Journal of Luminescence 131 (2011) 1864–1868

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Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Influence of Li dopants on thermoluminescence spectra of CaSO4 dopedwith Dy or Tm

Y. Wang a,n, N. Can b, P.D. Townsend c

a School of Materials Science and Engineering, China University of Geosciences, Beijing 100083, Chinab Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Manisa, 45140, Turkeyc School of Science and Technology, University of Sussex, BN1 9QH, UK

a r t i c l e i n f o

Article history:

Received 21 January 2011

Received in revised form

15 April 2011

Accepted 20 April 2011Available online 4 May 2011

Keywords:

Thermoluminescence

CaSO4:Dy, Li

CaSO4:Tm, Li

Phosphor

13/$ - see front matter & 2011 Elsevier B.V. A

016/j.jlumin.2011.04.042

esponding author. Tel.: þ861082322379.

ail address: wyfemail@gmail.com (Y. Wang).

a b s t r a c t

Thermoluminescence emission spectra are presented for lithium doped variants of CaSO4:Dy or

CaSO4:Tm dosimetry material. All three dopants (Li, Dy and Tm) variously introduce different changes

in both the glow peak temperatures and the luminescence efficiency. In every case the emission signals

display the line emission characteristic of the rare earth ions. At temperatures below �50 K the relative

peak intensities differ for Dy and Tm doped samples, and there are small temperature shifts between

the Dy:Li and Tm:Li co-doped materials. Above room temperature the rare earth ions do not show peak

temperature movements when co-doped with lithium. However they do influence the peak tempera-

ture by �5 1C when they are the sole dopant. Inclusion of lithium dramatically moves the high

temperature glow peak from �200 1C down to 120 1C. All these changes are consistent with a single

defect model in which the trapping sites and luminescence occur within the complexes formed of the

rare earth ion, an intrinsic sulphate defect and lithium. The evidence and rationale for such a model are

presented. There is discussion which suggests that such defect complexes are the norm in thermo-

luminescence.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

For more than four decades, thermoluminescence (TL or TSL)dosimetry has been crucial for both individual and environmentalradiation monitoring. Many successful materials have emergedthat include aspects of high sensitivity, reliability, chemical andthermal stability, as well as ease of production and economiccommercial availability. Among the many phosphors used orconsidered as TL radiation dosimeters are variants of rare earth(RE) doped calcium sulphates, especially CaSO4:Dy or CaSO4:Tm.These sulphates are widely used for both individual and environ-mental radiation monitoring [1–3]. Their popularity has led to anextensive series of studies aimed at elucidating their behavior,attempting to assess the underlying mechanisms, and efforts topredict, or empirically develop new variants of these phosphors[4–10]. One particularly interesting result was reported by DrProkic who found that the addition of a lithium co-dopant duringpreparation of the CaSO4:Dy (or Tm) results in a major increase ofluminescence efficiency. Co-doping with Li impurities alters thethermoluminescence response of these TL phosphors and theeffects have been studied as a function of Li concentration [11].

ll rights reserved.

However, the detailed thermoluminescence spectra of CaSO4:Li,Dy (or Li:Tm) have not been reported. Secondly, the way in whichinclusion of lithium alters the details of the thermoluminescenceprocesses have not been satisfactorily discussed. Consideration ofnew mechanisms is therefore required. This present work is anattempt to address these issues, and in particular to includespectrally resolved data of the thermoluminescence and comparesuch spectra for Li as a co-dopant in CaSO4:Dy with earlier datafor Li free material.

2. Experimental

The phosphors used here were prepared at the institute ofNuclear Sciences—Vinca, Belgrade, Serbia [11,12]. Unusually, thelithium co-dopant was not added during the initial growth andcrystallization stage. Instead, lithium was successfully introducedat a concentration of 0.06% during a subsequent step of pressingand sintering of the TLD pellets. A number of alternative lithiumcompounds have been used in this role and they include Li2CO3,LiCl, LiF, Li2B4O7 and Li2SO4. Optimization trials are still inprogress.

The TL data presented here were the result of initial radiationtreatments with an X-ray radiation dose of 5 Gy in each case. Theirradiations and spectral measurements were made in-situ, so it

Fig. 1. (a) An isometric plot of thermoluminescence spectra of CaSO4:Tm, Li at

high temperature. (b) An isometric plot of thermoluminescence spectra of

CaSO4:Dy, Li at high temperature.

2000

2500

nits

)

CaSO4: Dy, LiCaSO4: Tm, Li

Y. Wang et al. / Journal of Luminescence 131 (2011) 1864–1868 1865

was feasible to monitor TL after low temperature irradiation, aswell as after room temperature irradiation.

Rather than just measuring an integrated luminescence signalvia a broad band optical filter and a blue sensitive photomultipliertube (as used in commercial TL dosimetry systems) we havebenefitted from the use of spectrally resolved TL data. In order todo this we used a high sensitivity system employing wavelengthmultiplexed spectrometers [13,14]. The spectral resolution usedhere was �5 nm. After data collection the signals were processedto correct the wavelength sensitivity of the spectrometer anddetector system. Only corrected spectra are presented here. Notealso that the wavelength multiplexing allows spectral resolutionwithin a temperature range of about one degree. This is essential,both to check if different spectral components occur at differenttemperatures (as can occur in doubly doped materials), and alsoto note if there are small temperature shifts in signals fromdifferent samples.

TL spectra were collected either at a high temperature rangefrom 30 to 400 1C, or at low temperature from 25 to 280 K.Samples irradiated at room temperature were then heated at arate of 0.5 1C/s during TL recording. For low temperature data thesamples were irradiated at 25 K and the TL was recorded with aheating rate of 0.1 K/s.

These are relatively very low heating rates compared withthose used in commercial dosimeters. However, the spectrometersystem is sufficiently sensitive that one can readily record thechanging spectra. Of greater importance is that the low heatingrates ensure that there are only small temperature gradients fromthe thermocouple, at the heater, to the surface of the samples.Tighter thermal coupling avoids the problem encountered in thecommercial systems where the apparent signal temperature(measured at the heater stage) differs from the actual tempera-ture of the emitting surface of the TL sample [15–19]. The glowpeak data could be replicated within about one degree.

200Wavelength (nm)

0

500

1000

1500

Inte

nsity

(arb

. u

300 400 500 600 700 800

Fig. 2. Emission spectra from CaSO4:Dy, Li and CaSO4:Tm, Li at 120 1C.

30Temperature (°C)

0

20

40

60

80

100

Nor

mal

ised

Int(a

rb. u

nits

)

470 nm570 nm

80 130 180 230 280 330 380

Fig. 3. High temperature glow curves recorded at two Dy emission wavelengths

from CaSO4:Dy, Li. The intensities are normalized at 120 1C.

3. Results

3.1. High temperature TL

Isometric presentations of high temperature thermolumines-cence spectra of the doubly doped samples are shown in Fig. 1.TL temperature dependent spectra from a CaSO4:Dy, Li sample areshown in Fig. 1a. The equivalent TL spectra from a CaSO4:Tm,Li sample are shown in Fig. 1b. It is obvious that both the samplesexhibited only sharp line luminescence signals characteristic ofthe rare earth dopants (either Dy or Tm). There is negligibleevidence for any broad band emission, as would be the character-istic of the pure host lattice. The details of the emission spectra areshown in Fig. 2 for data taken at 120 1C. For the CaSO4:Dy,Li material the main emission lines are at 470, 570 and 655 nm.These are the familiar characteristic emission lines from Dy3þ

transitions and they are respectively assigned to the electronictransitions between 4F9/2 and 6H15/2, 6H13/2, 6H11/2 states. Theemission lines of CaSO4:Tm, Li are at 340, 360, 455 and 480 nmand are similarly often assigned to Tm3þ transitions associatedwith 3P0-

3F4, 1D2-3H6, 1D2-

3F4 and 1G4-3H6 transitions.

Although the spectra are characteristic of the rare earthdopant, the temperature dependence of the high temperatureglow curves are identical within experimental error, as is shownin Fig. 3 for Tm signals at 470 nm and Dy signals at 570 nm. For asingle dopant the different lines have glow temperatures whichare identical, but in intensity terms there are small differences asa function of emission line. This is reasonable as the transitionprobabilities are a function of temperature.

25 75 125 175 225 275Temperature (K)

0

25

50

75

100

Nor

mal

ised

Int (

arb.

uni

ts)

470 nm570 nm

25

50

75

100

rmal

ised

Int (

arb.

uni

ts)

470 nm Dy450 nm Tm

Y. Wang et al. / Journal of Luminescence 131 (2011) 1864–18681866

Fig. 3 indicates that in addition to a glow peak centered near120 1C there is a higher temperature signal evident near 380 1C.The temperature values of these glow peaks differ considerablyfrom the values cited for calcium sulphates which contain rareearth dopant ions, but no addition of lithium. When these areemployed in commercial dosimetry systems, at very fast heatingrates, the main dosimetry peak is expected to be at 220 1C. Thetemperature dependence, and emission spectra of the originalcommercial material (i.e. without lithium) have been recorded asa function of heating rate [20] and this indicates that for the lowerheating rates, as used here, the peak temperatures will occur near200 1C. The presence of lithium has thus had a dramatic effect inreducing the glow peak temperature from around 200 1C down to120 1C in accordance with the simpler polychromatic data [11,12].

3.2. Low temperature TL

An isometric view of low temperature thermoluminescencespectra of CaSO4:Dy, Li is shown in Fig. 4. TL spectra once againexhibits sharp line luminescence, which is characteristic of therare earth dopants. A comparison of emission spectra ofCaSO4:Tm, Li recorded at two widely different temperatures isshown in Fig. 5. In this figure the spectral intensity obtained at120 1C was increased threefold for ease of a visual comparison(i.e. because it is much weaker than the low temperature emis-sion). The peak wavelengths are constant within the experimentalmeasurements over this temperature range.

At low temperature, for the dysprosium doped samples, thepeaks at 470 and 570 nm display the same shape of their glow

Fig. 4. An isometric plot of TL signal of CaSO4:Dy, Li at low temperature.

200Wavelength (nm)

0

1500

3000

4500

6000

7500

Inte

nsity

(arb

. uni

ts)

120 °C200 K

300 400 500 600 700 800

Fig. 5. Emission spectra of CaSO4:Tm, Li with temperature.

25 75 125 175 225 275Temperature (K)

0

No

Fig. 6. (a) TL glow curve of CaSO4:Dy, Li at low temperature. (b) Comparison of TL

glow curve of CaSO4:Dy, Li and CaSO4:Tm, Li at low temperature.

curves. This is shown in Fig. 6a, which overlays wavelengthselected glow curves for the dysprosium lines. The data werenormalized at the higher temperature glow peak. There is anobvious difference in the relative low and higher temperatureluminescence intensities of the peaks generated by Dy and Tmbelow 280 K. In terms of signal intensity there is a bonus thatthulium doping offers an improved sensitivity below about 50 K.Consequently there are some differences in the form of the lowtemperature glow curves when comparing Dy and Tm dopedsamples. This effect is apparent as shown in Fig. 6b. The twosignals were normalized near 200 K.

Fig. 6b emphasizes that there are a number of low temperaturefeatures located approximately near 40, 100, 200 and 240 K. Oncloser inspection one notes that they differ in relative intensityand there are small temperature differences between the Dy and Tmdoped signals. For example, there is about a 5 K shift at 240 Kbetween the two types of sample. The benefits of the wavelengthmultiplexed spectral recording are that the entire spectral rangeis captured within one degree and thus one can be confident thatthe changes in peak temperature seen above �200 K are realfeatures. A further peak just above 280 K may exist, but experi-mentally this was inconvenient to access with this version of theTL heating system.

4. Discussion

High and low temperature thermoluminescence measure-ments (Fig. 2) confirm the earlier data for rare earth doped

Y. Wang et al. / Journal of Luminescence 131 (2011) 1864–1868 1867

calcium sulphates that emission spectra of CaSO4:Li, Dy or Tm aredominated by characteristic emission lines of Dy3þ or Tm3þ , asreported in other literature [4,8]. A possible process of TL in CaSO4

(RE) phosphors was discussed by Lapraz et al. [21]. They proposeda simple model to explain why emission spectra are characteristicof Dy3þ or Tm3þ , even when the lithium impurity was added. Amore general model has been suggested by Mathews et al. andMorgan et al. [22,23], in which the RE ions need act directly at therecombination center, but rather, there could be an energytransfer between the recombination site and the rare earth ion.Whilst these were sensible proposals based on the then availabledata, both models have weaknesses in the light of data such asthat presented here with lower heating rates, and TL dataobtained at very low temperature. In particular the low tempera-ture TL data show temperature sensitivity to the choice of Dy orTm. This implies that the minor differences in lattice distortionscaused by different rare earth ions must be directly influencingthe original charge release phase of the TL process as reported byBos et al. [24,25]. Such obvious effects then imply that the rareearth ions and the charge traps are intimately connected. Thisargues against either models of independent rare earth lumines-cence sites, and/or an energy transfer process from distant sites.Energy transfer within a more complex defect package is stillviable.

The concept of complex groups of lattice defects is strength-ened by the fact that in the lithium doped material the hightemperature dosimetry peak (originally at �200 1C for our heat-ing rate) has fallen to just 120 1C. Such a major lowering of trapstability emphasizes that there is an intimate and close connec-tion between the charge trap and the lithium dopant.

In lithium free material it had clearly been demonstrated [20]that there is a temperature displacement between Dy and Tmdoped samples at the 220 1C dosimetry peak. This feature waselegantly confirmed by using doubly doped material (i.e. bothDy and Tm) and the difference was again apparent in thespectrally resolved signals from Dy and Tm. Such a resultconfirms that the rare earth ion is also intimately linked to thecharge trapping site for this high temperature glow peak. Coupledwith the evidence that addition of lithium drops the trap stabilitydown to 120 1C, means that the TL dosimetry signals must bebased on a very complex set of defect components which involvethe rare earth, the lithium, and an intrinsic feature of the hostlattice, all packaged within a single overall defect complex.

This concept of a multi-component defect complex in calciumsulphates is in agreement with the earlier suggestions that all thedopants distort the charge trapping sites, and by implication arenot totally independent of them [11,12,20,26]. In the current datawe have noted a similar effect for low temperature TL (Fig. 6b) forsignals above �200 K. Overall this implies that the traps that arebeing thermally deactivated may be host lattice related, but theyare not independent of the RE and lithium dopants.

In this material the interlinked nature of the charge capture,trapping, thermal release and thermoluminescence offer bothgains and disadvantages. The benefit from addition of lithium isthat the overall efficiency has been increased [11]. However, thereis a negative feature in that the lower temperature implies agreatly reduced long term storage capability for the dosimeter.Longer storage times might still be obtained via the new glowpeak reported near 380 1C.

Whilst there is clear evidence of a multiplicity of componentsin the defect complex, which provides thermoluminescence andthe data do not identify the intrinsic components [23,27,28].However, there is general agreement that SO4

� related ions couldplay an important role in the TL. It was suggested by Huzimuraand Asahi [28], that the 120 1C TL peak may be caused by therecombination of charge carriers from SO4

� , whereas the peak at

220 1C may be produced by a stimulated relaxation of SO3� ions

(i.e. the lithium has altered the charge state of the intrinsic site).There are several variations on models for the SO4

� centers, whichcan be stabilized by a nearest-neighbor Ca vacancy, and formationof Ca vacancies can be prevented by co-doping with monovalentcations [23,29]. The fact that Li dopants shift the 220 1C peak tolower temperature in CaSO4:Dy or Tm may be explained this way.In terms of local charge equilibrium the added Li centers could actas Liþ at Ca2þ sites, which would provide charge compensationas needed when Dy3þ or Tm3þ ions replace adjacent Ca2þ ions.The Ca2þ vacancies created by the incorporation of Dy3þ or Tm3þ

would be then automatically removed along with a sulphateradical related to the hole traps. Such a change might stabilizethe vacancies of SO3

� , as suggested by Huzimura. Variants on suchmechanisms can be used to explain the change of the 220 1C peakwith other monovalent co-dopants, such as Naþ and Agþ [10,30].The models imply removal of Ca2þ ions, and their replacementwith Tm3þ or Dy3þ ions. These new sites would favor associationwith monovalent lithium ions as this would achieve a local chargebalance [31].

The current suggestion of thermoluminescence totally basedwithin a large complex of components of intrinsic site, rare earthand lithium dopants has numerous precedents. As has beendiscussed by many authors, the original concept of TL in whichthere are independent charge traps and luminescence recombina-tion sites is very naive and in reality lattice energy is minimizedby having the various types of defect distortions associated inlarge complexes [32]. Indeed, the early classic examples for TLdosimetry complexes were discussed in terms of the way Mg, Tiand O are incorporated into LiF in a single package. The equivalentcomplex for the doubly doped CaSO4, together with a RE andlithium, not only offers local charge compensation but minimizesdistortional energy of the lattice sites. The presence of lithiumprovides a preferred site for charge trapping and reduces the trapdepth for charge and energy transfer to the RE ion. These bothraise the overall efficiency and lowers the glow peak temperature.(i.e. from 220 to 120 1C).

Such a model is not exclusive of a role for the SO4� radical [33],

which could equally be stabilized by the Liþ ion occupying a Ca2þ

site. All such features could result in the observed temperatureshift to 120 1C, and enhancement of the signal.

It is only possible to make a brief comment on the nature ofthe newly detected glow peak at 380 1C. It certainly has atemperature similarity with that reported at 375 1C in CaSO4:Dy,Ag [34]. One speculative model for this signal was that it might becorrelated with holes released from Ag2þ , together with recom-bination centers produced by the SO3

2� radical. If in fact the samedefect site is relevant in each case then the Ag2þ model must bediscarded, since no silver was added in the present work. How-ever, further data are needed to clarify such models.

For commercial dosimetry, thermoluminescence must beabove room temperature. Consequently there is a very limitedliterature for low temperature TL of CaSO4 based phosphors,although low temperature TL normally gives more intensity. Glowcurves at low temperature range for the current two phosphorsare shown in Fig. 6. Other low temperature TL has been reportedfor CaSO4 co-doped with Ce and Mn [35]. There were signals,which could be spectrally separated and related to Ce and Mnsites. Both impurities revealed intense TL near 40 K and a weakerbroad emission just below 100 K. Such a very low temperaturesignal is in line with the current low temperature data shown inFig. 6b. However, in the earlier work there was no indication ofthe features from Dy and Tm seen here above 175 K.

Our focus has been on resolution of changes in the spectra sowe have not assessed the activation energies for the low tem-perature peaks, nor have we attempted to resolve how many

Y. Wang et al. / Journal of Luminescence 131 (2011) 1864–18681868

different peaks are hidden within the overall low temperatureglow curve. In general for low heating rates one may estimate theactivation energies (E) by noting the peak temperature (Tmax) andusing the rough guide that E is approximately 25 kTmax. Such anapproximation has the assumption that the relevant frequencyfactor is of the order 1012 or 1013 per second, which occurs inmany cases. Nevertheless TL, which involves several componentsof a complex association of trapping and emission sites, candisplay very different apparent frequency factors and conse-quently unexpected values for the activation energies. Such asituation has been noted for LiF:Mg:Ti dosimeters, where peaks1–4 are normal but peak 5 has extremely high values, which offerinsights into the mechanism [36].

From the ever expanding literature on thermoluminescencethere is an emerging pattern, in which the most probable TLprocess is one where there is close association of the trap andemission center. Such models minimize the lattice energies fromseparated types of defect, allow local charge equilibrium and veryefficiently feed the energy from charge release to the rare earthemission sites. We therefore favor a model where the entireprocess occurs within a single defect complex [32,37]. Suchcomplexes automatically result in features of peak temperatureshifts caused by differences in the size of different rare earth.There are equivalent changes ensuing from increases in dopantconcentration and consequent pairing or precipitation of impu-rities in order to minimize the lattice distortions. Indeed a peakshift for CaSO4 has also been reported with both dopant concen-tration and radiation dose [38,39]. Parallel experiments with rareearth ions in Bi4Ge3O12 or LaF3 similarly indicate clear linksbetween the ionic sizes and temperature differences with thevarious RE glow peaks [40,41]. All such observations lead us tobelieve that the trap and recombination center are not indepen-dent and the TL process intimately involves the trap, lithium andthe rare earth ion in the same extended defect complex.

5. Conclusion

Both high and low temperature thermoluminescence ofCaSO4:Dy and CaSO4:Tm co-doped with lithium are reported.Addition of lithium enhances the overall luminescence efficiencyof the material, but significantly shifts the dosimetry peak from220 1C down to 120 1C for both the materials. This is in contrastwith the lithium free material where there is a 5 1C difference inpeak temperature. However, low temperature data still showvariations between the Dy and Tm dopants. All the evidencepresented here is compatible with the models of thermolumines-cence, in which the impurities added can introduce high lumines-cence efficiency, and/or control the glow peak temperatures, anddo so through the formation of large complex structures. Thebenefits of a complex involving all the components of trapping,charge storage and luminescent site include (a) an intimatecoupling between the components, and thus a high efficiency;(b) the ability to vary the trap stability and thus the temperatureof the glow peaks; (c) a minimization of the lattice distortions andstrains within the host material; and (d) localized charge equili-brium. We are therefore suggesting that such complex formationis both desirable and inevitable. Appreciation of this situation is akey to understanding the TL processes.

For the investigations of TL processes the work underlines boththe value of low heating rates (which aid temperature control andreproducibility) and spectrally resolved TL (as this enables one todistinguish effects from different dopants and emission sites). Infuture studies where the luminescence and trapping sites are

potentially linked it would be helpful to have materials dopedwith more than one rare earth ion, or a range of dopant concentra-tions, as these should reveal the presence of complexes bytemperature shifts of the spectrally resolved components. Further,such evidence can be particularly obvious in low temperature TL,even though applications are above room temperature.

Acknowledgments

One of the authors, Y. Wang thanks the support of theFundamental Research Funds for the Central Universities of China.

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