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Journal of Luminescence 130 (2010) 1834–1840

Contents lists available at ScienceDirect

Journal of Luminescence

0022-23

doi:10.1

n Corr

E-m

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

Dosimetric properties of rare earth doped LiCaBO3

thermoluminescence phosphors

S.R. Anishia b, M.T. Jose a,n, O. Annalakshmi a, V. Ponnusamy b, V. Ramasamy b

a Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamilnadu, Indiab Department of Physics, Annamalai University, Chidambaram, Tamilnadu, India

a r t i c l e i n f o

Article history:

Received 6 October 2009

Received in revised form

9 April 2010

Accepted 28 April 2010Available online 4 May 2010

Keywords:

Thermoluminescence

LiCaBO3

Radiation dosimetry

Phosphor

Emission spectra

13/$ - see front matter & 2010 Elsevier B.V. A

016/j.jlumin.2010.04.019

esponding author. Tel.: +91 44 27480352; fa

ail address: mtj@igcar.gov.in (M.T. Jose).

a b s t r a c t

Lithium Calcium borate (LiCaBO3) polycrystalline thermoluminescence (TL) phosphor doped with rare

earth (RE3 +) elements has been synthesized by high temperature solid state diffusion reaction. The

reaction has produced a very stable crystalline LiCaBO3:RE3 + phosphors. Among these RE3 + doped

phosphors thulium doped material showed maximum TL sensitivity with favorable glow curve shape.

TL glow curve of gamma irradiated LiCaBO3:Tm3 + samples had shown two major well-separated glow

peaks at 230 and 430 1C. The glow peak at 430 1C is almost thrice the intensity of the glow peak at

230 1C. The TL sensitivity of the phosphor to gamma radiation was about eight times that of TLD-100

(LiF). Photoluminescence and TL emission spectra showed the characteristic Tm3 + peaks. TL response to

gamma radiation dose was linear up to 103 Gy. Post-irradiation TL fading on storage in room

temperature and elevated temperatures was studied in LiCaBO3:Tm3 + phosphor.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

There is a continuous growing interest in TL materials forradiation dosimetry, especially in the fields of environmental,personal and clinical applications. Though several phosphors arebeing used in these fields today, much efforts are in progress inorder to develop new TL materials, having better tissue equiva-lence, neutron and gamma sensitivity [1]. TL materials based onborates have an important place in dosimetry for their near-tissue-equivalent absorption coefficient and neutron sensitivity[2,3]. Thermoluminescence (TL) characteristics of the mixedborate LiCaBO3 (LCB) doped with rare earth elements [4,5] havebeen studied recently for the application as radiation dosimetryphosphor. Jiang et al. [5] synthesized this phosphor by high-temperature solid state reaction. The influence of different rareearth dopants, i.e. Tm3 +, Tb3 +, Ce3 + and Dy3 +, on thermolumines-cence of LiCaBO3, is also investigated. The effective atomicnumber of this material is 14.86 slightly lesser than CaSO4:Dy,which is presently used as a dosimeter material for personnelmonitoring in nuclear industry. Luminescence properties ofLiCaBO3 (LCB) phosphor doped with rare earth elements such asEu3 +, Sm3 +, Tb3 +, Ce3 + and Dy3 + have been studied for use inwhite light emitting diode by Panlai et al. [6,7].

Synthesis of borate compounds is quite difficult. Based on thepreparation technique borate compounds may form crystalline as

ll rights reserved.

x: +91 44 27480235.

well as glassy phase. For obtaining efficient luminescent materialscrystalline forms are required. In this study, we have synthesizedimpurity doped polycrystalline LiCaBO3 phosphor powder by asimple solid state sintering technique. LCB phosphors areprepared with Mn, Cu, Zn, Ag and rare earth elements as dopantsand they are tested for the use of radiation dosimetric application.In LCB, RE doped materials showed better TL properties whencompared to other dopants and silver codoping further improvedit. Dosimetric investigation of LiCaBO3 doped with thulium (Tm)and terbium (Tb) revealed the possibility of using it as apromising TL phosphor in radiation dosimetry. The importantdosimetric properties like glow curve structure, dose response forgamma radiation, TL fading, photoluminescence, TL spectra, etc.for LiCaBO3:Tm/Tm,Ag phosphors are discussed in this paper.

2. Experimental

Lithium calcium borate phosphor powder samples are pre-pared by solid-state diffusion reaction at high temperature. Forthe synthesis analytical grade lithium carbonate, calcium carbo-nate, boric acid, silver nitrite and rare earth oxides are used as rawmaterials. The amounts of chemicals taken for the preparation of atypical batch of LiCaBO3 phosphor with dopant thulium and silverare Li2CO3—0.73 g, CaCO3—2 g, H3BO3—1.36 g, Tm2O3—20 mgand AgNO3–20 mg. Initially these chemicals are mixed with triplydistilled water and the resultant paste is dried by controlledheating in a mantel heater. The dry powder was ground, placed in

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S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–1840 1835

an alumina crucible, and sintered at 750710 1C for 3 h in an airatmosphere in a muffle furnace. The phosphor was cooled to500 1C inside the closed furnace itself; subsequently, the aluminacrucible was removed from the furnace and cooled to roomtemperature. For the characterization studies the phosphorpowder was made into uniform grain size of �100 mm bygrinding in an agate mortar. Phosphors with different dopantsat different doping concentrations are prepared by the samesynthesis technique and recipes.The phosphor was irradiated atroom temperature in a gamma chamber containing Co-60 sourceof dose rate 270 Gy/h. The single phase formation of the lithiumcalcium borate (LCB) mixed crystal was confirmed by XRD patternrecorded in STOE diffractometer using Cu-Ka radiation ofwavelength 1.5406 A.

Thermoluminescence measurements were done using Nucleo-nix make TLD reader at a linear heating rate of 10 1C/s byelectrical heating in a kanthal tray. TL intensity for each glow peakwas calculated by taking the area under that peak. TL emissionspectra of the irradiated phosphor powders at each glow peaktemperature are obtained by controlled heating of the samples ina kanthal tray enclosed inside a light proof metallic cover. Theemitted light from the irradiated sample during heating wascollected through an optical cable placed over the sample. For therecording of the spectra, the optical cable outlet is fed to theemission monochromator of Jobin Yvon-Spex make Spectro-fluorometer (Fluorolog version-3; Model FL3-11) with a xenonlamp in switched-off condition. Photoluminescence (PL) spectraof the phosphor powder have been recorded using the samespectrofluorometer with 450 W high-pressure xenon lamp asexcitation source. PL excitation and emission spectra wereacquired at room temperature for a spectral resolution of0.2 nm and slit width kept at 0.25 mm (band pass¼1 nm).Spectral corrections were not made for the instrumental responsein the acquired spectra.

1000

x10 f

d

Temperature (°C)200 300 400

Fig. 2. Gamma induced TL glow curves of LiCaBO3 powder doped with different RE

dopants: (a) LiCaBO3:Tb; (b) LiCaBO3:Tb,Ag; (c) LiCaBO3:Ce; (d) LiCaBO3:Ce,Ag;

(e) LiCaBO3:Dy and (f) LiCaBO3:Gd (multiplied by 10) at a linear heating rate of

10 1C/s (60Co gamma dose—21 Gy).

3. Results and discussion

3.1. XRD data

The X-ray powder diffraction pattern of the compound is givenin Fig. 1. The pattern is in good agreement with the reported XRDdata of LiCaBO3 in Wu et al. [8] and Jiang et al. [5]. Wu et al. [9]prepared LiCaBO3 from analytically pure Li2O, CaO and H3BO3 in

200

100

200

300

400

Inte

nsity

(a.u

.)

Angle (2θ)40 60 80

Fig. 1. X-ray powder diffraction data of LiCaBO3 powder prepared by solid state

diffusion method at 750 1C.

the molar ratio of 1:1:1 with sintering at 790 1C. In the Li2O–CaO–B2O3 system two new ternary compounds with composition closeto LiCaBO3 and Li6CaB3O8.5 were identified. Wu et al. [8] obtainedsingle crystals of LiCaBO3 by solid state reaction from the startingmaterials, pure analytical reagents Li2CO3, CaCO3 and H3BO3 inthe homogeneous mixer with initial roasting at 790 1C and a finalheating at 950 1C in a platinum crucible, then the slow cooling.Structural pattern of these crystals showed an orthorhombic unitcell with different lattice parameters: a¼13.227 A, b¼6.1675 Aand c¼6.026 A and the space group is Pbca [8].

3.2. TL glow curves

TL glow curves of gamma irradiated LCB phosphors synthe-sized with different dopants are given in Figs. 2–4. A comparisonof dosimetric peak intensities of gamma irradiated LiCaBO3

phosphors doped with different RE3 + ions isgiven in Table 1.Tm3 + and Tb3 + doped LCB phosphor with codopant Ag showed 5–9 times higher sensitivity than TLD-100. TL glow curves with REdopants Tb3 +, Ce3 +, Dy3 +, Gd3 +, Tm3 + and codopant Ag are shownin Figs. 2 and 3. Cerium and Ag codoped LCB phosphor showed

00

1

2

3

c

c

b b

d

d

c

a

TL in

tens

ity (a

.u.)

Temperature (°C)100 200 300 400 500

Fig. 3. TL glow curves of gamma irradiated (dose—21 Gy) LiCaBO3 phosphors at a

linear heating rate of 10 1C/s. (a) LiCaBO3 (undoped); (b) LiCaBO3:Ag; (c)

LiCaBO3:Tm and (d) LiCaBO3:Tm,Ag.

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1

2

3c

da

a

x10

e

e

d c

b

TL in

tens

ity (a

.u.)

Temperature (°C)100 150 200 250 300 350 400

Fig. 4. TL glow curves of g-irradiated (dose—21 Gy) LiCaBO3 phosphor powder

with different non-RE dopants: (a) LiCaBO3:Mn; (b) LiCaBO3:Zn; (c) LiCaBO3:Ag;

(d) LiCaBO3:Cu (multiplied by 10) and (e) LiCaBO3:Cu.Ag.

Table 1Comparison of TL sensitivities at the main glow peak temperatures of gamma-

irradiated LiCaBO3 phosphors doped with different rare earth elements and Ag

codoping (60Co gamma dose¼21 Gy and linear heat rating¼10 1C/s).

TL phosphor

material

Dosimetric peak

temperature (1C)

TL sensitivity

comparison

with TLD—100

Temperature

of other glow

peaks (1C)

TLD 100 214 1

LiCaBO3:Tm 233 2.9 60

430 8.3 100

LiCaBO3:

Tm,Ag

186 7.2 56

250 6.2 126

420 5.5

LiCaBO3:Tb 170 5 72, 106

LiCaBO3:Tb,Ag 230 9 81, 112, 170

LiCaBO3:Ce 247 1.7 83, 115, 357

LiCaBO3:Ce,Ag 251 3.3 71,102, 359

LiCaBO3:Dy 232 1 71, 93, 126

S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–18401836

about three times TL intensity than TLD-100. Glow curves ofgamma irradiated RE3 +-doped (except Tb3 +) LCB phosphor havingintense TL emission are given in Fig. 2. All these phosphors have incommon the undesirable low temperature peak at �100 1C.Cerium and dysprosium doped samples have useful glow peaks at�240 1C but their intensities are relatively less. With dopants Tband Ce, codoping of Ag improved the glow curve shape andintensity; codoping of Ag in LCB:Tb3 + created a new glow peak at240 1C, which showed the highest TL intensity among all thedopants tried.

Jiang et al. [5] reported that the main TL peak of LCB:Tm3 +

phosphor was about 291 1C and had a great over lap with itsshoulder; therefore, although the TL sensitivity of this phosphor isthe strongest, it did not carry further investigation on its potentialuse for dosimetric material. However, the TL dosimetric char-acteristics of LCB:Ce3 + phosphor were studied because the TLintensity was considerably strong and the peak shape is simpleand the main peak locates at 196 1C. But in our studies among theRE3 + dopants Tm3 + doped phosphor showed very good TLsensitivity for the desirable dosimetric peak about 200 1C andthe best glow curve structure. Glow curves of gamma irradiatedLiCaBO3 phosphors for undoped, Tm, Ag and Tm,Ag phosphors aregiven in Fig. 3. Contrary to Jiang’s observation, our LiCaBO3:Tmsamples have shown two well separated intense glow peaks at

around 230 and 430 1C with two low temperature peaks around60 and 120 1C. When LiCaBO3:Tm was codoped with Ag, somechanges in the glow curve structure were observed. The intensityof the high temperature peak at 430 1C decreased to half whereasthe intensity of 230 1C peak doubled, a new intense shoulder peakappeared at 180 1C and the low temperature peaks almostdisappeared. TL efficiency of these phosphors to gamma radiationwas about eight times that of TLD-100. High temperature peak at430 1C in these phosphors could be highly suitable for radiationdose measurements at elevated temperature environments. Formeasuring low doses using the peak at 430 1C we neededsuppression of the infra-red background emitted from the heaterstrip. This could be achieved by introducing a suitable band passfilter between sample and PM tube that matches the wavelengthof light emitted from the sample. Since the glow peak is at ahigher temperature, there is only negligible fading observed forthe irradiated samples on storage at room temperature. Theeffects of dopants like Mn, Zn, Cu and Ag are investigated inLiCaBO3 host for radiation dosimetry and the recorded TL glowcurves are given in Fig. 4. Glow curves of these phosphors aredominated by low temperature peaks at �100 1C, which are notuseful for TL dosimetry. Therefore, Tm3 + doped LiCaBO3 phosphorwas selected for the purpose of further dosimetry studies.

3.3. Gamma dose response

TL response of LiCaBO3:Tm to different gamma doses for thepeaks at 430 and 230 1C are shown in Fig. 5 separately. Glow peakat 230 1C has a linear dose response from 1 to 103 Gy. But for thecase of high temperature peak, doses in the milliGray could not bemeasured due to the interference with IR radiation from theKanthal heater strip and it showed linear response from 2 to103 Gy. Lower dose measurements, in the milliGray levels, couldbe improved for both glow peaks using a blue band pass filter thatmatches the Tm3 + emission at 460 nm, which would necessarilycut off the infra-red emission from the sample holder heatingstrip. Since both the glow peaks in LiCaBO3:Tm/Tm,Ag phosphorswere well separated and equally sensitive, dose measurementfrom 430 1C peak could be used for dose confirmation as well. TLglow curve structure changes for LCB:Tm3 + phosphor to differentgamma doses are shown in Fig. 6. Peak at 230 1C is very stable,which showed negligible (�10 1C) variations in the peak positionwith doses. But the high temperature peak showed large shifting;with increase of dose it moved downward from 450 1C at 2 Gy to320 1C at 103 Gy and in saturation region it looks like a shoulderto the 230 1C peak. The relative intensity of 430 peak to 230 1Cshows decrease with an increase of gamma dose. Precaution isneeded in fixing the region of interest for dose measurement atdifferent dose ranges using high temperature peak.

3.4. TL fading on storage

Gamma irradiated LCB:Tm/Tm,Ag samples packed in ordinarypaper were stored at room temperature to study TL fading.Residual TL at each individual dosimetric peak at 230 and 430 1Cis measured at different storage durations. The integrated areasunder each peak by normalization with the immediate measure-ment after irradiation are given in Fig. 7. In both phosphors nosignificant reduction in the original TL signal was observed forboth the peaks for 10 days storage. But both the phosphorsshowed about 15% fading for one month storage at the 230 1Cpeak. At the same conditions of storage TL signal at the 430 1Cpeak showed lesser fading (5–10%) when compared to the230 1C peak. One reason for the relatively higher fadingcompared to literature values of other materials might be due

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102

103

104

TL in

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Dose (Gy)

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TL in

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Dose (Gy)101 102 103 104

101

102

103

104

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Fig. 5. TL response LiCaBO3:Tm phosphor at different gamma doses for the dosimetric glow peaks at (a) 420 and (b) 230 1C.

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f

f

f

ee

e

d

dd

c

cb

b

a

a

a

TL In

tens

ity (a

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Temperature (°C)200 300 400 500

Fig. 6. TL glow curves of LiCaBO3:Tm at different gamma doses at a heating rate of

10 1C/s: (a) 3.7 Gy; (b) 22.5 Gy; (c) 75 Gy; (d) 675 Gy; (e) 1175 Gy; (f) 17625 Gy

(intensities are not in same scale for comparison).

1

0.83

1

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1

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1

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0.99

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0Storage Time (days)

Nor

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ised

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Inte

nsity

LiCaBO3:Tm (230°C)LiCaBO3:Tm, Ag (230°)LiCaBO3:Tm (430°)LiCaBO3:Tm, Ag (430°C)

10 20 30 40

Fig. 7. Post-irradiation TL fading in the gamma irradiated LiCaBO3:Tm/Tm,Ag

powder samples for different duration of storage in room temperature; irradiation

dose—21 Gy.

1

0.96

0.77

10.97

0.63

0.990.960.98

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0Temperature (°C)

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ised

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Inte

nsity

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Fig. 8. TL intensity changes in gamma irradiated (21 Gy) LiCaBO3:Tm phosphor

powder stored for 1 h at various temperatures.

S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–1840 1837

to the absence of light-proof packing of the irradiated sampleduring storage. No degradation in the phosphor quality wasobserved due to storage in laboratory conditions.

The post-irradiation storage stability of the peak at 430 1C inLiCaBO3:Tm phosphor was tested at different higher temperaturesto establish the utility of this phosphor as a dosimeter at elevatedtemperatures. The amount of TL remaining in LiCaBO3:Tm for 1 hpost-irradiation storage in an oven kept at different temperaturesis given in Fig. 8. As expected at elevated temperatures, the firstpeak at 230 1C showed very fast fading, 25% of the TL was erasedafter 1 h storage at 150 1C and TL was fully erased at 200 1C. Butthe high temperature peak at 430 1C exhibited very good TLstorage stability; it showed negligible loss of TL (�3%) for 1 hstorage at 250 1C and about 40% at 300 1C. Loss in TL with storagetime for the irradiated LCB:Tm phosphor stored at 300 1C is givenin Fig. 9. Half an hour storage at 300 1C erased the original TL by7% only, but much faster fading is observed with increase in timeand half of the TL was removed within 2 h storage. A similar typeof study was reported in CaSO4:Tm,Ag, which has the dosimetricpeak at �375 1C; negligible fading was seen for the post-irradiation storage duration of 6 h at 250 1C [10]. Results of postirradiation storage stability studies in LiCaBO3:Tm phosphorconfirmed that like CaSO4:Tm,Ag phosphor it would be a verypromising TL dosimeter to use at elevated temperatures formeasurement of gamma as well as neutron.

3.5. Reusability

Annealing procedure to reuse irradiated LiCaBO3:Tm phosphorfor many cycles is studied. Before reuse, the irradiated readout

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0.51

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0.2

0.4

0.6

0.8

1

1.2

0Storage Duration (min)

Nor

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ised

TL

Inte

nsity

430°C

50 100 150 200

Fig. 9. TL reduction in the g-irradiated (21 Gy) LiCaBO3:Tm powder for different

durations of storage at 300 1C.

1 0.991

0.93

1

0.910.89

0.940.96

1.03

0.97

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0.6

0.8

1

1.2

1Number of Cycles

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ised

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Inte

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230°C430°C

2 3 4 5 6 7

Fig. 10. TL sensitivity of LiCaBO3:Tm phosphor at repeated gamma irradiation and

annealing at 500 1C for 1 h.

3000

30

60

90

120

150 100°C 230°C 420°C

TL in

tens

ity (a

.u.)

Wavelength (nm)400 500 600 700

Fig. 11. TL emissions in g-irradiated LiCaBO3:Tm3+ phosphor at different glow

peak temperatures.

3000

10

20

30

40 120°C 180°C 250°C 420°C

TL in

tens

ity (a

.u.)

Wavelength (nm)400 500 600 700

Fig. 12. TL emissions in g-irradiated LiCaBO3:Tm,Ag phosphor at different glow

peak temperatures.

S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–18401838

phosphor is to be annealed at 500 1C for 1 h to erase the residualTL signals. TL sensitivity and glow curve structure of the phosphorpowder were checked for six cycles of gamma irradiations andannealing at 500 1C for 1 h and these results are given in Fig. 10.Decrease in the TL sensitivity of dosimetric peak at 230 1C evenafter six cycles is negligible. However, the high temperature peakat 430 1C showed 10% decrease after the completion of six cycles.Reusability study showed that no complicated annealingprocedure is needed to keep the glow curve structure and TLsensitivity of the phosphor for repeated use in radiationdosimetry.

3.6. TL emission spectra

The TL emission spectra of gamma irradiated LCB:Tm andLCB:Tm,Ag phosphors heated at each glow peak temperature aregiven in Figs. 11 and 12. LCB:Tm3 + have shown characteristicTm3 + emissions at 365 and 455 nm for the peaks at 100, 230 and430 1C (see Fig. 11). Similarly irradiated LCB:Tm,Ag on heating atthe glow peak temperatures emitted light at 365 and 455 nm(Fig. 12). Even though the emission peaks are the same in bothphosphors, the relative intensities of 365 and 455 nm are differentin both phosphors. In LCB:Tm blue light emission at 455 nm isthree times more than 365 nm emission but in Ag codopedLiCaBO3 phosphor 365 nm is the dominant emission. However, allthe spectra are characteristic of Tm3 +, which shows that the Tm3 +

is the emission centre for all the peaks. The multilevel spectraltransitions corresponding to Tm3 + in the free ion state are3P0-

3H6¼285 nm, 3P0-3F4¼341 nm, 1D2-

3H6¼365 nm,1D2-

3F4¼455 nm, 1G4-3H6¼472 nm, 1G4-

3F4¼645 nm and1G4-

3H5¼769 nm. Most of these peaks are observed in the TLemission spectra and similar spectra were observed in the Tm3 +

doped CaSO4 TL phosphor [11]. TL emission spectra of gammairradiated LCB phosphors doped with terbium, silver codoped andundoped samples recorded at the main glow peak temperaturesare given in Fig. 13. Undoped phosphor showed broad emission at550 nm, Tb and Ag codoped samples showed Tb3 + characteristicemissions as Tm3 + doped samples. With Ag single doping showedless intense emission peak at 350 nm. The TL light emission peaksrecorded in the spectra of irradiated phosphors at 413, 436, 487,545, 583 and 621 nm correspond to electronic transitions5D3-

7F5, 5D3-7F4,5D4-

7F6, 5D4-7F5,5D4-

7F4 and 5D4-7F3,

respectively, of Tb3 + ions and emission at 378 nm matches theexcitation wavelength of Tb3 +. Characteristic emissions of Tm3 +

or Tb3 + in the TL emission spectra of the Tm or Tb doped and Agcodoped LCB propose that the luminescence emission centres inthese phosphors are Tm3 + or Tb3 + ions. TL emission of thesephosphors at blue or green region matches the peak response ofcommon photomultiplier tubes and are an advantage.

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200 250 300 350 400 450 500 550 600 6500

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10

15

20

25

30

35

a3

a2

b2b1

a1

EmissionExcitation

PL

Inte

nsity

(a.u

.)

Wavelength (nm)

Fig. 15. PL excitation spectra of unirradiated phosphors: (a1) LiCaBO3:Tb and (b1)

LiCaBO3:Tb,Ag for emission at 546 nm and emission spectra of phosphors; (a2)

LiCaBO3:Tb; (b2) LiCaBO3:Tb,Ag (both unirradiated) and (a3) LiCaBO3:Tb (irra-

diated at 126 Gy) for l at 379 nm.

S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–1840 1839

3.7. Photoluminescence spectra

Photoluminescence (PL) excitation and emission spectra ofLiCaBO3:Tm /Tm,Ag are shown in Fig. 14. The characteristicemission bands of Tm3 + corresponds to 1D2-

3F4 observed as themajor emission at 455 nm along with a minor band at 480 nm tothe transition 1G4-

3H6 under the 360 nm excitation correspondsto 3H6-

1D2 transition. No reduction in Tm3 + emission was seenin the gamma irradiated sample. This shows that no conversion ofTm3 + to Tm2 + takes place due to gamma irradiation. Addition ofAg in LiCaBO3:Tm caused drastic decrease in the intensity of Tm3 +

emission but a new peak appeared at 533 nm. Reduction in the PLis not reflected in TL intensity. However, Ag codoping inLiCaBO3:Tm changed the TL glow curve shape; a new TL peakappeared at 190 1C. Similar PL measurements were performed inLiCaBO3:Tb/Tb,Ag phosphors also to substantiate the observationsin Tm doped samples (Fig. 15). The electronic configuration ofsilver (4d105 s1) permits single valancy (4d10) as well as doublevalancy (4d9) as codopants for charge compensation. Ag codopingin LCB:Tb3 + reduced the PL intensity by one order, similar to Tmdoped samples. This shows that silver does not act as charge

300 400 500 600 7000

5

10

15

20

25

30

35

x100

undoped (144oC) Tb (170oC) Tb,Ag (230oC) Ag (200oC)

TL In

tens

ity (a

.u.)

Wavelength (nm)

Fig. 13. TL emissions in g-irradiated LiCaBO3 powder for various dopants at the

main glow peak temperatures (heating temperature in bracket).

200 250 300 350 400 450 500 5500

5

10

15

20

25

30

35

b2

a3a2

x10

b1

a1

EmissionExcitation

PL

inte

nsity

(a.u

.)

Wavelength (nm)

Fig. 14. PL excitation spectra for unirradiated phosphors: (a1) LiCaBO3:Tm; (b1)

LiCaBO3:Tm,Ag for the emission at 455 nm and emission spectra of phosphors: (a2)

LiCaBO3:Tm; (b2) LiCaBO3:Tm,Ag (both unirradiated) and (a3) LiCaBO3:Tm

(irradiated at 42 Gy) for excitation at 360 nm.

ex

compensator. Almost all the Tb3 + characteristic peaks were seenin the excitation spectra of both samples but the peak at 241 nmcorresponding to 4 f75d1 excitation appeared as the major peakcontrary to observations in most of the Tb3 + doped phosphorswhere the main peak appears at 379 nm from the 7F6-

5D4

transition. Other Tb3 + characteristic excitation peaks appear at350 and 366 corresponding to transition from ground state7F6-

5D2 and 7F6-5D3. Excitation by 377 nm light produces

dominant Tb3 + green emissions at 485 and 546 nm. Gammairradiations showed only negligible reduction in the Tb3 +/Tm3 +

PL intensity, which means that redox processes do not play anysignificant role in the TL of LCB phosphors.

3.8. Effect of Ag codoping in LiCaBO3:RE3 + (RE¼Tb, Tm and Ce)

When the divalent cation Ca in LiCaBO3 host material issubstituted by a trivalent rare earth ion such as Tm3 +, Tb3 +, Ce3 +,it will essentially create vacancy related defect in the host. TLemission intensities depend on the number of trapped charges inthe defect centers. Glow curve changes with Ag codoping are notseen, similar to different LiCaBO3:RE3 + phosphors. Glow peakaround 200 1C is related to Ag ions as seen in LCB:Ag andLCB:RE3 +,Ag samples and not seen for Cu, Ce, Tb, Tm on singledoping. In LCB:Tm3 +, codoping of Ag enhanced the TL emission at360 nm for 230 1C peak as well as a new peak appeared at 180 1Crelated to Ag ion. Addition of Ag in LCB:Tb created change in theglow curve shape, enhanced the intensity of existing peaks and anew useful dosimetric peak appeared at 230 1C. In the case of PL,addition of Ag causes a drastic reduction of about 30 times to RE3 +

emission intensity. Reduction in the PL shows that Ag does not actas a charge compensator because it may be going as Ag2 + in thecrystal lattice. Panlai et al. [7] reported that LCB:RE3 + phosphorsshowed increases in PL emission intensities by monovalentcodopants like Li+, Na+ and K+ due to charge compensation.Codoping effect of Ag was studied in CaSO4:Tm and other rareearth doped CaSO4 TL phosphors, which showed change in theglow peak position [11,12]. With addition of Ag in CaSO4:RE3 + thecharacteristic dosimetric peak at 230 1C in all samples wereshifted upwards by about 150 1C [12]. In CaSO4:Ag the main TLglow peak has been reported at �370 1C and its intensity wasincreased drastically in CaSO4:RE,Ag phosphors. ESR studies inCaSO4:Dy,Ag phosphor confirmed that an Ag2 + centre, observableonly at low temperatures, appears to correlate with the TL peak at

ARTICLE IN PRESS

S.R. Anishia et al. / Journal of Luminescence 130 (2010) 1834–18401840

375 1C. Holes released from this centre initiate the TL process inCaSO4:Dy,Ag phosphor [13]. Ag codoping in LiCaBO3:Tm/Tbsamples also showed a similar increase in main TL peak of LCB:Agobserved at �200 1C (Fig. 4).

4. Conclusions

Different rare earth and transition metals are tried in LiCaBO3

as dopants and doping of Tm gave the best result. Silver codopingin LiCaBO3:Tm3 + phosphor enhanced the intensity of the 230 1Cpeak. This paper reveals the findings on the radiation dosimetriccharacteristics of Tm-doped LiCaBO3 phosphor synthesized by thehigh temperature solid state diffusion reaction. Two major wellseparated useful dosimetric peaks are observed at 230 and 430 1Calong with a low temperature peak in LiCaBO3:Tm3 +. TLsensitivity of these peaks to gamma radiation is about 8 timesthat of TLD-100. Both the TL peaks showed near-linear responseup to a dose of �103 Gy. A simple procedure of annealing at500 1C for 1 h is enough for the repeated use of the phosphor forradiation dosimetry. High temperature peak at 430 1C in theirradiated sample showed negligible fading up to 250 1C storage.Post-irradiation TL storage stability of LiCaBO3:Tm phosphorshows that it would be a very promising TL dosimeter at elevatedtemperatures for gamma as well as neutron. Enrichment of 6Liand 10B in LiCaBO3 could improve the neutron sensitivity. In thecontent of the paper, importance is given on the characterizationof the phosphor for radiation dosimetric applications. However,

the present studies are not enough to propose any concrete andmeaningful TL mechanism in these materials. Hence, furtherstudies to understand the mechanism of the TL process and TLparameter calculations for these phosphors are under progress inour group and will be published as a separate paper.

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