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Development of a large area silicon pad detector for the identification of cosmic ions
Nuclear Physics B (Proc. Suppl.) 172 (2007) 162–164 www.elsevierphysics.com
Development of a large area silicon pad detector for the identification of cosmic ions M.Y. Kimab∗ , P.S. Marrocchesia, C. Avanzinib , M.G. Bagliesia , G. Bigongiaria, A. Caldaronea , R. Cecchia , P. Maestroa , N. Malakhovb, F. Morsanib and R. Zeia a
Department of Physics, University of Siena and INFN, Via Roma 56, 53100 Siena, Italy b
INFN Sezione di Pisa, Edificio C-Polo Fibonacci Largo Bruno Pontecorvo 3, 56127 Pisa, Italy A silicon sensor with 64 large area pads (“pad” of 1 cm2 area) was developed to identify relativistic ions in direct measurements of the elemental composition of cosmic rays. A single-element discrimination can be achieved via an accurate measurement of the electric charge Z, taking advantage of the Z 2 dependence of specific ionization in silicon. Space-based or balloon-borne cosmic ray experiments of the next generation require the coverage of large sensitive areas with arrays of this kind of detectors. Preliminary results on the performance of the sensor are presented. Key words: cosmic ray, silicon sensor PACS: 29.40.Wk, 95.55.Vj, 95.85.Ry
1. INTRODUCTION Above the atmosphere, direct measurements of charged cosmic rays are performed with balloonborne or space based instruments. In contrast with ground based experiments, where the identification of the primary particle is performed on a statistical basis and is affected by large systematic errors, direct detection by modern instruments has achieved the single-element discrimination [1,2]. In most cases, this is done via an accurate measurement of the electric charge of the incoming (fully ionized) nucleus. Silicon sensors, with adequate pixel size, provide an excellent charge discrimination taking advantage of the saturated specific ionization for relativistic particles on the Fermi plateau and its Z2 dependence. Unfortunately, direct measurements of cosmic ray fluxes at high energy are limited by collection power which is severely constrained by the instrument weight and size. At present, a lot of effort is going into the development of large ar∗ [email protected]
0920-5632/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2007.08.148
rays of silicon sensors. A seamless sensitive area is achieved by a suitable mechanical arrangement of partially overlapping sensors. Another important issue is the specification of the front-end electronics [3,4] which should provide low-noise in order to identify Z=1 particle with a S/N possibly approaching 10 and a dynamic range of about 103 MIP in order to identify nuclei up to Iron and above. In this paper, we report on a large area silicon sensor (64 cm2 of active area with 8 × 8 pads) which was developed on a 6 inch wafer (thickness 500 μm and resistivity >10000 Ω·cm) to provide single-element charge identification of highenergy cosmic rays. The characterization and performance of this sensor as a silicon PIN diode device was presented in [5]. Tests with atmospheric muons demonstrated its excellent performance to identify single charged particles. A dedicated low-noise front-end electronics with large dynamic range [4] was used during the test. The results are reported.
M.Y. Kim et al. / Nuclear Physics B (Proc. Suppl.) 172 (2007) 162–164
2. SILICON SENSOR TEST WITH ATMOSPHERIC MUONS A lab test was performed where the large area silicon sensor was triggered by atmospheric muons using an external coincidence of 3 scintillators. 2.1. Silicon sensor The electrical characteristics of the sensor were measured at 25◦ C. Typically pads are fully depleted at 100 V and they do not exhibit breakdown up to 200 V. For the over-depletion operation, 130 V is biased through 1 MΩ bias resistor on the n-side of the sensor, during the atmospheric muon test. The leakage current and the capacitance of 64 pads at operating voltage are shown in Fig.1.
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probable input of the front-end electronics (it approximately corresponds to the most probable charge signal generated by 1 MIP transversing the 500 μm thick silicon sensor). This measurement was repeated with an equivalent capacitor connected to each input channel. The result in Fig.2 was used for the channel equalization in the analysis.
Figure 2. Charge injection result of 64 electronics channels(Expected value: 25 ADU counts, Measured value: 25.2 ± 0.2, Relative error < 1 %)
Figure 1. Leakage current and Capacitance of 64 pads at the operating voltage (Measurement accuracy ± 1 %) The variation of the capacitance between pads is not exceeding 5 % from the nominal capacitance of each pad (1 cm2 ), 22 pF. The average leakage current at the operating voltage is 2nA, the shot noise generated by random fluctuations of the current flowing through the pad being negligible. 2.2. Front-end Electronics The sensor is equipped with two VA32HDR14.2 ASICs for the readout of 32 channels each, as described in [4]. To optimize the signalto-noise ratio in the silicon detector, one should take into account the different gains of the electronics channels. For the gain measurement, a charge pulse of 6.4 fC was injected as the most-
2.3. Atmospheric muon test setup During the muon test, three scintillators are arranged as described in Fig.3. The illuminated area on the silicon sensor is limited by the geometry of the trigger counters and is about 2 cm × 5 cm. On average 12 pads have a significant statistics with a typical run duration of 12 hours. Measurements of individual channel pedestals are done only before and after each atmospheric muon run because pedestals were found to be remarkably stable.
Figure 3. Trigger setup geometry
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M.Y. Kim et al. / Nuclear Physics B (Proc. Suppl.) 172 (2007) 162–164
3. ANALYSIS and RESULT 3.1. Analysis The rms value of the pedestals, measured during a stand alone test of the front-end electronics, is typically around 3 ADU counts. It increases to ∼ 5 ADU counts when the silicon sensor is connected to the front-end input. A common-noise correction (CNC) per event is applied. The average pulse height of 20 reference channels from each ADC is used for the correction. After CNC, the individual rms values of the pedestals decrease to the level of the electronics stand alone test. For each trigger, the pixel with the maximum pulse height is identified as the muon signal and plotted in Fig.4b. In the following, ∼ 3400 events are analyzed where on average 12 pads were illuminated due to the trigger geometry.
(a) Pulse height distribution from 12 pads: pedestal fitting
3.2. Result To calculate the S/N ratio of the system, we first fit the rms of the inclusive pedestal distribution from the 12 pads (Fig.4a). Then, the muon signal in (Fig.4b) is fitted with a Landau distribution convoluted with a Gaussian. The most probable value of the energy loss ∼ 24 ADC is ∼ 75 % of the mean loss of a MIP as expected for a 500 μm thick silicon absorber. As a result, the most probable signal-to-noise ratio of the system is ∼ 8. 4. CONCLUSION A large area pixelated silicon sensor, developed for cosmic ray research, is described. Test results show that signals from Z=1 charged particles can be separated from the noise with an excellent S/N ratio. REFERENCES 1. V.I. Zatsepin, et al. Nucl. Instr. and Meth. A 524 (2004) 195. 2. E.S. Seo, et al. Adv. Space Res. 30 (5)(2002) 1263. 3. S.Torii et. al. Nuclear Physics B Proc. Suppl. Vol.150, 390-303, 2006. 4. ”Front-end electronics with large dynamic range for space-borne experiments”, M.G.
(b) Muon signal Figure 4. Atmospheric Muon test result Bagliesi, et al. 10th Topical Seminar on Innovative Particle and Radiation Detectors, Siena, Italy, October 1 - 5, 2006. 5. ”A large area silicon pixel array for the identification of relativistic nuclei in cosmic ray experiments”, P.S. Marrocchesi, et al 10th Pisa Meeting on Advanced Detectors, La Biodola, Isola d’Elba, Italy, May 21 - 27, 2006 (in press).
Development of a large area silicon pad detector for the identification of cosmic ions M.Y. Kimab∗ , P.S. Marrocchesia, C. Avanzinib , M.G. Bagliesia , G. Bigongiaria, A. Caldaronea , R. Cecchia , P. Maestroa , N. Malakhovb, F. Morsanib and R. Zeia a
Department of Physics, University of Siena and INFN, Via Roma 56, 53100 Siena, Italy b
INFN Sezione di Pisa, Edificio C-Polo Fibonacci Largo Bruno Pontecorvo 3, 56127 Pisa, Italy A silicon sensor with 64 large area pads (“pad” of 1 cm2 area) was developed to identify relativistic ions in direct measurements of the elemental composition of cosmic rays. A single-element discrimination can be achieved via an accurate measurement of the electric charge Z, taking advantage of the Z 2 dependence of specific ionization in silicon. Space-based or balloon-borne cosmic ray experiments of the next generation require the coverage of large sensitive areas with arrays of this kind of detectors. Preliminary results on the performance of the sensor are presented. Key words: cosmic ray, silicon sensor PACS: 29.40.Wk, 95.55.Vj, 95.85.Ry
1. INTRODUCTION Above the atmosphere, direct measurements of charged cosmic rays are performed with balloonborne or space based instruments. In contrast with ground based experiments, where the identification of the primary particle is performed on a statistical basis and is affected by large systematic errors, direct detection by modern instruments has achieved the single-element discrimination [1,2]. In most cases, this is done via an accurate measurement of the electric charge of the incoming (fully ionized) nucleus. Silicon sensors, with adequate pixel size, provide an excellent charge discrimination taking advantage of the saturated specific ionization for relativistic particles on the Fermi plateau and its Z2 dependence. Unfortunately, direct measurements of cosmic ray fluxes at high energy are limited by collection power which is severely constrained by the instrument weight and size. At present, a lot of effort is going into the development of large ar∗ [email protected]
0920-5632/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2007.08.148
rays of silicon sensors. A seamless sensitive area is achieved by a suitable mechanical arrangement of partially overlapping sensors. Another important issue is the specification of the front-end electronics [3,4] which should provide low-noise in order to identify Z=1 particle with a S/N possibly approaching 10 and a dynamic range of about 103 MIP in order to identify nuclei up to Iron and above. In this paper, we report on a large area silicon sensor (64 cm2 of active area with 8 × 8 pads) which was developed on a 6 inch wafer (thickness 500 μm and resistivity >10000 Ω·cm) to provide single-element charge identification of highenergy cosmic rays. The characterization and performance of this sensor as a silicon PIN diode device was presented in [5]. Tests with atmospheric muons demonstrated its excellent performance to identify single charged particles. A dedicated low-noise front-end electronics with large dynamic range [4] was used during the test. The results are reported.
M.Y. Kim et al. / Nuclear Physics B (Proc. Suppl.) 172 (2007) 162–164
2. SILICON SENSOR TEST WITH ATMOSPHERIC MUONS A lab test was performed where the large area silicon sensor was triggered by atmospheric muons using an external coincidence of 3 scintillators. 2.1. Silicon sensor The electrical characteristics of the sensor were measured at 25◦ C. Typically pads are fully depleted at 100 V and they do not exhibit breakdown up to 200 V. For the over-depletion operation, 130 V is biased through 1 MΩ bias resistor on the n-side of the sensor, during the atmospheric muon test. The leakage current and the capacitance of 64 pads at operating voltage are shown in Fig.1.
163
probable input of the front-end electronics (it approximately corresponds to the most probable charge signal generated by 1 MIP transversing the 500 μm thick silicon sensor). This measurement was repeated with an equivalent capacitor connected to each input channel. The result in Fig.2 was used for the channel equalization in the analysis.
Figure 2. Charge injection result of 64 electronics channels(Expected value: 25 ADU counts, Measured value: 25.2 ± 0.2, Relative error < 1 %)
Figure 1. Leakage current and Capacitance of 64 pads at the operating voltage (Measurement accuracy ± 1 %) The variation of the capacitance between pads is not exceeding 5 % from the nominal capacitance of each pad (1 cm2 ), 22 pF. The average leakage current at the operating voltage is 2nA, the shot noise generated by random fluctuations of the current flowing through the pad being negligible. 2.2. Front-end Electronics The sensor is equipped with two VA32HDR14.2 ASICs for the readout of 32 channels each, as described in [4]. To optimize the signalto-noise ratio in the silicon detector, one should take into account the different gains of the electronics channels. For the gain measurement, a charge pulse of 6.4 fC was injected as the most-
2.3. Atmospheric muon test setup During the muon test, three scintillators are arranged as described in Fig.3. The illuminated area on the silicon sensor is limited by the geometry of the trigger counters and is about 2 cm × 5 cm. On average 12 pads have a significant statistics with a typical run duration of 12 hours. Measurements of individual channel pedestals are done only before and after each atmospheric muon run because pedestals were found to be remarkably stable.
Figure 3. Trigger setup geometry
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M.Y. Kim et al. / Nuclear Physics B (Proc. Suppl.) 172 (2007) 162–164
3. ANALYSIS and RESULT 3.1. Analysis The rms value of the pedestals, measured during a stand alone test of the front-end electronics, is typically around 3 ADU counts. It increases to ∼ 5 ADU counts when the silicon sensor is connected to the front-end input. A common-noise correction (CNC) per event is applied. The average pulse height of 20 reference channels from each ADC is used for the correction. After CNC, the individual rms values of the pedestals decrease to the level of the electronics stand alone test. For each trigger, the pixel with the maximum pulse height is identified as the muon signal and plotted in Fig.4b. In the following, ∼ 3400 events are analyzed where on average 12 pads were illuminated due to the trigger geometry.
(a) Pulse height distribution from 12 pads: pedestal fitting
3.2. Result To calculate the S/N ratio of the system, we first fit the rms of the inclusive pedestal distribution from the 12 pads (Fig.4a). Then, the muon signal in (Fig.4b) is fitted with a Landau distribution convoluted with a Gaussian. The most probable value of the energy loss ∼ 24 ADC is ∼ 75 % of the mean loss of a MIP as expected for a 500 μm thick silicon absorber. As a result, the most probable signal-to-noise ratio of the system is ∼ 8. 4. CONCLUSION A large area pixelated silicon sensor, developed for cosmic ray research, is described. Test results show that signals from Z=1 charged particles can be separated from the noise with an excellent S/N ratio. REFERENCES 1. V.I. Zatsepin, et al. Nucl. Instr. and Meth. A 524 (2004) 195. 2. E.S. Seo, et al. Adv. Space Res. 30 (5)(2002) 1263. 3. S.Torii et. al. Nuclear Physics B Proc. Suppl. Vol.150, 390-303, 2006. 4. ”Front-end electronics with large dynamic range for space-borne experiments”, M.G.
(b) Muon signal Figure 4. Atmospheric Muon test result Bagliesi, et al. 10th Topical Seminar on Innovative Particle and Radiation Detectors, Siena, Italy, October 1 - 5, 2006. 5. ”A large area silicon pixel array for the identification of relativistic nuclei in cosmic ray experiments”, P.S. Marrocchesi, et al 10th Pisa Meeting on Advanced Detectors, La Biodola, Isola d’Elba, Italy, May 21 - 27, 2006 (in press).