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Thermoluminescence properties of rare earth doped lithium magnesiumborate phosphors

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

a Radiological Safety Division, Radiological Safety & Environmental Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, Tamilnadu, Indiab Department of Physics, Annamalai University, Chidambaram, Tamilnadu, India

a r t i c l e i n f o

Article history:

Received 24 August 2010

Received in revised form

29 May 2011

Accepted 13 June 2011Available online 20 June 2011

Keywords:

Thermoluminescence

Dosimetry

Borates

Phosphor

Photoluminescence

a b s t r a c t

This paper reports the thermoluminescence (TL) properties of rare earth doped lithium magnesium

borate (LMB) polycrystalline phosphor. LMB phosphor has been prepared by high temperature solid

state diffusion method. Among all the rare earth doped LMB phosphors, terbium doped material has

shown maximum TL sensitivity with a broad dosimetric glow peak at 240 1C. near the tissue equivalent

TL phosphor with terbium dopant has about four times the TL sensitivity of TLD-100. The main

dosimetric properties such as glow curve stability, TL response versus absorbed dose, post-irradiation

storage stability, and reusability are investigated. This TL material has a linear dose response up to

103 Gy, negligible storage fading and a simple annealing procedure for reuse. The TL emission spectra of

LMB:Tb3þ showed broad green emission at 544 nm, which merged with host emission. The

characteristic Tb3þ emissions are seen in the photoluminescence (PL) spectra.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Luminescence studies on borate compounds have been startedfrom 1967. Borate compounds are mainly used for nonlinearoptics and radiation dosimeters based on thermo-stimulatedluminescence generally known as thermoluminescence (TL). TLdosimeters have been widely used for clinical, personal, andenvironmental monitoring of ionizing radiation. Many research-ers have studied the TL properties of tissue equivalent lithiumtetra borates and magnesium tetra borates doped with Mn, Cu, orrare earth elements [1–3]. In radiation dosimetry point of viewsome undesirable qualities such as hygroscopic nature, lightinduced fading, etc. are reported for these borates [3,4]. Anotherpossible important application of the borates is as a birefringentcrystal for the manufacturing of optical communication compo-nents. The boron atom can coordinate with either three or fouroxygen atoms to form a variety of atom groups; polarizationscaused by light perpendicular and parallel to the [BO3]� planeatoms are expected to be very different and can induce a largebirefringence. Moreover, due to their good heat stability and lowmelting temperature, borate crystals are easy to grow.

Luminescence study of mixed metal borates has been startedvery recently. TL studies are reported in impurity doped mixedmetal borates such as SrMg(BO3)2 [5], LiBa2B5O10:RE3þ [6],

LiSr4(BO3)3:Ce [7], LiKB4O7 [8], KSr(BO3)3:Ce [9], NaSr(BO3)3:Ce [10], LiSrBO3:RE3þ [11], and LiCaBO3 [12–14]. Wu et al. [15]prepared lithium–magnesium borate compounds and determinedits structure. The phase equilibria for Li2O–MgO–B2O3 weremeasured by Bazarova et al. [16]. The effective atomic numberfor LMB material is about 8, which is near to the human tissue Zeff

value. Studies on the dosimetric properties of mixed metalborates are very few. Using the solid state diffusion reactionmethod, we have prepared the lithium - magnesium borate (LMB)polycrystalline phosphor doped with various rare earth (RE)elements. Among all the rare earth doped LMB samples, Tb3þ

doped sample revealed the highest TL intensity. Hence, dosi-metric properties of terbium doped LMB phosphor is discussed inthis paper.

2. Experimental details

2.1. Procedure for the synthesis of LMB phosphor

Impurity doped lithium magnesium borate (LMB) polycrystal-line phosphor materials are synthesized by the solid statediffusion reaction. For synthesis, analytical grade chemicals,lithium carbonate, magnesium oxide, boric acid, rare earth oxides(other dopants), and silver nitrate are used as raw materials.LMB phosphor of different borate compositions are synthesizedby varying the concentration of boric acid. The weights ofstarting chemicals for the synthesis of LMB compound viz.,

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

Journal of Luminescence

0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.jlumin.2011.06.019

n Corresponding author. Tel.: þ91 44 27480352; fax: þ91 44 27480235.

E-mail address: mtj@igcar.gov.in (M.T. Jose).

Journal of Luminescence 131 (2011) 2492–2498

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Li2CO3¼0.73 g, MgO¼0.88 g, H3BO3¼5.44 g, Tb4O7¼20 mg, andAgNO3¼10 mg for Ag co-doping. Initially these raw materials aremixed uniformly using a mortar and pestle. This mixture issintered in an alumina crucible at 850 1C for 3 h inside a mufflefurnace in atmospheric air. After initial cooling to 500 1C insidethe furnace it is removed and suddenly cooled to room tempera-ture. Phosphors with different dopants or doping concentrationsare synthesized by the same technique and recipes. For furthercharacterization studies the phosphor powder was grinded in anagate mortar and made into uniform grain of �100 mm size.

2.2. Characterization

The crystalline formation of the materials are characterizedfrom X-ray diffraction (XRD) pattern in powder recorded in STOEdiffractometer using Cu–Ka radiation of wavelength 1.5406 A atroom temperature. The g-ray induced TL glow curves of thepolycrystalline powder are recorded using a computerized TSLreader (PMT EMI 9924B) made by Nucleonix India Ltd., India,model TSL Analyzer (TL 1007) designed for powder samples. TLglow curves of the samples are recorded at linear heating rate of10 1C/s by electrical heating in kanthal tray. TL intensity wascalculated by taking the area under dosimetric peak in the regionof interest 150–340 1C. For low and high doses of gamma irradia-tion, two different gamma sources are used. For low dose, a 137Cs(gamma energy of 0.662 MeV) point source of strength 42 GBq atdose rate of 6.3 mGy/h and for high dose, a 60Co (gamma energiesof 1.73 and 1.33 MeV) source of strength 3.8 TBq in a gammachamber at a dose rate of 270 Gy/h are employed in our experi-ments. TL emission spectra of the gamma irradiated samples areobtained by isothermal heating of the samples at each glow peaktemperature in a TL reader heating tray. The light emitted fromthe sample is collected using a fiber optical cable and it is sent tothe emission monochromator of the spectrofluorometer to recordthe spectrum, with xenon lamp in switched-off condition [14].Photoluminescence (PL) spectra of the phosphor powder havebeen recorded using a Jobin Yvon-Spex make Spectrofluorometer(Fluorolog version-3; Model FL3-11) with 450 W high-pressurexenon lamp as excitation source. PL excitation and emissionspectra were acquired at room temperature for a spectral resolu-tion of 0.2 nm and slit width kept at 0.25 mm (band pass¼1 nm).Spectral corrections were not made for the instrumental responsein the acquired spectra.

3. Results and discussions

3.1. XRD data

Lithium magnesium borate samples are prepared with differ-ent amounts of boric acid to have borate radicals with two or fourboron atoms. For the preparation of these materials the startingchemical, which supplies the lithium, magnesium, and boronatoms in the final product are kept in the ratio of 2:2: x (x¼4or 8). The XRD patterns of these materials are given in Fig. 1. Inspite of using different amounts of boric acid it showed similarXRD lines for both materials. Hence even with different boric acidconcentrations in the starting recipe the borate radical formed inLMB phosphor may be same. However for recorded XRD patternsno matching files are seen in the JCPDS library (limited numbersof files are only available for mixed lithium magnesium boratecrystals). The boron atoms can coordinate with either three orfour oxygen atoms to form a variety of atom groups. Thesubsolidus phase relations of the ternary system, Li2O–MgO–B2O3, have been studied by the powder X-ray diffraction method[15,16]. Wu et al. [15] prepared 52 samples with different

compositions by solid state reaction methods and their phaseidentifications are listed. There are 16 three-phase regions iden-tified in that system and no solid solution regions are found in allthe binary and ternary compounds; two phase regions are joint-lines of the two compounds [15]. Our material produced welldefined X-ray diffraction lines for the powder samples thatconfirm its crystalline nature. The chemical formula of thecompound is most probably Tb-doped lithium -magnesium tetra-borate. A comprehensive study based on X-ray diffraction is underway for the identification of crystal structure and exact chemicalformula.

3.2. Thermoluminescence in LMB phosphors

Fig. 2 shows TL glow curves of gamma irradiated LMBphosphors with the doping of some rare earth elements and alsoundoped one. These are recorded after exposure to 60Co gammarays at room temperature to a dose of 21.4 Gy. All the glow curvescontain two or more peaks. Undoped sample has TL peaks at 115,235 and 290 1C. Doping of Tb3þ , Gd3þ , Dy3þ , Mn2þ , Pr3þ or Ce3þ

in LMB phosphor has enhanced the intensity of dominant

10 20 30 40 50 60 70 800

5

10

15

b

a

Inte

nsity

(a.u

.)

2θ

Fig. 1. X-ray diffraction patterns of LMB:Tb3þ phosphor synthesized with differ-

ent mole concentrations of boric acid: (a) 4 and (b) 8 taken at room temperature

by Cu-Ka radiation.

50 100 150 200 250 300 3500

10

20

30

40

50

d

h

g

f e

d

cb

a

TL In

tens

ity (a

.u)

Temperature (°C)

Fig. 2. TL glow curves of LMB with different dopants: (a) LMB:undoped,

(b) LMB:Tb, (c) LMB:Gd, (d) LMB:Dy, (e) LMB:Pr, (f) LMB:Mn, (g) LMB:Ce and

(h) LMB:Eu (gamma dose¼21.4 Gy, heating rate¼10 1C/s).

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dosimetric peak seen around 220 1C. Relative TL sensitivities ofthe dosimetric peaks observed in LMB for various dopants havebeen given in Table 1. Among these phosphors Tb3þ doped LMBphosphor has the highest TL with a favorable dosimetric peak at250 1C. For further improvement of TL sensitivity or peak struc-ture by co-doping of Ag or Na charge compensators have notyielded better result in terms of intensity or glow curve structure.Two intense peaks are observed at 180 and 350 1C in LMB: Dy3þ

phosphor and peak at 180 1C is about three times more intensethan at the latter. Even though the peak at 180 1C is 3 times moreintense, it is not better than LMD:Tb3þ for long duration dosi-metry due to expected higher signal fading. Gd3þ or Pr3þ dopantsalso have produced reasonably intense and useful dosimetrypeaks around 220 1C. Eu doped sample is not good because theintensity of the peaks observed at 117 and 292 1C are very less.Besides RE3þ dopants, transition metal manganese, a successfuldopant for various types of phosphors and in borates of lithiumand magnesium, has been tried in this mixed borate. The ionicradius and oxidation state of Mn2þ are better match than RE3þ

dopants for replacing Li or Mg in LMB crystal. In LMB:Mn2þ arelatively intense dosimetic peak is formed at 215 1C with shoulderpeaks at 112 and 276 1C. Dosimetric properties of LMB:Tb3þ havebeen studied and reported in the following sections.

3.3. TL glow curve of LMB:Tb3þ

Among LMB phosphors Tb doping has given the highest TLintensity with dosimetric peak at 250 1C, which is best suited fordosimetric application. Further detailed dosimetric and thermolu-minescence characterization studies are performed in Tb dopedphosphor to establish the utility of this phosphor for radiationmeasurements. TL glow curves for the LMB:Tb3þ powder irradiatedto a dose of 18 Gy by 1.17 and 1.3 MeV gammas of 60Co or 2.7 MeVbetas of 90Sr–90Y are given in Fig. 3. Shapes of both the glow curvesare similar but TL intensity to betas are almost twice that of samegamma dose. The enhanced TL intensity may be due to higherenergy of betas. Deconvoluted peaks of LMB:Tb3þ glow curveappeared at 122, 212, 284, and 390 1C are also shown in Fig. 3.De-convolution revealed that the main dosimetric peak of LMB:Tbconstituted of a major peak at 284 1C and a shoulder peak at 212 1C.

The heating rate is a fundamental experimental variable inthermoluminescence measurements. TL glow curves of irradiatedLMB:Tb3þ powder at different heating rates from 0.1 to 10 1C/sare shown in Fig. 4. No change in the glow curve structure hasbeen observed at various heating rates. Peak temperature (Tmax)as a function of heating rates LMB:Tb3þ are given in the inset ofFig. 4. A general increase in Tmax with heating rate is observed. Atlower heating rates a faster increase in Tmax is observed withincreasing heating rates; peak seen at 203 1C for 0.1 1C/s has beenshifted to 256 1C at 4 1C/s and it is followed by a slower increasein higher heating rates. Similar changes in Tmax as a function of

heating rates have been reported in other TL materials as well[17–19]. The shift of Tmax versus heating rate can be empiricallyexplained in the following way. At the low heating rate b1, thetime spent by the phosphor at a temperature T1 is long enough sothat an amount of thermal release of electrons depending on half-life at this temperature could take place. As heating rate increasesto b24b1 the time spent at the same temperature T1 decreasesand therefore the thermal release of electrons is also decreased.Then a higher temperature T2 is needed for the same amount ofthermal release to take place at b2. In this way the whole glowpeak is shifted to higher temperatures as heating rate increases ina manner depending on the half-life and time spent at eachtemperature. We used very small amount (about 10 mg) ofpolycrystalline powder of size �100 mm for each measurementsand it is further spread as a thin uniform layer over the heatingelement for good thermal contact with the powder. Temperaturelag is expected in thick samples at higher heating rates. But in ourstudies, higher shift for peak temperature (Tmax) is observed atlow heating rates compared to the heating rates above 4 1C/s.

Table 1Relative TL sensitivity of dosimetric peaks for different LMB phosphors normalized

to LMB:Tb3þ and temperature of other glow peaks (60Co gamma dose¼21 Gy and

linear heat rating¼10 1C/s).

TL phosphor

material

Dosimetric peak

temperature (1C)

Relative TL

sensitivity

Temperature of other

glow peaks (1C)

LMB:undoped 230 0.3 115, 280

LMB:Tb 240 1 –

LMB:Gd 235 0.8 117, 338

LMB:Dy 350 0.8 178

LMB:Pr 260 0.5 290

LMB:Mn 212 0.6 120, 280

LMB:Ce 290 0.3 124

LMB:Eu 217 0.03 115

50 100 150 200 250 300 350 400 4500

10

20

30

40

4

3

2

1

b

a

TL In

tens

ity (a

.u.)

Temperature (°C)

Fig. 3. TL glow curves of LMB:Tb3þ irradiated to 18 Gy: (a) gamma and (b) beta

radiation (deconvoluted glow peaks are given).

50 100 150 200 250 300 350 400 4500

10

20

30

40

50

60

70

80

gfed

c

ba

TL In

tens

ity (a

.u.)

Temperature (°C)

Fig. 4. Glow curve shapes of LMB:Tb phosphor obtained at heating rates in 1C/s:

(a) 0.1, (b) 0.2, (c) 0.4, (d) 1, (e) 2, (f) 4 and (g) 10. Inset: position of the maximum

glow peak temperature as a function of heating rate in LMB:Tb phosphor (gamma

dose¼125 Gy).

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It shows that temperature lag effect may be negligible because ofthe use of less amount of small size powder for measurements.

Like Tmax, an increase in the full width at half-maximum(FWHM) has been observed with heating rate in LMB:Tb3þ butthe change in FWHM was less compared to Tmax. Dependence ofglow peak integral and peak height on heating rates is studied forthe main glow peak of irradiated LMB:Tb. Fig. 5 shows thechanges in the integral area and peak height of the main glowpeak with heating rate. No systematic change appeared in LMB:Tbfor both the parameters as against the general phenomenon ofreduction in TL reported in literature for many materials [18,19].LMB:Tb, integral area and peak heights are not decreased butshowed variations within þ20% and 77%, respectively. Kitis et al.[17] have reported increase of the TL response as a function of theheating rate in lithium tetraborate material . Piters and Bos [20]reported an increase in the sensitivity of glow peak two ofLiF:Mg,Ti, as heating rate increases and they explained the effectin a framework of the defect interaction model during TL readout .Many authors explained the decrease of both integral and peakheight versus heating rate assigned to the possibility of thermalquenching whose efficiency increases as the temperatureincreases [17–19]. There are view points against the thermalquenching effects for explaining the decrease in TL sensitivity insome materials with increasing heating rates [21].

3.4. TL intensity versus dose

An important property of a TL dosimeter material is that itexhibits a linear relation between TL intensity and absorbed dose.Many TL materials exhibit a nonlinear growth of TL intensity withabsorbed dose at over certain dose region. The relationshipbetween the TL intensity and gamma dose of Tb3þ doped LMBphosphor has been shown in Fig. 6. To record the dose –responsecurve, phosphor has been irradiated to different gamma doses atroom temperature and the TL intensity is taken from glow curvearea. For irradiation two sources are used, which are havingdifferent gamma energies and dose rates. In lower doses up to2 Gy a 137Cs gamma source of energy 0.662 Mev at different doserates 4, 6, 16 and 256 mGy/h is used. At higher doses above 6 Gydosimeters are irradiated at dose rate of 270 Gy/h in a gammachamber containing 60Co source. A near linear response isobtained for the 240 1C peak from low dose in mGy till thesaturation dose at 103 Gy. In terms of dose linearity and

saturation dose, LMB:Tb3þ material has higher values, whencompared with the reported values of other tissue equivalent TLphosphors such as LiF and Li/ Mg borates [22].

TL glow curves of LMB:Tb3þ powder exposed to differentamount of gamma doses are shown in (Fig. 7. TL intensities arenot in same scale for comparison and these glow curves are givento illustrate the changes in shape and peak temperature as afunction of absorbed dose. No major glow curve distortions orpeak shift is observed in this material. However, the glow peakseen near 230 1C at low doses (235 1C at 0.5 Gy) shifts upwardswith an increase in exposure and reached 250 1C at 26 Gy. Withfurther increase in dose beyond 26 Gy the glow peak shifts movesdownward and reached 210 1C at 6 kGy. A similar glow peaktemperature shifting behavior is reported in CaSO4:Dy andCaSO4:Dy,Zn with increasing dose by Jose et al. [23].

It is observed from the glow curves that the peak shift is notaccompanied by major change in the glow curve shape exceptslight increase in the low temperature shoulder peak. The variationin the peak shape has been assessed by the full width at half-maximum (o), low temperature half-width (d1), high temperature

0 1 2 3 4 5 6 7 8 9 10 110.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2b

a

TL in

tens

ity (

a.u.

)

Heating rate (°C/s)

Fig. 5. Change in (a) peak height and (b) integrated TL, for the dosimetric peak of

LMB:Tb3þ with heating rates, normalized to the heating rate of 0.1 1C/s.

10-1 100 101 102 103 104 10510-1

100

101

102

103

104

105

TL in

tens

ity (a

.u)

Dose (Gy)

Fig. 6. Integral TL versus gamma doses for the dosimetric glow peaks at �240 1C

(heating rate¼10 1C/s).

50 100 150 200 250 300 350 4000

4

8

12

16

20

e

d

c

b

a

TL In

tens

ity (a

.u)

Temperature (°C)

Fig. 7. TL glow curve of LMB:Tb3þ at different doses (a) 0.5 Gy, (b) 26 Gy,

(c) 770 Gy, (d) 11.2 kGy and (e) 63 kGy and (heating rate¼10 1C/s). (TL intensities

are not in same scale for comparison.)

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half-width (d2), and the symmetry factor (m) of the glow peak,which is estimated by m¼d2/o. These glow peak parameters ofthe phosphor with increasing exposure are given in Table 2.Peak width o has shown about 20 1C variation between differentexposures with minimum value in the exposure range 102–103 Gyand higher values at lower and higher doses. Further analysis ofFWHM in terms of low and high temperature half-widths revealsthat the increase in o is due to an appreciable increase in d1 up to amaximum of 24 1C , whereas d2 shows a gradual decrease with theincrease of dose. The higher values of o and d1 are essentially dueto the growth of low temperature shoulder peak at doses below10 Gy and above 103 Gy. One of the simplest methods for checkingthe kinetics order of an isolated peak is based on the symmetryproperties of the glow peak. According to Chen’s criterion,LMB:Tb3þ phosphor shows almost symmetrical peak with mhaving value of about 0.52 at exposures below 103 Gy, suggestingthat the glow peak follows second order kinetics. With a furtherincrease in dose beyond 103 Gy, the glow curve becomes more andmore asymmetrical with value of m decreasing to 0.4 and belowwith an increase of the dose. The value of m may suggest thatLMB:Tb3þ follows first order kinetics or lower non-integer valuebeyond dose of 103 Gy, which is the saturation dose for thismaterial. The TL processes with negligible re-trapping are char-acteristic of first order kinetics whereas TL processes with domi-nant re-trapping are characteristics of second order kinetics. Thisimplies that the kinetics responsible for thermoluminescence inLMB:Tb3þ is second order at all the doses up to the saturation dosebeyond that it follows a lower order kinetics. Reported studies inCaSO4:Dy TL phosphor revealed the changes in the glow curveshape mainly at high dose regions and it is confirmed that thechanges are due to variations in the order of kinetics in thematerial with the increase in absorbed dose [24].

The effective atomic number, Zeff is an indicator of the TL materialsenergy response. For absorbed dose measurement it is useful tocompare these figures with that of air, namely Zeff(air)¼7.64 and fortissue Zeff(tissue)¼7.42. The Zeff for some of the popular TLD phosphorssuch as LiF¼8.14, CaSO4:Dy¼15.3, CaF2¼ 16.3, Li2B4O7¼ 7.4 andMgB4O7¼8.4. Ratio of TL response to X-ray of 30 keV and 60Cogamma rays is equal to 1 for materials of Zeff values near to 8. In thecase of LMB:Tb3þ phosphor Zeff is about 8, which can have energyindependent TL response for X-rays and high energy gamma rays.Hence it can have application both in medical dosimetry and nuclearindustry. Peak shift of about 40 1C was observed for the dosimetricpeak in LMB:Tb3þ without any major distortion in glow curvestructure for the dose ranges from mGy to kGy level. This showsthat the material is very stable at wide range of doses. Compared toTLD- 100, its TL sensitivity is about four times higher and dosimetricglow peak is observed at �25 1C higher temperature.

3.5. Reusability

In the radiation dosimetry application reusability of the dosi-metric material is another important property. Thermal annealing isneeded to remove the residual dose in readout material before the

reuse of the phosphor for another measurement. This thermalannealing can change the defect distribution of the phosphor andthereby alter its sensitivity. To confirm the reusability property wehave carried out eight cycles of repeated annealing, irradiation, andreadout for same material. In the test, freshly prepared powder wasirradiated to 21.4 Gy, readout and annealed at 500 1C for 1 h beforesubsequent irradiation. Annealing temperature and duration areoptimized after different trails. Annealed samples on testing showedany residual TL or change in sensitivity. The sensitivity of phosphorpowder obtained for eight test cycles is given in Fig. 8. No change inTL intensity or glow peak shape has been observed even after eightcycles of annealing, irradiation, and readout. In the case of LiFphosphors, it required a complex and long duration annealingprocedure to maintain the glow peak structure and sensitivity. Forpractical dosimetry a complicated annealing procedure is a majordisadvantage in the routine operation of the detector. It is mainly forthis reason that a search for alternative to LiF has been under wayfor a long time. In this regard LMB:Tb3þ phosphor has the advantageover LiF by the simple annealing procedure for reuse, a heattreatment at 500 1C for 1 h duration in oven.

3.6. Post-irradiation storage stability

An important factor in the choice of a material for TL dosimeter isthe stability of the stored signal for the environment at which thedosimeter is employed. Thus, it is necessary to assess if the trappedcharges in the phosphor can be lost before readout by thermal,optical, or any other anomalous fading. The post-irradiation storagestability of TL signal in LMB:Tb3þ and CaSO4:Dy powder at roomtemperature with storage time is given in Fig. 9. Two sets of gammairradiated powder samples packed one in white other in black paperare stored in a cupboard in air conditioned room to a maximum of62 days. Negligible reduction in original TL intensity was observedfor both of the samples for 2 month storage. For comparison,irradiated standard CaSO4:Dy powder stored in same storage con-ditions also showed same amount of fading. Storage stability ofLMB:Tb3þ material is comparable with other standard TL phosphorsemployed in laboratory environment.

3.7. Light induced fading

Some stored charges in the traps can be emptied by opticalstimulation. A dosimeter that is exposed to sunlight or toflorescent lamps or other light sources may lose part of its signalby stimulation of the trapped charges by light photons. For light

Table 2TL glow peak parameters of LMB:Tb3þ phosphor with increasing gamma dose

(linear heat rating¼10 1C/s).

Absorbed

dose (Gy)

Tmax

(1C)

T1

(1C)

T2

(1C)d1 d2 FWHM (o) m¼d2/o

0.5 233 184 275 49 42 91 0.47

26 247 210 289 37 42 79 0.52

770 235 201 273 34 38 72 0.53

11,175 208 155 243 53 35 88 0.40

63,325 213 155 245 58 32 90 0.35

Fig. 8. TL sensitivity of LMB:Tb3þ phosphor at repeated gamma irradiation and

annealing at 500 1C for 1 h.

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fading studies we have exposed the irradiated samples underroom light and sunlight. The TL signal losses of irradiatedLMB:Tb3þ and CaSO4:Dy powder with time for exposure underroom light and sunlight have been shown in the Fig. 10. Forcomparison, optical fading property of LMB:Tb3þ material isstudied along with CaSO4:Dy TL phosphor under continuousexposure sunlight up to maximum 5 h. The irradiated powdersare exposed to direct sunlight during mid-day in the month ofOctober at an ambient temperature of �37 1C. We have exposedthe samples under sunlight in two ways, one set of samplespacked in black paper and next set of samples in open condition.Both the phosphors, which are exposed to sunlight directly, hadvery fast fading, after 1 h the residual TL was only �35% and in3 h the residual TL had come down below 10% in LMB:Tb3þ but inCaSO4:Dy it remained about 20% after 5 h. Irradiated powdersstored in black paper under sunlight exposure for 5 h packed inblack paper had shown 80% residual TL in LMB:Tb3þ sample andnegligible fading in CaSO4:Dy powder. Continuous exposure of theirradiated LMB:Tb3þ to room light from fluorescent tube light ofintensity 545 lx reduced the TL signal by 20% in 1 h, 30% in 3 h,and 40% in �8 h. In both materials, stored TL is removed underhigh intensity light but the light induced fading is higher inLMB:Tb3þ than CaSO4:Dy. Hence LMB:Tb3þ used in intense lightenvironment needs light protection cover.

3.8. TL emission spectra

Spectral measurement of TL provides useful information on thenature of luminescence centers, which are responsible for TLemission and may be formed by the presence of impuritiesand/or imperfections in the crystal. TL spectra of pure LMBphosphor recorded at its glow peak temperatures 110, 173, 234and 286 1C are shown in Fig. 11. Undoped host material exhibited agreen emission band around 560 nm, which is probably not fromany impurity of trivalent lanthanide ions but most likely from hostlattice or due to uncontrolled impurity of Mn in the startingchemicals (TL spectra of LMB:Mn showed main emission bandaround 570 nm). TL spectra of LMB:Tb3þ/Tb3þ , Ag recorded for240 1C glow peak is shown in Fig. 12. LMB:Tb3þ has the character-istic Tb3þ emission bands, minor peaks at 379, 412, 435, 487 andmajor emission at 544 nm due to electronic transitions 5D3-

7F6,5D3-

7F5, 5D3-7F4,

5D4-7F6 and 5D4-

7F5 , respectively [25,26].After silver co-doping, minor emissions of Tb3þ are disappearedand major emission at 544 merged with host emission. With

Fig. 9. Comparison of post-irradiation fading for TL as a function of storage time in

LMB:Tb3þ powder packed in white and black papers with CaSO4:Dy (in white

paper) all stored in room temperature; gamma dose¼21 Gy.

Fig. 10. Fading of TL with time under sun light exposure (a)–(d): (a) CaSO4:Dy

(black paper), (b) LMB:Tb (black paper), (c) CaSO4:Dy (open), (d) LMB:Tb (open)

and (e) LMB:Tb (open in room light); gamma dose �21 Gy.

350 400 450 500 550 600 650 7000

1

2

3

4

5

6

7d

c

ba

TL In

tens

ity (a

.u.)

Wavelength (nm)

Fig. 11. TL spectra recorded in irradiated undoped LMB powder at peak

temperatures: (a) 110 1C, (b) 173 1C, (c) 234 1C and (d) 286 1C.

350 400 450 500 550 600 650 7000

4

8

12

16 ba

TL In

tens

ity (a

.u.)

Wavelength (nm)

Fig. 12. TL spectra recorded at peak temperature (240 1C) in irradiated (a) LMB:Tb

and (b) LMB:Tb, Ag.

S.R. Anishia et al. / Journal of Luminescence 131 (2011) 2492–2498 2497

Author's personal copy

Ag co-doping emission intensity remained the same but it isshifted to 557 nm and becomes broad. By Tb3þ doping in LMB,host related peak at 240 1C (TL emission at 557 nm) has increasedby �4 times from the transfer of the recombination energyemitted during TL readout process to Tb3þ ions which producedits characteristic emissions. The glow peak at 240 1C may be due tothe host related defect because it is present in pure, Tb doped andAg co-doped LMB. Further studies on the TL process and mechan-ism are under way. From dosimetry point of view, LMB:Tb3þ andAg co-doped phosphors have broad emissions at green thatmatches the sensitivity of photomultiplier tubes.

3.9. Photoluminescence spectra

Photoluminescence spectra of LMB:Tb3þ have all major peaksassigned to the parityand spin-allowed transitions from the 7F6

ground state of Tb3þ (4f8) to the different excited states of (4f7)5dconfiguration (see Fig. 13). The narrow band excitation peaksfrom parity forbidden f–f transitions at 379 nm from 7F6-

5D4,370 nm from 7F6-

5D3, 350 nm from 7F6-5D2 and 318 nm from

7F6-5D2 are intense in LMB:Tb3þ . The narrow emission bands

are characteristic of the spectra of trivalent rare earth ions in thecrystals, because the 4en valance electrons of RE3þ ions areelectrically shielded by the 5S2 5P6 electron clouds. The weak andstrong series are attributed to the electronic transitions from 5D3

and 5D4, respectively, to 7FJ (J¼6, 5, 4, 3, 2, 1 and 0) within 4e8

configuration in Tb3þ . However, in LMB:Tb3þ by 377 nm excita-tion, emissions the in green region at 488 nm (5D4-

7F6), 541 nm(5D4-

7F5), 586 nm (5D4-7F4) and 619 nm (5D4-

7F3) are onlyseen. The intensity of the 5D3 (blue) series is quenched by thecross-relaxation process between 5D3-

5D4 of the emitting Tb3þ

and 7F6-7F0 of the neighboring Tb3þ [25,26]. Gamma irradia-

tions showed only negligible change in Tb3þ PL intensity, whichmeans that redox process do not play any significant role in theTL of LMB:Tb3þ phosphors.

4. Conclusions

Lithium magnesium borate phosphor doped with different rareearth dopants has been synthesized by the simple solid statediffusion technique. TL glow curves have revealed that LMB:Tb3þ ,

which is the best among the RE dopants, has a very stabledosimetric peak at 240 1C. XRD diffraction pattern of the phos-phors showed a crystalline structure but any matching JCPDS filewas identified. A comprehensive study based on X-ray diffractionis under progress for the identification of crystal structure andexact chemical formula. Dosimetric studies in LMB:Tb3þ revealeda near linear TL response up to the saturation gamma dose of103 Gy. Another advantage of this near tissue equivalent phos-phor is its simple annealing procedure followed for the reuse thatkeeps the glow curve structure and sensitivity stable for manycycles. Post-irradiation storage stability of LMB:Tb3þ phosphor iscomparable with other standard TL phosphors but the lightinduced fading is twice as that of CaSO4:Dy powder. TL emissionspectra have shown a PMT sensitive broad green emission bandaround 550 nm from dopant Tb3þ and host material. PL spectrarevealed the characteristic Tb3þ lines and no redox process wasobserved on irradiation. Further detailed studies are needed inthis material for proposing any model for the TL process. Thispaper mainly emphasized on the radiation dosimetric character-ization of the material and further investigation on the TLmechanism and parameter calculations are under way in ourlaboratory. LMB:Tb3þ phosphor can be a very promising TLdosimetry material for gamma as well as neutrons and its neutronsensitivity can be enhanced using 6Li and 10B enriched chemicals.

Acknowledgments

The authors are grateful to Dr. P. Chellapandi, Director, SafetyGroup and Dr. B. Venkatraman, Head, Radiological Safety Divisionfor their support and encouragement and Dr. U. Madhusoodananof Radiological Safety Division for the useful discussions. Theauthors also wish to acknowledge Dr. G Panneerselvam of FuelChemistry Division for XRD measurements.

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200 250 300 350 400 450 500 550 600 6500

5

10

15

20

25Emission

PL

Inte

nsity

(a.u

)

Wavelength (nm)

Excitationλex = 379 nm

λem = 541 nm

Fig. 13. PL spectra of unirradiated LMB:Tb3þ powder for excitation at 379 nm and

emission at 541 nm.

S.R. Anishia et al. / Journal of Luminescence 131 (2011) 2492–24982498