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A microstrip gas counter for single VUV photons
Nuclear Instruments and Methods m Physics Research A 351 (1994) 585-587
ELSEVIER
Letter to the Editor
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A microstrip gas counter for single VUV photons K. Zeitelhack *, J. Friese, R. Gernhäuser, P. Kienle, H.-J. Körner, P. Maier-Komor, S. Winkler Physzk-Department E12, TU München, Germany
Received 3 June 1994 ; revised form received 5 August 1994 Abstract A microstrip gas counter (MSGC) for the detection of single VUV photons is described. An additional CsI layer evaporated onto the MSGC plate provides high efficiency for the conversion of photons in the wavelength region 160 nm < A < 210 nm into photoelectrons . Using Ar-C2He (95-5) at p = 950 mbar as detector gas, a gain g = 4 x 10 4 was achieved . An overall detection efficiency s = 2.5% for single VUV photons at A = 170 ± 0.2 nm was measured . Since their introduction in 1988 [1] the number of activities in the development of microstrip gas counters (MSGC) has grown rapidly. Many applications in the fields of particle, astro and nuclear physics have been suggested [2]. Due to the high count rate capability as well as the good energy and position resolution, these devices are particularly well suited as particle tracking [3] or X-ray detectors [4]. We report on a further application of microstrip gas counters . At our laboratory we have built and successfully operated a MSGC coated with a solid CsI photon converter for the detection of single VUV photons. The use of CsI layers as efficient photon converters in gaseous detectors has already been successfully demonstrated in Refs . [5,6] and is suggested for many applications of fast photon detectors [7]. Fig. 1 shows a schematic view of the VUV-sensitive MSGC. The anode and cathode strips (aluminium, thickness = 1 .5 wm) are deposited on a 440 pm thick borosilicate glass substrate (DESAG D263 1 ) using conventional photolithography and wet etching technique. Within an active area A = 15 x 30 mm 2 the anode and cathode strips have a width of 10 and 250 Wm, respectively, with an anode pitch of 460 li,m . A grid of gold-plated tungsten wires (transmission T = 98%) mounted 5 mm above the MSGC plate forms the drift electrode, the metallized backplane of the glass substrate provides the back electrode of the MSGC . The whole setup was mounted in a small gas tight box with a CaFZ entrance window and filled with Ar-CZH, (95-5) at p = 950 mbar . As an efficient photon converter a 500 nm thick CsI layer was homogeneously * Corresponding author . 1 Trademark Deutsche Spezialglas AG, D-31074 Griinenplan, Germany.
evaporated onto the MSGC plate under high vacuum conditions (p < 10 -h mbar) at T = 60 °C. Before evaporation the MSGC plate had to be heated to T = 100°C for several hours to remove water vapour and other contaminants. After evaporation the MSGC was immediately mounted in the detector box and flushed with Ar-C Z H6, the total exposure to air being less than 10 min. In connection with the construction of a fast RICH detector [8] we have studied in detail the quantum efficiency QE of evaporated CsI layers and measured the VUV transmission T of the window material and the detector gases (see Fig. 2) . The quantum efficiency of our CsI layers is in good agreement with data from literature [6]. According to Fig. 2 the spectral sensitivity of the MSGC is restricted to the wavelength region 160 nm < A < 210 nm due to the low quantum efficiency of CsI at A > 210 nm and the absorption of C, H t, at A < 160 rim . UV-photon
CaFZ
Srnm
(-5301)
Fig. 1. Schematical view of the microstrip gas counter for VUV photons. The MSGC plate is homogeneously covered by a thin photo sensitive CsI layer. The detector is equipped with a CaFZ entrance window and is operated with Ar-CZH6 (95-5) at atmospheric pressure .
0168-9002/94/$07.00 © 1994 Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00994-5
586
K. Zeitelhack et al. /Nucl. Instr . and Meth. 1n Phys. Res . A 351 (1994) 585-587
If the MSGC is illuminated by VUV light, the photons absorbed in the Csl layer create photoelectrons . In the region of the cathode strips the photoelectrons are extracted by the electric field from the CsI layer into the gas volume and then drift towards the anode strips (see Fig. 1) . Close to the strips the electrons initiate an avalanche multiplication process in the gas and the charge collected by the anode strips is read out with a charge integrating circuit . In our measurements photons of wavelength A = 170 ± 0.2 nm from a deuterium arc lamp were focused (spot size Q) = 4 mm) by a monochromator onto the entrance window of the MSGC. The rate of incident photons Nph = 10 kHz was determined independently by a calibrated photomultiplier tube . At this incident rate the MSGC was operated in single photon counting mode using Ar-C 2 H, (95-5) at p = 950 mbar as detector gas. Cathode strips and back electrode were set to a potential U= -540 V, whereas the drift electrode and the anode strips were set to U = -1 kV and ground potential, respectively . All anode strips were electrically connected together and read out with a low-noise charge integrating preamplifier, shaping amplifier and ADC. The gain of the electronic chain was calibrated using a pulser and a calibrated input capacitor. Fig. 3 shows a measured calibrated pulse height distribution of single photoelectrons created by the incident VUV photons. The distribution fits well to a Polya function [9]: . 9 e -(1 + n)(q/gm_ ) ~ P(q) a ~(1 + O) gmean
with q and gmean the total and the average number of electrons in the avalanche and where O is a parameter. A fit of the distribution yields O= -0 .13 and a gain gmean = 3 .6 X 10 °. This is in good agreement with measurements of the gain performed with the MSGC operated at the same conditions but without Csl layer using an E, -5.8 MeV ot-source and an EX-ray = 5 .9 keV 55 Fe source . The negative value of O indicates an insufficient amount of quenching [10]. 1 0 E-
c
0 .8
0
m 0 .6 é~m 04 r.
02 00
140
160
180
200
wavelength [nm]
220
Fig. 2. Quantum efficiency QE of Csl and VUV transmission T of the CaF2 window and the detector gases as a function of wavelength .
10 2
00
100000
200000
300000
charge q [ e ] Fig 3. Calibrated pulse height distribution of single photoelectrons resulting from illumination of the MSGC by photons of wavelength A= 170±0.2 am . The distribution fits well to a Polya function with O = -0 .13 and a gas amplification gmean = 3 .6 X 10 °. From a comparison of the incident photon flux Nph with the photon flux measured with the MSGC NMSCc an overall detection efficiency Eexp : Eexp
=
cc
NNph
= 2.5%,
for single VUV photons at A = 170 ± 0.2 nm was deduced. However this has to be compared to an expected overall detection efficiency Etheo' = QE cs1(170 nm)T(170
= 9 .6%,
(3) assuming QEcg1(170 nm) = 39%, T(170 nm) = 70% the VUV transmission of entrance window and gas, EA = 54% the fraction of the active area of the module (i .e . the area covered by the cathode strips) and Ee = 65% the single electron detection efficiency . Ee is calculated from Ee = fq P(q) dq with q0 = 1 .4 X 10' electrons the threshold of the electronic readout and the Polyia function P(q) normalized to unity. The large discrepancy between measured and expected overall detection efficiency is not yet understood and is subject to further investigations . First of all we have to study in detail the long term stability of the CsI photon converter in the MSGC . In order to improve the single electron detection efficiency ee we aim to increase the gas gain by using a modified design with a larger gap between anode and cathode strips . The photo sensitive area of the detector could be increased by choosing a configuration with the Csl photon converter evaporated onto the entrance window [11] . However, in this geometry the production of secondary photoelectrons created by photons from the avalanche process is not suppressed . In addition, the quantum efficiency of semitransparent photocathodes is considerably lower than that of reflective photon converters . Encouraged by our results we plan to build a new VUV-sensitive MSGC with an active area A = 5 X 5 cm 2 . A pad electrode mounted on the rear side of the thin substrate will provide a two-dimensional position informaE theo
nm) EA Ee
K. Zertelhack et al./Nucl. Instr. and Meth . in Phys. Res. A 351 (1994) 585-587 tion by single pad readout. There is a series of interesting applications for such a device . In particular in future RICH detectors it might be an advantageous alternative to photosensitive MWPCs.
Acknowledgement This work has been supported by the Bundesministerium für Forschung und Technologie .
[3] [4] [5] [6] [7] [8]
References [1] A. Oed, Nucl . Instr. and Meth . A 263 (1988) 351. [2] P. Geltenbort (ed.), Proc . Int. Workshop on Progress in
[9] [10] [11]
587
Gaseous MicroStrip Proportional Chambers, Grenoble, 21-23 June 1993. RD-28 Collaboration (Spokesman : F. Sauli): RD-28 Status Report, CERN/DRDC/93-94 (1993) . C. Budtz-Jorgensen et al ., Nucl . Instr. and Meth . A 310 (1991) 82 . V. Dangendorf et al ., Nucl . Instr. and Meth . A 289 (1990) 322. J. S6guinot et al ., Nucl. Instr. and Meth. A 297 (1990) 133. RD-26 Collaboration (Spokesmen : G. Paic, F. Piuz): RD-26 Status Report, CERN/DRDC/93-36, Geneva (1993) . HADES-collaboration (Spokesman : W. Kühn): HADES : A proposal for a high acceptance di-electron spectrometer, Darmstadt (1994). H. Gertz et al., Nucl . Instr. and Meth. 112 (1973) 83 . J. Va'vra et al ., Nucl. Instr. and Meth . A 324 (1993) 113. A. Breskin et al ., Nucl . Instr. and Meth . A 345 (1994) 205.
ELSEVIER
Letter to the Editor
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A microstrip gas counter for single VUV photons K. Zeitelhack *, J. Friese, R. Gernhäuser, P. Kienle, H.-J. Körner, P. Maier-Komor, S. Winkler Physzk-Department E12, TU München, Germany
Received 3 June 1994 ; revised form received 5 August 1994 Abstract A microstrip gas counter (MSGC) for the detection of single VUV photons is described. An additional CsI layer evaporated onto the MSGC plate provides high efficiency for the conversion of photons in the wavelength region 160 nm < A < 210 nm into photoelectrons . Using Ar-C2He (95-5) at p = 950 mbar as detector gas, a gain g = 4 x 10 4 was achieved . An overall detection efficiency s = 2.5% for single VUV photons at A = 170 ± 0.2 nm was measured . Since their introduction in 1988 [1] the number of activities in the development of microstrip gas counters (MSGC) has grown rapidly. Many applications in the fields of particle, astro and nuclear physics have been suggested [2]. Due to the high count rate capability as well as the good energy and position resolution, these devices are particularly well suited as particle tracking [3] or X-ray detectors [4]. We report on a further application of microstrip gas counters . At our laboratory we have built and successfully operated a MSGC coated with a solid CsI photon converter for the detection of single VUV photons. The use of CsI layers as efficient photon converters in gaseous detectors has already been successfully demonstrated in Refs . [5,6] and is suggested for many applications of fast photon detectors [7]. Fig. 1 shows a schematic view of the VUV-sensitive MSGC. The anode and cathode strips (aluminium, thickness = 1 .5 wm) are deposited on a 440 pm thick borosilicate glass substrate (DESAG D263 1 ) using conventional photolithography and wet etching technique. Within an active area A = 15 x 30 mm 2 the anode and cathode strips have a width of 10 and 250 Wm, respectively, with an anode pitch of 460 li,m . A grid of gold-plated tungsten wires (transmission T = 98%) mounted 5 mm above the MSGC plate forms the drift electrode, the metallized backplane of the glass substrate provides the back electrode of the MSGC . The whole setup was mounted in a small gas tight box with a CaFZ entrance window and filled with Ar-CZH, (95-5) at p = 950 mbar . As an efficient photon converter a 500 nm thick CsI layer was homogeneously * Corresponding author . 1 Trademark Deutsche Spezialglas AG, D-31074 Griinenplan, Germany.
evaporated onto the MSGC plate under high vacuum conditions (p < 10 -h mbar) at T = 60 °C. Before evaporation the MSGC plate had to be heated to T = 100°C for several hours to remove water vapour and other contaminants. After evaporation the MSGC was immediately mounted in the detector box and flushed with Ar-C Z H6, the total exposure to air being less than 10 min. In connection with the construction of a fast RICH detector [8] we have studied in detail the quantum efficiency QE of evaporated CsI layers and measured the VUV transmission T of the window material and the detector gases (see Fig. 2) . The quantum efficiency of our CsI layers is in good agreement with data from literature [6]. According to Fig. 2 the spectral sensitivity of the MSGC is restricted to the wavelength region 160 nm < A < 210 nm due to the low quantum efficiency of CsI at A > 210 nm and the absorption of C, H t, at A < 160 rim . UV-photon
CaFZ
Srnm
(-5301)
Fig. 1. Schematical view of the microstrip gas counter for VUV photons. The MSGC plate is homogeneously covered by a thin photo sensitive CsI layer. The detector is equipped with a CaFZ entrance window and is operated with Ar-CZH6 (95-5) at atmospheric pressure .
0168-9002/94/$07.00 © 1994 Elsevier Science B.V . All rights reserved SSDI0168-9002(94)00994-5
586
K. Zeitelhack et al. /Nucl. Instr . and Meth. 1n Phys. Res . A 351 (1994) 585-587
If the MSGC is illuminated by VUV light, the photons absorbed in the Csl layer create photoelectrons . In the region of the cathode strips the photoelectrons are extracted by the electric field from the CsI layer into the gas volume and then drift towards the anode strips (see Fig. 1) . Close to the strips the electrons initiate an avalanche multiplication process in the gas and the charge collected by the anode strips is read out with a charge integrating circuit . In our measurements photons of wavelength A = 170 ± 0.2 nm from a deuterium arc lamp were focused (spot size Q) = 4 mm) by a monochromator onto the entrance window of the MSGC. The rate of incident photons Nph = 10 kHz was determined independently by a calibrated photomultiplier tube . At this incident rate the MSGC was operated in single photon counting mode using Ar-C 2 H, (95-5) at p = 950 mbar as detector gas. Cathode strips and back electrode were set to a potential U= -540 V, whereas the drift electrode and the anode strips were set to U = -1 kV and ground potential, respectively . All anode strips were electrically connected together and read out with a low-noise charge integrating preamplifier, shaping amplifier and ADC. The gain of the electronic chain was calibrated using a pulser and a calibrated input capacitor. Fig. 3 shows a measured calibrated pulse height distribution of single photoelectrons created by the incident VUV photons. The distribution fits well to a Polya function [9]: . 9 e -(1 + n)(q/gm_ ) ~ P(q) a ~(1 + O) gmean
with q and gmean the total and the average number of electrons in the avalanche and where O is a parameter. A fit of the distribution yields O= -0 .13 and a gain gmean = 3 .6 X 10 °. This is in good agreement with measurements of the gain performed with the MSGC operated at the same conditions but without Csl layer using an E, -5.8 MeV ot-source and an EX-ray = 5 .9 keV 55 Fe source . The negative value of O indicates an insufficient amount of quenching [10]. 1 0 E-
c
0 .8
0
m 0 .6 é~m 04 r.
02 00
140
160
180
200
wavelength [nm]
220
Fig. 2. Quantum efficiency QE of Csl and VUV transmission T of the CaF2 window and the detector gases as a function of wavelength .
10 2
00
100000
200000
300000
charge q [ e ] Fig 3. Calibrated pulse height distribution of single photoelectrons resulting from illumination of the MSGC by photons of wavelength A= 170±0.2 am . The distribution fits well to a Polya function with O = -0 .13 and a gas amplification gmean = 3 .6 X 10 °. From a comparison of the incident photon flux Nph with the photon flux measured with the MSGC NMSCc an overall detection efficiency Eexp : Eexp
=
cc
NNph
= 2.5%,
for single VUV photons at A = 170 ± 0.2 nm was deduced. However this has to be compared to an expected overall detection efficiency Etheo' = QE cs1(170 nm)T(170
= 9 .6%,
(3) assuming QEcg1(170 nm) = 39%, T(170 nm) = 70% the VUV transmission of entrance window and gas, EA = 54% the fraction of the active area of the module (i .e . the area covered by the cathode strips) and Ee = 65% the single electron detection efficiency . Ee is calculated from Ee = fq P(q) dq with q0 = 1 .4 X 10' electrons the threshold of the electronic readout and the Polyia function P(q) normalized to unity. The large discrepancy between measured and expected overall detection efficiency is not yet understood and is subject to further investigations . First of all we have to study in detail the long term stability of the CsI photon converter in the MSGC . In order to improve the single electron detection efficiency ee we aim to increase the gas gain by using a modified design with a larger gap between anode and cathode strips . The photo sensitive area of the detector could be increased by choosing a configuration with the Csl photon converter evaporated onto the entrance window [11] . However, in this geometry the production of secondary photoelectrons created by photons from the avalanche process is not suppressed . In addition, the quantum efficiency of semitransparent photocathodes is considerably lower than that of reflective photon converters . Encouraged by our results we plan to build a new VUV-sensitive MSGC with an active area A = 5 X 5 cm 2 . A pad electrode mounted on the rear side of the thin substrate will provide a two-dimensional position informaE theo
nm) EA Ee
K. Zertelhack et al./Nucl. Instr. and Meth . in Phys. Res. A 351 (1994) 585-587 tion by single pad readout. There is a series of interesting applications for such a device . In particular in future RICH detectors it might be an advantageous alternative to photosensitive MWPCs.
Acknowledgement This work has been supported by the Bundesministerium für Forschung und Technologie .
[3] [4] [5] [6] [7] [8]
References [1] A. Oed, Nucl . Instr. and Meth . A 263 (1988) 351. [2] P. Geltenbort (ed.), Proc . Int. Workshop on Progress in
[9] [10] [11]
587
Gaseous MicroStrip Proportional Chambers, Grenoble, 21-23 June 1993. RD-28 Collaboration (Spokesman : F. Sauli): RD-28 Status Report, CERN/DRDC/93-94 (1993) . C. Budtz-Jorgensen et al ., Nucl . Instr. and Meth . A 310 (1991) 82 . V. Dangendorf et al ., Nucl . Instr. and Meth . A 289 (1990) 322. J. S6guinot et al ., Nucl. Instr. and Meth. A 297 (1990) 133. RD-26 Collaboration (Spokesmen : G. Paic, F. Piuz): RD-26 Status Report, CERN/DRDC/93-36, Geneva (1993) . HADES-collaboration (Spokesman : W. Kühn): HADES : A proposal for a high acceptance di-electron spectrometer, Darmstadt (1994). H. Gertz et al., Nucl . Instr. and Meth. 112 (1973) 83 . J. Va'vra et al ., Nucl. Instr. and Meth . A 324 (1993) 113. A. Breskin et al ., Nucl . Instr. and Meth . A 345 (1994) 205.