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Study of the radiation hardness of the pure CsI crystals
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 598 (2009) 273–274
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the radiation hardness of the pure CsI crystals I.V. Bedny, A.E. Bondar, V.V. Cherepkov, D.A. Epifanov, M.G. Golkovsky, A.S. Kuzmin, S.B. Oreshkin, V.E. Shebalin , B.A. Shwartz, Yu.V. Usov Budker Institute of Nuclear Physics, Novosibirsk 630090, Russian Federation
a r t i c l e in fo
abstract
Available online 27 August 2008
This study was initiated by the Belle detector upgrade plan including the replacement of the CsI(Tl) crystals in the endcaps by the pure CsI. Within this work five samples of pure CsI crystals were irradiated by the beam of 0:6 MeV g-quanta. The absorbed dose accumulated in several runs reached 55 krad for two samples and 13 krad for three others. Four of these samples were found to satisfy the super B-factory conditions. The light output reduction for them does not exceed 15% at 13 krad of absorbed dose. One sample had poor radiation resistance. & 2008 Published by Elsevier B.V.
Keywords: Scintillators Crystals Pure CsI Radiation hardness
1. Introduction The Belle detector [1] has been operating at KEKB B-factory since 1999. The electromagnetic calorimeter of this detector consists of about 9000 CsI(Tl) crystals coupled with silicon photodiodes. During 10 years of operation the calorimeter had demonstrated high performance. This work is initiated by the KEKB upgrade plan which implies the luminosity gain up to more than 1035 cm 2 =s [2]. High electron and positron beam currents will unavoidably cause an increase of the background especially in the endcap regions. To keep good performance the upgrade of all detector systems is necessary. Within the detector upgrade slow CsI(Tl) crystals are planned to be replaced with the pure CsI crystals. As photodetectors for pure CsI the vacuum photopenthodes (Hamamatsu Photonics) are proposed [3]. Such a combination allows us to suppress the high pile-up noise which will increase substantially at high background. The crystals used in the calorimeter should tolerate the increased radiation background. By now the absorbed dose in the endcap crystals is about 250 rad which cause about 10% decrease of the crystal light output. The expected radiation dose in the endcaps after accelerator upgrade will be about 5 krad for five years of operation. A study of the pure CsI radiation hardness was performed in this work to prove that these crystals are able to withstand the absorbed dose expected at the super B-factory.
2. Studied samples and irradiation procedure In this work five pure CsI samples were studied. All of them had a shape of the truncated pyramid with the base of about Corresponding author. Tel.: +7 383 3 29 47 60; fax. +7 383 3 30 71 63.
E-mail addresses: [email protected], [email protected] (V.E. Shebalin). 0168-9002/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.08.106
6 6 cm2 and height of about 30 cm. One of these crystals was assembled with a photopenthode and packed as a standard Belle counter. All crystals were covered by the porous teflon sheet of 100 mm and aluminized mylar foil 50 mm thick. The characteristics of pure CsI crystal in comparison with those of CsI(Tl) are presented in Table 1. Studied samples were irradiated by bremsstrahlung photons as shown in Fig. 1. We used an 1.4 MeV electron accelerator ELV-6 (BINP). Bremsstrahlung photons produced by the electrons in the converter irradiate the samples located about 1 m below the converter. The resulting photon beam has a wide spectrum up to 1.4 MeV with an average value of about 0.6 MeV. The absorbed dose was measured by a special sensor based on CsI(Tl) crystal coupled with a silicon photodiode [4].
3. The light output measurements To measure the scintillation parameters of the studied crystals we used g-quanta from a collimated 137 Cs radioactive source, which could move along the crystal axis. The scintillation light was measured by the PM tube with UV input window. The setup stability monitoring was performed using the reference crystal which has not been irradiated. Unlike crystals light output measurement, for measurements of assembled counter characteristics the scheme described below was used. The layout of the Belle counter prototype and readout electronics are shown in Fig. 2. To detect the light from the crystal the 2 in. photopenthode developed by Hamamatsu Photonics is used. It has been coupled with the crystal using optical grease. The signal from the counter after shaping is digitized by the flash ADC with clock rate of 43 MHz. The digitized data are recorded in the circle buffer. Sixteen measurements around the signal maximum are fit to the function with a standard shape and two parameters—the pulse height and the arrival time of the signal.
ARTICLE IN PRESS 274
I.V. Bedny et al. / Nuclear Instruments and Methods in Physics Research A 598 (2009) 273–274
Table 1 The characteristics of CsI(Tl) and pure CsI scintillators at room temperature Crystal
r (g=cm3 )
X0 (cm)
lem (nm)
n
N ph =MeV
t (ns)
dN at 20 C (%) dT
CsI(Tl) CsI
4.51 4.51
1.86 1.86
550 305/400
1.8 2
52 000 5000
1000 20/1000
0.4 1:3
ELV- 6 e-
water
E=1.4 MeV
Cr.1
1
Pb onverter
counter
Al
Cr.2
ampl (ref.units)
0.8
V
Cr.4 0.6
0.4
Dose sensor
Cr.3 0.2
Fig. 1. The irradiation scheme.
0
HV
1
Photopentode
10
102
103 104 dose, rad
105
106
Fig. 3. The light output dependencies on the absorbed dose.
Pure CsI Voltage devider and preamplifier IN
FADC
Shaper Discriminator
Delay line
STOP
Fig. 2. The layout of registration of the signal from assembled counter.
In order to monitor the light output of the counter the spectrum of cosmic ray muons crossing the crystal from top to bottom surface was measured. It has a Landau-like shape with a distinctive peak which was used to determine the counter light output. The light output of the pure CsI depends on its temperature. Therefore the crystal temperature is measured using a sensor attached to the crystal surface and the light output was corrected. After temperature correction the accuracy of light output measurement was better than 1.5%.
4. The results Within this work two sessions of radiation hardness study were performed. At the first one two pure CsI crystals marked as cr.1 and cr.2 were irradiated with doses of 280, 980, 4250, 10 300, 32 000 rad. The second study was performed with crystals marked as cr.3 and cr.4 and one assembled counter. In this case doses of
irradiations were 890, 3200, 8500 rad. The results are shown in Fig. 3 and represent the dependencies of crystal light output on the absorbed dose. It should be noted that the effect of partial recovery was observed for all studied samples. The light output values shown in Fig. 3 are after 5–7 days after irradiation.
5. Conclusion Four pure CsI crystals and one assembled counter were studied. Two samples were irradiated with a dose up to 55 krad and three others got an absorbed dose up to 13 krad. We found that four of studied samples have high radiation hardness satisfying the requirements of the super B-factory. For these samples (included one counter) the light output degradation did not exceed 15% at the 13 krad. However, one of the samples showed much larger light output decrease. This indicates to the possible uncontrolled contaminations in the growing process and makes necessary to test each crystal. References [1] A. Abashian, et al., Nucl. Instr. and Meth. A 479 (2002) 117. [2] T. Iijima, et al., in: 10th International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Russia, 2008, these Proceedings. [3] B.A. Shwarz, et al., in: 10th International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Russia, 2008, these Proceedings. [4] I.V. Bedny, et al., Study of the radiation hardness of pure CsI scintillation crystals, preprint INP 2007-27, Novosibirsk, 2007.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Study of the radiation hardness of the pure CsI crystals I.V. Bedny, A.E. Bondar, V.V. Cherepkov, D.A. Epifanov, M.G. Golkovsky, A.S. Kuzmin, S.B. Oreshkin, V.E. Shebalin , B.A. Shwartz, Yu.V. Usov Budker Institute of Nuclear Physics, Novosibirsk 630090, Russian Federation
a r t i c l e in fo
abstract
Available online 27 August 2008
This study was initiated by the Belle detector upgrade plan including the replacement of the CsI(Tl) crystals in the endcaps by the pure CsI. Within this work five samples of pure CsI crystals were irradiated by the beam of 0:6 MeV g-quanta. The absorbed dose accumulated in several runs reached 55 krad for two samples and 13 krad for three others. Four of these samples were found to satisfy the super B-factory conditions. The light output reduction for them does not exceed 15% at 13 krad of absorbed dose. One sample had poor radiation resistance. & 2008 Published by Elsevier B.V.
Keywords: Scintillators Crystals Pure CsI Radiation hardness
1. Introduction The Belle detector [1] has been operating at KEKB B-factory since 1999. The electromagnetic calorimeter of this detector consists of about 9000 CsI(Tl) crystals coupled with silicon photodiodes. During 10 years of operation the calorimeter had demonstrated high performance. This work is initiated by the KEKB upgrade plan which implies the luminosity gain up to more than 1035 cm 2 =s [2]. High electron and positron beam currents will unavoidably cause an increase of the background especially in the endcap regions. To keep good performance the upgrade of all detector systems is necessary. Within the detector upgrade slow CsI(Tl) crystals are planned to be replaced with the pure CsI crystals. As photodetectors for pure CsI the vacuum photopenthodes (Hamamatsu Photonics) are proposed [3]. Such a combination allows us to suppress the high pile-up noise which will increase substantially at high background. The crystals used in the calorimeter should tolerate the increased radiation background. By now the absorbed dose in the endcap crystals is about 250 rad which cause about 10% decrease of the crystal light output. The expected radiation dose in the endcaps after accelerator upgrade will be about 5 krad for five years of operation. A study of the pure CsI radiation hardness was performed in this work to prove that these crystals are able to withstand the absorbed dose expected at the super B-factory.
2. Studied samples and irradiation procedure In this work five pure CsI samples were studied. All of them had a shape of the truncated pyramid with the base of about Corresponding author. Tel.: +7 383 3 29 47 60; fax. +7 383 3 30 71 63.
E-mail addresses: [email protected], [email protected] (V.E. Shebalin). 0168-9002/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.nima.2008.08.106
6 6 cm2 and height of about 30 cm. One of these crystals was assembled with a photopenthode and packed as a standard Belle counter. All crystals were covered by the porous teflon sheet of 100 mm and aluminized mylar foil 50 mm thick. The characteristics of pure CsI crystal in comparison with those of CsI(Tl) are presented in Table 1. Studied samples were irradiated by bremsstrahlung photons as shown in Fig. 1. We used an 1.4 MeV electron accelerator ELV-6 (BINP). Bremsstrahlung photons produced by the electrons in the converter irradiate the samples located about 1 m below the converter. The resulting photon beam has a wide spectrum up to 1.4 MeV with an average value of about 0.6 MeV. The absorbed dose was measured by a special sensor based on CsI(Tl) crystal coupled with a silicon photodiode [4].
3. The light output measurements To measure the scintillation parameters of the studied crystals we used g-quanta from a collimated 137 Cs radioactive source, which could move along the crystal axis. The scintillation light was measured by the PM tube with UV input window. The setup stability monitoring was performed using the reference crystal which has not been irradiated. Unlike crystals light output measurement, for measurements of assembled counter characteristics the scheme described below was used. The layout of the Belle counter prototype and readout electronics are shown in Fig. 2. To detect the light from the crystal the 2 in. photopenthode developed by Hamamatsu Photonics is used. It has been coupled with the crystal using optical grease. The signal from the counter after shaping is digitized by the flash ADC with clock rate of 43 MHz. The digitized data are recorded in the circle buffer. Sixteen measurements around the signal maximum are fit to the function with a standard shape and two parameters—the pulse height and the arrival time of the signal.
ARTICLE IN PRESS 274
I.V. Bedny et al. / Nuclear Instruments and Methods in Physics Research A 598 (2009) 273–274
Table 1 The characteristics of CsI(Tl) and pure CsI scintillators at room temperature Crystal
r (g=cm3 )
X0 (cm)
lem (nm)
n
N ph =MeV
t (ns)
dN at 20 C (%) dT
CsI(Tl) CsI
4.51 4.51
1.86 1.86
550 305/400
1.8 2
52 000 5000
1000 20/1000
0.4 1:3
ELV- 6 e-
water
E=1.4 MeV
Cr.1
1
Pb onverter
counter
Al
Cr.2
ampl (ref.units)
0.8
V
Cr.4 0.6
0.4
Dose sensor
Cr.3 0.2
Fig. 1. The irradiation scheme.
0
HV
1
Photopentode
10
102
103 104 dose, rad
105
106
Fig. 3. The light output dependencies on the absorbed dose.
Pure CsI Voltage devider and preamplifier IN
FADC
Shaper Discriminator
Delay line
STOP
Fig. 2. The layout of registration of the signal from assembled counter.
In order to monitor the light output of the counter the spectrum of cosmic ray muons crossing the crystal from top to bottom surface was measured. It has a Landau-like shape with a distinctive peak which was used to determine the counter light output. The light output of the pure CsI depends on its temperature. Therefore the crystal temperature is measured using a sensor attached to the crystal surface and the light output was corrected. After temperature correction the accuracy of light output measurement was better than 1.5%.
4. The results Within this work two sessions of radiation hardness study were performed. At the first one two pure CsI crystals marked as cr.1 and cr.2 were irradiated with doses of 280, 980, 4250, 10 300, 32 000 rad. The second study was performed with crystals marked as cr.3 and cr.4 and one assembled counter. In this case doses of
irradiations were 890, 3200, 8500 rad. The results are shown in Fig. 3 and represent the dependencies of crystal light output on the absorbed dose. It should be noted that the effect of partial recovery was observed for all studied samples. The light output values shown in Fig. 3 are after 5–7 days after irradiation.
5. Conclusion Four pure CsI crystals and one assembled counter were studied. Two samples were irradiated with a dose up to 55 krad and three others got an absorbed dose up to 13 krad. We found that four of studied samples have high radiation hardness satisfying the requirements of the super B-factory. For these samples (included one counter) the light output degradation did not exceed 15% at the 13 krad. However, one of the samples showed much larger light output decrease. This indicates to the possible uncontrolled contaminations in the growing process and makes necessary to test each crystal. References [1] A. Abashian, et al., Nucl. Instr. and Meth. A 479 (2002) 117. [2] T. Iijima, et al., in: 10th International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Russia, 2008, these Proceedings. [3] B.A. Shwarz, et al., in: 10th International Conference on Instrumentation for Colliding Beam Physics, Novosibirsk, Russia, 2008, these Proceedings. [4] I.V. Bedny, et al., Study of the radiation hardness of pure CsI scintillation crystals, preprint INP 2007-27, Novosibirsk, 2007.