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Radiation resistivity of BGO crystals due to low-energy gamma-rays
Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
Radiation resistivity of BGO crystals due to low-energy gamma-rays Peter Kozmaa,*, Petr Kozma Jrb a b
Institute of Technological Investigations, StranWice, Prague Eastern District, Prague 251 63, Czech Republic Yxell Ltd., Hi-Tech and Engineering Co., StranWice, Prague Eastern District, Prague 251 63, Czech Republic Received 21 August 2002; received in revised form 1 November 2002; accepted 18 December 2002
Abstract Radiation resistivity of 4 4 30 mm3 BGO crystals from three suppliers has been studied for doses 104 Gy (106 rad) and 105 Gy (107 rad). Radiation hardness was examined by the measurement of optical transmission through BGO crystals before and after 60Co gamma-ray irradiations. The absolute degradation of transmission for 104 and 105 Gy doses at 480 nm wavelength of the peak emission of BGO, was found to be lower than 3.4% and 7.5%, respectively. The results have been also compared with radiation hardness measurements for a large volume +30 30 mm3 BGO crystal as well as another heavy scintillation crystals: fluorides and tungstates. Complete recovery of BGO radiation damage was observed only after few days. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40.M; 61.80.E; 61.80.x Keywords: BGO scintillation crystals; Gamma-ray irradiations; Optical transmission; Radiation damage; Recovery of damaged crystals
1. Introduction Scintillation crystals of bismuth germanate BGO (Bi4Ge3O12) have been considered as a scintillation material for electromagnetic calorimeters in high-energy physics experiments [1]. A large amount of BGO scintillation crystals have been grown by the SIC (Shanghai Institute of Ceramics) for the L3 experiment on LEP at CERN [2].
*Corresponding author. Tel.: +42-204-640176; fax: +42204-640438. E-mail address: [email protected] (P. Kozma).
Small volume BGO crystals are widely used in nuclear medicine diagnostic systems, particularly PE tomographs (PET) and CT Scanners (CTS) [3]. In PET systems the scintillation crystals are used to detect pairs of the 511 keV gamma-rays produced when a positron emitted from the positron emitter (11C, 13Na, 15O, etc.) annihilates with an atomic electron. In earlier stage of CTS or gamma camera systems, NaI(Tl) crystals have been used as the scintillation detectors of gammarays. After BGO was invented in the late 1970s, it gradually took the place of NaI(Tl) as the scintillation detector in most PET and CTS systems because of its high stopping power, light yield and decay time, as well. The BGO block
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00617-X
500
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
detectors [4], which was invented in the 1980s, are also used in a multi-slice PET systems with high spatial resolution. Radiation damage of BGO scintillation crystals due to low-energy gamma-rays is known to be the most crucial parameter [5,6], and therefore, we have also studied radiation hardness of these crystals by measuring the change in transmittance of light [7–12]. Light yield dependence on irradiation dose seems to be one of the most decisive parameter for practical using of these scintillators in various applications. The main goal of our investigation was to compare the irradiation measurements performed with small BGO samples from three different suppliers (see Table 1). In this work, we report on the radiation damage of small BGO crystals at accumulated doses of low-energy gamma-rays at 104 Gy (106 rad) and 105 Gy (107 rad), respectively. The results are also compared with those obtained for the large volume BGO crystal (+30 30 mm3) and measurements for other scintillation crystals (see Table 2) published earlier [7–12]. We have also estimated the recovery time of irradiated crystals. 2. Experimental results BGO crystal surfaces were optically polished to enable transmission measurements. The crystal samples were irradiated by a 60Co gamma-ray source. The radiation damage was examined by the measurement of transmission spectra before
and after irradiations. Irradiation conditions and optical transmission spectra measurements were identical to those described in Ref. [7]. The activity at the time of the experiment was A ¼ 2:27 105 GBq (i.e. 2.27 1015 s1). The BGO crystals were placed longitudinally at a distance of r ¼ 50 cm from the 60Co gamma-ray source The dose fluence Dexp (Gy s1) was calculated from A X m Dexp ¼ ð1Þ pi Ei i : 4pr2 i r For the two energies E1 ¼ 1:173 MeV and E2 ¼ 1:332 MeV of 60Co, the branching ratios of both lines p1 ¼ p2 ¼ 1 and the values of the mass-energy absorption coefficients [13] m1 / r ¼ 2:68 1022 cm2 g1 and m2 /r ¼ 2:59 102 Table 2 Properties of the most prominent heavy scintillation crystals Scintillation crystal 3
Density (g/cm ) Melting point ( C) Radiation length (cm) Moliere radius (cm) Refractive indexa Wavelength (nm) Decay time (ns) Light yieldc (%)
BGO
CeF3 PbF2 PbWO4 CdWO4
7.13 1050 1.1 2.7 2.1 480 300 15–20
6.16 1285 1.7 2.6 1.6 300 25 10
7.77 1250 0.9 2.2 1.8 Chb Chb 20
8.30 1123 0.9 2.0 2.2 420 6 0.6
7.90 1325 1.1 2.2 2.3 470 — 25–30
a
At the wavelength of the maximum. Cherenkov radiator. c Relative to NaI(Tl) for gamma rays. b
Table 1 Transmission and absolute degradation of BGO samples Sample
Manufacturer
CRISMATECc CRYTUR Ltd.d ISC Co.e CRISMATECc
1 2 3 4 a
Size (mm3)
4 4 30 4 4 30 4 4 30 +30 30
Transmissiona
Absolute degradationb
0 Gy (%)
104 Gy (%)
105 Gy (%)
104 Gy (%)
105 Gy (%)
74.0 70.0 71.0 75.0
71.2 67.1 68.0 71.9
67.8 63.8 64.8 68.0
3.4 3.7 3.9 3.8
7.6 7.9 7.5 8.4
Transmission at peak emission (l ¼ 480 nm). Absolute degradation of transmission at peak emission (l ¼ 480 nm) per unit radiation length (X0 ¼ 1:1 cm). c Saint-Gobain Crystals and Detectors, France. d Turnov, Czech Republic. e Institute for Single Crystals, Kharkov, Ukraine. b
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
501
BGO (sample #2)
BGO (sample #1) 80 60
TRANSMISSION [%]
40 20
before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
1.0 x 104 Gy 1.0 x 105 Gy
BGO (sample #3)
BGO (sample #4)
80 60 40
before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
4
1.0 x 10 Gy 1.0 x 105 Gy
20
400
500
400
500
WAVELENGHT [nm]
Fig. 1. Comparison of transmission spectra before and after longitudinally.
60
Co gamma-ray irradiations. The optical transmissions were measured
BGO (sample #4) Ø 3.0 x 3.0 cm3
80
PbF2
60
2.2 x 2.2 x 24.0 cm
3
TRANSMISSION [%]
40 before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
1.0 x 104 Gy 5 1.0 x 10 Gy
20
CeF3 3.0 x 3.0 x 6.0 cm3
PbWO4
before irradiation
before irradiation
3
2.2 x 2.2 x 25.0 cm
80 60 40
1.0 x 104 Gy 1.0 x 105 Gy
20
400
1.0 x 104 Gy 1.0 x 105 Gy
500
400
500
WAVELENGHT [nm]
Fig. 2. Comparison of transmission spectra before and after 60Co gamma-ray irradiations for large volume crystals: BGO-sample 4 (this work), PbWO4 [11], PbF2 [12] and CeF3 [7]. The optical transmissions were measured longitudinally.
cm2 g1, respectively, we calculated Dexp ¼ 0:795 Gy s1 (i.e. 79.5 rad s1). The appropriate irradiation runs were calculated to be 3.65 and
36.5 h for 104 and 105 Gy accumulated doses, respectively. After the irradiation the transmission spectra were measured several times.
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P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
Table 3 Absolute degradation of transmission for scintillation crystals at peak emission Scintillation crystal
Absolute degradation per unit radiation length Accumulated dose=104 Gy (%)
BGO (l ¼ 480 nm, X0 ¼ 1:1 cm) PbWO4 (l ¼ 420 nm, X0 ¼ 0:9 cm) CdWO4 (l ¼ 470 nm, X0 ¼ 1:1 cm) CeF3 (l ¼ 300 nm, X0 ¼ 1:7 cm)
The transmission spectra before and after absorption of 104 and 105 Gy accumulated doses for samples 1–4 are displayed in Fig. 1. The transmission at peak emission and the absolute degradation of transmission at peak emission per unit radiation length are compared in Table 1. As can be seen, the degradation of transmission for 104 Gy at the wavelength of the peak emission, l ¼ 480 nm, was found to be better than 3.4% per the unit radiation length X0 ¼ 1:1 cm. The degradation of the transmission for 105 Gy at the same wavelength was found to be better than 7.5% per X0 : The results obtained for the large volume +30 30 mm3 BGO crystal (sample 4) have also been compared with those published earlier [7,11,12] for fluoride and tungstate large volume scintillation crystals (Fig. 2) The absolute
Accumulated dose=105 Gy (%)
3.4
7.5
7.5
12.5
4.8
8.0
2.8
10.0
1
0.9
BGO (sample #1) λ = 480 nm
0.8
0.7
Tirr(t)/T(0)
0.6
0.5 1 0
2
4
6
8
10
12
14
10
12
14
0.9
BGO (sample #4) λ = 480 nm
0.8
0.7
0.6
0.5 0
2
4
6
8
Time [day] Fig. 3. Recovery time for optical transmission of small (sample 1) and large (sample 4) BGO crystals after 60Co gamma-ray irradiation 1.0 105 Gy.
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
degradation of transmissions for PbWO4, CdWO4 and CeF3 scintillation crystals for doses 104 and 105 Gy at the appropriate peak emission wavelength (420, 470 and 300 nm, respectively) are compared in Table 3. The complete recovery from the radiation damage due to gamma-rays is one of the significant features of scintillation detectors. The recovery time has been estimated from measurements of transmission spectra during 0.5–15 days after irradiations at 105 Gy. The recovery time estimated from data displayed in Fig. 3 is of order of only few days. The experimental points at the wavelength l ¼ 480 nm have been least-squares fitted by using the relation Tirr ðtÞ=Tð0Þ ¼ 1 expðbtÞ
ð2Þ
Table 4 Parameters of recovery time dependence Tirr ðtÞ=Tð0Þ ¼ 12ebt at l ¼ 480 nm BGO sample 1
BGO sample 4
(4 4 30 mm3) b=(2.1770.31) day1
(+30 30 mm3) b=(2.5670.38) day1
in which Tirr ðtÞ is the transmission measured at time t; Tð0Þ is the transmission before irradiations, the fitting parameter b represents the recovery factor. The results of least-squares fitting procedure are summarized in Table 4. The comparison of the induced absorption coefficients Dk; defined by Dk ¼ ð1=LÞ logðTbefore irr =Tafter irr Þ
3. Conclusions Radiation damage of small 30 4 4 mm3 BGO crystals at 104 Gy accumulated low-energy gamma-rays dose was found to be negligible. However, radiation damage of small BGO crystals can be significant at roughly about 105 Gy accumulated low-energy gamma-rays dose. The absolute degradation of transmission per radiation unit for 105 Gy accumulated dose was found to be lower than 7.5%. This value was found to be lower than the same value for large volume fluoride and tungstate crystals. Radiation damage of large
BGO (sample #2)
INDUCED ABSORPTION COEFFICIENT ∆ k [m-1]
4 3 1.0 x 105 Gy
1.0 x 105 Gy 1.0 x 104 Gy
1
1.0 x 104 Gy
BGO (sample #3)
BGO (sample #4)
4 3 2
5 1.0 x 10 Gy
1
4 1.0 x 10 Gy
400
ð3Þ
L being the length of crystal, is displayed in Fig. 4.
BGO (sample #1)
2
503
500
1.0 x 105 Gy 1.0 x 104 Gy
400
500
WAVELENGHT [nm]
Fig. 4. Comparison of the induced absorption coefficients.
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P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
+30 30 mm3 BGO crystals was found to be approximately the same as for small BGO crystals at 104 Gy accumulated dose. Radiation damage of the large volume BGO sample due to 105 Gy gamma-rays was found to be consistent with data published earlier for fluoride [7,12] and tungstate [8–11] large volume scintillation crystals. The recovery as a function of time does not show two regimes, one fast and one slow, as analysed in Ref. [6]. Complete recovery of radiation damage of small BGO crystals irradiated by 105 Gy low-energy gamma-rays was observed only after few days. It can be concluded that small BGO crystals, widely used in nuclear medicine diagnostic PET and CTS systems, were found to have good radiation resistivity due to irradiation by lowenergy gamma-rays.
Acknowledgements The authors would like to express their sincere thanks to Dr. A. Gektin, Dr. V. Ryzhikov and Dr. S. Burachas from Institute for Single Crystals, Ukraine, Dr. K. Blamek from CRYTUR Ltd., Czech Republic, and Dr. F. Kniest from CRISMATEC, France.
References [1] M. Kobayashi, S. Kurokawa, S. Sugimoto, Y. Yoshimura, M. Chiba, H. Yoshida, M. Ishii, S. Akiyama, H. Ishibashi, Nucl. Instr. and Meth. 189 (1981) 629. [2] L3 Collaboration: B. Adeva, et al., Nucl. Instr. and Meth. A289 (1990) 35. [3] Lu Yusheng, Mao Chengsheng, Tang Xiaowei, in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference SCINT’97, Shanghai, People’s Republic of China, September 22–26, 1997, p. 18. [4] J.G. Rogers, et al., IEE Trans. Nucl. Sci. NS-39 (1992) 1063; J.G. Rogers, et al., IEE Trans. Nucl. Sci. NS-41 (1994) 1423. [5] M. Kobayashi, et al., Nucl. Instr and Meth. 206 (1983) 107. [6] P. Lecoq, P.J. Li, B. Rostaing, Nucl. Instr. and Meth. A300 (1990) 240. [7] P. Kozma, S. Afanasiev, A. Malakhov, A. Povtoreiko, Nucl. Instr. and Meth. A328 (1993) 599. [8] P. Kozma, R. Bajgar, P. Kozma Jr., in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference SCINT’97, Shanghai, People’s Republic of China, September 22–26, 1997, p. 248. [9] P. Kozma, R. Bajgar, P. Kozma Jr., in: S. Baccaro, et al. (Eds.), Proceedings of the International Workshop on Tungstate Crystals, Roma, October 12–14, University La Sapienza Press, 1998, p. 199. [10] P. Kozma, R. Bajgar, P. Kozma Jr., Radiat. Phys. Chem. 59 (2000) 377. [11] P. Kozma, R. Bajgar, P. Kozma Jr., Radiat. Phys. Chem. 65 (2002) 127. [12] P. Kozma, R. Bajgar, P. Kozma Jr., Nucl. Instr. and Meth. A484 (2002) 149. [13] J.R. Jarger, W. Hubner, . Dosimetrie und Straghlenschutz, Springer, Berlin, 1974.
Radiation resistivity of BGO crystals due to low-energy gamma-rays Peter Kozmaa,*, Petr Kozma Jrb a b
Institute of Technological Investigations, StranWice, Prague Eastern District, Prague 251 63, Czech Republic Yxell Ltd., Hi-Tech and Engineering Co., StranWice, Prague Eastern District, Prague 251 63, Czech Republic Received 21 August 2002; received in revised form 1 November 2002; accepted 18 December 2002
Abstract Radiation resistivity of 4 4 30 mm3 BGO crystals from three suppliers has been studied for doses 104 Gy (106 rad) and 105 Gy (107 rad). Radiation hardness was examined by the measurement of optical transmission through BGO crystals before and after 60Co gamma-ray irradiations. The absolute degradation of transmission for 104 and 105 Gy doses at 480 nm wavelength of the peak emission of BGO, was found to be lower than 3.4% and 7.5%, respectively. The results have been also compared with radiation hardness measurements for a large volume +30 30 mm3 BGO crystal as well as another heavy scintillation crystals: fluorides and tungstates. Complete recovery of BGO radiation damage was observed only after few days. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40.M; 61.80.E; 61.80.x Keywords: BGO scintillation crystals; Gamma-ray irradiations; Optical transmission; Radiation damage; Recovery of damaged crystals
1. Introduction Scintillation crystals of bismuth germanate BGO (Bi4Ge3O12) have been considered as a scintillation material for electromagnetic calorimeters in high-energy physics experiments [1]. A large amount of BGO scintillation crystals have been grown by the SIC (Shanghai Institute of Ceramics) for the L3 experiment on LEP at CERN [2].
*Corresponding author. Tel.: +42-204-640176; fax: +42204-640438. E-mail address: [email protected] (P. Kozma).
Small volume BGO crystals are widely used in nuclear medicine diagnostic systems, particularly PE tomographs (PET) and CT Scanners (CTS) [3]. In PET systems the scintillation crystals are used to detect pairs of the 511 keV gamma-rays produced when a positron emitted from the positron emitter (11C, 13Na, 15O, etc.) annihilates with an atomic electron. In earlier stage of CTS or gamma camera systems, NaI(Tl) crystals have been used as the scintillation detectors of gammarays. After BGO was invented in the late 1970s, it gradually took the place of NaI(Tl) as the scintillation detector in most PET and CTS systems because of its high stopping power, light yield and decay time, as well. The BGO block
0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-9002(03)00617-X
500
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
detectors [4], which was invented in the 1980s, are also used in a multi-slice PET systems with high spatial resolution. Radiation damage of BGO scintillation crystals due to low-energy gamma-rays is known to be the most crucial parameter [5,6], and therefore, we have also studied radiation hardness of these crystals by measuring the change in transmittance of light [7–12]. Light yield dependence on irradiation dose seems to be one of the most decisive parameter for practical using of these scintillators in various applications. The main goal of our investigation was to compare the irradiation measurements performed with small BGO samples from three different suppliers (see Table 1). In this work, we report on the radiation damage of small BGO crystals at accumulated doses of low-energy gamma-rays at 104 Gy (106 rad) and 105 Gy (107 rad), respectively. The results are also compared with those obtained for the large volume BGO crystal (+30 30 mm3) and measurements for other scintillation crystals (see Table 2) published earlier [7–12]. We have also estimated the recovery time of irradiated crystals. 2. Experimental results BGO crystal surfaces were optically polished to enable transmission measurements. The crystal samples were irradiated by a 60Co gamma-ray source. The radiation damage was examined by the measurement of transmission spectra before
and after irradiations. Irradiation conditions and optical transmission spectra measurements were identical to those described in Ref. [7]. The activity at the time of the experiment was A ¼ 2:27 105 GBq (i.e. 2.27 1015 s1). The BGO crystals were placed longitudinally at a distance of r ¼ 50 cm from the 60Co gamma-ray source The dose fluence Dexp (Gy s1) was calculated from A X m Dexp ¼ ð1Þ pi Ei i : 4pr2 i r For the two energies E1 ¼ 1:173 MeV and E2 ¼ 1:332 MeV of 60Co, the branching ratios of both lines p1 ¼ p2 ¼ 1 and the values of the mass-energy absorption coefficients [13] m1 / r ¼ 2:68 1022 cm2 g1 and m2 /r ¼ 2:59 102 Table 2 Properties of the most prominent heavy scintillation crystals Scintillation crystal 3
Density (g/cm ) Melting point ( C) Radiation length (cm) Moliere radius (cm) Refractive indexa Wavelength (nm) Decay time (ns) Light yieldc (%)
BGO
CeF3 PbF2 PbWO4 CdWO4
7.13 1050 1.1 2.7 2.1 480 300 15–20
6.16 1285 1.7 2.6 1.6 300 25 10
7.77 1250 0.9 2.2 1.8 Chb Chb 20
8.30 1123 0.9 2.0 2.2 420 6 0.6
7.90 1325 1.1 2.2 2.3 470 — 25–30
a
At the wavelength of the maximum. Cherenkov radiator. c Relative to NaI(Tl) for gamma rays. b
Table 1 Transmission and absolute degradation of BGO samples Sample
Manufacturer
CRISMATECc CRYTUR Ltd.d ISC Co.e CRISMATECc
1 2 3 4 a
Size (mm3)
4 4 30 4 4 30 4 4 30 +30 30
Transmissiona
Absolute degradationb
0 Gy (%)
104 Gy (%)
105 Gy (%)
104 Gy (%)
105 Gy (%)
74.0 70.0 71.0 75.0
71.2 67.1 68.0 71.9
67.8 63.8 64.8 68.0
3.4 3.7 3.9 3.8
7.6 7.9 7.5 8.4
Transmission at peak emission (l ¼ 480 nm). Absolute degradation of transmission at peak emission (l ¼ 480 nm) per unit radiation length (X0 ¼ 1:1 cm). c Saint-Gobain Crystals and Detectors, France. d Turnov, Czech Republic. e Institute for Single Crystals, Kharkov, Ukraine. b
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
501
BGO (sample #2)
BGO (sample #1) 80 60
TRANSMISSION [%]
40 20
before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
1.0 x 104 Gy 1.0 x 105 Gy
BGO (sample #3)
BGO (sample #4)
80 60 40
before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
4
1.0 x 10 Gy 1.0 x 105 Gy
20
400
500
400
500
WAVELENGHT [nm]
Fig. 1. Comparison of transmission spectra before and after longitudinally.
60
Co gamma-ray irradiations. The optical transmissions were measured
BGO (sample #4) Ø 3.0 x 3.0 cm3
80
PbF2
60
2.2 x 2.2 x 24.0 cm
3
TRANSMISSION [%]
40 before irradiation
before irradiation
1.0 x 104 Gy 1.0 x 105 Gy
1.0 x 104 Gy 5 1.0 x 10 Gy
20
CeF3 3.0 x 3.0 x 6.0 cm3
PbWO4
before irradiation
before irradiation
3
2.2 x 2.2 x 25.0 cm
80 60 40
1.0 x 104 Gy 1.0 x 105 Gy
20
400
1.0 x 104 Gy 1.0 x 105 Gy
500
400
500
WAVELENGHT [nm]
Fig. 2. Comparison of transmission spectra before and after 60Co gamma-ray irradiations for large volume crystals: BGO-sample 4 (this work), PbWO4 [11], PbF2 [12] and CeF3 [7]. The optical transmissions were measured longitudinally.
cm2 g1, respectively, we calculated Dexp ¼ 0:795 Gy s1 (i.e. 79.5 rad s1). The appropriate irradiation runs were calculated to be 3.65 and
36.5 h for 104 and 105 Gy accumulated doses, respectively. After the irradiation the transmission spectra were measured several times.
502
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
Table 3 Absolute degradation of transmission for scintillation crystals at peak emission Scintillation crystal
Absolute degradation per unit radiation length Accumulated dose=104 Gy (%)
BGO (l ¼ 480 nm, X0 ¼ 1:1 cm) PbWO4 (l ¼ 420 nm, X0 ¼ 0:9 cm) CdWO4 (l ¼ 470 nm, X0 ¼ 1:1 cm) CeF3 (l ¼ 300 nm, X0 ¼ 1:7 cm)
The transmission spectra before and after absorption of 104 and 105 Gy accumulated doses for samples 1–4 are displayed in Fig. 1. The transmission at peak emission and the absolute degradation of transmission at peak emission per unit radiation length are compared in Table 1. As can be seen, the degradation of transmission for 104 Gy at the wavelength of the peak emission, l ¼ 480 nm, was found to be better than 3.4% per the unit radiation length X0 ¼ 1:1 cm. The degradation of the transmission for 105 Gy at the same wavelength was found to be better than 7.5% per X0 : The results obtained for the large volume +30 30 mm3 BGO crystal (sample 4) have also been compared with those published earlier [7,11,12] for fluoride and tungstate large volume scintillation crystals (Fig. 2) The absolute
Accumulated dose=105 Gy (%)
3.4
7.5
7.5
12.5
4.8
8.0
2.8
10.0
1
0.9
BGO (sample #1) λ = 480 nm
0.8
0.7
Tirr(t)/T(0)
0.6
0.5 1 0
2
4
6
8
10
12
14
10
12
14
0.9
BGO (sample #4) λ = 480 nm
0.8
0.7
0.6
0.5 0
2
4
6
8
Time [day] Fig. 3. Recovery time for optical transmission of small (sample 1) and large (sample 4) BGO crystals after 60Co gamma-ray irradiation 1.0 105 Gy.
P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
degradation of transmissions for PbWO4, CdWO4 and CeF3 scintillation crystals for doses 104 and 105 Gy at the appropriate peak emission wavelength (420, 470 and 300 nm, respectively) are compared in Table 3. The complete recovery from the radiation damage due to gamma-rays is one of the significant features of scintillation detectors. The recovery time has been estimated from measurements of transmission spectra during 0.5–15 days after irradiations at 105 Gy. The recovery time estimated from data displayed in Fig. 3 is of order of only few days. The experimental points at the wavelength l ¼ 480 nm have been least-squares fitted by using the relation Tirr ðtÞ=Tð0Þ ¼ 1 expðbtÞ
ð2Þ
Table 4 Parameters of recovery time dependence Tirr ðtÞ=Tð0Þ ¼ 12ebt at l ¼ 480 nm BGO sample 1
BGO sample 4
(4 4 30 mm3) b=(2.1770.31) day1
(+30 30 mm3) b=(2.5670.38) day1
in which Tirr ðtÞ is the transmission measured at time t; Tð0Þ is the transmission before irradiations, the fitting parameter b represents the recovery factor. The results of least-squares fitting procedure are summarized in Table 4. The comparison of the induced absorption coefficients Dk; defined by Dk ¼ ð1=LÞ logðTbefore irr =Tafter irr Þ
3. Conclusions Radiation damage of small 30 4 4 mm3 BGO crystals at 104 Gy accumulated low-energy gamma-rays dose was found to be negligible. However, radiation damage of small BGO crystals can be significant at roughly about 105 Gy accumulated low-energy gamma-rays dose. The absolute degradation of transmission per radiation unit for 105 Gy accumulated dose was found to be lower than 7.5%. This value was found to be lower than the same value for large volume fluoride and tungstate crystals. Radiation damage of large
BGO (sample #2)
INDUCED ABSORPTION COEFFICIENT ∆ k [m-1]
4 3 1.0 x 105 Gy
1.0 x 105 Gy 1.0 x 104 Gy
1
1.0 x 104 Gy
BGO (sample #3)
BGO (sample #4)
4 3 2
5 1.0 x 10 Gy
1
4 1.0 x 10 Gy
400
ð3Þ
L being the length of crystal, is displayed in Fig. 4.
BGO (sample #1)
2
503
500
1.0 x 105 Gy 1.0 x 104 Gy
400
500
WAVELENGHT [nm]
Fig. 4. Comparison of the induced absorption coefficients.
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P. Kozma, P. Kozma Jr / Nuclear Instruments and Methods in Physics Research A 501 (2003) 499–504
+30 30 mm3 BGO crystals was found to be approximately the same as for small BGO crystals at 104 Gy accumulated dose. Radiation damage of the large volume BGO sample due to 105 Gy gamma-rays was found to be consistent with data published earlier for fluoride [7,12] and tungstate [8–11] large volume scintillation crystals. The recovery as a function of time does not show two regimes, one fast and one slow, as analysed in Ref. [6]. Complete recovery of radiation damage of small BGO crystals irradiated by 105 Gy low-energy gamma-rays was observed only after few days. It can be concluded that small BGO crystals, widely used in nuclear medicine diagnostic PET and CTS systems, were found to have good radiation resistivity due to irradiation by lowenergy gamma-rays.
Acknowledgements The authors would like to express their sincere thanks to Dr. A. Gektin, Dr. V. Ryzhikov and Dr. S. Burachas from Institute for Single Crystals, Ukraine, Dr. K. Blamek from CRYTUR Ltd., Czech Republic, and Dr. F. Kniest from CRISMATEC, France.
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