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2014-02
Feasibility study of CaSO4:Tb,Yb as a
thermoluminescent dosimeter
Radiation Physics and Chemistry, Amsterdam : Elsevier, v. 95, p. 119-121, Feb. 2014
http://www.producao.usp.br/handle/BDPI/50814
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Radiation Physics and Chemistry 95 (2014) 119–121
Contents lists available at ScienceDirect
Radiation Physics and Chemistry
journal homepage: www.elsevier.com/locate/radphyschem
Feasibility study of CaSO4:Tb,Yb as a thermoluminescent dosimeter
Danilo O. Junot a,n, Max A. Santos a, Marcos A.P. Chagas a, Marcos A. Couto dos Santos a,
Luiz A.O. Nunes b, Divanizia N. Souza a
a
b
Departamento de Fı́sica, Universidade Federal de Sergipe; Campus Prof. José Aloı́sio de Campos, Av. Marechal Rondon, S/N; 49.100-000, Sa~ o Cristóva~ o, SE, Brazil
~ Carlos, Universidade de Sa~ o Paulo, CP 369, 13560-970, Sao
~ Carlos, SP, Brazil
Instituto de Fı́sica de Sao
H I G H L I G H T S
c
c
c
c
c
A new dosimeter based on CaSO4 and doped with Tb and Yb is proposed.
The samples were grown using a production route based in the Yamashita method.
CaSO4:Tb,Yb glow curves exhibited a TL emission in temperatures from 100 to 270 1C.
The TL responses of the composites produced are proportional to the dose absorbed.
The CaSO4:Tb,Yb has potential to be used as a thermoluminescent dosimeter.
a r t i c l e i n f o
abstract
Article history:
Received 30 September 2012
Accepted 11 January 2013
Available online 21 January 2013
A new composite based on CaSO4, using terbium as dopant and ytterbium as co-dopant (CaSO4:Tb,Yb),
was developed for employment as a thermoluminescent (TL) dosimeter. The crystals used in this work
were grown using a production route based on the Yamashita method (Yamashita et al., 1968). Crystal
powder was calcined at 600 1C for 1 h. Pellets were made by adding commercial and colorless glass to
improve physical resistance and sintered at 700 1C for 6 h. All samples were irradiated by a beta source
(90Sr/90Y) and received doses from 1 Gy to 5 Gy. TL analyses have been performed and characteristics
such as sensitivity, reproducibility, linearity, and fading have been studied. The CaSO4:Tb,Yb pellets
glow curves presented two peaks, the first at around 115 1C, and the second at around 200 1C. The
highest intensity was shown for CaSO4:Tb,Yb with a concentration of 0.1 mol% of Tb and Yb together.
In all the samples the TL response was proportional to the absorbed dose. Therefore, the CaSO4:Tb,Yb
has potential to be used as a thermoluminescent dosimeter.
& 2013 Published by Elsevier Ltd.
Keywords:
TL
Dosimetry
CaSO4:Tb,Yb
1. Introduction
Calcium sulphate (CaSO4) with different dopants is one of the
most investigated materials to be used as a thermoluminescent
dosimeter (TLD) (Campos, 1983). Thus, it is important to search
for new doping elements for the base material in order to improve
the sensitivity and the thermoluminescent (TL) response of the
material to radiation (Mckeever et al., 1995).
Rare earth (RE) elements are often used as dopants for the
production of CaSO4 thermoluminescent detectors. Yamashita
et al. (1968), (1971) developed a relatively simple and efficient
method for doping CaSO4 using rare earths. This method has been
widely used and modified by various researchers (Kása et al.,
2007; Yang et al., 2004; Junot et al., 2011).
In this work we consider a new composite based on CaSO4,
using terbium as dopant and ytterbium as co-dopant (CaSO4:
n
Corresponding author. Tel.: þ55 79 98233937.
E-mail address: [email protected] (D.O. Junot).
0969-806X/$ - see front matter & 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.radphyschem.2013.01.012
Tb,Yb), to be employed as a TLD for the measurement of ionizing
radiation. The TL of CaSO4:Tb,Yb has been studied with the aim of
evaluating its applicability to dosimetry, considering characteristics such as sensitivity, dose response, reproducibility, and
fading. In order to characterize the samples by this technique,
composites were produced with the addition of glass.
2. Materials and methods
The crystals used in this work were grown using a production
route based on the Yamashita method (Yamashita et al., 1968).
Calcium carbonate (CaCO3) (J.T. Baker 99%) was mixed with
terbium oxide (Aldrich 99.9%) at a concentration of 0.1 mol%
and with ytterbium oxide (Aldrich 99.9%) at concentrations of
0.1 mol%, 0.2 mol%, and 0.5 mol%, which will result in samples
with terbium to ytterbium ratios of 1:1, 1:2 and 1:5, respectively.
After this, the compounds were placed in a beaker with an excess
of sulfuric acid (Vetec) and mixed by a magnetic stirrer at 350 1C
D.O. Junot et al. / Radiation Physics and Chemistry 95 (2014) 119–121
3. Results
TL analysis of CaSO4:Tb,Yb pellets, with terbium to ytterbium
ratios of 1:1, 1:2, and 1:5, respectively, exhibited glow curves
with TL emission in the temperature range from 100 to 270 1C.
Fig. 1 presents a comparison among the glow curves of these
three samples after exposure to 1 Gy of beta radiation. Two peaks
can be observed: one at 130 1C, whose maximum intensity is
obtained for the samples with 0.1 mol% of Tb and Yb, and the
other at 240 1C, whose maximum intensity is obtained for the
samples with 0.1 mol% of Tb and 0.2 mol% of Yb. This demonstrates that the traps of CaSO4:Tb,Yb with 0.2 mol% of Yb are
deeper than those of the other compounds studied.
Fig. 2 shows the calibration curves of CaSO4:Tb,Yb at doses from
1 Gy to 5 Gy deposited with beta radiation (90Sr/90Y). It can be
observed that the thermoluminescent responses of the composites
produced are proportional to the dose absorbed in this dose range.
With the goal of studying the reproducibility of the CaSO4:
Tb,Yb, five samples were irradiated with 1 Gy of beta radiation,
read, annealed, and irradiated again, creating a cycle that was
performed eight times. As can be seen in Fig. 3, all TL responses
were similar with only around 5% deviation.
Table 1 shows the decay of the TL signal of the CaSO4:Tb,Yb
produced. Although the peak centered at 130 1C practically fades
TL Intensity (arb. unit)
0.6
CaSO4 :Tb,Yb (1:1)
CaSO4 :Tb,Yb (1:2)
CaSO4 :Tb,Yb (1:5)
4
TL Intensity (arb. unit)
under constant pressure of 1 atm until all the acid had evaporated
(about 48 h), leaving only doped calcium sulfate.
After growing process, the doped crystals were rinsed three
times with cold and hot distilled water, calcined at 600 1C for 1 h,
crumbled, rinsed again, and grain-selected to obtain grain sizes
between 75 and 180 mm.
Pellets were made using a 2:1 (wt) mixture of the phosphor and
powdered commercial and colorless glass to improve physical
resistance. The pellets were compacted by application of uniaxial
pressing of 100 kgf and sintered at 700 1C for 6 h. After each
irradiation-readout cycle the pellets were annealed for 1 h at 4001 C.
The samples were irradiated with a 90Sr/90Y source at a rate of
0.35 Gy per minute and received doses from 1 Gy to 5 Gy.
TL analyses were made with a Harshaw 3500 TL reader using a
heating rate of 10 1C/s. The maximum shunting line found in the
measures was 76.5%.
To study the fading of the composites, TL analyses were
measured at the following times: immediately after irradiation,
one week after irradiation, two weeks after irradiation, and one
month after irradiation. Percentage decay of the TL intensity
during storage period was calculated by comparison with the TL
signal of the sample read immediately after irradiation.
CaSO4 :Tb,Yb (1:1)
CaSO4 :Tb,Yb (1:2)
CaSO4 :Tb,Yb (1:5)
3
2
1
0
1
2
3
4
5
Absorbed Dose (Gy)
Fig. 2. TL response of CaSO4:Tb,Yb samples as a function of dose absorbed from
beta radiation.
4.0
Peak 1
Peak 2
3.5
TL Intensity (arb. unit)
120
3.0
2.5
2.0
1.5
1.0
0.5
0
1
2
3
4
5
6
7
8
9
Cycle
Fig. 3. Normalized TL response of CaSO4:Tb,Yb sample after each irradiation-readanneal cycle.
Table 1
TL response of composites after irradiation with beta radiation.
Material
TL peak TL intensity Fading
(arb. unit)
in 24 h
temp.
(%)
(1C)
Fading
in
7 days
(%)
Fading
in 15
days (%)
Fading
in 30
days (%)
CaSO4:Tb,Yb
(1:1)
CaSO4:Tb,Yb
(1:2)
CaSO4:Tb,Yb
(1:5)
130
240
130
240
130
240
18.69 7 0.93
2.68 7 0.13
18.48 7 0.92
4.57 7 0.23
5.25 7 0.26
2.72 7 0.13
70 75
11 75
69 75
15 75
38 75
22 75
777 5
157 5
797 5
257 5
657 5
277 5
84 75
19 75
85 75
32 75
78 75
35 75
527 5
87 5
497 5
87 5
217 5
107 5
out after one month, the fading of CaSO4:Tb,Yb appeared acceptable because in 30 days the peak centered at 240 1C, which is the
dosimetric peak, fades by 35% for a 1:5 ratio of Tb to Yb, 32% for a
1:2 ratio, and 19% for a 1:1 ratio.
It is important to consider that the fading is related not only to
the composition of the dosimeter but also to the storage conditions during the time between irradiation and reading.
0.4
0.2
0.0
50
100
150
200
250
Temperature (°C)
Fig. 1. Typical TL glow curves of CaSO4:Tb,Yb samples.
300
4. Conclusions
The CaSO4:Tb,Yb phosphorous analyzed in this work showed
TL glow curves with two peaks. The peak centered at 240 1C,
D.O. Junot et al. / Radiation Physics and Chemistry 95 (2014) 119–121
although not the most intense, could be used as the dosimetric
peak due to its good sensitivity, reproducibility, and fading
acceptable. The highest intensity was shown for CaSO4:Tb,Yb
with a concentration of 0.1 mol% of both Tb and Yb. In all samples
the TL response was proportional to the absorbed dose. Commercial colorless glass was shown to be appropriate for incorporation
with the phosphors for the production of dosimeters in the form
of pellets. Due the previously mentioned properties associated
with their low cost, CaSO4:Tb,Yb has potential for use as a
thermoluminescent dosimeter.
Acknowledgments
The authors thank FAPITEC, CNPq, and CAPES for partial
financial support.
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