RESPONSE OF NEW TLDs TO MEDIUM AND LOW ENERGY X- RAYS
Željka Knežević1, Saveta Miljanić1, Mária Ranogajec-Komor1, Branko Vekić1, Matjaž
Štuhec2, Gorazd Lakovič2 and Rafael Martinčič2
1
Ruđer Bošković Institute, Bijenička c. 54, POB 180, 10002 Zagreb, Croatia
2
Jožef Stefan Institute, Jamova 39, 1001 Ljubljana, Slovenia
Abstract
In order to determine the energy response characteristics, several types of TLDs
previously calibrated with 137Cs gamma rays, were irradiated with X-ray beams in the range
of effective energies between 33 and 116 keV.
The following types of TLDs were investigated: two types of the recently developed
highly sensitive LiF:Mg,Cu,P material: TLD-700H and GR 200A, two new types on the basis
of lithium borate: Li2B4O7:Cu,In and Li2B4O7:Cu,In,Ag (developed in the Institute of Nuclear
Science, Vinča), two well known types of LiF:Mg,Ti detectors: TLD-100 and TLD-700 and
the new, highly sensitive Al2O3:C.
Measured energy responses relative to air, normalised to 137Cs photons were compared
with calculated data. Although the deviations of the measured data from the “theoretical”
predictions are different for all investigated TLDs, there are no large differences in “tissue
equivalency” between them. The only exception is Al2O3:C, which (due to large energy
dependence) is not suitable for personal dosimetry in an unknown photon radiation field.
Introduction
The energy dependence of dosimeters is important in the evaluation of the overall
uncertainty of measurements for personal dosimetry and for environmental monitoring. It is
desirable that the energy response of the dosimeter is equal or close to that of biological
tissue. For thermoluminescent dosimeters (TLD), the calculation method of energy
dependence, using the ratios of the mass energy absorption coefficients, is not satisfactory.
Namely, the type and the concentration of different activators may influence the energy
dependence in a way which is not yet completely understood.
In order to determine the energy response characteristics, several types of TLDs
previously calibrated with 137Cs gamma rays, were irradiated with X-ray beams in the range
of effective energies between 33 and 116 keV.
The following types of TLDs were investigated:
a) two types of the recently developed highly sensitive LiF:Mg,Cu,P material: TLD-700H
and GR 200A,
b) two new types on the basis of lithium borate: Li2B4O7:Cu,In and Li2B4O7:Cu,In,Ag
(developed in the Institute of Nuclear Science, Vinča),
c) two well known types of LiF:Mg,Ti detectors: TLD-100 and TLD-700,
d) the new, highly sensitive Al2O3:C.
Material and methods
The properties of the investigated TLDs as well as the parameters used for the readout
and pre-irradiation annealing are presented in Table 1. After irradiation, dosimeters stayed at
room temperature at least one day. Before reading, a 100oC post-irradiation external annealing
for 20 min was used for all types except dosimeters on the basis of lithium borate.
Table 1. Characteristics of TL detectors and evaluation parameters
Li2B4O7
Li2B4O7
Al2O3
Material
Li2B4O7
Li2B4O7
Al2O3
Doping
Cu, In
Cu, In, Ag
Manufacturer
Vinča
TLD-700H
7
GR 200A
LiF
LiF
C
Cu, Mg, P
Vinča
Russia
chip
chip
 4.5x0.95
2.38
TLD-700
7
TLD-100
LiF
LiF (nat)
Cu, Mg, P
Mg, Ti
Mg, Ti
Harshaw
China
Harshaw
Harshaw
chip
chip
chip
chip
chip
 4.5x0.95
5x1
3 x 3 x 0.5
4.5 x x0.8
3.2 x 3.2x0.9
3.2 x 3.2x0.9
2.43
3.97
2.49
2.65
2.64
2.64
270
210
270
210
210
400 + 100
400 + 100
readout
readout
readout
readout
readout
1+2
1+2
Temp.max.(C)
270
210
270
210
210
270
270
Time (s)
35
35
35
35
35
35
35
Form
Size (mm)
Density (g cm-1)
Pre-irradiation annealing
Temp (C)
Time (h)
Reading
a
Institute of Nuclear Science, Vinča,Yugoslavia
Harshaw: Harshaw Chemical Company, Solon, OH, USA
b
The reading of TL signal was carried out using a modified TOLEDO 654 (Vinten) reader
which enables the integration of the glow curves with variable integration limits. Detectors
were preheated at the temperature of 100oC for 6 s and then heated with a constant heating
rate of 10oC per second.
TL detectors were irradiated in polymethyl methacrylate (PMMA) holders containing
recesses for detectors. The holders with 3 mm and 1.5 mm wall thickness were used for 137Cs
and X-ray irradiations, respectively. In every irradiation run, 5 detectors of each type were
irradiated at the same time.
For calibration, the irradiations with 137Cs gamma rays were performed at a distance of
1 m from the source; the dose rate was 0.62 mGy/h (specified as “kerma in air”). The
individual sensitivity of each detector was previously determined by irradiations with the
same137Cs gamma ray source.
The energy dependence was determined for X-ray beams with effective energies
between 33 and 116 keV, generated by a Pantac HF 160C Series-2 Constant Potential X Ray
Unit (150 kV) with narrow spectra and heavy filtration (used in radiation protection). The
specified effective energies were obtained by varying the operating potential and added
filtration. The characteristics of the X-ray spectra obtained were shown in previously
published paper [1]. The irradiations were performed at a distance of 1 m from the tube. Dose
rates in terms “kerma in air” were about 50 mGy/h. The irradiation conditions were with
holders placed “free in air” in both cases, for calibration as well as for X-ray irradiation.
There were 3 irradiations in the dose range 5 - 15 mGy, for every effective energy.
Control samples also containing 5 dosimeters each, were used for determination of
background radiation.
Results and Discussion
The results are shown in Figs. 1 - 5. Measured energy responses relative to air,
normalised to 137Cs photons, were compared with calculated data of the ratios of mass-energy
absorption coefficients for a given TLD and air. The values of mass-energy absorption
coefficients were taken from ref. [2] for LiF and Li2B4O7. For Al2O3:C, Li2B4O7:Cu,In and
Li2B4O7:Cu,In,Ag were calculated according to the Bragg rule.
Results for Li2B4O7-based detectors are shown in Figs 1 and 2 In Fig. 1 calculated
values of ratios of mass-energy absorption coefficients for Li2B4O7:Cu,In and
Li2B4O7:Cu,In,Ag are compared with a “pure” Li2B4O7. The activator concentrations were
taken to be 0.07 wt. % Cu, 0.07 wt. % In and 0.05 wt. % Ag [3]. According to Fig. 1, the
addition of activators increases the mass-energy absorption coefficients of Li2B4O7 the more
so the concentration and atomic number are higher. Actually, calculated values of effective
atomic numbers are 7.2, 7.8 and 8.04 for Li2B4O7, Li2B4O7:Cu,In and Li2B4O7:Cu,In,Ag,
respectively. In this respect Li2B4O7-based TL detectors can be considered to be nearly tissue
equivalent (Zeff = 7.4 for soft tissue). The measured values in Fig. 2 show about 10% higher
energy dependence in the investigated energy range than the theoretically predicted curve.
1.15
Relative TL response
1.10
1.05
c
1.00
0.95
b
0.90
0.85
a
0.80
0
50
100
150
200
Photon energy (keV)
Fig. 1: Relative ratios of mass-energy absorption coefficients for Li2B4O7 and air normalised
to 137Cs photons as a function of photon energy: “pure” Li2B4O7 (curve a); Li2B4O7:Cu,In
(curve b); Li2B4O7:Cu,In,Ag (curve c).
1.40
Relative TL response
1.30
1.20
Bexp
1.10
B
1.00
Aex
p
A
0.90
0.80
0
20
40
60
80
100
120
140
160
Photon energy (keV)
Fig. 2: Measured photon energy dependence of Li2B4O7:Cu,In (Aexp) and Li2B4O7:Cu,In,Ag
(Bexp) compared with the calculations for Li2B4O7:Cu,In (A) and for Li2B4O7:Cu,In,Ag (B).
In Fig. 3, the measured photon energy dependence of LiF-based TL detectors is
compared with the calculations for the “pure” natural LiF. The results for TLD-100 and TLD700 are practically equal within the experimental error, and only slightly higher than the
calculated curve. This means that the relative TL efficiency for the conventional LiF:Mg:Ti is
approximately equal to 1 for the measured photon energy range with respect to the response
to 137Cs gamma rays, and also that the addition of the activators did not influence the energy
absorption characteristics. The results for two types of LiF:Mg,Cu,P show the anomalous
photon energy response which has been explained as an ionisation density effect [4].
The effect is more pronounced for the TLD-700H for which the response is about 12%
lower at the maximum (about 50 keV) and 4% lower at the minimum (80-100 keV) than for
GR 200A.
Previously published results for the third type of LiF:Mg,Cu,P [5] (the commercial
name MCP, produced in the Institute of Nuclear Physics, Krakow) showed the same energy
dependence in this energy range as GR 200A.
1.40
a
Relative TL response
1.30
b
1.20
1.10
c
1.00
d
0.90
0.80
0.70
0
20
40
60
80
100
120
140
160
Photon energy (keV)
Fig. 3: Measured photon energy dependence of LiF-based detectors: TLD-100 (■) and TLD700 () (curve a is shared by both types), GR 200A ( and curve c), TLD-700H (o and curve
d) compared with the calculation (curve b).
Relative TL response
3.50
3.00
a
2.50
2.00
b
1.50
1.00
0.50
0.00
0
20
40
60
80
100
120
140
160
Photon energy (keV)
Fig. 4: Measured photon energy dependence of Al2O3:C (■ and curve a) compared with the
calculation (curve b).
The measured photon energy dependence of Al2O3:C (Fig. 4) shows a good agreement
with the calculation based on data given in ref. [6], but due to the high energy dependence,
this detector is inconvenient for personal dosimetry without the use of correcting filters.
In Fig. 5. the measured energy dependence for all investigated types (except Al 2O3:C)
normalised to 137Cs photons, is shown together with the calculated ratios of mass-energy
absorption coefficients for water and air (also normalised to 137Cs photons). The relation
between particular experimental curve and calculated water/air curve gives the approximation
of the “tissue equivalency” for the particular type of detector. The both LiF:Mg,Ti types have
a maximum at about 30 keV and about 47% higher energy absorption than water at this
energy. The both LiF:Mg,Cu,P types also have a maximum at about 30 keV (about 8% and
16% higher than water for TLD-700H and GR 200A, respectively) and have a minimum
between 80 and 100 keV (about 26% and 22% lower than water for TLD-700H and GR
200A, respectively). Lithium borate based detectors have a maximum at about 50 keV (about
16% and 35% higher than water for Li2B4O7:Cu,In and Li2B4O7:Cu,In,Ag, respectively).
Relative TL response
1.55
1.40
1.25
1.10
0.95
a
0.80
0.65
0
40
80
Photon energy (keV)
120
160
Fig. 5: Measured energy dependence of TLDs compared with the calculated ratios of massenergy absorption coefficients for water and air – curve a (all normalised to 137Cs photons):
TLD-100 (□), TLD-700 (■), GR 200A (▲), TLD-700H (○), Li2B4O7:Cu,In ()and
Li2B4O7:Cu,In,Ag ().
Conclusions
Measured energy responses relative to air, normalised to 137Cs photons were compared
with calculated data. Although the deviations of the measured data from the “theoretical”
predictions are different for all investigated TLDs, there are no large differences in “tissue
equivalency” between them. The only exception is Al2O3:C which (due to a large energy
dependence) is not suitable for personal dosimetry in an unknown photon radiation field.
The newly prepared detector based on lithium borate, Li2B4O7:Cu,In (with the effective
atomic number of 7.8), shows the closest energy absorption characteristics to the tissue of all
investigated TLDs in this work.
Acknowledgements
The authors are very grateful to Dr. Mirjana Prokić who gave us samples of newly
prepared detectors based on lithium borate.
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