- Email: [email protected]
Radiation damage studies of multipixel Geiger-mode avalanche photodiodes
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437 www.elsevier.com/locate/nima
Radiation damage studies of multipixel Geiger-mode avalanche photodiodes Y. Musienkoa,,1, D. Renkerb, S. Reucrofta, R. Scheuermannb, A. Stoykovb, J. Swaina a
Northeastern University, Boston, USA Paul Scherrer Institut, Villigen PSI, Switzerland
b
Available online 7 August 2007
Abstract Results on the radiation hardness of multipixel Geiger-mode avalanche photodiodes (G-APDs) are presented. Recently developed GAPDs from three manufacturers (Hamamatsu (Japan), CPTA(Russia) and Mikron/Dubna(Russia)) were exposed to 28 MeV positrons with fluences up to 8 1010 positrons=cm2 at the Paul Scherrer Institute. The effects of this radiation on many G-APD parameters such as gain, photon detection efficiency, dark current and count rate for these devices are shown and discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Wk Keywords: Photodiodes; Silicon radiation detectors
1. Introduction
2. Irradiation set-up and G-APDs under study
Recently developed multipixel Geiger-mode avalanche photodiodes—G-APDs (also known as SiPMs, SSPMs, MRS APDs, AMPDs, MPPCs) are very promising candidates for high energy physics (HEP) applications. Many of the high energy physics experiments where G-APDs are planned to be used (ILC detectors, the CMS detector at the LHC, etc.) operate in harsh radiation environments which produce damage in many materials. As has been shown by many investigators, radiation (gamma rays, electrons, neutrons, charged hadrons, etc.) can produce defects in silicon. As a result, parameters of G-APDs such as leakage current, dark count rate, gain, and photon detection efficiency may change during irradiation. How these parameters change during irradiation becomes one of the most important questions for the application of the G-APDs in high energy physics experiments. To answer this question several types of GAPDs (produced by Hamamatsu, CPTA, Mikron/Dubna) were irradiated with a 28 MeV positron beam at PSI.
Eight G-APDs from three producers (see Table 1) were irradiated with a 7 cm diameter high intensity beam of 28 MeV positrons. Two G-APDs were produced by CPTA(Moscow), three devices were produced by Mikron (developed by the Dubna APD group [1]) and two were from Hamamatsu. Most of these G-APDs were experimental devices, except the CPTA-t1, a 4:41 mm2 active area G-APD, which is a commercial device (SSPM0606BG4MM-PCB) supplied by Photonique [2]. All the APDs were placed in a PCB board and connected in parallel to a Keithley 487 pico-ammeter/source and biased at 10 V. The total current delivered to the APDs was monitored during irradiation. The G-APDs were positioned to be in the center of the beam. Non-uniformity of the beam intensity over the whole area of APD location was measured to be less than 15%.
Corresponding author. 1
E-mail address: [email protected] (Y. Musienko). On leave from INR (Moscow).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.021
3. Experimental results Such parameters of the G-APDs as the photon detection efficiency ðPDEÞ, gain, dark current and count were measured as a function of bias voltage before and 2 days
ARTICLE IN PRESS 434
Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
Table 1 G-APDs and their parameters before irradiation ðT ¼ 22 CÞ G-APDs
Producer’s reference
Substrate
Area (mm2 Þ
# of pixels
U op (V)
Gain, 106 (Gate ¼ 60 ns)
PDE (515 nm) (%)
Dark count (MHz)
CPTA-t1 CPTA-t2 Dubna-t1 Dubna-t2#1 Dubna-t2#2 Dubna-t3 Hamamatsu-t1 Hamamatsu-t2
SSPM-0606BG F1707 MW-3 p-INT-2 p-INT-2 pMP-3d-11 311-31A-001 311-53-1A-001
p-type p-type n-type p-type p-type p-type n-type n-type
4.41 1 1 3.24 3.24 1 1 1
1748 556 10 000 2436 2436 1024 1600 400
21 52.5 119 26.5 26 45.5 69.5 69.5
0.2 1.2 0.05 1.5 1.5 0.9 0.5 3.5
32 20 19 18 18 12 12 27
20 4 7 8 8 5 0.5 1.3
40
CPTA_t1, U=20.3 V
35
CPTA_t2, U=52.5 V
PDE [%]
30 25 20 15 10 5
PDE [%]
0 400 20 18 16 14 12 10 8 6 4 2 0 350
450
500
550 600 650 Wavelength [nm]
700
750
800
Dubna/Mikron_t1, U=119 V Dubna/Mikron_t2#1, U=26.3 V Dubna/Mikron_t3, U=45 V 400
450
500
550
600
650
700
750
800
750
800
Wavelength [nm]
PDE [%]
40 35
Hamamatsu-t2, U=69.5 V
30
Hamamatsu-t1, U=69.8 V
25 20 15 10 5 0 350
400
450
500
550
600
650
700
Wavelength [nm] Fig. 1. G-APDs spectral responses—measured at T ¼ 22 C before irradiation.
after irradiation at the CERN APD Lab. The temperature during the measurements was stabilized to 22 C with the variation during the measurements not exceeding 1 C. The
most important parameters of the G-APDs used in this study (including the active area, number of pixels and silicon substrate type) can be found in Table 1. The values
ARTICLE IN PRESS Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
of PDE, gain, and dark count rate for each APD presented in this table were measured at the maximum bias voltage reachable. Further increase of the bias voltage was impossible due to a significant increase of the APD noise. For the spectral response measurements, an ‘‘Optometrics’’ SDMC1-03 spectrophotometer was used. Fig. 1 shows the dependence of photon detection efficiency on the wavelength measured for eight non-irradiated G-APDs. More details about the measurement technique used in this study can be found in [3]. The photon detection efficiency and gain as functions of bias voltage measured for Hamamatsu-t1 and Hamamatsut2 G-APDs before and after irradiation are shown in Fig. 2. The APDs were illuminated with small green (515 nm) LED pulses via a 0.5 mm collimator. The signals from the G-APDs were amplified with a fast transimpedance amplifier (gain ¼ 60) and digitised with a LeCroy 2249W ADC (gate 60 ns). One can see rather good agreement with the PDE and gain versus bias voltage before and after irradiation. No change of the PDE and gain vs. voltage were found for the other six irradiated devices within the errors of our measurements (estimated to be 15%).
435
A significant increase in leakage current and dark count rate was measured for all the devices. Fig. 3 shows the dependences of the leakage current (top figure) and dark count (bottom figure) on the bias voltage measured with the Hamamatsu G-APDs before and after irradiation. 4. Discussion of the results From Table 1 one can see that the G-APDs studied have different areas, numbers of pixels, operating voltages, gains, PDE’s, etc. To compare the damages produced by positrons in different G-APDs, for each G-APD we calculated the increase of the dark count DNðUÞ ¼ ðN after ðUÞ N before ðUÞÞ for each measurement point (the set of the bias voltages used to measure the parameters of non-irradiated and irradiated G-APDs was the same). Then we divided this increase by the active area of the device and by its PDE measured with green (515 nm) LED: DN norm ðUÞ ¼ DNðUÞ=Area=PDEð515 nmÞðUÞ.
(1)
The reason we used the G-APD PDE value for normalization was that it is proportional to the G-APD fill factor
40 before irr.
35 PDE(515 nm) [%]
after irr.
Hamamatsu-t2
30 25 20 15 10
Hamamatsu-t1
5 0
67
68
69 Bias [V]
70
10
71
before irr.
Hamamatsu-t2
after irr.
Gain*106
1
Hamamatsu-t1 0.1
0.01
67
68
69 Bias [V]
70
71
Fig. 2. Photon detection efficiency vs. bias voltage (top) and gain vs. bias voltage (bottom), measured before and after 8 1010 positrons=cm2 for two Hamamatsu G-APDs.
ARTICLE IN PRESS 436
Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
100 before irr. after irr.
Hamamatsu-t2 Dark Current [μA]
10 1 0.1 0.01 0.001
Hamamatsu-t1 67
68
69 Bias [V]
70
71
10000
Dark Count [kHz]
8000
before irr.
Hamamatsu-t2
after irr. 6000 4000
Hamamatsu-t1
2000 0
67
68
69 Bias [V]
70
71
Fig. 3. Dark current and dark count vs. bias voltage measured before and after irradiation for two Hamamatsu G-APDs.
and to the pixel avalanche breakdown probability [4]. Dividing the DNðUÞ by the PDE we tried to reduce the dependence of DNðUÞ on these G-APD parameters. The results of these calculations for eight APDs are presented in Fig. 4, where the normalized dark count increases (DN norm ) are plotted as functions of the PDE measured at 515 nm. From this figure one can see that five G-APDs out of eight have similar values of DN norm (DN norm ¼ 60 110 kHz= %=mm2 Þ over wide range of PDE (from 5% to 25% for some G-APDs) and these depend rather weakly on PDE. It may be that these devices have similar depletion layer thicknesses which would explain the close values of DN norm found for these G-APDs. From Fig. 4 one can also see that DN norm calculated for the CPTA-t2, Dubna/ Mikron-t3 and Hamamatsu-t1 G-APDs are rather sensitive functions of PDE, and that the DN norm reaches rather high values (DN norm ¼ 3502700) for highest values of PDE measured with these G-APDs (from 12% to 20% depending on the APD type). The high values of DN norm found for these devices still need to be understood. We plan to explore this further in future irradiation tests.
5. Irradiation studies of the Hamamatsu S8148 APD and estimation of the damage produced by neutrons in G-APD The S8148 APD was developed for the CMS electromagnetic calorimeter by Hamamatsu Photonics Inc. in close cooperation with the CMS ECAL group. It has a 5 5 mm2 sensitive area and a depletion region with an effective thickness of approximately 5:6 mm [5]. Positron irradiations of the S8148 APD were made at the same time and in the same beam as was used to irradiate the G-APDs. The gain and dark current were measured for this APD as functions of bias before irradiation, and two days afterwards. Following this, the APD was annealed at 80 1C for 2 days and these measurements were made again. Dark current divided by gain should be a good measure of bulk dark current, which expected to dominate at high gains. It was found that the bulk dark current increased by 80 pA after irradiation. This increase dropped to 40 pA after annealing. Knowing the APD area and effective thickness as well as the total positron flux one can calculate the dark current damage constant for silicon for 28 MeV positrons. This was found to be 7 1017 A=cm after irradiation and
ARTICLE IN PRESS Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
800
CPTA-t1 CPTA-t2 Hamamatsu-t1 Hamamatsu-t2 Dubna/Mikron-t1 Dubna/Mikron-t2#1 Dubna/Mikron-t2#2 Dubna/Mikron-t3
700 600 ΔNnorm [kHz/%/mm2]
437
500 400 300 200 100 0
0
5
10
15
20
25
30
35
PDE(515 nm) [%] Fig. 4. Normalized dark count increase (see text) vs. photon detection efficiency calculated for eight irradiated G-APDs.
half that ð3:5 1018 A=cmÞ after annealing. This damage constant can be compared with the corresponding damage constant for 1 MeV neutrons which was previously measured after annealing to be 1016 A=cm (see [6]). This means that 28 MeV positrons should produce about 30 times less damage than 1 MeV neutrons. This is similar to the conclusions drawn in reference [7] considering nonionising energy losses for 28 MeV electrons (as contrasted with positrons in our case). For G-APDs then, the damage from irradiation with ð8 1010 Þ 28 MeV positrons=cm2 is approximately equivalent to the damage produced by ð2:7 109 Þ 1 MeV neutrons=cm2 . 6. Summary Eight G-APDs (produced by CPTA, Mikron/Dubna and Hamamatsu) were irradiated with a 28 MeV positron beam at PSI. The G-APD’s gain, photon detection efficiency,
dark current and count rate were measured before and after irradiation. The change of gain and photon detection efficiency as a function of voltage was found to be small (less than 15%). Measurements using a Hamamatsu S8148 APD in the same beam, together with earlier studies of proton and neutron damage to the same device, make this positron exposure equivalent to a 30 times lower dose of neutrons of 1 MeV.
References [1] Dubna APD group, hhttp://sunhe.jinr.ru/struct/neeo/apd/i. [2] S.A. Photonique, Geneva hhttp://www.photonique.chi. [3] Y. Musienko, S. Reucroft, J. Swain, Nucl. Instr. and Meth. A 567 (2006) 57. [4] D. Renker, Nucl. Instr. and Meth. A 567 (2006) 48. [5] K. Deiters, et al., Nucl. Instr. and Meth. A 453 (2000) 223. [6] Y. Musienko, et al., Nucl. Instr. and Meth. A 447 (2000) 437. [7] G. Lindstrom, Nucl. Instr. and Meth. A 512 (2003) 30.
Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437 www.elsevier.com/locate/nima
Radiation damage studies of multipixel Geiger-mode avalanche photodiodes Y. Musienkoa,,1, D. Renkerb, S. Reucrofta, R. Scheuermannb, A. Stoykovb, J. Swaina a
Northeastern University, Boston, USA Paul Scherrer Institut, Villigen PSI, Switzerland
b
Available online 7 August 2007
Abstract Results on the radiation hardness of multipixel Geiger-mode avalanche photodiodes (G-APDs) are presented. Recently developed GAPDs from three manufacturers (Hamamatsu (Japan), CPTA(Russia) and Mikron/Dubna(Russia)) were exposed to 28 MeV positrons with fluences up to 8 1010 positrons=cm2 at the Paul Scherrer Institute. The effects of this radiation on many G-APD parameters such as gain, photon detection efficiency, dark current and count rate for these devices are shown and discussed. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.Wk Keywords: Photodiodes; Silicon radiation detectors
1. Introduction
2. Irradiation set-up and G-APDs under study
Recently developed multipixel Geiger-mode avalanche photodiodes—G-APDs (also known as SiPMs, SSPMs, MRS APDs, AMPDs, MPPCs) are very promising candidates for high energy physics (HEP) applications. Many of the high energy physics experiments where G-APDs are planned to be used (ILC detectors, the CMS detector at the LHC, etc.) operate in harsh radiation environments which produce damage in many materials. As has been shown by many investigators, radiation (gamma rays, electrons, neutrons, charged hadrons, etc.) can produce defects in silicon. As a result, parameters of G-APDs such as leakage current, dark count rate, gain, and photon detection efficiency may change during irradiation. How these parameters change during irradiation becomes one of the most important questions for the application of the G-APDs in high energy physics experiments. To answer this question several types of GAPDs (produced by Hamamatsu, CPTA, Mikron/Dubna) were irradiated with a 28 MeV positron beam at PSI.
Eight G-APDs from three producers (see Table 1) were irradiated with a 7 cm diameter high intensity beam of 28 MeV positrons. Two G-APDs were produced by CPTA(Moscow), three devices were produced by Mikron (developed by the Dubna APD group [1]) and two were from Hamamatsu. Most of these G-APDs were experimental devices, except the CPTA-t1, a 4:41 mm2 active area G-APD, which is a commercial device (SSPM0606BG4MM-PCB) supplied by Photonique [2]. All the APDs were placed in a PCB board and connected in parallel to a Keithley 487 pico-ammeter/source and biased at 10 V. The total current delivered to the APDs was monitored during irradiation. The G-APDs were positioned to be in the center of the beam. Non-uniformity of the beam intensity over the whole area of APD location was measured to be less than 15%.
Corresponding author. 1
E-mail address: [email protected] (Y. Musienko). On leave from INR (Moscow).
0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.08.021
3. Experimental results Such parameters of the G-APDs as the photon detection efficiency ðPDEÞ, gain, dark current and count were measured as a function of bias voltage before and 2 days
ARTICLE IN PRESS 434
Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
Table 1 G-APDs and their parameters before irradiation ðT ¼ 22 CÞ G-APDs
Producer’s reference
Substrate
Area (mm2 Þ
# of pixels
U op (V)
Gain, 106 (Gate ¼ 60 ns)
PDE (515 nm) (%)
Dark count (MHz)
CPTA-t1 CPTA-t2 Dubna-t1 Dubna-t2#1 Dubna-t2#2 Dubna-t3 Hamamatsu-t1 Hamamatsu-t2
SSPM-0606BG F1707 MW-3 p-INT-2 p-INT-2 pMP-3d-11 311-31A-001 311-53-1A-001
p-type p-type n-type p-type p-type p-type n-type n-type
4.41 1 1 3.24 3.24 1 1 1
1748 556 10 000 2436 2436 1024 1600 400
21 52.5 119 26.5 26 45.5 69.5 69.5
0.2 1.2 0.05 1.5 1.5 0.9 0.5 3.5
32 20 19 18 18 12 12 27
20 4 7 8 8 5 0.5 1.3
40
CPTA_t1, U=20.3 V
35
CPTA_t2, U=52.5 V
PDE [%]
30 25 20 15 10 5
PDE [%]
0 400 20 18 16 14 12 10 8 6 4 2 0 350
450
500
550 600 650 Wavelength [nm]
700
750
800
Dubna/Mikron_t1, U=119 V Dubna/Mikron_t2#1, U=26.3 V Dubna/Mikron_t3, U=45 V 400
450
500
550
600
650
700
750
800
750
800
Wavelength [nm]
PDE [%]
40 35
Hamamatsu-t2, U=69.5 V
30
Hamamatsu-t1, U=69.8 V
25 20 15 10 5 0 350
400
450
500
550
600
650
700
Wavelength [nm] Fig. 1. G-APDs spectral responses—measured at T ¼ 22 C before irradiation.
after irradiation at the CERN APD Lab. The temperature during the measurements was stabilized to 22 C with the variation during the measurements not exceeding 1 C. The
most important parameters of the G-APDs used in this study (including the active area, number of pixels and silicon substrate type) can be found in Table 1. The values
ARTICLE IN PRESS Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
of PDE, gain, and dark count rate for each APD presented in this table were measured at the maximum bias voltage reachable. Further increase of the bias voltage was impossible due to a significant increase of the APD noise. For the spectral response measurements, an ‘‘Optometrics’’ SDMC1-03 spectrophotometer was used. Fig. 1 shows the dependence of photon detection efficiency on the wavelength measured for eight non-irradiated G-APDs. More details about the measurement technique used in this study can be found in [3]. The photon detection efficiency and gain as functions of bias voltage measured for Hamamatsu-t1 and Hamamatsut2 G-APDs before and after irradiation are shown in Fig. 2. The APDs were illuminated with small green (515 nm) LED pulses via a 0.5 mm collimator. The signals from the G-APDs were amplified with a fast transimpedance amplifier (gain ¼ 60) and digitised with a LeCroy 2249W ADC (gate 60 ns). One can see rather good agreement with the PDE and gain versus bias voltage before and after irradiation. No change of the PDE and gain vs. voltage were found for the other six irradiated devices within the errors of our measurements (estimated to be 15%).
435
A significant increase in leakage current and dark count rate was measured for all the devices. Fig. 3 shows the dependences of the leakage current (top figure) and dark count (bottom figure) on the bias voltage measured with the Hamamatsu G-APDs before and after irradiation. 4. Discussion of the results From Table 1 one can see that the G-APDs studied have different areas, numbers of pixels, operating voltages, gains, PDE’s, etc. To compare the damages produced by positrons in different G-APDs, for each G-APD we calculated the increase of the dark count DNðUÞ ¼ ðN after ðUÞ N before ðUÞÞ for each measurement point (the set of the bias voltages used to measure the parameters of non-irradiated and irradiated G-APDs was the same). Then we divided this increase by the active area of the device and by its PDE measured with green (515 nm) LED: DN norm ðUÞ ¼ DNðUÞ=Area=PDEð515 nmÞðUÞ.
(1)
The reason we used the G-APD PDE value for normalization was that it is proportional to the G-APD fill factor
40 before irr.
35 PDE(515 nm) [%]
after irr.
Hamamatsu-t2
30 25 20 15 10
Hamamatsu-t1
5 0
67
68
69 Bias [V]
70
10
71
before irr.
Hamamatsu-t2
after irr.
Gain*106
1
Hamamatsu-t1 0.1
0.01
67
68
69 Bias [V]
70
71
Fig. 2. Photon detection efficiency vs. bias voltage (top) and gain vs. bias voltage (bottom), measured before and after 8 1010 positrons=cm2 for two Hamamatsu G-APDs.
ARTICLE IN PRESS 436
Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
100 before irr. after irr.
Hamamatsu-t2 Dark Current [μA]
10 1 0.1 0.01 0.001
Hamamatsu-t1 67
68
69 Bias [V]
70
71
10000
Dark Count [kHz]
8000
before irr.
Hamamatsu-t2
after irr. 6000 4000
Hamamatsu-t1
2000 0
67
68
69 Bias [V]
70
71
Fig. 3. Dark current and dark count vs. bias voltage measured before and after irradiation for two Hamamatsu G-APDs.
and to the pixel avalanche breakdown probability [4]. Dividing the DNðUÞ by the PDE we tried to reduce the dependence of DNðUÞ on these G-APD parameters. The results of these calculations for eight APDs are presented in Fig. 4, where the normalized dark count increases (DN norm ) are plotted as functions of the PDE measured at 515 nm. From this figure one can see that five G-APDs out of eight have similar values of DN norm (DN norm ¼ 60 110 kHz= %=mm2 Þ over wide range of PDE (from 5% to 25% for some G-APDs) and these depend rather weakly on PDE. It may be that these devices have similar depletion layer thicknesses which would explain the close values of DN norm found for these G-APDs. From Fig. 4 one can also see that DN norm calculated for the CPTA-t2, Dubna/ Mikron-t3 and Hamamatsu-t1 G-APDs are rather sensitive functions of PDE, and that the DN norm reaches rather high values (DN norm ¼ 3502700) for highest values of PDE measured with these G-APDs (from 12% to 20% depending on the APD type). The high values of DN norm found for these devices still need to be understood. We plan to explore this further in future irradiation tests.
5. Irradiation studies of the Hamamatsu S8148 APD and estimation of the damage produced by neutrons in G-APD The S8148 APD was developed for the CMS electromagnetic calorimeter by Hamamatsu Photonics Inc. in close cooperation with the CMS ECAL group. It has a 5 5 mm2 sensitive area and a depletion region with an effective thickness of approximately 5:6 mm [5]. Positron irradiations of the S8148 APD were made at the same time and in the same beam as was used to irradiate the G-APDs. The gain and dark current were measured for this APD as functions of bias before irradiation, and two days afterwards. Following this, the APD was annealed at 80 1C for 2 days and these measurements were made again. Dark current divided by gain should be a good measure of bulk dark current, which expected to dominate at high gains. It was found that the bulk dark current increased by 80 pA after irradiation. This increase dropped to 40 pA after annealing. Knowing the APD area and effective thickness as well as the total positron flux one can calculate the dark current damage constant for silicon for 28 MeV positrons. This was found to be 7 1017 A=cm after irradiation and
ARTICLE IN PRESS Y. Musienko et al. / Nuclear Instruments and Methods in Physics Research A 581 (2007) 433–437
800
CPTA-t1 CPTA-t2 Hamamatsu-t1 Hamamatsu-t2 Dubna/Mikron-t1 Dubna/Mikron-t2#1 Dubna/Mikron-t2#2 Dubna/Mikron-t3
700 600 ΔNnorm [kHz/%/mm2]
437
500 400 300 200 100 0
0
5
10
15
20
25
30
35
PDE(515 nm) [%] Fig. 4. Normalized dark count increase (see text) vs. photon detection efficiency calculated for eight irradiated G-APDs.
half that ð3:5 1018 A=cmÞ after annealing. This damage constant can be compared with the corresponding damage constant for 1 MeV neutrons which was previously measured after annealing to be 1016 A=cm (see [6]). This means that 28 MeV positrons should produce about 30 times less damage than 1 MeV neutrons. This is similar to the conclusions drawn in reference [7] considering nonionising energy losses for 28 MeV electrons (as contrasted with positrons in our case). For G-APDs then, the damage from irradiation with ð8 1010 Þ 28 MeV positrons=cm2 is approximately equivalent to the damage produced by ð2:7 109 Þ 1 MeV neutrons=cm2 . 6. Summary Eight G-APDs (produced by CPTA, Mikron/Dubna and Hamamatsu) were irradiated with a 28 MeV positron beam at PSI. The G-APD’s gain, photon detection efficiency,
dark current and count rate were measured before and after irradiation. The change of gain and photon detection efficiency as a function of voltage was found to be small (less than 15%). Measurements using a Hamamatsu S8148 APD in the same beam, together with earlier studies of proton and neutron damage to the same device, make this positron exposure equivalent to a 30 times lower dose of neutrons of 1 MeV.
References [1] Dubna APD group, hhttp://sunhe.jinr.ru/struct/neeo/apd/i. [2] S.A. Photonique, Geneva hhttp://www.photonique.chi. [3] Y. Musienko, S. Reucroft, J. Swain, Nucl. Instr. and Meth. A 567 (2006) 57. [4] D. Renker, Nucl. Instr. and Meth. A 567 (2006) 48. [5] K. Deiters, et al., Nucl. Instr. and Meth. A 453 (2000) 223. [6] Y. Musienko, et al., Nucl. Instr. and Meth. A 447 (2000) 437. [7] G. Lindstrom, Nucl. Instr. and Meth. A 512 (2003) 30.