- Email: [email protected]
An ESR study of thermoluminescent processes in CaSO4 phosphors
Nuclear Instruments and Methods 175 (1980) 8-9 © North-Holland Publishing Company
AN ESR S T U D Y OF T H E R M O L U M I N E S C E N T P R O C E S S E S IN CaSO4 PHOSPHORS Ryoitiro HUZIMURA and Kazuaki ASAHI Nara University o f Education, 630 Nara, Japan
and Mutsuo TAKENAGA Matsushita Electric Industrial Co. Ltd., Central Research Laboratory, 570 Moriguchi, Osaka, Japan
The mechanism of TL of CaSO4 : Tm is discussed through the investigation of thermal decay of radical ions which were detected by single crystal ESR measurements.
Rare earth activated CaSO4 phosphors are recently acquiring an increasing interest in application in radiation dosimetry. The mechanism of the TL remains, however, still unclear although some works have been done by using powder materials [1,2]. In order to elucidate the processes, we report paramagnetic and thermal properties of radical ions which were produced by X-irradiation in undoped and Tin-doped CaSO4 anhydrite single crystals (orthorhombic, a = 0.622 nm, b = 0.696 nm, c = 0.697 nm). The methods of crystal preparation and ESR measurements are described in refs. 3 and 2, respectively. There are six prominent lines (group A) in the ESR spectra of undoped, irradiated CaSO4, which can be attributed to SO; from the angular variation of ESR spectra; the direction of the smallest g value is found to coincide with that of the S - O bond in the ab and ac planes. Three other lines appear in the spectra of CaSO4 : T m . Two of them (B) showed the angular variation corresponding to an orthorhombic g tensor and the other (C) that of axial symmetry with the smallest g value in the a axis direction. The g values are listed in table 1. The signal B may be assigned to SO2 and the signal C to SO~- paired with a substitutional monovalent cation [4]. The thermal decay of the ESR signals of CaSO4 : Tm by isochronal (5 min) annealing is shown in fig. 1, where z2d is the amount of the signal decrease at each temperature. In table 1 are listed the thermal activation energies for these centers which could be estimated by assuming the e q u a t i o n l n [ - l n ( 1 - Ad//)] = c o n s t . - E / k T . The A signal began to decrease above 120°C and decayed out at
about 240°C. E was obtained only from the final stage of the decay. The thermal decay of SO3 occurred almost in the same way in undoped CaSO4 as in CaSO4 " Tm. At temperatures above 200°C an isotropic signal (D) was found to grow, which decayed above 300°C. This might be attributed to SO; of relaxed state at different lattice sites as the SO; signal was decreasing in that temperature range.
300
10
200
100 (°C)
~ ' - "
100
~--
_
"~ 1
~
10 E
I--
I
1
.li ....
2.0
I
2.5 1000/ T (K -~)
3.0
Fig. l. TL glow curves and isochronal annealing of radical ions of irradiated CaSO4 : Tm. Curve 1, full flow curve; 2 and 3, decomposed ones. e, • and A, plots of -In(1 £x///) for the change of ESR intensity 1 of signals A, B and C. o shows the increase of D, i.e., 1 is replaced by I ' = I m - I, where I m is the maximum intensity of D.
R. Huzimura et al. / ESR study o f TL processes
Table 1 Parameters of radical ions in CaSO4 : Tm Principal g value
Assigned center
Decay temperature (°C)
Activation energy (eV)
A
2.0011 2.0021 2.0061
SO~
120-140
2.0
B
2.011 2.022 2.005
SO~
-90
0.86
0.75
Group
C
2.005 2.020
Axial center (SO~ with -110 cation)
D
2.0037
relaxed SO~
-300
In fig. 1 the TL glow curves o f CaSO4 : Tm after about 40 R exposure are shown to be decomposed into three peaks b y the partial bleaching method. F r o m the slopes of initial rises the thermal activation energies were obtained to be 1.0 eV, 1.2 eV and 1.5 eV for the peaks at 1 1 5 , 2 2 0 and 285°C, respectively. The remarkable differences between the TL of Tin-doped CaSO4 and that of undoped CaSO4 are the very high efficiency of TL o f the former and the absence of the main peak at 200°C in the latter. The
9
significant features in ESR spectra o f CaSO4 : Tm are the formation o f SO4 and C center and the growth o f relaxed SOy above 200°C which did not occur in undoped CaSO4. Contrary to these, the formation and decay of SO3 seem to be little affected by presence o f Tm dopants. Considering the above results, we may infer the TL mechanism as follows: 1) The 1 15°C peak may be caused by recombination of charge carriers from S07~ and C centers at Tm 3+" 2) In the processes o f the main TL peaked at 220°C, Tm 3÷ ions may be excited b y receiving energy quanta which are released during the stimulated relaxation o f SO~ ions. 3) The 285°C peak may be related to the relaxed SO~ centers which may be responsible for the supralinear response to radiation.
References [1] K.V.S Nambi, V.N. Bapat and A.K. Ganguly, J. Phys. C: Sol. Stat. Phys. 7 (1974) 4403. [2] R. Huzimura, Jap. J. Appl. Phys. 18 (1979) 2031. [3] T. Yamashita, N. Nada, H. Onishi and S. Kitamura, Health Phys. 21 (1971) 295. [4] Detailed discussions on these assignments will be given elsewhere.
I. MECHANISMS 1
AN ESR S T U D Y OF T H E R M O L U M I N E S C E N T P R O C E S S E S IN CaSO4 PHOSPHORS Ryoitiro HUZIMURA and Kazuaki ASAHI Nara University o f Education, 630 Nara, Japan
and Mutsuo TAKENAGA Matsushita Electric Industrial Co. Ltd., Central Research Laboratory, 570 Moriguchi, Osaka, Japan
The mechanism of TL of CaSO4 : Tm is discussed through the investigation of thermal decay of radical ions which were detected by single crystal ESR measurements.
Rare earth activated CaSO4 phosphors are recently acquiring an increasing interest in application in radiation dosimetry. The mechanism of the TL remains, however, still unclear although some works have been done by using powder materials [1,2]. In order to elucidate the processes, we report paramagnetic and thermal properties of radical ions which were produced by X-irradiation in undoped and Tin-doped CaSO4 anhydrite single crystals (orthorhombic, a = 0.622 nm, b = 0.696 nm, c = 0.697 nm). The methods of crystal preparation and ESR measurements are described in refs. 3 and 2, respectively. There are six prominent lines (group A) in the ESR spectra of undoped, irradiated CaSO4, which can be attributed to SO; from the angular variation of ESR spectra; the direction of the smallest g value is found to coincide with that of the S - O bond in the ab and ac planes. Three other lines appear in the spectra of CaSO4 : T m . Two of them (B) showed the angular variation corresponding to an orthorhombic g tensor and the other (C) that of axial symmetry with the smallest g value in the a axis direction. The g values are listed in table 1. The signal B may be assigned to SO2 and the signal C to SO~- paired with a substitutional monovalent cation [4]. The thermal decay of the ESR signals of CaSO4 : Tm by isochronal (5 min) annealing is shown in fig. 1, where z2d is the amount of the signal decrease at each temperature. In table 1 are listed the thermal activation energies for these centers which could be estimated by assuming the e q u a t i o n l n [ - l n ( 1 - Ad//)] = c o n s t . - E / k T . The A signal began to decrease above 120°C and decayed out at
about 240°C. E was obtained only from the final stage of the decay. The thermal decay of SO3 occurred almost in the same way in undoped CaSO4 as in CaSO4 " Tm. At temperatures above 200°C an isotropic signal (D) was found to grow, which decayed above 300°C. This might be attributed to SO; of relaxed state at different lattice sites as the SO; signal was decreasing in that temperature range.
300
10
200
100 (°C)
~ ' - "
100
~--
_
"~ 1
~
10 E
I--
I
1
.li ....
2.0
I
2.5 1000/ T (K -~)
3.0
Fig. l. TL glow curves and isochronal annealing of radical ions of irradiated CaSO4 : Tm. Curve 1, full flow curve; 2 and 3, decomposed ones. e, • and A, plots of -In(1 £x///) for the change of ESR intensity 1 of signals A, B and C. o shows the increase of D, i.e., 1 is replaced by I ' = I m - I, where I m is the maximum intensity of D.
R. Huzimura et al. / ESR study o f TL processes
Table 1 Parameters of radical ions in CaSO4 : Tm Principal g value
Assigned center
Decay temperature (°C)
Activation energy (eV)
A
2.0011 2.0021 2.0061
SO~
120-140
2.0
B
2.011 2.022 2.005
SO~
-90
0.86
0.75
Group
C
2.005 2.020
Axial center (SO~ with -110 cation)
D
2.0037
relaxed SO~
-300
In fig. 1 the TL glow curves o f CaSO4 : Tm after about 40 R exposure are shown to be decomposed into three peaks b y the partial bleaching method. F r o m the slopes of initial rises the thermal activation energies were obtained to be 1.0 eV, 1.2 eV and 1.5 eV for the peaks at 1 1 5 , 2 2 0 and 285°C, respectively. The remarkable differences between the TL of Tin-doped CaSO4 and that of undoped CaSO4 are the very high efficiency of TL o f the former and the absence of the main peak at 200°C in the latter. The
9
significant features in ESR spectra o f CaSO4 : Tm are the formation o f SO4 and C center and the growth o f relaxed SOy above 200°C which did not occur in undoped CaSO4. Contrary to these, the formation and decay of SO3 seem to be little affected by presence o f Tm dopants. Considering the above results, we may infer the TL mechanism as follows: 1) The 1 15°C peak may be caused by recombination of charge carriers from S07~ and C centers at Tm 3+" 2) In the processes o f the main TL peaked at 220°C, Tm 3÷ ions may be excited b y receiving energy quanta which are released during the stimulated relaxation o f SO~ ions. 3) The 285°C peak may be related to the relaxed SO~ centers which may be responsible for the supralinear response to radiation.
References [1] K.V.S Nambi, V.N. Bapat and A.K. Ganguly, J. Phys. C: Sol. Stat. Phys. 7 (1974) 4403. [2] R. Huzimura, Jap. J. Appl. Phys. 18 (1979) 2031. [3] T. Yamashita, N. Nada, H. Onishi and S. Kitamura, Health Phys. 21 (1971) 295. [4] Detailed discussions on these assignments will be given elsewhere.
I. MECHANISMS 1