phys. stat. sol. (a) 202, No. 14, 2800–2806 (2005) / DOI 10.1002/pssa.200521199

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Thermoluminescence characteristics of terbium-doped

Ba2Ca(BO3)2 phosphor

Liyan Liu1, Yanli Zhang2, Jingquan Hao1, Chengyu Li*, 1, Qiang Tang3,

Chunxiang Zhang3, and Qiang Su**, 1, 4

1 Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, P.R. China 2 Ionizing Radiation Division, National Institute of Metrology, Beijing 130000, P.R. China 3 Department of Physics, Sun Yat-sen University, Guangzhou 510275, P.R. China 4 State Key Laboratory of Optoelectronic Materials and Technology,

School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275,

P.R. China

Received 9 March 2005, revised 2 August 2005, accepted 18 August 2005

Published online 12 October 2005

PACS 61.80.Ed, 78.60.Kn, 87.58.Sp

Thermoluminescence (TL) characteristics and dosimetric properties of Ba2Ca(BO

3)2 phosphor doped with

Tb3+ exposed to gamma-ray irradiation are reported for the first time. Firstly, the influence of different

rare earth dopants, i.e. Dy3+, Tb3+ and Tm3+, on TL of Ba2Ca(BO

3)2 phosphor is discussed. All glow curves

consist of two TL peaks located at 109–120 °C and 199–220 °C and Tb3+-doped Ba2Ca(BO

3)2 phosphor

exhibits the highest TL sensitivity. Secondly, the effects of concentration of Tb3+ and gamma-ray irradia-

tion dose on TL are investigated. The optimum Tb3+ concentration is 2 mol% and the TL kinetic parame-

ters of Ba2Ca(BO

3)2:0.02Tb are calculated by the peak shape method. The high-temperature peak for this

sample shifts to higher temperature with increasing irradiation dose. Furthermore, the dosimetric proper-

ties of Ba2Ca(BO

3)2:0.02Tb phosphor, i.e. the pre-irradiation annealing treatment, reproducibility and

linearity, are studied, the results of which indicate that Tb3+-doped Ba2Ca(BO

3)2 phosphor has a potential

application in the personal protection dosimetry field. TL emission of the phosphor is observed to peak at

about 485, 542, 580 and 625 nm originating from the characteristic transitions of Tb3+.

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Ionizing radiation has been widely used in various fields, such as in industry for water purification and organic polymer material crosslinking, in medicine for radio-sterilization and in agriculture for disinfes-tation or inhibition of sprouting. Unfortunately, ionizing radiation can induce cancer and genetic defects, possibly even at very low doses. Thus the measurement of the radiation becomes a science of increasing importance for estimating the risks and benefits inherent in the use of ionizing radiation. Thermolumi-nescence (TL) is the most widely used method for detecting the dose of various energetic rays such as X-, β- or γ-rays. Borate thermoluminescent materials have attracted much attention due to their tissue-equivalent char-acteristics [1, 2] and many investigations on them have been performed. Sangeeta and Sabharwal studied the thermally stimulated luminescence behaviors of alkaline earth borates XB2O4 (X = Ca, Sr, Ba) irradi-ated by γ-rays [3] and evaluated their kinetic parameters employing both isothermal decay and peak

* Corresponding author: e-mail: cyli@ciac.jl.cn, Phone: +86 431 5262208, Fax: +86 431 5698041

** e-mail: suqiang@ciac.jl.cn, Phone: +86 431 5262208, Fax: +86 431 5698041

phys. stat. sol. (a) 202, No. 14 (2005) 2801

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Original

Paper

shape methods [4]. Santiago et al. reported the TL properties of pure strontium tetraborate and found that its TL efficiency was similar to that of the commercial TLD-700 dosimeter [5], which prompted them to explore the TL properties of dysprosium-doped strontium tetraborate exposed to a γ-ray source, for which the TL intensity was usually increased by adding activators [6]. Recently our group also investi-gated the β-ray-induced TL characteristics of the dysprosium-doped strontium tetraborate and suggested that it has potential application for high-dose β-ray dosimetry [7]. In addition, the TL properties of single crystals of BaB2O4 [8] and LiB3O5 [9] have been reported, as well as those of polycrystalline powder samples of BaB4O7 :Dy [10], Li2B4O7 :Cu [11, 12] and MgB4O7 :Dy,Na [13, 14]. Although not all of these materials can be utilized in practice, at least the results obtained provide useful information and insight concerning the defects and trap structure that are helpful in the search for new borate TLD mate-rials and show that borates are attractive TLD materials. To our knowledge, the preparation and TL properties of mixed sulfate compounds K2Ca2(SO4)3 acti-vated by europium have been described, the sensitivity of which is five times as that of CaSO4 :Dy [15]. However, the TL characteristics and dosimetric properties of mixed alkaline earth borates have never been reported. In this paper, we investigate the TL characteristics and dosimetric properties of Ba2Ca(BO3)2 doped with terbium, based on which the potential of Ba2Ca(BO3)2 :Tb phosphor as a TLD material is proposed.

2 Experimental

Powder samples of Ba2Ca(BO3)2 were synthesized by solid-state reaction at high temperature. The raw materials were BaCO3 (AR), CaCO3 (AR), B2O3 (AR), Tb4O7 (99.99%), Dy2O3 (99.99%) and Tm2O3 (99.99%). The concentrations of Dy3+ and Tm3+ dopants were fixed at 4 mol% and that of Tb3+ dopant ion at 0.2, 0.5, 1, 2, 4 and 8 mol%. The stoichiometric amounts of the corresponding raw materials were thoroughly mixed and ground in an agate mortar. The mixtures were than transferred to an alumina cru-cible and fired at 600 °C for 30 min. After slowly cooling to room temperature, the pre-fired samples were thoroughly reground and then calcined at 1000 °C for 24 h in CO atmosphere for samples doped with Tb3+ and ambient atmosphere for samples doped with Dy3+ and Tm3+. X-ray diffraction (XRD) pat-terns of the samples were obtained using a Rigaku D/max-ПB X-ray diffractometer with CuKα1 (λ = 1.5405 Å) radiation and the data were coincident with those of Ba2Ca(BO3)2 (JCPDS: 85-2268). TL glow curve measurements were carried out using an FJ-427A TL meter (Beijing Nuclear Instru-ment Factory, Beijing, China) at a linear heating rate of 2 °C/s. The three-dimensional (3D) TL emission spectrum was measured using a spectrometer consisting of a home-built linear heater and EG & G PAR optical multichannel analyzer with a linear heating rate of 5 °C/s. The dose response measurement was performed using a Toledo 654TLD reader (Vinten Instruments, Weybridge, England) with a heating rate of 20 °C/s and nitrogen flow protection. Before measurements, the powder samples were first pressed into pellets (5 mm in diameter and 0.5 mm in thickness) and then exposed to 60Co γ-ray irradiation at room temperature.

3 Results and discussion

3.1 Effect of different dopants on TL

TL glow curves of Ba2Ca(BO3)2 phosphor doped with Dy3+, Tb3+ or Tm3+ exposed to a 50 Gy irradiation dose are shown in Fig. 1. The background signal has been subtracted from the TL glow curves. It is ob-vious that Ba2Ca(BO3)2 phosphor doped with Tb3+ exhibits the strongest TL sensitivity compared with those doped with Dy3+ or Tm3+. The TL glow curves of the samples are all composed of two peaks des-ignated PL and PH for the lower- and higher-temperature peak, respectively. For the three samples in the sequence of the activator Dy3+, Tb3+ and Tm3+, the peak temperatures of PL are situated at about 120, 111 and 109 °C, and those of PH at 199, 200 and 220 °C, respectively. The maximum shifts for PL and PH among the Ba2Ca(BO3)2 phosphors with Dy3+, Tb3+ and Tm3+ are ~11 and ~21 °C, indicating that the

2802 Liyan Liu et al.: Thermoluminescence characteristics of terbium-doped Ba2Ca(BO

3)2 phosphor

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

300 350 400 450 500 550 600 650 700 750

0

2

4

6

8

10

12

0 2 4 6 8

TL

inte

nsit

y(a

.u.)

f

e

dc

b

TL

Inte

nsit

y(a.

u.)

Temperature(K)

a

concentration of Tb3+ (mol%)

dopant rare earth ion can affect the intrinsic trap of the host. As a matter of fact, there exist many illustra-tions of the dependence of peak temperature on the dopant rare earth ionic radii. For example, for CaSO4 :Tm and CaSO4 :Dy, the glow peaks have a difference of 8 °C [16]; and for magnesium borate the differences turns to 30 °C [17]. For LaF3 [18], the variation of the TL peak at –145 °C (128 K) is 13 °C as a function of the dopant rare earth ionic radius.

3.2 Effect of concentration of Tb3+ on TL

TL glow curves of Ba2Ca(BO3)2 :Tb phosphor with different Tb3+ concentrations under 50 Gy γ-ray irra-diation at room temperature are shown in Fig. 2. It is clear that the peak shape and temperature of main dosimetry peak (PH) keep constant in the concentration range studied. As shown in the inset of Fig. 2, the TL output of the sample firstly increases with increasing concentration of Tb3+ from 0.2 to 2 mol%, reaches a maximum value at 2 mol% and then gradually decreases from 2 to 8 mol%. This is the well-known concentration quenching phenomenon. Similar results have been observed for CaSO4 :Dy [19], CaF2 :Ce [20] and CaF2 :Tm (TLD-300) [21]. Therefore, the following studies are focused on Ba2Ca(BO3)2 with 2 mol% Tb3+, i.e. Ba2Ca(BO3)2 :0.02Tb.

3.3 Studies of Ba2Ca(BO3)2 :0.02Tb phosphor

3.3.1 Kinetic parameters

Figure 3 shows the TL bands of Ba2Ca(BO3)2 :0.02Tb as well as the deconvoluted curves. Two deconvo-luted peaks are obtained, namely PL and PH, and the overlapping part of PL and PH is very small. Fur-thermore, it is apparent that PH fits very well with the main peak of the non-deconvoluted curve with the same peak temperature and similar shape. Therefore, when we apply the peak shape method, based on the symmetrical property of the TL peak, to calculate the TL parameters of PH, e.g. activation energy (E) and frequency factor (s), the effect of PL and its overlapping part with PH can be neglected. Here it should be noted that the aim of deconvolving the glow curve with Gaussian curves is to prove when the peak shape method is used to study the main TL peak, i.e. the higher-temperature TL peak, the peak at lower temperature has little effect on the results. The small overlapping part of the two deconvoluted peaks

200 300 400 500 600 7000

2

4

6

8

10

12

cb

TL

Inte

nsit

y(a,

u,)

Temperature(K)

P1

P2

a

Fig. 1 TL glow curves of Ba2Ca(BO

3)2 phosphor

doped with (a) 4 mol% Tb3+, (b) 4 mol% Dy3+ (ampli-

fied 10 times) and (c) 4 mol% Tm3+ (amplified 10

times) exposed to 50 Gy γ-ray irradiation at room

temperature.

Fig. 2 TL glow curves of Ba2Ca(BO

3)2 phosphor with

different Tb3+ concentrations irradiated with a 50 Gy γ-ray

dose at room temperature: (a) 0.2 mol%, (b) 0.5 mol%,

(c) 1 mol%, (d) 2 mol%, (e) 4 mol%, (f) 8 mol%. The

inset shows the dependence of TL response on Tb3+ con-

centration.

phys. stat. sol. (a) 202, No. 14 (2005) 2803

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400 6 00

0

4

8

12

PL

Tm T2

experimental curveFitted curvedeconvoluted curvedeconvoluted curve

Inte

nsity

(a.u

.)

Temperature(K)

T1

Ph

gives a direct proof for our assumption. According to Chen’s method, the symmetry factor µ g is deter-mined by the equation µ g = δ /ω = (T2 – Tm)/(Tm – T1), and the value of µ g thus obtained is 0.52 which indicates that the present TL peak obeys second-order kinetics. Therefore, the activation energy (E) of PH can be estimated with the following equation [22]:

E = cγ(kTm2 /γ) – bγ(2kTm) . (1)

Here γ stands for τ, δ, or ω which are respectively determined by low-temperature half-width (τ = Tm – T1), high-temperature half width (δ = T2 – Tm) and full width (ω = T2 – T1); k is the Boltzmann constant. For second-order kinetics, the values of cγ and bγ depending on τ, δ, or ω are given in Table 1 [22]. Therefore, according to Eq. (1), the values of E of PH with different peak parameters can be calculated, as given in Table 2. The value of s can be obtained by inserting E and the known values of b (2 for second-order kinetics) and β (heating rate, 2 K/s) into the following equation [22]:

m

2

m m

21 ( 1) exp

E kT Es b

kT E kT

β Ê ˆÈ ˘= + - -Á ˜Í ˙ Ë ¯Î ˚. (2)

The as-calculated frequency factors are also given in Table 2.

3.3.2 TL peak dependence on dose

The TL glow curves of Ba2Ca(BO3)2 :0.02Tb phosphor exposed to various γ-ray irradiation doses are shown in Fig. 4. The high-temperature TL peaks are located at around 200, 206 and 211 °C, increasing with increasing irradiation dose. However, for a second-order kinetics model this is confusing because the result should be the opposite, i.e. peak temperature (Tm) decreases with increasing dose. The reason for this anomaly is due to the fact that for the assumption of the known second-order kinetics model for TL, the existence of the deeper trap (termed the ‘competing trap’) is usually omitted, whereas, with an increase of the radiation dose, the pre-omitted competing trap is filled and plays an important role in the

Table 1 Values of cγ and b

γ depending on τ, δ, or ω.

τ δ ω

cγ 1.81 1.71 3.54 bγ 2.0 0 1.0

Fig. 3 TL curve and deconvoluted curves of

Ba2Ca(BO

3)2:0.02Tb. P

L and P

H are the deconvoluted

peaks. T1, T

m and T

2 are the temperatures of half-

intensity on the low-temperature side, maximum inten-

sity and half-intensity on the high temperature side of

PH, respectively.

2804 Liyan Liu et al.: Thermoluminescence characteristics of terbium-doped Ba2Ca(BO

3)2 phosphor

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

Table 2 Trap depth and frequency factor of PH peak for Ba2Ca(BO

3)2:0.02Tb phosphor.

τ δ ω mean value

Eα (eV) 0.86 0.89 0.88 0.88 s (s–1) 1.12 × 108 2.87 × 108 2.00 × 108 2.0 × 108

TL, causing Tm to move to higher temperature with increasing radiation dose, as shown in Fig. 4. Chen et al. have numerically analyzed the dose dependence of the TL peak [23, 24]. In addition, the phenomenon of the TL peak increasing with radiation dose has been observed for different kinds of TL materials [24, 25] and a detailed study of this phenomenon is beyond the scope of the present article. In Section 3.3.1, the kinetic parameters of the sample are calculated via the second-order kinetics model; however, the peak shifting to higher temperature with increasing dose implies that the model assumed, i.e. second-order kinetics, is not strictly the right one. Therefore, the calculated values for the activation energy and the frequency factor should be regarded as sort of ‘effective values’, since they do not describe rigor-ously the physical process of the TL of the sample.

3.3.3 Pre-irradiation annealing, reproducibility and linearity studies

One of the factors affecting the TL dosimetric properties is the pre-irradiation annealing treatment with the aim of obtaining high TL sensitivity and eliminating the effect of the previous irradiation. Figure 5 shows the TL outputs of Ba2Ca(BO3)2 :0.02Tb phosphor after different annealing temperatures from 300 to 550 °C with different annealing time, i.e. 30 min or 1 h. The sample was firstly exposed to 50 Gy irradiation at room temperature. It can be seen that the sample has the strongest TL intensity when an-nealed at 350 °C for 30 min and on increasing the annealing temperature higher than 350 °C the TL intensity decreases. Therefore it is important to carry out a suitable annealing procedure for the use of the material. In order to check the reproducibility of the dose measurement using Ba2Ca(BO3)2 :0.02Tb phosphor, eight annealing–radiation–readout repeat cycles were carried out for the same 50 Gy γ-ray exposure. Figure 6 displays the results obtained after the eight cycles, which shows the good reproducibility of the material for dose measurements.

350 400 450 500 550 600 650

0

2

4

6

8

10c

b

Inte

nsit

y(a.

u.)

Temperature(K)

a

550 600 650 700 750 800 8500.0

5.0x104

1.0x105

1.5x105

2.0x105

2.5x105

3.0x105 30min1h

TL

Inte

nsit

y(a

.u.)

Temperature(K)

Fig. 4 TL glow curves of Ba2Ca(BO

3)2:0.02Tb phos-

phor exposed to (a) 50 Gy, (b) 125 Gy and (c) 200 Gy

γ-ray irradiation at room temperature.

Fig. 5 Behavior of the TL response for

Ba2Ca(BO

3)2:0.02Tb phosphor after annealing treat-

ment exposed to 50 Gy irradiation at room temperature.

phys. stat. sol. (a) 202, No. 14 (2005) 2805

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Figure 7 shows the relationship between the TL response and the absorbed dose for Ba2Ca(BO3)2 :0.02Tb. For the purpose of comparison, the TL response is also shown for LiF:Mg,Ti (TLD-100) under the same conditions. For each dose level three dosimeters are irradiated simultaneously and each data point corresponds to the mean of the three readouts. Linearity is well observed in the range of protection dose level from 1 to 1000 mGy. The linearity correlation coefficients are 0.998 and 0.999 for Ba2Ca(BO3)2 :0.02Tb and TLD-100 phosphors, respectively. It is apparent that Ba2Ca(BO3)2 :0.02Tb is more sensitive than TLD-100 in the studied dose range.

3.3.4 3D TL emission spectrum

Figure 8 shows the 3D TL emission spectrum of Ba2Ca(BO3)2 :0.02Tb phosphor irradiated at a dose of 50 Gy at room temperature. It is observed that TL emission consists of four peaks at about 485, 542, 580 and 625 nm due to the characteristic 5D4 → 7F6,

5D4 → 7F5, 5D4 → 7F4 and 5D4 → 7F3 transitions of the

Tb3+ ion. It is obvious that the strongest emission, i.e. 5D4 → 7F5 transition (542 nm), contributes greatly to the TL output.

Fig. 8 (online colour at: www.pss-a.com) 3D TL emission spectrum of Ba2Ca(BO3)2 :0.02Tb phosphor

irradiated at a dose of 50 Gy at room temperature.

0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

TL

Inte

nsit

y(a

.u.)

Number of mesurement 0 200 400 600 800 1000 1200-1x107

0

1x107

2x107

3x107

4x107

5x107

TL

Inte

nsit

y(a

.u.)

dose(mGy)

Ba2Ca(BO3)2:0.02Tb

TLD-100

Fig. 6 Reproducibility of the TL signal from

Ba2Ca(BO

3)2:0.02Tb phosphor over eight repeated

cycles of annealing–irradiation–readout.

Fig. 7 TL dose response of Ba2Ca(BO

3)2:0.02Tb and

TLD-100 phosphors.

2806 Liyan Liu et al.: Thermoluminescence characteristics of terbium-doped Ba2Ca(BO

3)2 phosphor

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

4 Conclusions

Borates based on Ba2Ca(BO3)2 were synthesized by the solid-state reaction. Ba2Ca(BO3)2 phosphor with Tb3+ shows higher TL sensitivity compared with that doped by Dy3+ or Tm3+. Further, the optimum Tb3+ dopant concentration was determined to be 2 mol%. At the optimum Tb3+ concentration, the high-temperature TL peak of the phosphor shifts to higher temperatures with increasing irradiation dose. By resorting to the peak shape method the following effective values were estimated for the activation en-ergy and the frequency factor of the second peak: 0.88 eV and 2.0 × 108 s–1, respectively. Pre-irradiation annealing procedure adopted for Ba2Ca(BO3)2 :0.02Tb is at 350 °C for 30 min and the reproducibility investigation reveals it is reusable for TL measurements. The TL response is linear in a protection dose level range of 1–1000 mGy and is about 7 times more sensitive than LiF:Mg,Ti. The TL emission was also observed peaking at about 485, 542, 580 and 625 nm originating from the characteristic Tb3+ transi-tions. The characteristics imply the potential of Ba2Ca(BO3)2 :Tb phosphors as γ-ray TL materials in the personal protection dosimetry field.

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