- Email: [email protected]
Photosensitive gaseous detectors for cryogenic temperature applications
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305 www.elsevier.com/locate/nima
Photosensitive gaseous detectors for cryogenic temperature applications L. Perialea, V. Peskova,b, C. Iacobaeusc,, B. Lund-Jensend, P. Picchia, F. Pietropaoloa, I. Rodionove a CERN, Geneva, Switzerland Pole University Leonard de Vinci, Paris, France c Karolinska Institute, Stockhom, Sweden d Royal Institute of Technology, Stockhom, Sweden e Reagent Research Center, Moscow, Russia b
Available online 21 November 2006
Abstract There are several proposals and projects today for building LXe time projection chambers (TPCs) for dark matter search. Important elements of these TPCs are the photomultipliers operating either inside LXe or in vapours above the liquid. We have recently demonstrated that photosensitive gaseous detectors (wire type and hole type) can operate perfectly well, until temperatures of LN2. In this paper, results of systematic studies of operation of the photosensitive version of these detectors (combined with reflective or semi-transparent CsI photocathodes) in the temperature interval of 300–150 K are presented. In particular, it was demonstrated that both sealed and flushed by a gas detectors could operate at a quite stable fashion in a year/time scale. Obtained results strongly indicate that they can be cheap and simple alternatives to photomultipliers or avalanche solid-state detectors in LXe TPC applications. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40 Keywords: TPC; WIMP; CsI photocathode; Noble liquids; Capillary plates
1. Introduction The origin of dark matter is one of the fundamental problems of modern physics. There are theoretical predictions that dark matter consists of weakly interacting massive particles (WIMPs). Several WIMP detector concepts were developed and tested, for example Refs. [1–9]. One of the most promising detectors could be the one, which is based on a LXe TPC, which has potential for a unique rejection power [7–9]. An important element of this TPC is the large area of the photmultipliers (PMs) operating inside the liquid or placed in vapours above the liquid. The use of these detectors considerably increases the overall cost of the LXe TPCs and may also bring some additional radioactive background. There have been several efforts to replace PMs Corresponding author. Tel.: +468 729 7206; fax: +468 729 7108.
E-mail address: [email protected] (C. Iacobaeus). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.264
by avalanche solid-state detectors [10]; however, the cost of these new devices is still high. In our recent pilot study works, we have experimentally demonstrated that gaseous detectors, single wire counters or hole type detectors, could operate until LN2 [11,12]. The aim of this work is to perform systematic studies of gaseous detectors combined with reflective or semitransparent CsI photocathodes in the temperature interval of 300–165 K, in order to check if their characteristics match the requirements for the LXe TPC applications. A special focus in these studies was put on long-term stability, which is an essential point in practical applications. The other important issue to address was the operation of the sealed detectors and the operation of the semitransparent CsI photocathodes, which may also improve the light collection from LXe compared to the reflective one. This paper gives a brief description of the main results. More comprehensive responses can be found in Ref. [13].
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
2. Experimental set up
MgF2 window
a
Anode wire
Reflective CsI photocathode MgF2 window
b
Removable mesh Hole-type detector
Amplifier Fig. 2. Schematic drawing of the detectors used (a) single-wire counter and (b) hole-type detector installed in the gas chamber with the MgF2 window.
1.00E+07 1.00E+06 Am, Af,V, QE
Our experimental set up is shown schematically in Fig. 1. It contains a cryostat (see Ref. [12] for more details) inside which a ‘‘scintillation chamber’’ was installed, a gaseous detector and a PM or a UV source. The ‘‘scintillation chamber’’ was a cylindrical vessel with two MgF2 windows on opposite flanges to each other. The chamber was filled either with an Ar or a Xe gas at a pressure of 1 atm. Inside the ‘‘scintillation chamber,’’ an a source of 241Am was installed. The ‘‘scintillation chamber’’ could be independently cooled and this allowed, if necessary, LXe to be obtained inside the scintillation chamber [13]. To one of the MgF2 windows, a gaseous detector was attached and to the opposite MgF2 window, a stainless steel tube was connected and flushed, depending on measurements either with Ar or CH4 at p ¼ 1 atm. To the opposite end of this tube, a PM (Schlumberger 541F09-17) or a pulsed H2 lamp was mounted [13]. Two types of photosensitive gaseous detectors were tested and studied: wire-type and hole-type detectors. The wire-type detectors were either of a single-wire counter (SWC) flushed with Ar+CH4 (‘‘P10’’ gas) at a pressure of p ¼ 1 atm (see Fig. 2a) or a sealed SWC filled with the same gas at a pressure of p ¼ 1 atm. These detectors were combined with reflective or semitransparent CsI photocathodes (see Ref. [13] for more details). The following hole-type detectors were tested: the gas electron multiplier (GEM), capillary plates (CPs) and home-made CPs (HMCPs). The description of the HMCPs, GEM and CPs used in our experiment is given in [13–15], respectively. The cathode of the hole-type detectors facing the scintilla-
303
1.00E+05 1.00E+04 1.00E+03 1.00E+02 1.00E+01 1.00E+00 150
200 250 Temperature (T)
300
Fig. 3. Changes of SWC characteristics with the temperature: triangles— Am, open squares—Af, rhombus—Vc, filled dots-QE of the reflective CsI photocathodes, transparent dots-QE of the semi-transparent CsI photocathode.
PM or H2 lamp Flushed tube
tion chamber were coated by a 0.4-mm thick CsI layer; their anodes were mechanically and electrically connected to the readout plate, see Fig. 2b. As in the case of the SWC, these detectors were either flushed by the gas (P10 or Xe) or filled by a gas and sealed. Some tests were also done with semitransparent CsI photocathodes [13]. The quantum efficiency (QE) measurements procedure is described in Ref. [13]. MgF2 windows
Dewar
Cryostat
Gaseous detectors
Fig. 1. Schematic drawing of the experimental set up.
3. Results Fig. 3 shows the gains (Am and Af) and the QE variations with the temperature (T) for the SWC with reflective CsI photocathodes flushed by P10 at p ¼ 1 atm. The gain Am was defined as the maximum achievable gain at which a corona discharge appeared; the gain Af is the gain at which photon feedback pulses appear, 10% compared to the main pulse (see Ref. [16] for more details). The ‘‘characteristic’’ voltage Vc on the plot is the voltage at which a gain of 104 was achieved. From the data presented in the
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
figure, one can see that Vc, Am and Af increased with a decrease of the T, whereas the QE dropped with the decrease of T. These changes were the result of the gas density that increased with the cooling (see discussion in Ref. [13]); indeed in the case of the sealed SWC the values of Vc, Am, Af, and the QE did not change with the temperature. The important conclusion one can draw from such data is that SWCs combined either with reflective or with semitransparent CsI photocathodes are able to operate stably at LXe temperatures at gains high enough to detect single photoelectrons. These detectors were used by us [13] and by the Berkeley group [17] to detect the scintillation light from the LXe. Detailed description of the results obtained with CPs, GEMs and HMCPs can be found in Ref. [13]. In this paper, only the summary of the main results is presented. Fig. 4 shows the maximum achievable gains Am for all tested hole-type detectors. For example, the maximum achievable gain of the HMCP operating in Xe was 104, however after many cooling cycles some noise pulses appeared at low temperatures and gains41000. Note that in these measurements Am were not exactly the gain at which the breakdown appeared, but the gain at which first signs of unstable behaviour were recoded. On the same plot, the QE of the CP and the HMCP are presented as well. The main conclusions one can make from the data presented in this paper and in Ref. [13] are the following: (1) Bare hole-type detectors operate at gains less than the SWC combined with CsI photocathode, (2) in P10 gas mixtures, CPs are able to operate at higher gains than the GEMs, (3) HMCPs can operate stably and without noise pulses in pure Xe at gains of 300–1000, (4) the maximum achievable gain of the hole-type detectors combined with CsI photocathdes is almost 10 times less than in the case of bare detectors. Thus hole-type detectors could, in principle, be used for the detection of the LXe scintillation light, but several detectors operating in tandem are necessary to achieve 10000
Am, QE
1000 100 10 1 150
170
190
210 230 250 Temperature (K)
270
290
310
Fig. 4. Comparison of the main characteristics of all hole-type detectors combined with reflective CsI photocathodes: rhombus—Am for the CP operating in P10 at 1 atm, open squares—Am for the GEM operating in P10 at 1 atm, triangles—Am for the HMCP operating in Xe, open circles— QE of the CP in P10, filled circles—QE of the HMCP in Xe.
30 25 20 QE (%)
304
15 10 5 0 0
100
200
300
400
500
600
Time (days)
Fig. 5. Results of long-term stability tests filled rhombus—QE of the sealed SWC with reflective CsI photocathodes flushed with P10, open rhombus—QE of the flushed SWC cooled to LXe temperatures, filled squares—QE of the sealed SWC at room temperature, open squares—QE of the same detector cooled to LXe temperatures, open circles—QE of the SWC combined with semi-transparent CsI photocathodes and flushed with P10, dots—QE of the CP covered by CsI photocathodes, triangles— QE of the HMCP with reflective CsI photocathodes in Xe.
gains of A4104 sufficient for the single photoelectron detection. Fig. 5 shows some results of the long-term stability tests. One can see that in the case of the flushed SWC with the reflective CsI photocathode, its QE dropped from 25% to 20% during 520 days of continuous operation. In the case of the semitransparent ones, the QE dropped rather quickly during the first few days after which it degraded rather slowly. A very good stability was achieved with a sealed detector. The hole-type detectors were tested for 150–180 days only; during this period, fast degradation was observed in the beginning but was then considerably slowed down [13]. 4. Conclusions Several new results were obtained in this work: (1) For the first time, it was demonstrated that sealed SWC and hole-type detectors combined with semitransparent CsI photocathodes can operate at LXe temperatures. (2) SWC combined with semitransparent CsI photocathodes can reach gains sufficient enough to detect single photoelectrons. (3) HMCPs with reflective CsI photocathodes can operate in pure Xe and thus in principle, could be used in vapours above the liquid Xe. However, special tests are required to verify that the cascaded detector will be able to operate in turbulent vapours above LXe. (4) For the first time, long-term tests (of up to 1.5 years) for photosensitive detectors (sealed and flushed by a gas) were performed. The obtained results indicate that photosensitive gaseous detectors (with windows and without) could be cheap and
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
simple alternatives to PMs or avalanche solid-state detectors in LXe TPCs. The other potential advantage could be the possibility of manufacturing them from materials having low levels of radioactivity. References [1] H.V. Klapdor-Kleingrothaus, et al., in: Proceeding of the Third International Workshop on the Identification of Dark Matter, World Scientific, Singapore, 2000, p. 415. [2] L. Baudis, et al., Phys. Rep. 307 (1998) 301. [3] R. Bernabei, et al., Phys. Lett. B. 480 (2000) 23. [4] R. Bernabei, et al., Nucl. Instr. and Meth. B 436 (1998) 379. [5] A. Benoit, et al., Phys. Lett B. 513 (2001) 15.
305
[6] M. Bravin, et al., Astropart. Phys. 12 (1999) 104. [7] D. Cline, et al., Astropart. Phys. 12 (2000) 373. [8] T. Summer, et al., in: Proceedings of the Sd Internatonal Workshop on the Identifcation of Dark Matter, World Scientific, Singapore, 2000, p. 452. [9] A. Aprile, et al., XENON: A Liquid Xenon Experiment for Dark Matter, proposal to the NSF # 0201740 September 29, 2001. [10] E. Aprile, et al., Phys/0501002 and 0502071, 005. [11] L. Periale, et al., Nucl. Instr. Meth. A 535 (2004) 517. [12] L. Periale, et al., IEEE Trans. Nucl. Sci. NS-52 (2005) 927. [13] L. Periale, et al., Preprint Physics/0509077, 2005. [14] V. Peskov, et al., IEEE Trans. Nucl. Sci. NS-48 (2001) 1070. [15] J. Ostling, et al., IEEE Trans. Nucl. Sci. NS-50 (2003) 809. [16] G. Charpak, et al., Nucl. Instr. and Meth. A 307 (1991) 63. [17] J.G. Kim, et al., Nucl. Instr. and Meth. A 534 (2004) 376.
Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305 www.elsevier.com/locate/nima
Photosensitive gaseous detectors for cryogenic temperature applications L. Perialea, V. Peskova,b, C. Iacobaeusc,, B. Lund-Jensend, P. Picchia, F. Pietropaoloa, I. Rodionove a CERN, Geneva, Switzerland Pole University Leonard de Vinci, Paris, France c Karolinska Institute, Stockhom, Sweden d Royal Institute of Technology, Stockhom, Sweden e Reagent Research Center, Moscow, Russia b
Available online 21 November 2006
Abstract There are several proposals and projects today for building LXe time projection chambers (TPCs) for dark matter search. Important elements of these TPCs are the photomultipliers operating either inside LXe or in vapours above the liquid. We have recently demonstrated that photosensitive gaseous detectors (wire type and hole type) can operate perfectly well, until temperatures of LN2. In this paper, results of systematic studies of operation of the photosensitive version of these detectors (combined with reflective or semi-transparent CsI photocathodes) in the temperature interval of 300–150 K are presented. In particular, it was demonstrated that both sealed and flushed by a gas detectors could operate at a quite stable fashion in a year/time scale. Obtained results strongly indicate that they can be cheap and simple alternatives to photomultipliers or avalanche solid-state detectors in LXe TPC applications. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40 Keywords: TPC; WIMP; CsI photocathode; Noble liquids; Capillary plates
1. Introduction The origin of dark matter is one of the fundamental problems of modern physics. There are theoretical predictions that dark matter consists of weakly interacting massive particles (WIMPs). Several WIMP detector concepts were developed and tested, for example Refs. [1–9]. One of the most promising detectors could be the one, which is based on a LXe TPC, which has potential for a unique rejection power [7–9]. An important element of this TPC is the large area of the photmultipliers (PMs) operating inside the liquid or placed in vapours above the liquid. The use of these detectors considerably increases the overall cost of the LXe TPCs and may also bring some additional radioactive background. There have been several efforts to replace PMs Corresponding author. Tel.: +468 729 7206; fax: +468 729 7108.
E-mail address: [email protected] (C. Iacobaeus). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.10.264
by avalanche solid-state detectors [10]; however, the cost of these new devices is still high. In our recent pilot study works, we have experimentally demonstrated that gaseous detectors, single wire counters or hole type detectors, could operate until LN2 [11,12]. The aim of this work is to perform systematic studies of gaseous detectors combined with reflective or semitransparent CsI photocathodes in the temperature interval of 300–165 K, in order to check if their characteristics match the requirements for the LXe TPC applications. A special focus in these studies was put on long-term stability, which is an essential point in practical applications. The other important issue to address was the operation of the sealed detectors and the operation of the semitransparent CsI photocathodes, which may also improve the light collection from LXe compared to the reflective one. This paper gives a brief description of the main results. More comprehensive responses can be found in Ref. [13].
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
2. Experimental set up
MgF2 window
a
Anode wire
Reflective CsI photocathode MgF2 window
b
Removable mesh Hole-type detector
Amplifier Fig. 2. Schematic drawing of the detectors used (a) single-wire counter and (b) hole-type detector installed in the gas chamber with the MgF2 window.
1.00E+07 1.00E+06 Am, Af,V, QE
Our experimental set up is shown schematically in Fig. 1. It contains a cryostat (see Ref. [12] for more details) inside which a ‘‘scintillation chamber’’ was installed, a gaseous detector and a PM or a UV source. The ‘‘scintillation chamber’’ was a cylindrical vessel with two MgF2 windows on opposite flanges to each other. The chamber was filled either with an Ar or a Xe gas at a pressure of 1 atm. Inside the ‘‘scintillation chamber,’’ an a source of 241Am was installed. The ‘‘scintillation chamber’’ could be independently cooled and this allowed, if necessary, LXe to be obtained inside the scintillation chamber [13]. To one of the MgF2 windows, a gaseous detector was attached and to the opposite MgF2 window, a stainless steel tube was connected and flushed, depending on measurements either with Ar or CH4 at p ¼ 1 atm. To the opposite end of this tube, a PM (Schlumberger 541F09-17) or a pulsed H2 lamp was mounted [13]. Two types of photosensitive gaseous detectors were tested and studied: wire-type and hole-type detectors. The wire-type detectors were either of a single-wire counter (SWC) flushed with Ar+CH4 (‘‘P10’’ gas) at a pressure of p ¼ 1 atm (see Fig. 2a) or a sealed SWC filled with the same gas at a pressure of p ¼ 1 atm. These detectors were combined with reflective or semitransparent CsI photocathodes (see Ref. [13] for more details). The following hole-type detectors were tested: the gas electron multiplier (GEM), capillary plates (CPs) and home-made CPs (HMCPs). The description of the HMCPs, GEM and CPs used in our experiment is given in [13–15], respectively. The cathode of the hole-type detectors facing the scintilla-
303
1.00E+05 1.00E+04 1.00E+03 1.00E+02 1.00E+01 1.00E+00 150
200 250 Temperature (T)
300
Fig. 3. Changes of SWC characteristics with the temperature: triangles— Am, open squares—Af, rhombus—Vc, filled dots-QE of the reflective CsI photocathodes, transparent dots-QE of the semi-transparent CsI photocathode.
PM or H2 lamp Flushed tube
tion chamber were coated by a 0.4-mm thick CsI layer; their anodes were mechanically and electrically connected to the readout plate, see Fig. 2b. As in the case of the SWC, these detectors were either flushed by the gas (P10 or Xe) or filled by a gas and sealed. Some tests were also done with semitransparent CsI photocathodes [13]. The quantum efficiency (QE) measurements procedure is described in Ref. [13]. MgF2 windows
Dewar
Cryostat
Gaseous detectors
Fig. 1. Schematic drawing of the experimental set up.
3. Results Fig. 3 shows the gains (Am and Af) and the QE variations with the temperature (T) for the SWC with reflective CsI photocathodes flushed by P10 at p ¼ 1 atm. The gain Am was defined as the maximum achievable gain at which a corona discharge appeared; the gain Af is the gain at which photon feedback pulses appear, 10% compared to the main pulse (see Ref. [16] for more details). The ‘‘characteristic’’ voltage Vc on the plot is the voltage at which a gain of 104 was achieved. From the data presented in the
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
figure, one can see that Vc, Am and Af increased with a decrease of the T, whereas the QE dropped with the decrease of T. These changes were the result of the gas density that increased with the cooling (see discussion in Ref. [13]); indeed in the case of the sealed SWC the values of Vc, Am, Af, and the QE did not change with the temperature. The important conclusion one can draw from such data is that SWCs combined either with reflective or with semitransparent CsI photocathodes are able to operate stably at LXe temperatures at gains high enough to detect single photoelectrons. These detectors were used by us [13] and by the Berkeley group [17] to detect the scintillation light from the LXe. Detailed description of the results obtained with CPs, GEMs and HMCPs can be found in Ref. [13]. In this paper, only the summary of the main results is presented. Fig. 4 shows the maximum achievable gains Am for all tested hole-type detectors. For example, the maximum achievable gain of the HMCP operating in Xe was 104, however after many cooling cycles some noise pulses appeared at low temperatures and gains41000. Note that in these measurements Am were not exactly the gain at which the breakdown appeared, but the gain at which first signs of unstable behaviour were recoded. On the same plot, the QE of the CP and the HMCP are presented as well. The main conclusions one can make from the data presented in this paper and in Ref. [13] are the following: (1) Bare hole-type detectors operate at gains less than the SWC combined with CsI photocathode, (2) in P10 gas mixtures, CPs are able to operate at higher gains than the GEMs, (3) HMCPs can operate stably and without noise pulses in pure Xe at gains of 300–1000, (4) the maximum achievable gain of the hole-type detectors combined with CsI photocathdes is almost 10 times less than in the case of bare detectors. Thus hole-type detectors could, in principle, be used for the detection of the LXe scintillation light, but several detectors operating in tandem are necessary to achieve 10000
Am, QE
1000 100 10 1 150
170
190
210 230 250 Temperature (K)
270
290
310
Fig. 4. Comparison of the main characteristics of all hole-type detectors combined with reflective CsI photocathodes: rhombus—Am for the CP operating in P10 at 1 atm, open squares—Am for the GEM operating in P10 at 1 atm, triangles—Am for the HMCP operating in Xe, open circles— QE of the CP in P10, filled circles—QE of the HMCP in Xe.
30 25 20 QE (%)
304
15 10 5 0 0
100
200
300
400
500
600
Time (days)
Fig. 5. Results of long-term stability tests filled rhombus—QE of the sealed SWC with reflective CsI photocathodes flushed with P10, open rhombus—QE of the flushed SWC cooled to LXe temperatures, filled squares—QE of the sealed SWC at room temperature, open squares—QE of the same detector cooled to LXe temperatures, open circles—QE of the SWC combined with semi-transparent CsI photocathodes and flushed with P10, dots—QE of the CP covered by CsI photocathodes, triangles— QE of the HMCP with reflective CsI photocathodes in Xe.
gains of A4104 sufficient for the single photoelectron detection. Fig. 5 shows some results of the long-term stability tests. One can see that in the case of the flushed SWC with the reflective CsI photocathode, its QE dropped from 25% to 20% during 520 days of continuous operation. In the case of the semitransparent ones, the QE dropped rather quickly during the first few days after which it degraded rather slowly. A very good stability was achieved with a sealed detector. The hole-type detectors were tested for 150–180 days only; during this period, fast degradation was observed in the beginning but was then considerably slowed down [13]. 4. Conclusions Several new results were obtained in this work: (1) For the first time, it was demonstrated that sealed SWC and hole-type detectors combined with semitransparent CsI photocathodes can operate at LXe temperatures. (2) SWC combined with semitransparent CsI photocathodes can reach gains sufficient enough to detect single photoelectrons. (3) HMCPs with reflective CsI photocathodes can operate in pure Xe and thus in principle, could be used in vapours above the liquid Xe. However, special tests are required to verify that the cascaded detector will be able to operate in turbulent vapours above LXe. (4) For the first time, long-term tests (of up to 1.5 years) for photosensitive detectors (sealed and flushed by a gas) were performed. The obtained results indicate that photosensitive gaseous detectors (with windows and without) could be cheap and
ARTICLE IN PRESS L. Periale et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 302–305
simple alternatives to PMs or avalanche solid-state detectors in LXe TPCs. The other potential advantage could be the possibility of manufacturing them from materials having low levels of radioactivity. References [1] H.V. Klapdor-Kleingrothaus, et al., in: Proceeding of the Third International Workshop on the Identification of Dark Matter, World Scientific, Singapore, 2000, p. 415. [2] L. Baudis, et al., Phys. Rep. 307 (1998) 301. [3] R. Bernabei, et al., Phys. Lett. B. 480 (2000) 23. [4] R. Bernabei, et al., Nucl. Instr. and Meth. B 436 (1998) 379. [5] A. Benoit, et al., Phys. Lett B. 513 (2001) 15.
305
[6] M. Bravin, et al., Astropart. Phys. 12 (1999) 104. [7] D. Cline, et al., Astropart. Phys. 12 (2000) 373. [8] T. Summer, et al., in: Proceedings of the Sd Internatonal Workshop on the Identifcation of Dark Matter, World Scientific, Singapore, 2000, p. 452. [9] A. Aprile, et al., XENON: A Liquid Xenon Experiment for Dark Matter, proposal to the NSF # 0201740 September 29, 2001. [10] E. Aprile, et al., Phys/0501002 and 0502071, 005. [11] L. Periale, et al., Nucl. Instr. Meth. A 535 (2004) 517. [12] L. Periale, et al., IEEE Trans. Nucl. Sci. NS-52 (2005) 927. [13] L. Periale, et al., Preprint Physics/0509077, 2005. [14] V. Peskov, et al., IEEE Trans. Nucl. Sci. NS-48 (2001) 1070. [15] J. Ostling, et al., IEEE Trans. Nucl. Sci. NS-50 (2003) 809. [16] G. Charpak, et al., Nucl. Instr. and Meth. A 307 (1991) 63. [17] J.G. Kim, et al., Nucl. Instr. and Meth. A 534 (2004) 376.