In the current work the glow curves for LiF:Mg,Cu,Na,Si thermoluminescent material were measured at various heating rates in the range from 1 K/s to 30 K/s. In the thermoluminescent measurements a contact heating was used to heat the powder samples. The temperature lag between the heating element and the dosimeter was estimated and corrected by applying Kitis-Tuyn's method. Some kinetic parameters of the traps in LiF:Mg,Cu,Na,Si were evaluated using a variable-heating-rate method

Ha, Vu Thi Thai

Hai, Nguyen Thi Quy

Long, Nguyen Ngoc

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 21, No. 2 (2011), pp. 145-152
ESTIMATION AND CORRECTION FOR THE TEMPERATURE
LAG IN THERMOLUMINESCENT MEASUREMENTS FOR
LiF:Mg,Cu,Na,Si PHOSPHOR
VU THI THAI HA AND NGUYEN THI QUY HAI
Institute of Materials Science, VAST
NGUYEN NGOC LONG
Center for Materials Science, Hanoi University of Science, VNU
Abstract. In the current work the glow curves for LiF:Mg,Cu,Na,Si thermoluminescent material
were measured at various heating rates in the range from 1 K/s to 30 K/s. In the thermolumi-
nescent measurements a contact heating was used to heat the powder samples. The temperature
lag between the heating element and the dosimeter was estimated and corrected by applying Kitis-
Tuyn’s method. Some kinetic parameters of the traps in LiF:Mg,Cu,Na,Si were evaluated using a
variable-heating-rate method.
I. INTRODUCTION
Nowadays, it is well-known that thermoluminescence dosimetry (TLD) is a dosi-
metric technique with applications in areas such as personnel, environmental and clinical
dosimetry. TLD is based on thermoluminescence (TL) materials which after exposure to
ionising radiation emit light when they are heated. It is found that the impurities doped in
TL material create localized energy levels (trapping levels) in the forbidden energy band
gap and that these are crucial to the TL process. The study of the TL glow curve, which
is a plot TL intensity versus temperature, is used to determine the trapping parameters.
However, the TL response strongly depends on heating rate [1–10]. In addition, in TL
measuring experiment the temperature is usually that of the heating element measured
by the thermocouple fixed on it, but it is not exactly the temperature of the TL material.
However, in order to extract physical information from TL glow curve, it is essential to
know the sample temperature rather than that of the heating element. Contact heating
is the most commonly used type of heating. In this case, the temperature of the sample
usually differs from the temperature of the heating element measured by thermocouple
placed on it. This difference is caused by temperature gradients in the heating element,
non-ideal thermal contact between the heater element and the sample (temperature lag
(TLA)), temperature gradients (TG) across the sample. The difference between the ac-
tual temperature of the sample and the measured temperature obtained from the heating
element has been the object of study by different authors [5, 11–15]. The TLA and the
TG could influence the values of the trapping parameters of a glow peak [12, 13]. Kitis
and Tuyn [14, 15] proposed a simple method to correct for the temperature lag based on
TL measurements only.
146 VU THI THAI HA, NGUYEN THI QUY HAI, AND NGUYEN NGOC LONG
A powder-type of LiF doped with four dopants: Mg, Cu, Na and Si was first de-
veloped by Kim et al in 1989 [16, 17]. The LiF:Mg,Cu,Na,Si material, being a promis-
ing material for thermoluminescent dosimetry, has attracted the researchers’ attention
in the last ten years. The effect of the dopant concentrations on the TL glow curve in
LiF:Mg,Cu,Na,Si phosphor was investigated [18–21]. The influence of the heating rate on
TL response for LiF:Mg,Cu,Na,Si phosphor was reported by Ha et al [22]. However, to
the best of our knowledge, up to now, there is almost no work about the evaluation of the
trapping parameters in the LiF:Mg,Cu,Na,Si phosphor.
The aim of the present work is to estimate and correct the temperature lag between
the heating element and the dosimeter by applying Kitis-Tuyn’s method. Some kinetic
parameters of the traps for the main TL glow peak in LiF:Mg,Cu,Na,Si were evaluated
using a variable-heating-rate method.
II. EXPERIMENTAL
Phosphor preparation
Synthesis process for LiF:Mg,Cu,Na,Si TL powders described elsewhere [20, 21].
The host LiF material was prepared by using precursor chemicals such as lithium chloride
(LiCl) and fluohydric acid (HF). The doping with Mg, Cu, Na and Si was performed
by mixing the compounds containing required activator: MgCl2.6H2O, CuCl2.H2O and
Na2SiO3.9H2O in appropriate ratio. Our results of analysis of the dependence of the
intensity for the TL glow curves on the dopant concentration [21] indicated that the
optimum dopant concentrations were 0.2 mol% Mg, 0.6 mol% Cu and 2.0 mol% NaSi,
which in good agreement with data reported by Nam et al [18].
Thermoluminescence measurements
The LiF:Mg,Cu,Na,Si powders were irradiated by X-ray radiation. All the samples
were irradiated to the same dose of 0.1 Gy from an X-ray tube (RORIX, RFT, Germany)
1.5 kW in power with a cobalt target, operated at 20 kV and 1-5 mA. The total irradiation
duration was 3 minutes. The TL glow curves of the powders were subsequently measured
by using a commercial TLD reader (Harshaw TLD-3500) controlled by a computer. In the
TL readout, the 5 mg of the phosphor powders were filled in a hole of 3 mm in diameter
on a copper plate. The powder layer had a thickness below 1 mm. The TL glow curve
measurements were carried out with linear heating rates from 1 K/s up to 30 K/s in
temperature range from 323 K to 633 K. The TL measurements at a given heating rate
were repeated 5 times and the results are represented by the average values with the mean
deviation. The measurement data were processed using the software ORIGIN.
III. RESULTS AND DISCUSSION
As mentioned above, an important effect that has to be taken in to consideration to
avoid large error in the trapping parameter determination is the temperature lag (TLA).
There are various types of sample heating in the TL measurements: contact heating,
hot gas heating, laser heating and microwave heating. Then the TLA may be caused by
temperature gradients in the heating element, non-ideal thermal contact between heater
ESTIMATION AND CORRECTION FOR THE TEMPERATURE LAG IN TL MEASUREMENTS... 147
element and sample, temperature gradients across the sample. These processes are de-
termined by three physical heat transfers: conduction, convection, radiative loss. In the
case of contact heating, the heat transfer mechanism by conduction plays a dominant
role [12, 13] and the non-ideal thermal contact is the main cause for the temperature dif-
ferences between the heating element and the sample. In addition, the authors of the
works [13, 15] have found that for both LiF:Mg,Ti and Al2O3:C samples less than 1 mm
thick, the temperature gradient across the sample is relatively small.
The TLA, as well as the thermal gradient (TG) across the dosimeter itself, can
strongly influence on the trapping parameter evaluation. Kitis and Tuyn [14,15] proposed
a simple method to correct for the temperature lag based on TL measurements only. This
method is based on the following equation:
Tj = Ti − cln
( βi
βj
)
(1)
where Tj and Ti are the maximum temperatures of a glow peak with heating rates βj
and βi, respectively, and c is a constant, which is determined by using two very low
heating rates where the TLA can be considered a negligible. Kitis et al. [14] emphasized
that the measurements at the low rates of heating are reference measurements, so they
need a special attention. This could be achieved in two ways: first, using silicon oil of
high thermal conductivity between the heating element and sample; and second, for these
reference measurements one can use loose powder instead of chips, with or without silicon
oil, improving the accuracy further.
Using the equation (1) and the calculated value of c, the new maximum temperatures
Tm,corr with correction for the TLA at high heating rates are evaluated relative to the
lowest heating rate. The TLA (∆T ) in the position of the peak maximum is given by
∆T = Tm,raw − Tm,corr (2)
where Tm,raw is the glow-peak maximum temperature including the TLA.
The analysis results for the main glow-peak maximum are presented in Table 1.
Column 2 shows the main glow-peak maximum temperatures of the raw data at different
heating rates. Constant c was calculated by using the following expression
c =
Tm,raw,β=2 − Tm,raw,β=1
ln2
(3)
where Tm,raw,β=1 and Tm,raw,β=2 are the values of Tm,raw for heating rates of 1 K/s and 2
K/s. The value of c was found to be 11.48. Column 3 shows the values of the maximum
temperatures calculated by using equation (1), i.e. corrected Tm,corr. The temperature
lags at the positions of the peak maxima are shown in column 4. In our case, like as [1], the
samples are powder layers having less than 1 mm thickness, so the temperature gradient
across the sample is relatively small. Therefore, the TLA is mainly caused by the non-ideal
thermal contact between the heater element and the sample.
The effective heating rates βeff were calculated using the expression proposed by
Kitis and Tuyn [14]:
βeff =
Tm,raw − To −∆T
Tm,raw − To
β (4)
148 VU THI THAI HA, NGUYEN THI QUY HAI, AND NGUYEN NGOC LONG
Table 1. Results of the analysis for LiF:Mg,Cu,Na,Si.
β (K/s) Tm,raw (K) Tm,corr (K) ∆T (K) βeff (K/s) Tm,corr,eff (K)
1 490.4±1.0 490.4±1.0 0.0 1 490.4±1.0
2 498.4±1.0 498.4±1.0 0.0 2 498.4±1.0
5 510.9±1.0 508.9±1.0 2.0 4.94 508.7±1.0
10 522.6±1.0 516.8±1.0 5.8 9.74 516.5±1.0
15 529.8±1.2 521.5±1.2 8.3 14.39 521.0±1.2
20 536.0±1.7 524.8±1.7 11.2 18.91 524.1±1.7
30 546.2±1.0 529.4±1.0 16.8 27.82 528.6±1.0
where To is the starting temperature, ∼293 K. The values of βeff are listed in column 5.
Finally, the maximum temperatures Tm,corr,eff calculated with the effective heating rates
are listed in column 6.
Fig. 1. (a) Glow curves of the LiF:Mg,Cu,Na,Si powders measured at different
heating rates.(b) The main glow-peak maximum temperature as a function of
heating rate for LiF:Mg,Cu,Na,Si phosphor, curve Tm,raw is plotted from the data
including the TLA and curve Tm,corr is plotted from the data after correction for
the TLA.
Figure 1(a) shows typical glow curves of LiF:Mg,Cu,Na,Si powder measured at some
heating rates. As seen from the figure, the whole TL glow curve is shifted to higher
temperatures and the height of the main glow peak increases with increasing heating rate
in the range from 1 K/s to 30 K/s. We focused only on the main glow peak because
it usually used in the dosimetric measurements. Figure 1(b) shows the plot of the main
glow-peak maximum temperature (Tm) before and after correction for the TLA against
heating rate.
ESTIMATION AND CORRECTION FOR THE TEMPERATURE LAG IN TL MEASUREMENTS... 149
Like as [1], we suppose that the change of the Tm against heating rate follows an
equation of the form ln(Tm or Im) = A + Bln(β), where A and B are constants and β is
the heating rate. For the glow-peak maximum temperature including the TLA, denoted
by Tm,raw, A = 6.1906 ± 0.0034; B = 0.0312 ± 0.0015; for the glow-peak maximum
temperature corrected for the TLA, denoted by Tm,corr, A = 6.19559 ± 0.00024; B =
0.0225 ± 0.0001. Using these constants, we calculated the dependence of Tm on β. The
results are shown by the solid lines in figures 1(b). It can be noted that the fitted lines
are in rather good agreement with experimental data.
Fig. 2. (a) Peak maximum temperature of the main glow peak as a function
of logarithm of the heating rate for LiF:Mg,Cu,Na,Si. Curve Tm,raw is plotted
from the data including the TLA and curve Tm,corr is plotted from the data after
correction for the TLA. (b) The temperature lag as a function of the heating rate
for the main glow peak of LiF:Mg,Cu,Na,Si.
The behaviour of the main glow-peak temperature at the maximum plotted as a
function of logarithm of the heating rates is illustrated in figure 2(a). The results indicated
that a TLA of 2.0 K is already evident at a heating rate of 5 K/s. This TLA reaches a
value of 16.8 K at a heating rate of 30 K/s.
In addition, it is found that the TLA has a good linear relation to the heating
rate (see figure 2(b)), like that observed early by other authors [12–14] for LiF:Mg,Ti and
Al2O3:C. For our LiF:Mg,Cu,Na,Si samples the linear relationship is ∆T = (−0.72 ± 0.22)
+ (0.59 ± 0.01)β.
The variable heating rate method is based on the shift position of the glow-peak
maximum temperature as a function of the heating rate. From expression (4) of Ref. [22],
one obtains
ln
(T 2m
β
)
= ln
(E
sk
)
+
E
kTm
(5)
It can be noted that plot of ln(T 2m/β) versus 1/(kTm) must be a straight line with
slope E, and the intercept of this plot is ln(E/sk), which allows evaluating the frequency
factor s.
150 VU THI THAI HA, NGUYEN THI QUY HAI, AND NGUYEN NGOC LONG
Fig. 3. a) Plot of ln(T 2m/β) against 1/(kTm) for LiF:Mg,Cu,Na,Si. Curve Tm,raw
is plotted from the data including the TLA and curve Tm,corr is plotted from the
data after correction for the TLA. (b) Plot of ln(βi/βj) versus 1/(kTm,corr).
Figure 3(a) shows a plot of ln(T 2m/β) as a function of 1/(kTm) for LiF:Mg,Cu,Na,Si.
As seen from the figure, curve Tm,raw plotted with the data including the TLA is not a
straight line, but as expected, curve Tm,corr plotted with the corrected data is a very good
straight line with slope E = 1.86 ± 0.02 eV. The intercept of this straight line is found to
be −31.6 ± 0.4 from which one obtains s = 1.14×1018 s−1.
From the equation proposed by [14, 23]
Tj = Ti − (TjTi)
k
E
ln
(βi
βj
)
(6)
one finds out:
ln
(βi
βj
)
= −
E
kTi
+
E
kTj
(7)
Thus, plot of ln(βi/βj) versus 1/(kTm) must be a straight line with slope E, and the
intercept of this plot is −E/(kTm) which allows to evaluate the maximum temperature
Tm corresponding to the heating rate βi. The relationship between ln(βi/βj) and 1/(kTm)
is described in figure 3(b). It can be seen from the figure, the plot of ln(βi/βj) against
1/(kTm) is a very good straight line with slope E = 1.95 ± 0.02 eV. The intercept of this
plot is −46.07 ± 0.04, from which one finds out the glow-peak maximum temperature
corresponding to the heating rate of 1 K/s is 491.2 K, which is in good agreement with
the experimental data.
It must be noticed that the use of the data listed in columns 5 and 6 for the effective
heating rates gives the same results as well.
In the Table 2 are listed the values of the trapping parameters resulting from the
deconvolution of the glow curve in our previous report [22] and from the variable-heating-
rate method for the main glow peak in LiF:Mg,Cu,Na,Si in the current work. All errors
ESTIMATION AND CORRECTION FOR THE TEMPERATURE LAG IN TL MEASUREMENTS... 151
Table 2. Evaluated trapping parameters of the main glow peak in LiF:Mg,Cu,Na,Si.
Method E (eV) Intercept s (s−1)
Glow-curve deconvolution*, β = 1 K/s 1.89 ± 0.04 — 2.59× 1018
Variable-heating-rate method
ln(T 2m/β) versus 1/(kTm) 1.86 ± 0.02 −31.6 ± 0.4 1.14× 10
18
ln(βi/βj) versus 1/(kTm) 1.95 ± 0.02 −46.07 ± 0.04 —
*Data from our previous work [22]
were obtained by fitting the measured data. It must be noted that our values of activation
energy are lower than a value of 2.33 eV for the glow peak 4 in LiF:Mg,Cu,Na,Si (KAERI)
dosimeter [24], but the work [24] did not indicate which method was used to evaluate the
activation energy. The values of the trapping parameters obtained in the current work are
approximate to that from the work [14] for the glow peak 5 in LiF:Mg,Ti (E = 1.96 −
2.07 eV; s = 1018−1021 s−1).
IV. CONCLUSION
The influence of the heating rate in the range from 1 K/s to 30 K/s on the TL
response was investigated. It was found that the glow-peak maximum temperature Tm
increased in the whole range of heating rates studied. The change of the Tm against
heating rate follows an equation of the form ln(Tm) = A + Bln(β). The TLA between
the heating element and the dosimeter reached a value of 16.8 K at a heating rate of 30
K/s. This TLA was corrected by using Kitis-Tuyn’s method. The variable-heating-rate
method was as well applied to evaluate the kinetic parameters of the traps for the main
glow-peak in the TL of LiF:Mg,Cu,Na,Si.
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Received 25 August 2010.


Estimation and Correction for the Temperature Lag in Thermoluminescent Measurements for LiF: Mg, Cu, Na, Si Phosphor

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Estimation and Correction for the Temperature Lag in Thermoluminescent Measurements for LiF: Mg, Cu, Na, Si Phosphor

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