charge carrier trapping and thermoluminescence in calcium fluoride based phosphors

Radiat. Phys. Chem. Vol. 36, No. 1, pp. 47-57, 1990 Int. J. Radiar AppL lnstrum., Part C Printed in Great Britain. All rights reserved

0146-5724/90 $3.00 +0.00 Copyright © 1990 Pergamon Press plc

CHARGE CARRIER TRAPPING AND THERMOLUMINESCENCE IN CALCIUM

FLUORIDE BASED PHOSPHORS

V. K. JAIN Health Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India

Abstract--This paper reviews recent progress in the understanding and applications of the phenomenon of thermoluminescence (TL) in calcium fluoride based phosphors. The formation of charge carrier traps in rare earth and Mn doped CaF 2 and the effects of dose and the LET of radiation on the glow curve are discussed. The dosimetric properties and applications of the popular materials like CaF2:Tm, CaF2:Mn and CaF2:Dy are also described. The various explanations for the TL phenomenon in these materials are given to highlight the complications and difficulties in completely understanding the mechanisms involved. Optical absorption, ESR, electrical relaxation studies and detailed analysis of the TL emission spectrum lead to the identification of some charge carrier traps and luminescent centres, but one-to-one trap/luminescent centre correlation is very difficult.

INTRODUCTION

The fluorescence of mineral fluorite has been investigated for many decades. The mineral is found to contain a wide variety of impurities of which the rare earths (RE) are the most important from the standpoint of fluorescence. The study of RE ions in fluorite host lattice gained momentum when the need to develop materials for lasers and optical frequency converters became clear. Attention also shifted to the study of actinide ions in fluorite. Rare-earth ions in alkaline earth fluorides also provided an important avenue for spectroscopic and solid state studies (Anderson, 1974; Moore and Wright, 1981, Seelbinder and Wright, 1981).

Natural and Mn doped calcium fluoride were among the first materials used for the measurement of ionizing radiation dose through the phenomenon of thermoluminescence (Becker, 1973; Horowitz, 1984; McKeever, 1985). Rare-earth impurity ions and the associated defect centres, acting as charge carrier traps, are crucial to the observance of strong thermoluminescence in natural fluorite (Arkhangel'skaya 1964; Merz and Pershan, 1967; Sunta, 1984). This is also evidenced by the study of RE-ion doped syn- thetic calcium fluoride. The most successful dopants in this respect are dysprosium and thulium. Man- ganese doped calcium fluoride is also a widely used radiation dosimetry material.

CHARGE CARRIER TRAPS

In a crystal lattice, intrinsic and impurity related defects constitute charge carrier traps. These are formed in a variety of ways and are the subject of discussion of this section. The CaF 2 lattice is a cubic array of F - ions in which the Ca 2+ ion sublattice is

face-centred cubic (Ca ions occupy alternate body centre positions). Each cation has eight nearest neighbour F - ions such that an octahedral void exists between next nearest neighbour cation sites. When the crystals are grown with a rare-earth fluoride dopant, the RE ions readily dissolve, because of size compatibility, and occupy the divalent cation site as RE 3+. The necessary charge compensation takes place in a variety of ways leading to a number of different microscopic sites of defects in the crystal. F - i o n s provide compensation by occupying the interstitial voids. Depending on the location of the compensating F - ion, the site symmetry of the RE 3+ ion varies. If the charge compensating F - ion occupies the near neighbour position (a/2, 0, 0) the site symmetry is tegragonal C4v. It is trigonal, C3v, if the F - ion is situated at the next nearest neighbour location (a/2, a/2, a/2) (Heist and Fong, 1970). The lattice constant a has the value 5.46,~. The compensating F - i o n can also be well separated from the RE 3+, leaving it unassociated. This type of non- locally compensated site has cubic symmetry Oh. A few percent of the RE 3+ ions appear to be located at sites of perfect cubic symmetry. Figure 1 shows the CaF 2 crystal structure and some site symmetries after RE doping (Manthey, 1973).

In addition to the above, another trigonal (C3v) centre is formed when an 02 ion substitutes for a lattice F - ion at one of the nearest neighbour (NN) position (a/4, a/4, a/4). There are also reports of other centres of still lower symmetry (Heist and Fong, 1970). The observed equilibrium distribution of various centres are not those expected from theoretical considerations. The relative distribution of different sites depends upon the impurity concentration and the thermal treatment of the sample. At higher impurity concentration the ratio of associated

47

48 V.K. JAIN

<001 >

~ ( ~ C4 °xis~ <III~.C oxis ~

C 2 axis

PURE CaF 2 C4V CENTRE C3V CENTRE

Fig. l. Structure of the CaF 2 lattice showing that the Ca 2+ sites have O n symmetry. The eight nearest-neighbour F- sites and the 12 next nearest-neighbour Ca 2+ sites are shown. Cnv centre; a Ce 3+ ion substitutes for a Ca 2÷ ion, while an interstitial F- ion at (approx.) the centre of one of the nearest empty cubes of lattice F- ions reduces the point symmetry of Ce 3+ ion to C4v. C3v centre: here the F- ion is at (approx.) the centre of one of the next-nearest empty cubes of lattice F ions, and reduces the

point symmetry of the Ce 3+ ion to C3v (Manthey, 1973).

(RE3+-F-) pairs to unassociated centres should increase. However, several investigators have observed just the opposite. Similarly with increasing temperature the proportion of unassociated centres to associated centres should increase. While this has generally been observed at low concentrations of impurity ions (up to about 0.1%), at higher concentrations defects interact, forming dimers or larger clusters containing more than one ( R E 3 + - F ) pair. Clustering has been confirmed through observations of laser-induced changes in fluorescence spectra (Chrysochoos, 1984a,b; Moore and Wright, 1979) as well as through radiation-induced electrical relaxation in RE doped CaF 2 (Andeen et al., 1981; Fontanella et al., 1980, 1984). The changes in the relative concentration of associated and unassociated centres are understood on the basis of the dissocia- tion of clusters and pairs with increasing temperature. The EPR spectrum consists of a narrow line and a broad line. The broad line spectrum cannot originate in cubic sites, and therefore a stable form of dimer has been postulated formed from the association of two NN pairs symmetrically arranged in a square planar configuration. Local ion symmetry is lower, C2v, but isotropic averaging of the local ion symmetry takes place such that the EPR spectrum appears cubic, due to a lack of angular dependence of its components (Heist and Fong, 1970). The existence of a separate cubic phase REF3:3(CaF~ ), which provides a cubic symmetry site with local charge compensation for a RE 3+ ion, has also been suggested to explain the increase in cubic sites with increasing RE impurity concentration (O'Hare, 1972).

Another important centre, having trigonal symmetry, is the photochromic centre (PC). The model of the PC is a RE 3+ ion associated with an NN F- vacancy with two electrons added for charge neutral- ity (Anderson and Salinsky, 1971; Hayes and Staebler, 1974; Staebler and Schnatterly, 1971). This impurity based colour centre has been reported for

CaF 2 doped with La, Ce, Gd, Tb, Lu and Y. The colour centre is photoionized by UV light (2 <~ 400 nm) leading to a reversible colour change, i.e. the material is photochromic. UV light ionizes the PC and the freed electron is captured by an isolated RE 3+. Light of longer wavelength or thermal energy then release the electron from RE 2÷ to be re-captured by PC+:

PC + RE 3 + light , p c + + RE 2+. kT

The band gap of CaF 2 is 12.2 eV and its lattice is transparent to a very wide range of wavelengths. Impurities introduce discrete energy states within the band gap. But absorption in the unirradiated CaF 2 is not observed. Manganese doped CaF 2 has also been studied extensively. Mn 2+ enters the lattice substitu- tionally in a cation position. Its point symmetry is Oh (Van Gorkon, 1970). No absorption band has been found between 200 and 2000 nm in CaF 2:Mn.

Besides the impurity related defects described above, intrinsic colour centres are also found in additively coloured crystals of CaF 2. As opposed to alkali halides, the colourability of alkaline earth fluorides exposed to ionizing radiation is very poor, presumably because of the extremely slow rate of production of separated interstitial anion-vacancy pairs. However, impurities, heat and chemical treatment increase the colourability markedly. The well known electron centres F, M and other F aggregate centres are observed. The F band occurs at 376 nm and the M band at 520 rim. Absorption due to conglomerates of F, M and other F-aggregate centres are also reported (G6rlich et al., 1965). Among the hole centres, the Vk centre (self-trapped hole) and the H centre (a hole trapped at an interstitial anion) have been shown to exist in CaF2 and other alkaline earth fluorides. In CaF2:Tm the V k band occurs at 320 nm and the V u band at 308 nm. These bands decay in the temperature ranges of

Charge carrier trapping 49

77-136 and 136-195 K respectively (Beaumont et al., 1970). Beaumont et al. (1970) also reported that 50% of the trapped hole centres created at 77 K remain even after heating to room temperature, albeit in a non-paramagnetic form. Existence of hole centres stable at room temperature has also been demon- strated (Hayes and Staebler, 1974). EPR measurements of Beaumont et al. also showed the presence of a modified V k centre (VKA centre) in BaF2:Tm and SrF 2 :Tm, but not in CaF2 :Tm. However, the creation of VrA centres in CaF2:Mn irradiated at 77 K has been suggested to explain some features of its thermoluminescent behaviour, as we shall see later (Alcal~i et al., 1980; Jain and Jahan, 1985a).

RE 3÷ ions in CaF 2 can be reduced to RE 2÷ by X- or y-irradiation (Hayes and Twidell, 1961) or by chemical means such as additive colouration (Kiss and Yocom, 1964) and electrolysis (Fong, 1964). Most of the RE ions, except perhaps Gd, Tb and Lu, have been identified in the divalent state after X- or ?-irradiation at room temperature and below. There is some controversy as to which ions are reduced by ionizing radiation. But wide ranging experimental results demonstrate that only ions in cubic sites are reduced to the divalent state, because of the repulsive coulombic effect of a charge compensator in other sites. Radiation-induced environmental changes (for example change in site symmetry, motion of interstitial F, etc.) have also been reported (Batygov and Osikov, 1972; Kask and Korienko, 1968; Twidell, 1970). It is believed that the PC centres are involved in the thermoluminescent process (Jassemnejad and McKeever, 1987; Pai and Lee, 1981). The mechanism by which irradiation converts RE 3+ ions in the PC complexes is not understood, but it appears that the formation of RE :+ may be a first stage and that thermally-activated ionic motion is involved.

Thus we see that there is a wide range of defects-- impurity related and colour centres--which can act as charge carrier traps and are involved in the TL- phenomenon. RE 3÷ ions charge compensated locally or non-locally, F centres, F centre-impurity complexes and PC centres are some of the negative charge carrier traps, while the known positive charge carrier traps include Vk and H centres and perhaps some of their modified forms. The involvement of several such traps in the TL process is certain and there have been various speculations on the mechanism of TL emission. But it is not possible at present to correlate specfic TL glow peaks with specific traps.

THERMOLUMINESCENCE IN C a F z

In this section we discuss the thermoluminescence of extensively studied radiation dosimetry phosphors, viz. CaF2:Tm, CaF2:Dy and CaF2:Mn, and CaF: doped with some other RE ions. In Fig. 2 glow curves for the dosimetry phosphors manufactured by

/~,CaF 2 :Dy i l" '~c°rz "Tin

i tl ' " / ~ .r~CoFo :M. i I ' I : , . 11

0 IO0 2 0 0 3 0 0 4 0 0

Temperalure(*C)

Fig. 2. Glow curves of CaF2 :Tm, CaF 2 :Dy and CaF~ :Mn irradiated to 0.3 Gy 6°Co F-rays at room temperature, recorded 24h after irradiation (Pradhan and Rassow,

1987).

Harshaw/Filtrol Partnership, U.S.A., are given (Pradhan and Rassow, 1987). CaF2:Tm (TLD-300) ribbons, CaF2:Dy (TLD-200) powder and CaF2:Mn (TLD-400) powder were y-irradiated (6°Co) to 0.3 Gy y-dose at room temperature (RT), and the glow curves were recorded after a further 24 h at RT. Of these three materials, CaF2:Dy is the most complicated, in respect of glow curve structure and dose response.

CaFe:Dy

In addition to the peaks shown in Fig. 2, the glow curve of TLD-200 has two other peaks at about 320 and 380°C. All five peaks in this material grow at different rates with dose, rendering the glow curve structure highly dose dependent (Hasan and Charlambous, 1983; Horowitz, 1984, p. 141; Nakajima, 1977). Because of the dominant low temperature peaks, there is a considerable amount of fading of total TL output if the dosimeter is stored at RT before being heated. Various annealing recipes have been tried to stabilise the glow curve and obtain a linear dose response with minimal fading (Driscoll et al., 1986; Horowitz, 1984, p. 141; Lu et al., 1983). One such procedure includes preannealing at 300°C for 30 rain, when the dosimeters have not been used previously for very high doses, followed by holding for 16s at 160°C and for 32s at 240°C. This procedure isolates th~ high temperature peak (240°C) from the low temperature peak (140°C) and fading is less than 10% in 1-3 months (Driscoll et al., 1986). High temperature annealing (500°C for 3 h or 600°C for 1 h) is reported to stabilize the glow curve and ensure a linear response up to 103 Gy, albeit with reduced sensitivity (Horowitz, 1984). The luminescence spectrum (444, 474 and 568 nm) has been

50 V.K. JAIN

identified as due to the Dy 3+ ion, and to remain the same at all doses (Horowitz, 1984, Nakajima, 1977).

Intrinsically CaF2:Dy is not very sensitive to UV light. However, this sensitivity can be increased very greatly by repeated cycles of heat treatment at 900°C in air (Jain, 1984). The increase, which is accompa- nied by a decrease in 7-response, has been attributed to the formation of CaO. The decrease in 7-response has been linked variously to the permanent reduction of RE 3+ ions (Pradhan and Bhatt, 1983) and to the loss of interstitial fluoride ions (Jain and Jahan, 1985b).

Because of the problems associated with this material the high sensitivity of CaF 2:Dy to ionizing radiation has not been exploited very much in practice, but it is among the four most widely used TLDs for environmental dosimetry intercomparison (de Planque and Gesell, 1982). Tuyn (1982) reports employing TLD-200 detectors, with suitable filters, for dose determination due to low energy photons ( < 100 keV), charged particles and muons around a high energy proton accelerator. The possibility of discriminating different energy photons and different types of radiation (c~,/3, 7) through the use of ribbons annealed at 450°C 1 h and stored for 1 day after exposure has been suggested. At low energies (5-9 keV) only peaks at 140 and 200°C develop, whereas at higher energies a peak appears at 250°C (Wang et al., 1986). Strand and Stranden (1985) have suggested a method of using CaF2:Dy for the measurement of naturally occurring 7-radiation in houses. No energy compensation is used but it is noted that the usual increase in response below 200 keV is avoided. Calibration is done indoors using an air instrument, and fading is also taken care of through calibration. On the other hand Ben Shachar et al. (1985) have developed an energy compensating filter consisting of Ta (0.018cm), Pb (0.010cm) and AI (0.26cm) which satisfies the ANSI (1975) requirement for environmental monitoring vis-/t-vis its response to photons of energy below and above 80 keV. The response to /3-radiation of different maximum energies has also been determined (Driscoll et al., 1984).

C a F , : T m

Like CaF2:Dy, CaFz:Tm has several peaks in its glow curve. The two most prominent occur at temperatures of about 150 (peak 3) and 240°C (peak 5) as shown in Fig. 2. These two peaks are used for radiation dosimetry. The emission spectrum contains seven emission bands for each peak, the major emissions occurring at 345, 445 and 470 nm (Pradhan and Bhatt, 1987). However, the relative intensities of the different emission bands are not the same in each of the two peaks. Annealing and fading properties of CaF2:Tm have been studied quite extensively. (Driscoll et al., 1986; Furetta and Lee, 1983; Furetta and Tuyn, 1985a,b, 1986; Hsu et al., 1985; Pradhan and Rassow, 1987). Based on the constancy of residual thermoluminescence (RTL) and maximum TL output, an annealing regime of 400°C

for 90 min was found suitable (Pradhan and Bhatt, 1987). Pre-use anneal at 300°C for 30 min leads to 15% fading in 10 h or 20% in 38 days of dark storage at relative humidity 70-80% and 18-23°C. Storage while exposed to a 60W fluorescent lamp at a distance of 2 m for 25 days results in fading of all peaks up to a total of 55%. But the ratio of the peak 3/5 (150/240°C) heights remains constant up to 100 h (Furetta and Lee, 1983). It is also reported that heating and cooling rates may be important in the annealing procedure. Higher sensitivities and lower threshold dose may result from annealing at 400°C for 30 rain to 2 h, with heating and cooling rates of 6°C/min and 0.5°C/min respectively (Furetta and Tuyn, 1986). Various authors have studied linearity (10- ~-104 Gy), batch homogeneity, reproducibility, dose response and energy dependence and reusability of CaF2:Tm chips (Furetta and Lee, 1985; Furetta and Tuyn, 1986; Tsuda et al., 1982).

There are some very useful features of the TL response of these peaks (3 and 5) in CaF 2 :Tm. These concern their relative response with linear energy transfer (LET) and also with increasing dose (Tsuda et al., 1984; Hoffmann and Prediger, 1984). Firstly both peaks are very sensitive and can therefore be measured with high precision. The ratio (Pi /Pi 6°Co) of the height of these peaks for charged particles (P~) and for 6°Co 7-rays (Pi6°Co) is plotted against average LET in Fig. 3 (dose averaged lineal energy YD in Fig. 3 is related to LET and is derived from measurements of energy deposition in the counter gas and counter dimensions: Hoffmann and Prediger, 1984; Hoffman et al., 1980). The differences in the response of peaks 3 and 5 are clearly seen. While the response of peak 3 continues to fall rapidly with increasing LET, that of peak 5 remains constant even beyond 50 keV # - i. Thus, under heavy ion bombard- ment, CaFz:Tm delivers a two parameter signal which allows one to determine total dose and average LET from a single irradiation. As depicted in Fig. 3, for He 2+ ions the dose can be determined from the height of peak 5 and the average LET (YD) from the peak-height ratio; this was found to be independent of dose between 0.01 and 2.0 Gy. In Ne ~°÷ fields CaF2:Tm provides two LET dependent signals (Hoffmann and Prediger 1984). The ratio of the heights of glow peaks 5/3 changes from 0.41 for 0.55keV/~ -~ LET of 6°Co 7-rays to 2.50 for the 257keV# 1 LET of 5.45 MeV ~-particles from 241Am (Tsuda et al., 1984).

One of the very useful applications of differences in charge trapping as described above for CaF 2 :Tm is in the measurement of pion dose in radiation therapy (Hoffmann et al., 1980). The LET composition of the pion peam varies strongly with depth, particularly in the peak region, resulting in sharp depth dose changes as shown in Fig. 4 (Hoffmann and Prediger, 1984). However, if the decrease in sensitivity of the CaF2 :Tm peaks can be assumed to be only a function of LET, then from microdosimetric spectra and from


O tO o:- a'-

I.O

0.5

Fig. 3. LET dependence of CaF2:Tm (Hoffmann energy and is related to LET.

, peak 3 • peak 5

He +2 Ne +10 , , , , , 1 , , , , , , I , t I ~__

3 I0 30 I00 300

~D (KeV/jJ )

and Prediger, 1984). YD is the dose averaged lineal Pi is the height of peak 3 or 5.

Fig. 4, it should be possible to deduce the response for pion fields. Figure 5 shows the CaF2 :Tm glow curves in three regions of the pion dose distribution, viz. (1) plateau, (3) peak and (4) peak-fall off. Analysis of the data shows that the dependence of P = ( P 2 / P ~ ) / ( P 2 / P ] ) ~ ° C o on depth is the same as that of 17 D measured using a proportional counter, and the two are almost linearly related as

P = 1 +0.0167Y D. (1)

The ratio R of integrated TL response to H - dose (measured using an ion chamber) as a function of depth is empirically correlated with YD measurements:

R = 0.8 + 0.2 exp( - YD/22) (2)

= 0.8 + 0.2 exp[--(P -- 1)/0.367]. (3)

The ion chamber dose is then related to the integrated TL dose as

D I = DTL/R. (4)

Figure 4 shows the equality of the TL and ion chamber dose measurements using equation (4).

Neutron dose (D,) measurement can also be ac- complished using CaF2 :Tm, because the sensitivities of peaks 3 and 5 are very different (Hoffmann and Prediger, 1984):

Pt / (P l )~Co = D,~ + (EzK)D . (5)

where I = 3 and 5. The larger the difference in sensitivity E3 and Es, and

the larger the KERMA ratio K (dosimeter to tissue), the higher the accuracy. Therefore, it is better to use

3

2E

-- / ~4~ • Ion chamber measurement

2 0 : .

16

: i . . . . x

,2-

8 I0 12 14 16 18 20 22 24 26 28 30

Z (cm)

Fig. 4. Pion depth-dose measured using the integrated TL signal of CaF2:Tm and an ionisation chamber (Hoffmann et al., 1980).

52 V.K. JAIN

-o )- i J I-

I00

75

50

°\

i

i l';t i/ i i / "i @ ................. Plateau

ii11{ ® . . . . . .

~i / ! I ~ ) - - Peak- Falloff.

i .

i /..,..'.

C a F 2 :Tm

2 5 ;,

/ IOO 200 300

Temperature (°C)

Fig. 5. Glow curves of CaF: :Tm at different depths on the pion Bragg curve, Fig. 4 (Hoffmann et al., 1980).

CaF2:Tm encapsulated in polytetrafluoroethylene rather than exposed directly to the radiation. In fact encapsulation of the chips in polyethylene enhances the neutron sensitivity of peak 3 by about 40% and that of peak 5 by about 80% (Pradhan et al., 1984). The ratio of peak heights has also been found to rise linearly with the logarithm of average fl-energy (Furetta and Tuyn, 1985c).

CaF2 :Mn

CaF2:Mn has long been in use for radiation dosimetry (Horowitz, 1984). Its dosimetric characteristics are well described in Horowitz (1984, p. 141). The glow curve has generally been described as containing a single peak at about 250°C, as shown in Fig. 2. There are other lower temperature peaks, as shown in Fig. 6 (McMasters et al., 1987). which perhaps decayed during the 24h post-irradiation storage before the glow curves of Fig. 2 were recorded. In fact fading is an important characteristic of CaF2:Mn and has been studied in detail by de Planque (1984). She has reported a 3% decrease in TL response if the phosphor is stored for at least 17 h after irradiation. The decrease may be due to some optically related phenomenon. Also fading at each temperature is a linear function of the logarithm of the storage time (in the range of 17-161 h) following irradiation. The annealing regime recommended as an initialisation treatment and for re-use consists of 3 0 ~ 0 m i n at temperatures between 450 and 500°C. For measurements of low absorbed doses (<200 mGy) a re-use anneal at 400°C for 15-30 min

6

4

, I I ( a } l

I I I..~1

2

0 4

2

o

~ 4

~ 2

C

I.-

E

0 I00 200 300 400

T e m p e r a t u r e (*C)

Fig. 6. Thermoluminescence glow curves in CaF2:Mn following a ),-dose of 383Gy at room temperature. The Mn dopant concentrations are (a) 0; (b) 0.01%; (c) 0.1%; (d) 0.5%; (e) 1.0%; (f) 3.0% (McMasters et al.,

1987).


is adequate for the reduction of background signals (Driscoll et al., 1986). As shown in Fig. 6 the dopant concentration significantly affects the structure of the glow curve. At optimum concentration CaF2:Mn is also the most sensitive CaF2 based TL material, about 10 times as sensitive as the others. The emission spectrum consists of a single broad band with maximum at 495-500 nm. However, the possibility of exciting this emission by several wavelengths and the presence of even trace amounts of rare earth impurities complicate the understanding of the TL response (McMasters et al., 1987).

The TL response of CaF2:Mn has also been investigated below room temperature (Alonso and Alcal/t, 1980, 1981; Alonso et al., 1980; Jain and Jahan, 1985a,c). The glow curve of 0.1% Mn doped CaF 2 crystal is shown in Fig. 7. As many as five peaks can be easily seen and perhaps there are two more at 170 and 250 K (Jain and Jahan, 1985a). An investiga- tion of the influence of Mn concentration (0.1-5%) on the structure of the glow curve showed that the peak temperatures first increase and then decrease with increasing concentration. Also all the peaks eventually merge into one very broad peak with its maximum at 167 K (Jain and Jahan, 1985c). Kinetic parameters of the prominent glow peaks below RT in CaF2 containing 0.1% Mn (Jain and Jahan, 1985a) have also been obtained.

CaF 2" Gd, Tb,Lu

Glow curves of CaF 2 containing 0.1% RE (Gd, Tb, Lu) are shown in Fig. 8 in the temperature range 4-600 K. Both the Gd and the Tb doped samples have main glow peaks at 83 and 345 K. The

glow curve of CaF2:Lu is different. The emission spectrum contains lines due to the inner transitions of 4f-electrons in Gd and Tb. However the emission from CaF2 :Lu consists of broad bands (Kiessling and Scharmann, 1975). An interesting aspect of this study is the direct correlation of the glow peaks with the absorption spectrum which shows peaks at 315 and 540 nm. The thermal decay of the absorbing centre concentration in CaF2: Gd is shown in Fig. 9a and its negative derivative in Fig. 9h. Figure 9c shows the glow peaks. Similar correlations have been found in the Tb doped samples, but not in the Lu doped samples. Kiessling and Scharmann (1976) also carried out ionic thermocurrent measurements in these sam- pies. However, none of the ITC peaks is correlated with the TL glow peaks. The ITC peaks have been ascribed to the reorientation of the tetragonal R E 3 + - F - c o m p l e x e s .

T L MECHANISMS IN C a F 2

CaF2: R E

A popular explanation of the TL in RE doped CaF2 is as follows. X-ray irradiation reduces trivalent RE-ions to the divalent state. At the same time hole centres are produced which become mobile when the temperature is increased. If such a hole gets close to a divalent RE-ion, it may recombine with an electron. The ion is then in a excited trivalent state, and the decay to the ground state is observed as thermoluminescence. Merz and Pershan (1967) suggested that below room temperature most of the RE-ions involved in the TL process are those in cubic sites. The reduction of many RE 3÷ ions in cubic sites to the

A In "2

e u

o

1

97

\~,,, i I

O, 80

I I I 120

Caf 2 : Mn (0.1% )

X - IRRADIATED 1.6 X 104 R

7 X 104R

i/1 ' 'L._.-

160

194 . . . .

230

^

I !

, ~ I a 6 5 i I - / I I i I \ , \xo.3

200 240 280

Temperature ( K )

Fig. 7. Glow curve of CaF2:Mn (0.1%) for X-ray exposures of 1.6 x 104 rad ( ) and 7 x 104 rad ( - - ) . The sample was irradiated at 83 K and subsequently heated to 300 K (Jain and Jahan, 1985a).

54 V.K. JAIN

I0 C o F 2 : Gd

o J

? -

io

>,

-~ o

0 I00 200 300 400 500 600

T(K)

Fig. 8. Thermoluminescence glow curves of CaF2:Gd, Tb and Lu after X-ray excitation (10 4 rad at 4.2 K, heating rate 20 K rain -~) (Kiessling and Scharmann, 1975).

RE 2+ state has been reported at and below RT, including ions of interest in thermoluminescence, viz. Tm 3+, Dy 3+ and Sm 3+ (Hayes and Staebler, 1974, p. 419; Royce et al. , 1984). For RE 3+ ions in sites with

z_- c

t~

o

t 3

"?5

IO

aF2: Gd

_

.(5 o I-.-

I0 315 nm

5 - -

0 . . . . ..,,

I I

I I

\ b

0 I 0 0 2 0 0 :300 4 0 0

T ( K )

Fig. 9. (a) Temperature dependence of absorbing centre concentration in CaF 2 :Gd. (b) Negative derivative of (a). (c) TL glow curve of CaF 2 :Gd (Kiessling and Scharmann, 1975).

symmetry less than cubic giving TL about RT it has been suggested that hole traps consisting of fluorine aggregates break up, resulting in interstitial fluorine atoms diffusing to divalent RE-ions and converting them into RES+-Fi (Hayes and Staebler, 1974; Merz and Pershan, 1967). However, it may be that the TL emission at low temperature originates from ions in sites having other than cubic symmetry, and at high temperatures from cubic sites, because the compensators become mobile and leave the RE 3÷ ion in a cubic environment (Schlesinger and Whippy, 1969). There are variations from ion to ion from this general picture. For C a F 2 :Dy 3÷ it has been suggested that the emission originates from a mixture of cubic and other lower symmetry sites. (Schlesinger and Kwan, 1971). Another suggestion for the system CaF2:Er 3+ is that the high and low temperature TL emission is due mainly to cubic and tetragonal sites respectively.

Pai and Lee (1981) measured the fluorescence spectrum of Ho s+ doped CaF2 crystals at 77 and 300 K, and the TL emission spectrum after X-irradiation at the same temperatures. The high temperature (328 K) TL emission and low-temperature (77 K) luminescence spectra are similar, but the latter is dissimilar to the low-temperature (152K) TL emission although it covers the same spectral region. Pai and Lee have explained these observations as follows. Electron traps are created following the substitution of trivalent RE for divalent Ca. The effective potential energy of the 4felectrons possesses two minima, a deep one lying inside the 5s25p 6 closed shell and a shallow one lying outside this shell and forming a metastable RE 2+ state from which the


additional electron can be easily removed. Two types of TL centres result from irradiation at 77 K. Type I are formed by the usual reduction of RE 3+ to metastable RE 2+. Type II centres are a complex of RE 3+ with a nearest neighbour F-centre and the capture of an electron reduces the RE 3+ to RE 2+ (metastable). The presence of the F-centre distorts the symmetry and weakens the binding between the trapped electron and the RE 3÷. Thus the type II centres are less stable than the type I centres. On warming the crystal electrons are released from the metastable RE 2+, leaving behind an excited Re 3+ whose return to the ground state gives the observed emission. It is claimed that the same process occurs in CaF2 doped with other RE (Pai and Lee, 1981).

C a F 2: M n

In Mn doped CaF 2, Alcalfi et al., (1980), Alonso and Alcalfi (1980, 1981) and Alonso et al. (1980) attributed the emission observed at low temperatures to Mn 2+ and envisaged the formation of Mn ÷ and VKA (modified Vk) centres during X-irradiation. When the VKA centres are bleached thermally or optically, holes are released which recombine with Mn + ions and convert them to Mn 2+ in an excited state. The deexcitation of Mn 2+, from the 4T~g(4G) level to the 6Alg(6S ) level, leads to the 495nm emission. The experimental basis of this explanation is (i) an optical absorption (OA) band at 450 nm which is attributed to Mn +, (ii) destruction of this band during the appearance of the TL peak at 200 K, (iii) the activation energy for both the TL peak and the thermal decay of the OA is 0.51 eV. On the basis of ESR and OA studies, Baranov (1980) also inferred the presence of Mn + in CaF2:Mn irradiated at 77 K.

The inclusion of even trace amounts of RE impurities, in particular Ce, complicates the TL response of CaF 2 :Mn. The emission spectra of TL in CaF2:Mn (0.1%) and CaF2:Ce (0.1%) crystals obtained from Optovae Inc. is similar, in that both have two main bands at 345 nm and 495 nm. The glow curves of the two samples are also similar up to a temperature of 160 K (Jain and Jahan, 1985a). The 345nm emission in CaF2:Mn, attributed to Ce 3÷, disappears slowly when the irradiated sample is heated. During irradiation at 80 K free electrons and holes are produced. Mn 2+ present in different configurations captures these charge carriers and Mn + and Mn 3+ are formed. Electrons are trapped at other defect centres too (modified anion vacancies/complexes), and holes form modified VKA centres. Below 160 K, hole release leads to emission from Ce 3+ as well as from Mn 2+. In addition, as the temperature increases from 80 to 160K, Ce 3+ transfers increasingly its energy directly to Mn :+. Above about 160 K electron release and capture at Mn 3+ cause the 495 nm emission from Mn 2+ (Jain and Jahan, 1985a).

The TL of CaF2:Mn above RT is also affected by the presence of Ce (Jassemnejad and McKeever,

o.

¢(

4 0

30

20

I0

• - - - - - r r

(a)

30 (b)

2O

I o

4 0

3 0

20

IO

t J 400 600 800

Wavelength (nm)

Fig. I0. The optical absorption from CaF 2 samples doped with: (a) Ce, 0.5%; (b) Mn, 0.5%; (c) Ce, 0.5% and Mn,

0.5%. Dose 2.68 kGy (McMasters et aL, 1987).

1987; McMasters et al., 1987). CaF2:Mn exhibits several absorption bands upon irradiation (Fig. 10b). The strength and the thermal stability of this absorption increase with Mn content and are affected by the presence of unwanted trace impurities, notably RE. Figure 10a shows the OA obtained from a CaFE:Ce (0.5%) sample irradiated to 2.68 kGy of 6°Co 7-rays. The sharp peak at 303 nm is due to 4f -5d transitions within Ce 3+ ions in C4v symmetry; there are also absorption in the UV region (not shown) due to clusters of tetragonal centres. The absorption due to cubic Ce 2÷, PC and ionised PC centres has also been identified (Jassemnejad and McKeever 1987). Figure 10c shows the absorption in a sample doped with both Ce and Mn. A comparison of Figs 10a-c shows that Ce is able to suppress completely OA due to Mn. Even the TL glow curves have the characteristics of Ce doped CaF 2 . It has been suggested, on the basis of photoluminescence studies, that preferential pairing of Mn 2+ions with Ce 3+ions occurs in tetragonal sites, along with efficient energy transfer from Ce 3+ to Mn 2+ at these pairs. However, in X-ray luminscence and thermoluminescence any significant

56 V.K. JAIN

energy transfer is not observed. Instead tetragonal centres other than those of Ce 3+, cubic and PC centres appear to dominate these processes (McKeever et al,, 1986).

CONCLUSIONS

We have reviewed charge trapping centres in rare-earth doped CaF2 and CaF : :Mn , and also discussed the dosimetric properties of these and similar materials. While our knowledge of the defect centres formed as a result of RE doping is considerable, there is still controversy regarding the concentrat ion and temperature dependence of the relative abundance of different impurity sites. The model for the cubic symmetry of RE ions is also in dispute. RE ions in cubic sites (non-locally compensated) are probably converted by X-irradiation and play the main role in the TL emission. But the role of other defects, photochromic centres and clusters cannot be overlooked. The reduct ion-oxidat ion of RE ions model of the TL emission in CaF2:RE is probably largely correct but is certainly not the last word. Even for the seemingly simple CaF2:Mn the situation is so complicated that an accepted model has not yet emerged, Nevertheless the dosimetric characteristics of CaF 2 :Tin, CaF2 :Mn and CaF 2 :Dy are now well documented. The use of CaF2:Tm for the dosimetry of neutron and high LET radiations, and for mixed photon and particle fields, is noteworthy.

REFERENCES

Alcalfi R., Alonso P. J., Lalinde G. and Carretero A. (1980) Phys. Star. Sol. b 98, 315.

Alonso P. J. and Alcalfi R. (1980) J. Luminescence 21, 147. Alonso P. J. and Alcal~ R. (1981) J. Luminescence 22, 321. Alonso P. J., Orera V. M. and Alcal~. R. (1980) Phys. Stat.

Sol. b 99, 585. Andeen, C. G., Fontanella J. J., Wintersgill M. C., Welcher

P. J., Kimble R. J. Jr and Mathews G. E. Jr (1981) J. Phys. C: Solid State Phys. 14, 3557.

Anderson C. H. (1974) In Crystals with the Fluorite Structure (Edited by Hayes W.), p. 281. Clarendon Press, Oxford.

Anderson C. H. and Salinsky E. S. (1971) Phys. Rev. B3, 527. ANSI (1975) Performance Testing and Procedural Specifica-

tion for Thermoluminescence Dosimetry (Environmental Applications), ANSI N545-1975 American National Standards Institute, New York.

Arkhangel'skaya V. A. (1964) Opt. Spectrosc. 16, 343. Arkhangel'skaya V. A. (1965) Opt. Spectroc. 18, 46. Baranov P. G. (1980) Soviet Phys. Solid State 33, 133. Batygov S. Kh. and Osikov V. V. (1972) Soviet Phys. Solid

State 13, 1886. Beaumont J. H., Hayes W., Kirk D. L. and Summers G. P.

(1970) Proc. R. Soc. A315, 69. Becker K. (1973) Solid State Dosimetry. CRC Press,

Cleveland. Ben Shachar B., Laichter Y., German U. and Weiser G.,

(1985) Radiat. Protect. Dosim. 12, 333. Chrysochoos J. (1984a) J. Luminescence 31, 254. Chrysochoos J. (1984b) J. Luminescence 31, 256. Driscoll C. M. H., Francis T. M. and Richards D. J. (1984)

Radiat. Protect. Dosim. 9, 269.

Driscoll C. M. H., Barthe J. R., Oberhofer M., Busuoli G. and Hickman C., (1986) Radiat. Protect. Dosim. 14, 17.

Fong F. F. (1964) J. Chem. Phys. 41, 245. Fontanella J. J., Tracey D. J. and Andeen C. G. (1980)

J. Chem. Phys. 72, 2235. Fontanella J. J., Wintersgill M. C., Koladziejczak G. C. and

Figureoa D. R. (1984) Nucl. Instrum. Meth. Phys. Res. 1, 431.

Furetta C. and Lee Y. K. (1983) Radiat. Protect. Dosim. 5, 57. Furetta C. and Lee Y. K. (1985) Radiat. Protect. Dosim. 11,

101. Furetta C. and Tuyn J. W. N. (1985a) Int. J. Appl. Radiat.

Isotopes 36, 1000. Furetta C. and Tuyn J. W. N. (1985b) Int. J. Appl. Radiat.

Isotopes 36, 893. Furetta C. and Tuyn J. W. N. (1985c) Radiat. Protect

Dosim. 11, 263. Furetta C. and Tuyn J. W. N. (1986) Int. J. Appl. Radiat.

Isotopes 37, 458. G6rlich P., Karras H. and Koch A. (1965) Phys. Star. Sol.

12, 203. Hasan F. and Charalambous S. (1983) J. Phys. C: Solid

State Phys. 16, 5921. Hayes W. and Staebler D. L. (1974) In Crystals with the

Fluorite Structure (Edited by Hayes W.), p. 415. Clarendon Press, Oxford.

Hayes W. and Twidell J. W. (1961) J. Chem. Phys. 35, 1521. Heist R. H. and Fong F. K. (1970) Phys. Rev. BI, 2970. Hoffmann W. and Prediger B. (1984) Radiat. Protect.

Dosim. 6, 149. Hoffmann W., M611er G., Blattmann H. and Salzmann

(1980) Phys. Med. Biol. 25, 913. Horowitz Y. S. (Editor) (1984) Thermoluminescence and

Thermoluminescent Dosimetry, Vol. 1. CRC Press, Boca Raton.

Hsu P.-C, Jao J. C. and Weng P. S. (1985) Radiat. Protect. Dosim. I1, 123.

Jain V. K. (1984). In Thermoluminescence and Thermolu- minescent Dosimetry (Edited by Horowitz Y. S.), Vol. 2, p, 185. CRC Press, Boca Raton.

Jain V. K. and Jahan M. S. (1985a) Phys. Stat. Sol. a 92, 237.

Jain V. K. and Jahan M. S. (1985b) J. Phys. D: Appl. Phys. 18, L15.

Jain V. K. and Jahan M. S. (1985c) Bull. Am. Phys. Soc. 30, 1783.

Jassemnejad B. and McKeever S. W. S. (1987) J. Phys. D: Appl. Phys. 20, 323.

Kask N. E. and Korienko L. S. (1968) Soviet Phys. Solid State 9, 1670.

Kiessling J. and Scharmann A. (1975) Phys. Star. Sol. a 32, 459.

Kiessling J. and Scharmann A. (1976) Z. Naturforsch. 31a, 105.

Kiss Z. J. and Yocom P. N. (1964) J. Chem. Phys. 41, 1511. Lu J. K., Tung C. J. and Hsu P. C. (1983) Radiat. Effects

Lett. 76, 215. Manthey W. J. (1973) Phys. Rev. 8, 4086. McKeever S. W. S. (1985) Thermoluminescence of Solids.

Cambridge University Press, Cambridge. McKeever S. W. S., Jassemnejad B., Brown M. D., Mathur

V. K., Abundi R. J. and Chan H. (1986) Radiat. Effects 99, 15.

McMasters D. W., Jessemnejad B. and McKeever S. W. S. (1987) J. Phys. D: Appl. Phys. 20, 1182.

Merz J. L. and Pershan P. S. (1967) Phys. Rev. 162, 217,235. Moore D. S. and Wright J. C. (1979) Chem. Phys. Left. 66,

173. Moore D. S. and Wright J. C. (1981) J. Chem. Phys. 74,

1626. Nakajima T. (1977) Jap. J. Appl. Phys. 16, 1061. O'Hare J. M. (1972) J. Chem. Phys. 57, 3838. Pai S. T. and Lee K. C. (1981) Phys. Rev. B23, 2022.


Planque G. de (1984) U.S. DOE Report EML-418. Planque G. de and Gesell T. F. (1982) Int. J. Appl. Radiat.

Isotopes, 33, 1015. Pradhan A. S. and Bhatt R. C. (1983) J. Phys. D: AppL

Phys. 16, 53. Pradhan A. S. and Bhatt R. C. (1987) Radiat. Protect.

Dosim. 20, 237. Pradhan A. S. and Rassow J. (1987) Nucl. Instrum. Meth.

A255, 234. Pradhan A. S., Rassow J., Meissner P. and Olthoff Muenter

K. (1984) Radiat. Protect Dosim. 9, 269. Ratnam V. V. and Banerjee H. D. (1976) Phys. Stat. Sol. a

36, 693. Royce G. A., Mathur V. K., Abbundi R. J., Brown, M. D.

Fontanella J. J. and Wintersgill M. C. (1984) J. Lumines- cence 29, 205.

Schlesinger M. and Kwan C. T. (1971) Phys. Rev. B3, 2852.

Schlesinger M. and Whippy P. W. (1969) Phys. Rev. 177, 563.

Seelbinder M. B. and Wright J. C. (1981) J. Chem. Phys. 75, 5070.

Staebler D. L. and Schnatterly S. E. (1971) Phys. Rev. 133, 516.

Strand T. and Stranden E. (1985) Radiat. Protect. Dosim. 12, 261.

Sunta C. M. (1984) Radiat. Protect. Dosim. 8, 25. Tsuda M., Katsurada T., Ando F., Kawamata F. and

Yunogami S. (1982) Strahlentherapie 158, 563. Tsuda M., Katsurada T,, Ando F., Kawamata F. and

Yunogami S. (1984) Strahlentherapie 6, 175. Tuyn J. W. N. (1982) Radiat. Protect. Dosim. 2, 69. Twidell J. W. (1970) J. Phys. Chem. Solids 31, 799. Van Gorkon G. G. P. (1970) J. Phys. Chem. Solids 31, 905. Wang T. K., Hsu P.-C. and Weng P.-S. (1986) Radiat.

Protect. Dosim. 16, 225.

charge carrier trapping and thermoluminescence in calcium fluoride based phosphors

Documents