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High-dose behaviour of CaSO4: Dy thermoluminescent phosphors as deduced by a continuous model for trap depths
NUCLEAR INSTRUMENTS AND METHODS 154 (1978) 203-205 ; O
NORTH-HOLLAND PUBLISHING CO.
HIGH-DOSE BEHAVIOUR OF CaSO4 : Dy THERMOLUMINESCENT PHOSPHORS AS DEDUCED BY A CONTINUOUS MODEL FOR TRAP DEPTHS E. OLIVERI, O. FIORELLA and M. MANGIA
Istituto di Applicazioni ed hnpianti Nucleari, Universita di Palermo, Facohh d'lngegneria, 90128 Palermo, Italy
Viale delle Scienze, Ptlrco d'Orleans,
Received 3 March 1978 The high-dose behaviour of CaSO4 : Dy TL dosimeters observed by some authors is compared with the one deduced by the continuous distribution of trap depths previously proposed by us. A general agreement results.
In previous w o r k s 1'2) w e have investigated, by isothermal decay experiments at various temperatures, the energetic structure of the main glow peak of CaSO4 : D y phosphors 6°Co x-irradiated in the dose range 1 rad to 3× 10 5 rad*. It was shown that the experimental results might be explained by associating with the peak an approximately multi-Gaussian distribution of trap depths as the one in fig. 1, with the central energies Ej of the Gaussians and the related full widths at half maximum aJ reported in table 1. Besides, a different supralinear behaviour of the areas Aj of the Gaus-
TABLE 1 Parameters of the distribution of trap depths.
Gaussian
1 2
3 4
(eV)
1.28 1.38
0.033 0.043
1.50 1.62
0.025 0.021
s= 3× 1012s t
1
* The doses were measured by tissue equivalent dosimeters.
~
(eV)
I
1
I
7 Ii //
4
6
/
5
~
/
~
3 / / / "/ / / / / " / /
4
23
/ / ".
/ / / ' /
:2
~/11/
1.28 eV ~
1 1.38 ,,
1 1.50
1.1
1.2
1.3
1.4 1.5 Energy, eV
1.6
1.7
Fig. ]. Distribution of trap depths associated with the main glow peak of CaSO4 :Dy phosphors; frequency factor s = 3 x l 0 1 2 s - I ; dose ]00rad 60Co.
1
2
3
4
5
6
Log D Fig. 2. Areas (a.u.) of the first three Gaussians of fig. 1 as a function of dose D (rad). The dashed lines are extrapolations.
204
E. OLIVERI et al.
sians was found, as shown in fig. 2 for the three main Gaussians. In a recent paper Burgkhardt et al. 3) have reported their experiments on the supralinearity and the glow peak behaviour of some TL dosimeters 6°Co y-irradiated in the dose range 10R to 2 × 1 0 6 R . In particular for dosimeters of C a S O a : D y in teflon they observed the main following properties: - the supralinearity starts at about 100 R, reaching its m a x i m u m at about 5 × 10 4 R, with a m a x i m u m supralinearity factor of 3.1; - the glow peak temperature, which was found to be 230°C for low doses, shifts with increasing exposure to lower values up to 190°C for 10 6 R ; - the glow curve width changes with the exposure and shows a m a x i m u m almost at about 103 R. The conclusion was that CaSO4 dosimeters have multi-glow peaks and a complex behaviour. In this paper we show that there is a general agreement between the experimental results above mentioned and the predictions deducible from the continuous trap depths distribution obtained by US.
To this aim it is first necessary to obtain numerically the glow curves at various doses making use of table 1 and fig. 2. This was approximately made as follows. Only the first three Gaussians of fig. 1 were considered; the related energetic range was divided in such a way that ten equally spaced I
I
I
I
values of energy, E~, were obtained; the filled traps distribution at dose D was then represented by the corresponding ten lines given by:
÷
Li o(D) = ~ •
Aj(D)
j = z x / ( 2 ~ ) o'j
exp
J"
2o"i
(1)
Finally, the TL light intensity I vs. the absolute temperature T was numerically calculated by the equation4): 10
I ( T , D ) = ~ L~,o(D ) 2(Ei, T) x i=1
x exp
-
fl(r)
dr
.
(2)
As usual, the decay constant 2 is expressed by: 2 -- s x e x p ( - E / k r ) ,
(3)
where k is the Boltzmann constant and s the frequency, factor. The heating rate B = dT/dt, as in the paper of Burgkhardt et al.3), was assumed to be 5 °C/s. The resulting glow curves, plotted in a semi-log scale and normalized in height, are shown in fig. 3. The peak temperature is about 250°C at low doses and shifts up to about 220°C for 106 rad. The glow curve width vs. dose (fig. 4) shows a m a x i m u m at about 3 × 103 rad. The light sums vs. dose, compared to the response at 100rad, are reported in fig. 5. As one might expect from fig. 2, the supralinearity
I
90
rad -
I
I
I
80
\
\ \~
--3"104
\ \,® \ 3.1o \
~ ~n
70
106
6C
0 100
150
I I 4 5 6 Log D Fig. 4. Full width at half maximum vs. dose (rad) of 2
I
I
I
,
200
250
300
350
remper=ture,°C
Fig. 3. Glow curves of CaSO4 :Dy for different doses.
CaSO 4 : Dy.
I 3
HI'GH-DOSE B E H A V I O U R OF CaSO4 : Dy 10
I
I
I 2
I 3
I
I
I
I 5
I 6
c o ,-.,
1
~> ca
0.1
I 4 Log D
Fig. 5. Relative response of CaSO4:Dy vs. dose (rad).
starts very slowly at 100 rad, reaches meaningful values above 103 rad and attains its maximum of about 1.8 at about 5 x 104 rad. On the whole the experimental behaviour of the glow curves with dose is reproduced fairly well by the theoretical predictions, even though there are some numerical differences between the results, for instance in the values of the peak tempera-
205
tures or in the supralinearity factors. On the other hand, the phosphor used in order to investigate the trap depths distribution was prepared by us in the form of powder doped with 0.1 mol% of Dy, while Burgkhardt et al. 3) have used dosimeters of CaSO4 : Dy in teflon manufactured by Teledyne Isotopes for which the doping is not specified. Besides the different instrumentation and experimental procedures, for instance the post-irradiation treatment may have affected the results. In conclusion we think that the multiglow peaks and the complex behaviour of CaSO 4 : Dy may be explained in terms of the continuous distribution of trap depths and the different behaviour of the Gaussians with dose. References 1) o. Fiorella, M. Mangia and E. Oliveri~ Int. J. Rad. Phys. Chem. 8 (1976) 441. 2) M. Mangia, E. Oliveri and O. Fiorella, Proc. 5th int. Conf. on Luminescence dosimetry, Sao Paulo (1977) p. 29. 3) B. Burgkhardt, D. Singh and E. Piesch, Nucl. Instr. and Meth. 141 (1977) 363. 4) j. H. Schulmann, Proc. Int. Conf. on Luminescence dosimetry. Stanford (1965) p. 3.
NORTH-HOLLAND PUBLISHING CO.
HIGH-DOSE BEHAVIOUR OF CaSO4 : Dy THERMOLUMINESCENT PHOSPHORS AS DEDUCED BY A CONTINUOUS MODEL FOR TRAP DEPTHS E. OLIVERI, O. FIORELLA and M. MANGIA
Istituto di Applicazioni ed hnpianti Nucleari, Universita di Palermo, Facohh d'lngegneria, 90128 Palermo, Italy
Viale delle Scienze, Ptlrco d'Orleans,
Received 3 March 1978 The high-dose behaviour of CaSO4 : Dy TL dosimeters observed by some authors is compared with the one deduced by the continuous distribution of trap depths previously proposed by us. A general agreement results.
In previous w o r k s 1'2) w e have investigated, by isothermal decay experiments at various temperatures, the energetic structure of the main glow peak of CaSO4 : D y phosphors 6°Co x-irradiated in the dose range 1 rad to 3× 10 5 rad*. It was shown that the experimental results might be explained by associating with the peak an approximately multi-Gaussian distribution of trap depths as the one in fig. 1, with the central energies Ej of the Gaussians and the related full widths at half maximum aJ reported in table 1. Besides, a different supralinear behaviour of the areas Aj of the Gaus-
TABLE 1 Parameters of the distribution of trap depths.
Gaussian
1 2
3 4
(eV)
1.28 1.38
0.033 0.043
1.50 1.62
0.025 0.021
s= 3× 1012s t
1
* The doses were measured by tissue equivalent dosimeters.
~
(eV)
I
1
I
7 Ii //
4
6
/
5
~
/
~
3 / / / "/ / / / / " / /
4
23
/ / ".
/ / / ' /
:2
~/11/
1.28 eV ~
1 1.38 ,,
1 1.50
1.1
1.2
1.3
1.4 1.5 Energy, eV
1.6
1.7
Fig. ]. Distribution of trap depths associated with the main glow peak of CaSO4 :Dy phosphors; frequency factor s = 3 x l 0 1 2 s - I ; dose ]00rad 60Co.
1
2
3
4
5
6
Log D Fig. 2. Areas (a.u.) of the first three Gaussians of fig. 1 as a function of dose D (rad). The dashed lines are extrapolations.
204
E. OLIVERI et al.
sians was found, as shown in fig. 2 for the three main Gaussians. In a recent paper Burgkhardt et al. 3) have reported their experiments on the supralinearity and the glow peak behaviour of some TL dosimeters 6°Co y-irradiated in the dose range 10R to 2 × 1 0 6 R . In particular for dosimeters of C a S O a : D y in teflon they observed the main following properties: - the supralinearity starts at about 100 R, reaching its m a x i m u m at about 5 × 10 4 R, with a m a x i m u m supralinearity factor of 3.1; - the glow peak temperature, which was found to be 230°C for low doses, shifts with increasing exposure to lower values up to 190°C for 10 6 R ; - the glow curve width changes with the exposure and shows a m a x i m u m almost at about 103 R. The conclusion was that CaSO4 dosimeters have multi-glow peaks and a complex behaviour. In this paper we show that there is a general agreement between the experimental results above mentioned and the predictions deducible from the continuous trap depths distribution obtained by US.
To this aim it is first necessary to obtain numerically the glow curves at various doses making use of table 1 and fig. 2. This was approximately made as follows. Only the first three Gaussians of fig. 1 were considered; the related energetic range was divided in such a way that ten equally spaced I
I
I
I
values of energy, E~, were obtained; the filled traps distribution at dose D was then represented by the corresponding ten lines given by:
÷
Li o(D) = ~ •
Aj(D)
j = z x / ( 2 ~ ) o'j
exp
J"
2o"i
(1)
Finally, the TL light intensity I vs. the absolute temperature T was numerically calculated by the equation4): 10
I ( T , D ) = ~ L~,o(D ) 2(Ei, T) x i=1
x exp
-
fl(r)
dr
.
(2)
As usual, the decay constant 2 is expressed by: 2 -- s x e x p ( - E / k r ) ,
(3)
where k is the Boltzmann constant and s the frequency, factor. The heating rate B = dT/dt, as in the paper of Burgkhardt et al.3), was assumed to be 5 °C/s. The resulting glow curves, plotted in a semi-log scale and normalized in height, are shown in fig. 3. The peak temperature is about 250°C at low doses and shifts up to about 220°C for 106 rad. The glow curve width vs. dose (fig. 4) shows a m a x i m u m at about 3 × 103 rad. The light sums vs. dose, compared to the response at 100rad, are reported in fig. 5. As one might expect from fig. 2, the supralinearity
I
90
rad -
I
I
I
80
\
\ \~
--3"104
\ \,® \ 3.1o \
~ ~n
70
106
6C
0 100
150
I I 4 5 6 Log D Fig. 4. Full width at half maximum vs. dose (rad) of 2
I
I
I
,
200
250
300
350
remper=ture,°C
Fig. 3. Glow curves of CaSO4 :Dy for different doses.
CaSO 4 : Dy.
I 3
HI'GH-DOSE B E H A V I O U R OF CaSO4 : Dy 10
I
I
I 2
I 3
I
I
I
I 5
I 6
c o ,-.,
1
~> ca
0.1
I 4 Log D
Fig. 5. Relative response of CaSO4:Dy vs. dose (rad).
starts very slowly at 100 rad, reaches meaningful values above 103 rad and attains its maximum of about 1.8 at about 5 x 104 rad. On the whole the experimental behaviour of the glow curves with dose is reproduced fairly well by the theoretical predictions, even though there are some numerical differences between the results, for instance in the values of the peak tempera-
205
tures or in the supralinearity factors. On the other hand, the phosphor used in order to investigate the trap depths distribution was prepared by us in the form of powder doped with 0.1 mol% of Dy, while Burgkhardt et al. 3) have used dosimeters of CaSO4 : Dy in teflon manufactured by Teledyne Isotopes for which the doping is not specified. Besides the different instrumentation and experimental procedures, for instance the post-irradiation treatment may have affected the results. In conclusion we think that the multiglow peaks and the complex behaviour of CaSO 4 : Dy may be explained in terms of the continuous distribution of trap depths and the different behaviour of the Gaussians with dose. References 1) o. Fiorella, M. Mangia and E. Oliveri~ Int. J. Rad. Phys. Chem. 8 (1976) 441. 2) M. Mangia, E. Oliveri and O. Fiorella, Proc. 5th int. Conf. on Luminescence dosimetry, Sao Paulo (1977) p. 29. 3) B. Burgkhardt, D. Singh and E. Piesch, Nucl. Instr. and Meth. 141 (1977) 363. 4) j. H. Schulmann, Proc. Int. Conf. on Luminescence dosimetry. Stanford (1965) p. 3.