural

fluorites

from Spain (labelled

M-l, M-2, and M-3) and from the U.S.A. (M-4), had previously been characterized (Table 1) by AA

476

T. CALDERON

Table 1. Analysis by AA of major impurities in ppm Sample M-l

M-2

Element

Green

Yellow

Pink

Dark blue

Si

6125 485 371 316

6260 315 223

555 315 3272

5893 593 2808

Al Na Sr Mn Mg K Fe

M-3 Colour

539 120 249 156

-

M-4

the blue region the apparent noise level at the red end of the spectrum is worse than in the blue, i.e. once the appropriate corrections for the response of the system have been made. Absorption spectra were obtained with a CARY-17 spectrometer. Optically excited spectra were recorded with a JOBIN-YVON JY-3CS spectrofluorimeter. These PL measurements were made at low temperature. 3. RESULTS

120 155

et al.

AND DISCUSSION

3.1. Optical absorption

104

(atomic absorption spectrometry) and X-ray powder diffraction. Unfortunately these analysis methods are not suitable for quantitative data on the RE impurity content of the samples. The crystals were cut to 1 mm

thickness parallel to a (001) natural face. Irradiation was made with X-rays of 35 KVp at a dose rate of some 50 Gymin-‘. TL data were collected via a scanning monochromator system over the wavelength range 30&800 nm. A bandpass of 10 nm and a heating rate of 30°C min-’ were typically used. The data were corrected for the overall efficiency of the system but since both the diffraction grating and the S20 response photomultiplier are most sensitive in

Absorption, PL and TL measurements have been performed in the natural fluorite crystals and correlated with results formerly reported for synthetic crystals. The room temperature (RT) optical absorption spectra in the 2OW300 nm wavelength range are displayed in Fig. l(a-d). The green fluorite spectrum (Fig. l(a)) shows a number of lines which have been previously assigned to different transitions between the 4f6 and 4f5 to the 5d 1 electronic configuration of Sm2+ (Kaiser et al., 1961; Loh, 1968, 1969; Vagin et al., 1969). At this point it may be said that the absorption spectrum is very similar to that reported for other natural green fluorites (Bill and Galas, 1978) showing that samarium is an important ion in the coloration in this kind of fluorite.

b)

I

I 200

I

I

I

400 Xfnm)

Mnm)

I

I\

600 x(n m)

Xtnm

1

FIG. 1. Absorption spectra at RT of natural fluorite crystals, Spanish (a) green fluorite from Badajoz; (b) yellow fluorite from Segovia and (c) pink material from Asturias. The dark blue fluorite (d) originated in the U.S.A.

LUMINESCENCE 635

FROM NATURAL

FLUORITE

708

417

b)

0.6 -

3 Li ;r =

CRYSTALS

341

421

0.2-

200

400

200

600

400

600 Atrim)

Unm)

200

400

200

600 Mm

1

400

600

Ynm)

FIG. 2. Photoluminescence spectra of fluorite. Dashed lines correspond to excitation with (a) 708.5 nm; (b) 421 nm; (c) 340 nm and (d) 421 nm. Full lines are for excitation at (a) 426 nm; (b) 341 nm; (c) 310 nm and (d) 341 nm.

Absorption spectra of yellow fluorite (Fig. l(b)) is composed of two bands at 300 and 434 nm. The latter has been related (Bill et al., 1967) with the presence of 03- molecular ions substituting for two adjacent F- ions (this has been named the YC band or “Yellow Centre”). The former band at 300 nm originates from at least two types of impurity as will be demonstrated during the discussion of the excitation spectrum. As can be seen from Fig. l(c) the pink fluorite sample shows an absorption spectrum with bands at 220, 320, 410 and 510 nm. Similar results have been

reported by Hayes (1974) and Ehrlich et al. (1979) and related to the presence of (Y’+-F) dipolar molecular centres. Finally, the absorption spectrum of dark blue fluorite (Fig. l(d)) is composed of narrow bands at 315 and 395 nm and a broad band between 560 and 580 nm. The latter has been related to aggregated colloids of calcium atoms (Kubo, 1966; McLaughlam and Evans, 1968; Braithwaite et al., 1973). The colloid nature of the defect site was suggested by the fact that the width of the absorption band did not reduce during cooling from 300 to 77 K.

FIG. 3(a).

478

T. CALDERON

et al.

(b)

FIG. 3. Isometric plots recorded at a heating rate of 20°C min-’ of the natural TL from (a) green, (b) yellow and (c) dark blue fluorite samples.

3.2. Photoluminescence The PL of natural fluorite crystals has been studied under excitation in the optical absorption region corresponding to Fig. 1. Several kinds of emission spectra have been obtained. Fig. 2(a) shows the 77 K emission spectrum of green samples during an excitation with a wavelength of 426nm. This spectrum is quite similar to that previously reported for CaF, : SmZ+ synthetic crystals which show a maximum at 708.5nm superimposed on a broad and structured emission band. This spectrum corre-

sponds to the decay of the 5d and 4f transition of Sm2+. On the other hand, excitation on the high energy side of the absorption band (c 341 nm) produces an emission band peaking at 421 nm (Fig. 2(b)), that can be related with the signals for Eu2+ seen in doped synthetic crystals of CaF2:Eu2+ (Loh, 1969). Figures 2(c) and (d) show the emission spectra for yellow samples obtained under excitation of 77 K with light of wavelengths of 310 and 341 nm. Emissions at 340 and 421 nm produced with these wavelengths have been identified with the transitions

LUMINESCENCE

FROM NATURAL

Table 2. Glow peaks detected in natural fluorites; TLN refers to natural irradiation and storage; TL refers to laboratory X-irradiation at room temperature Sample colour Green Yellow Pink Dark blue

TLN (“C) 320 320 350.

FLUORITE

CRYSTALS

x103cm24 -

I ‘G1112 4115/2 +9/ 2

TL (“C) 320, 320,

250,

190,

20-

80

479

60 150, 150,

225

90

16-

90

‘h/2 12-

from 5d to 4f and from 4f 6 or 5 d 1 to 4f 7 of Ce3+ (Ehrlich et al., 1979) and Eu2+ (Murrieta et al., 1983),

respectively. No emission was detected for pink and

i ii

a-

ci X

s/Rz 7/2 912 11/2

4o-

FIG. 5. Energy level diagram of Dy3+ (Dieke G. H. (1968) Spectra and Energy Levels of Rare Earth Ions in Crystals. Wiley, Interscience, New York).

dark blue fluorites under excitation with the wavelengths studied.

1

400

3.3. Excitation spectra

I

I

I

500

600

700

WAVE LENGTH ( nm

) 12

12

b)

I(”

,‘O

f

b

z8 Z

E z6

400

500 WAVELENGTH

600

700

(nm 1

cl

WAVELENGTHt

FIG. 4. Thermoluminescence

nm

)

emission spectra taken at 320°C for (a) green, (b) yellow and (c) dark blue specimens.

In order to obtain additional information on the nature of the centres responsible for the luminescence spectra, the excitation spectra corresponding to the above emission bands were taken; these signals are displayed in Fig. 2. The excitation spectrum for the green fluorite sample (Fig. 2(a)) of the 708.5 nm emission band can be easily identified and the peaks corresponding to the ezgsymmetry component can be labelled in terms of the electronic levels of the 4fS configuration following the work of Loh (1968) in synthetic CaF, : Sm2+ crystals. However, a similar identification in the t,, component (high energy region) was not carried out because of strong defect absorption (Loh, 1968, 1969; Bill and Galas, 1978). Figure 2(b) also includes the excitation spectrum corresponding to the Eu2+ emission. A broad and structured band at 340 nm is shown. This band corresponds to one of the two absorption bands reported for synthetic Eu doped CaF, crystals (Kobayasi et al., 1980). It should be noted that in our crystals the high energy absorption band (e2g component) is not present in the excitation spectrum of europium emission. It probably means that excitation into this band decays non-radiatively or through another set of levels (Calderon et al., 1990). Finally, excitation spectra for yellow samples have been studied by monitoring the 340 and 421 nm emission wavelengths (Fig. 2(c, d)). Two kinds of excitation spectra have been obtained. The first related with emission at 340 nm is composed of one band at 3 10 nm that can be related with Sd to 4f ion transitions of Ce3+ (Ehrlich et al., 1979) and a second obtained with emission at 421 nm is centred at 341 nm and arises from the 4f 6 or 5d 1 to 4f 7 levels of Eu2+ (Murrieta et al., 1983). As has been

480

T. CALDERON

commented already, no absorption lines in the high energy region have been observed.

et al.

mechanism of excitation-emission describes many of the emission features relating to the impurities. Notwithstanding, there is evidence for additional decay schemes in which there is an energy transfer between different impurities as has been found earlier in these materials (Calderon et al., 1990). At the same time it should be noted that optical measurements with natural crystals reveal similar results to those observed for synthetic crystals.

3.4. Summary of PL data In summary, the absorption and luminescence of our natural fluorites have enabled us to detect the presence of several defects and ionic impurities, namely, Sm*+ and Eu*+ in green samples; Ce3+ and the “YC” defect in yellow ones; (Y3+-F) in pink; and Ca aggregated colloids for dark blue samples. These results emphasize the complexity of centres participating in luminescence phenomena in CaF,, but at the same time it is possible to see that a simple

3.5. Thermoluminescence Measurements of the emission spectra produced during TL of natural crystals, as well as subsequent

(4

(b) FIG. 6(a,b)

LUMINESCENCE

FROM NATURAL

FLUORITE

CRYSTALS

481

FIG. 6. Isometric plots recorded at a heating rate of 20°C min-’ of the laboratory TL from (a) green, (b) yellow, (c) pink and (d) dark blue natural fluorite crystals.

data from laboratory irradiated fluorites, are presented in Fig. 3(a-c). This shows typical isometric plots of the intensity as a function of wavelength and temperature of the natural crystals coloured green, dark blue and yellow, respectively (i.e. before irradiation). Such historically acquired signals, TLN, were not detected for pink fluorite samples with the available sensitivity. One sees that all three types of fluorites have similar TL behaviour. Thus, TL of green, yellow and dark blue samples are composed of high temperature glow peaks near 320°C the blue samples also have

a TL peak at 220°C (Table 2). Note that by 400°C there is a strong black body signal which dominates the spectra at the red end of the wavelength axis. Emission spectra at 320°C detected for these samples are very similar, as can be observed in Fig. 4(a-c). For simplicity the data will now be presented as emission spectra recorded at fixed temperatures. All samples studied have a broad emission band in the blue region between 350 and 450nm and a variety of line features extending into the red region. At wavelengths longer than 470 nm the

482

T. CALDERON

emission spectra, throughout the whole temperature range, consist of several intense lines and a number of weaker ones which can be easily identified as transitions between energy levels of Dy’+, which are indicated in Fig. 5. This rare earth is a common impurity both in natural and in undoped synthetic fluorites. The arrows at the top of Fig. 4(a) indicate the transitions marked in the energy level diagram of Dy3+ (see Fig. 5). The lines at 485, 580, 665 and 760 nm correspond to the transitions 4F,,, to 6H,,,z, 6H 1312, =%,,2 respectively. In addition, and -9129 weaker features (such as those at 387, 545 and 630 nm) correspond to transitions from the upper level labelled “I” in Fig. 5. The shorter arrows in Fig. 4 indicate those transitions, which are numbered 5-13 in the energy level diagram. Similar results can be observed for yellow and dark blue fluorites shown in Fig. 4(b, c). Heating the samples to 400°C to observe the natural TL empties the charge traps and may anneal some defects. Subsequently the natural samples were X-ray irradiated with a dose of 1000 Gy and the laboratory TL recorded. The TL curves of fluorites after RT X-irradiation were found to be sample dependent and results can be observed in Fig. 6(aad). In all cases the TL shows an intense glow peak between 80 and 90°C and a broad complex structure between 250 and 350°C with shoulders in the 120-l 50°C temperature region. However, the latter feature can be detected only in the green and yellow samples (Table 2). Thermoluminescence of “pure” synthetic fluorite samples has been carried out as shown in Fig. 7. Thermoluminescence spectra are composed of glow peaks near 90, 160 and 330°C.

et af.

3.6. Comparisons of PL and TL spectra In order to compare these isometric plots, slices taken at a fixed temperature (9O’C) are plotted for all five samples shown in Figs 6 and 7. These wavelength slices are given in Fig. 8(ae). Not only do they show strong similarities, despite the apparent colour differences between the samples, but the spectra resemble those previously detected in the NTL spectra of unirradiated fluorite samples in the high temperature range. This therefore suggests that the dominant recombination path in the thermally excited luminescence involves Dy3+ in all cases. One cannot describe the entire spectrum in terms of Dy3+ and it is necessary to account for the other line features by reference to a variety of other rare earth impurities. Nevertheless, to emphasize the prevalence of the Dy3+ signal the calculated position of 12 of the Dy transitions are indicated on each figure. It is apparent that the other lines contribute only a small fraction of the total signal. This is initially surprising since similar results have been obtained for glow peaks in both the high temperature region for green, yellow and dark blue fluorite samples from the NTL data and the lower temperature laboratory TL. This is despite the observations that the PL and optical absorption data had indicated different rare earth impurities as being dominant in each case. However, a reappraisal of Figs 2 and 8(a, b) does show consistency between PL and TL in the sense that for the green sample the PL data (Fig. 2(a, b)) reveal Sm lines near 708 nm and Eu emission at 421 nm which are also TL emission wavelengths for this sample. Similarly the yellow sample gives PL at 421 nm from Eu and 3 10 and 340 nm bands from Ce. These same wavelengths are matched by TL signals from this crystal.

FIG. 7. As for Fig. 6 but with a “pure”

synthetic

crystal.

LUMINESCENCE

FROM NATURAL

FLUORITE

CRYSTALS

483

7

A5 z t J -c3 J I1 400

500

600

700

WAVELENGTH 19 7-

.-2

d)

5-

z aJ 2

3_

-I Il400

500

,

700

600

400

500 WAVELENGTH

600

700

2I I9

e)

-I 8

h

.-

L!6

2% c

z4 !-

5

i

2

WAVELENGTH

(nm)

FIG. 8. Emission spectra taken at 90°C from Figs 6 and 7 for (a) green, (b) yellow, (c) pink, (d) dark blue natural and (e) synthetic samples. The wavelengths of the main Dy3+ transitions are indicated, as in Fig. 5.

The similarity in glow peak temperature in each sample suggests that the trapping sites are basically similar in each case. The peak temperature does not of course distinguish between intrinsic and extrinsic traps. However, the emission spectra show clearly that independent of the charge trap the rare earth impurities are the recombination sites. Further, in all cases Dy sites are the most effective radiative decay paths, with smaller signals contributed from other RE ions. The present data augment rather than conflict with earlier discussions of CaF, PL and TL. Earlier lu-

minescence models (Merz and Pershan, 1967; Kirsh and Kristianpoller, 1977) had proposed that during irradiation the RE3+ ions were reduced to RE2+ by electron capture. Holes released during heating migrate to the RE sites and return them to their original charge state with the excess energy emitted as characteristic RE luminescence. The models included the possibility of the RE*+ being stabilized by association with fluorine interstitials but, as indicated earlier, one may need to consider much more complex sites for the RE ions. Sunta (1977, 1979) had

T. CALDERON

484

concentrated on the role of Ce impurities since, over the range of wavelengths he investigated, Ce lines dominated the spectra. Bangert et al. (1982) similarly showed that Ce was a major luminescence impurity, but that heat treatments etc. could result in a variety of defects and, more importantly, the cerium influenced shallow traps as well as the recombination sites. However, other ions also modified both the trap and recombination sites. Jassemnejad and McKeever (1987) similarly noted the role of Dy3+ but realized that in the 3OWSO nm region cerium emission might be hidden within a broader emission band. Conventional discussions of TL charge movement are phrased as though the charges move from the shallow trap, via the conduction band, to the recombination centre. This is of course only one possibility. Direct recombination between the two components, i.e. if they are spatially linked in the lattice, or by charge transfer, e.g. between Ce3+ and Mn*+ (Sunta, 1984; Calderon et al., 1990) are equally viable alternatives. If transfer occurs then the dominant emission signal need not be the RE which exists in the largest concentration. This may be the case in the present samples in which the Dy signals are pronounced in the TL measurements whereas both Eu and Sm are evident in the PL data. Such differences in luminescence signals may merely reflect the fact that X-ray irradiation introduces band-to-band electronic transitions and hence a different charge distribution with metastable levels that are observed by TL,

whereas proton excitation can access only a limited range of excited states. Photoluminescence also reveals short-lived metastable transitions which may be of reduced importance during thermal excitation. Hence, charge transfer to a dominant, large capture cross-section recombination site is less obvious with PL than with TL. 3.7. Effects of thermal treatment The variations in the types of the defect sites in the various fluorite crystals is apparent not only in the emission spectra but also in annealing experiments, e.g. as described by Bangert et al. (1982). In the present work, Fig. 9 plots the retained TL signal for the 90°C glow peak as a function of RT storage time. In all cases the signals were characteristic of the Dy recombination centre but the simple model of an isolated defect as the origin of the 90°C TL peak is clearly unsatisfactory since the decay curves are highly sample dependent. The earlier model (e.g. Bangert et a/., 1982) of a cerium impurity ion thermally forming complexes with intrinsic and/or other impurity defects, would allow for alternative leakage paths for the charge from the trap. These decay routes might be by tunnelling or by surmounting internal barriers within a complex. One must assume that the perturbation of the defect structure by the secondary processes is small enough that the activation energy for thermal charge release is effectively

1

(1)

4

et al.

bl

GREEN

P .In

3 k

2

6 TIME

10 14 ( HOURS)

2

18

6 TIME

10

14

16

( HOURS)

DARK

BLUE

h3 c an c o 590nm

22 s

4 46Onm

I

I 2

I

I

I

I

6

10

14

16

TIME

(HOURS)

I

I 2

I

I

I

I

6

10

14

16

TIME

(HOURS)

FIG. 9. The intensity of TL at 90°C at 480 and 590 nm as a function of storage time after irradiation (a) green, (b) yellow, (c) pink and (d) dark blue natural fluorite crystals.

for

LUMINESCENCE

FROM

NATURAL

unchanged, i.e. the peak remains at 90°C although for alternative escape routes, such as tunnelling, the various impurities offer distinguishable stability. It should be recalled that in detailed studies of RE impurity defects in fluorites the “simple” point defects can involve a combination of impurity, interstitals and structural vacancies. The driving force for these complexes can be the minimization of the strain energy associated with the impurity. Hence, there can be preferential pairing of impurities which forms the core of a larger defect complex (e.g. Hayes and Boyce, 1982; Hayes and Stoneham, 1985; AgulloLopez et al., 1988). Consequently the term “point defect” may in reality define a block of more than 20 lattice ions. Within this more recent perspective of RE defect site structures in CaF,, all the problems of charge transfer and variations in TL signal stability are feasible. Finally, it is interesting to note that there are many close parallels between the present data for CaF, doped with RE ions and TL studies of zircon (Chee et al., 1988). Despite the difference in host material the RE impurity ions showed TL emission near 90°C which was frequently dominated by the Dy3+ emission, and further, there was a wide range of decay kinetics including signal growth and anomalous decay of higher temperature glow peaks. As for the present example such features could be revealed only by the detailed acquisition of the spectral information during the TL. Acknowledgements-We wish to thank the British Council for support within the Acciones Integradas programme and also the Iranian Government for a studentship.

REFERENCES Agullo-Lopez F., Catlow C. R. A. and Townsend P. D. (1988) Point Defects in Mureriuls. Academic Press, London. Bangert U., Thiel K., Ahmed K. and Townsend P. D. (1982) The emission spectra of TL produced by ion implanted CaF,. Radiat.- Effects 64, -143-151; ibid. Thermally induced changes in TL of ion imolanted CaF,. 153-160. Bill H. and Galas G. (1978). Color centres associated to rare earth ions and the origin of coloration in natural fluorites. Phys. Chem. Miner. 3, 117-131. Bill H., Sierro J. and Lacrois R. (1967) Origin of coloration of some fluorites. Am. Miner. 52, 1003-1008. Braithwaite R. S. W., Flowers W. T., Hazeldine R. N. and Russel M. (1973) The cause of the colour of Blue John and other purple fluorites. Miner. Mug. 36, 401411. Calderon T., Millan A., Jaque F. and Garcia-Sole J. (1990) Optical properties of Sm2+ and Et?+ in natural fluorite crystals. Nucl. Tracks. Radiaf. Meas. 17, 557-561. Chee J:, Ochkowski H. L., Kirsh Y., Scott A., Siyanbola W. 0. and Townsend P. D. (1988) TL snectra of natural zircons. Nucl. Tracks Ridiat. Meas. i4, 3542.

FLUORITE

CRYSTALS

485

Ehrlich D. J., Moulton P. F. and Osgood R. M. (1979) Ultraviolet solid state Ce:YLF laser at 325 nm. Opt. LeU. 4, 1811186. Hayes W. (Ed.) (1974) Crystals with the Fluorife Structure. Oxford University Press, Oxford. Hayes T. M. and Boyce J. B. (1982) Extended X-ray Absorption Fine Structure Spectroscopy. Academic Press, New York. Hayes W. and Stoneham A. M. (1985) Defects and Defect Processes in Non-metallic Solids. Wiley, New York. Jassemnejad B. and McKeever S. W. S. (1987) Photoreversible charge transfer processes and TL in CaF,:Ce. J. Phys. D20, 323-328. Kaiser W., Barnet C. G. B. and Wood D. L. (1961) Fluorescence and optical maser effects in CaF,:Sm. Phys. Rev. 123, 766716. Kaplyanskii A. A., Medvedev V. N. and Feofilov P. P. (1963) Spectra of trivalent cerium ions in alkaline earth fluoride crystals. Opt. Spektros. 14, 664675. Kirsh Y. and Kristianpoller N. (1977) U.V. induced processes in pure and doped SrF,. J. Lumin. 15, 3546. Kitts E. L. and Crawford J. H. (1974) Relaxation of type I dipolar complexes in CaF, containing rare earth impurities. Phys. Rev. B9, 52645267. Kobayasi T., Mroczkowski S. and Owen J. F. (1980) Fluorescence lifetime and quantum efficiency for 5d4f transitions in Eu*+ doued chloride and fluoride crvstals. J. Lumin. 21, 247:257. Kubo K. (1966) Effects of proton bombardment on CaF, crystals. J. Phys. Sot. Japan 21, 1300-1303. Loh E. (1968) 4f-5d spectra of rare earth ions in crystals. Phys. Rev. 175, 533-536. Loh E. (1969) U.V. absorption spectra of Eu and Yt in alkaline earth fluorides. Phys. Rev. 184, 348-352. McLaughlam S. D. and Evans H. W. (1968) Production of colloidal calcium by electron irradiation of CaF, crystals. Phys. Stat. Sol. 27, 695-700. Manthey W. J. (1973) Crystal field and site symmetry of trivalent cerium ions in CaF,. Phys. Rev. B8, 40864098. Merz J. L. and Pershan P. S. (1967) Charge conversion of irradiated rare earth ions in CaF,. Phys. Rev. 162, 217-247. Murrieta H., Hernandez J. and Rubio J. (1983) About optical properties of Eu ions in non-metallic crystals. Kinam. 5, 75-121. Nakata R., Khono K., Sumita M. and Higuchi E. (1976) New ESR center in X-irradiated CaF,. J. Phys. Sot. Japan 40, 1328-1332. Ratnam V. V. and Bose H. N. (1966) TL and TL spectra of CaF, crystals irradiated by X-rays. Phys. Srat. Sol. 15, 309-3 16. Sunta C. M. (1977) Associated luminescence centres and traps in TL of CaF,:Dy (TLD-2000). J. Phys. DlO, L47-L5 1. Sunta C. M. (1979). Mechanism of phototransfer of TL peaks in natural CaF,. Phys Stat. Sol. (a) 53, 127-135. Sunta C. M. (1984) A review of TL of calcium fluoride, calcium sulphate and calcium carbonate. Radial. Prof. Dosim. 8, 25-44. Vagin Yu. S., Marchenko V. M. and Prokhorov A. M. (1969) Spectrum laser based on electron-vibrational transitions in a CaF,: Sm crystal. Sov. Phys. JETP 28, 904909. Weber M. J. and Bierig R. W. (1964) EPR and relaxation of trivalent rare-earth ions in CaF,. Phys. Rev. 134, Al492-A1503.