IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 4, AUGUST 2002 1629

Dependence of the Performance of CsI-CoveredMicrostrip Plate VUV Photosensors on Geometry:

Experimental ResultsD. S. A. P. Freitas, J. F. C. A. Veloso, J. M. F. dos Santos, and C. A. N. Conde

Abstract—The behavior of a xenon-filled microstrip gaschamber instrumented with a microstrip plate with eight differentmicrostrip geometries is reported. The best energy resolutionachieved for 5.9-keV and for 22.1-keV X-rays was 13.6% and7.0%, respectively. The same microstrip plate covered with aCsI film was used in place of the photomultiplier tube to detectthe vacuum ultraviolet scintillation light produced in a driftlessxenon-filled gas proportional scintillation counter. The relativeamplitude, energy resolution, optical positive feedback, andphotoelectron extraction efficiency are discussed. The energyresolution obtained for 5.9-keV X-rays was 11.4%, which is betterthan that achieved with a standard proportional counter, about13% for argon based proportional counters.

Index Terms—Gaseous detector, microstrip plate, photosensor,X-ray.

I. INTRODUCTION

T HE gas proportional scintillation counter (GPSC) is usedfor X-ray spectrometry up to 100 keV [1]–[4]. This type

of detector is filled with a noble gas. The primary electronsproduced following the X-ray interaction with the noble gasatoms drift toward a strong electric field region, where they gainsufficient energy from the field to excite but not ionize the fillinggas. Subsequently, in the decaying processes, secondary vacuumultraviolet (VUV) light is emitted. Each primary electron givesrise to a large number of VUV photons. The light amplificationprocess characteristic of GPSCs, leads to a large-amplitudepulse with low statistical fluctuations. The traditional GPSCuses a photomultiplier tube (PMT) as the photosensor. However,these devices are expensive, fragile, power consuming, bulky,and inoperative in the presence of magnetic fields.

The microstrip gas chamber (MSGCs) were introduced byOed [5] in 1988 and are used in many important fields, likeX-ray astronomy, high-energy physics, and medicine. TheMSGC is also used as VUV photosensors [6]–[8].

A microstrip plate consists of alternate parallel electrodes(anodes and cathodes), with a small distance between them (fewtens of microns), photolithographed on an insulator substrate.Due to the small distance between the anodes and cathodes, it is

Manuscript received November 25, 2001; revised March 20, 2002. This workwas supported by Project CERN/P/FIS/15201/1999. The work of D. S. A. P.Freitas was supported by Grant SFRH/BD/3244/2000 from Fundação para aCiência e a Tecnologia (FCT). The work of J. F. C. A Veloso was supported byFCT. The travel of C. A. N. Conde was supported by Gulbenkian Foundation,Lisbon, Portugal.

The authors are with the Departamento de Física, Universidade de Coimbra,P-3004-516 Coimbra, Portugal (e-mail: jveloso@gian.fis.uc.pt).

Digital Object Identifier 10.1109/TNS.2002.801698

Fig. 1. Schematic of the E-field lines present above the MSP.

possible to achieve high electric fields strong enough to produceelectron multiplication in the gaseous medium. The positiveions are rapidly removed, which implies a reduced space-chargeeffect together with the possibility of having high counting rateswith gains up to 10 to 10 .

In [7], a CsI-covered microstrip plate (MSP) replaces thePMT as the VUV photosensor in a GPSC. Therefore, the GPSCbecomes less expensive, more rugged, more compact, and ca-pable of operating in presence of a magnetic field [8].

However, there are some factors that limit the performance ofthe CsI-covered MSP as photosensor, one of them being the op-tical positive feedback due to the VUV light production duringthe avalanche process. This effect does not allow high gains.While in a PMT the total area is photosensitive, in the MSP onlythe CsI area over the microstrip cathodes is photosensitive dueto the need of having a pointing-down electric field with a min-imum threshold value to extract the electrons from the CsI film(Fig. 1). When the cathodes and/or the distances between theelectrodes are very large, the electric field on the surface of theCsI above the cathode’s middle region is weak, causing the pho-toelectron efficiency to decrease.

MSPs can be produced in almost any geometry. There aresome comparative studies [9]–[11] for MSGCs operating asproportional counters but there is only one simulation work[12] that compares several microstrip geometries operating asa photosensor.

0018-9499/02$17.00 © 2002 IEEE

1630 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 4, AUGUST 2002

Fig. 2. The microstrip plate with the eight geometries studied.

In the present work, a MSP with eight different microstripgeometries is tested in a xenon gas chamber. The MSP is usedwith and without a CsI film. When the MSP is not covered withCsI, the detector operates as a proportional counter. When theMSP is covered with CsI film, the detector operates as a GPSC.

II. DESCRIPTION

The MSP has an area of 7676 mm , the electrodes and thebackplane (unstructured) are made of chromium 0.2m thickphotolithographed onto a Desag D263 glass 500-m-thick sub-strate. Fig. 2 shows the MSP schematically. The MSP accommo-dates eight cells with 11.3 23.6 mm area each. Each cell hasa different microstrip geometry as represented in Table I. TheMSP was produced byIMT Masken und Teilungen AG—8606Grefensee, Switzerland.

The CsI film is 500 nm thick and has been vacuum evaporatedonto the MSP. To reduce water contamination, the MSP washeated at 100C prior to evaporation and the CsI-covered MSPwas heated at about 60C under vacuum for several hours, afterdetector assembly.

The detector is represented schematically in Fig. 3. Thedetector body is built of stainless steel while the window andthe electrical contacts are electrically isolated with Macorpieces. The window is a 25-m-thick Mylar film, aluminizedon both sides. The distance between the window and the MSPis 11.5 mm. The detector is filled with pure xenon at 800 Torr(106.7 kPa).

In the first part of the work we used the MSP without CsI, op-erating as a proportional counter. When an X-ray enters the de-tector it deposits its energy ionizing the xenon gas and producinga number of primary electrons. Under the effect of a weak re-duced electric field ( ) ( 1 V cm Torr ) these electronsdrift toward the MSP. When the electrons are in the vicinity ofthe MSP the electric field is strong enough to start the avalancheprocess in the xenon gas (Fig. 1). The charge is collected in theanode.

TABLE IDIMENSIONS OF THEMICROSTRIPPLATES USED IN THEPRESENTSTUDY

Fig. 3. The test detector chamber for the eight microstrip plate geometries.

In a second stage of this study, we have evaporated CsI onthe MSP and used it as the photosensor. In this experiment, pri-mary electrons drift under a strong reduced electric field (6 V cm Torr ) toward the MSP. This field is below thexenon ionization threshold, so there is no charge multiplication,but it is strong enough for xenon excitation. In the de-excita-tion processes, secondary scintillation light ( nm) isemitted. When VUV scintillation light reaches the CsI-coveredMSP photosensitive area (cathodes), photoelectrons are ejected.These electrons will experience charge multiplication on theirpath to the anode. It was demonstrated that this hybrid systemfunctions as GPSC rather than an MSGC [7].

The anode pulses were pre-amplified with a CANBERRA2006 unit (1.5 V/pC sensitivity), linear amplified and pulse-height analyzed. The main amplifier shaping-time constant is4 s. The spectrum peak was fitted to a Gaussian function su-perimposed in a linear background to obtain the peak positionand the energy resolution.

III. EXPERIMENTAL RESULTS AND ANALYSIS

A. MSP Without CsI

The voltage applied to the window is500 V, the cathodevoltage is 0 V and the anode voltage is variable. In this partof the study pulse-height distributions were taken for two dif-ferent X-ray radiactive sources—Cd (22.1 keV) and Fe(5.9 keV). A chromium filter was used to absorb the MnX-rays (6.5 keV).

FREITASet al.: PERFORMANCE OF CsI-COVERED MICROSTRIP PLATE VUV PHOTOSENSORS ON GEOMETRY 1631

Fig. 4. Detector relative amplitude as a function of anode voltageV for22.1-keV X-rays for the different MSP geometries and for the MSPs workingas a proportional counter.

1) Relative Gain: Figs. 4 and 5 present the relativepulse-height amplitude as a function of for the 22.1-keVand 5.9-keV X-rays, respectively.

In Figs. 4 and 5 each line is labeled with a pair of num-bers—the first represents the gap between the anode and thecathode (in m) and the second represents the cathode width(in m); the anode width is 10m for all geometries.

For this study was maintained below the gas electricalbreakdown. The discharges would destroy the MSP that wasneeded for the second part of the work.

As expected, the relative pulse amplitude has an exponentialbehavior with increasing . Additionally, the data are groupedas a function of the gaps—it is noticeable that the microstripgeometries with 10-m and 25- m gaps are separated from theother geometries. As described in [11], the gain is larger at agiven , for the microstrip geometries that have smaller gap:the smaller the gap between the electrodes, the stronger the elec-tric field in the avalanche region. Note that for microstrip ge-ometries with the same gaps, those with larger cathodes havelarger gains.

2) Energy Resolution:Figs. 6 and 7 present the energy reso-lution as a function of for the 22.1-keV and 5.9-keV X-rays,respectively.

It is noticeable that the best energy resolution achieved foreach microstrip-geometry is not very different from each otherpresenting variations within 0.5% and 1% for 22.1 and 5.9 keV,respectively, with the exception of MS5 that presents the highestvalues. A compilation of best energy resolutions is presented inTable II.

B. MSP With CsI

The CsI-coated MSP is used as the photosensor of thedriftless GPSC. The voltage applied to the radiation window is

6000 V, the cathode voltage is 0 V, and the anode voltageis variable. In this study, pulse-height distributions were taken

Fig. 5. Detector relative amplitude as a function of anode voltageV for5.9-keV X-rays for the different MSP geometries and for the MSPs workingas a proportional counter.

Fig. 6. Detector energy resolution as a function of anode voltageV for22.1-keV X-rays for the different MSP geometries and for the MSPs workingas a proportional counter.

for a Fe (5.9 keV) X-ray source. A chromium filter was usedto absorb the Mn X-rays (6.5 keV).

Fig. 8 presents a typical pulse height distribution for 5.9-keVX-rays obtained with MS6 for V. The departure fromthe Gaussian shape in the low-energy region of the X-ray peakis due to the different penetration of the 5.9-keV X-rays withinthe detector. Since the light yield is constant along the electronpaths, the observed light amplitude depends on the distance cov-ered by the primary electron cloud in the detector and, hence, onthe depth of the interaction of the incoming X-ray photon.

1) Relative Gains:Fig. 9 presents the relative amplitudeas a function of for 5.9-keV X-rays and for the different

1632 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 49, NO. 4, AUGUST 2002

Fig. 7. Detector energy resolution as a function of anode voltageV for5.9-keV X-rays for the different MSP geometries and for the MSPs workingas a proportional counter.

TABLE IIBEST ENERGY RESOLUTION FOREACH MICROSTRIPGEOMETRY WORKING

IN THE PROPORTIONALCOUNTER MODE

MSPs, keeping the same voltage difference between the de-tector window and the cathode strips. Under these conditionsthe amount of light produced in the detector is approximateconstant.

The optical positive feedback is noticeable in Fig. 9. For lowgains, the amplitude increase is exponential but for largerthegain increases faster than exponential due to the optical positivefeedback.

There is some similarity between Figs. 5 and 9, but thereare also some differences. As shown, the data are grouped as afunction of the gap. The main differences are those for 55 : 320,55 : 160, and 25 : 140 microstrip line positions. The gain forthese curves is lower than expected, being the reason for thisthe low value of the electric field at the cathode’s surface due tothe large cathode widths, as predicted by numerical simulations[12]. We must point out that detector pulse amplitudes resultfrom primary electrons that come from the window regionfor the microstrip without CsI-coating case, while for themicrostrip with CsI case electron pulse amplitudes result from

Fig. 8. Typical pulse-height distribution for 5.9-keV X-ray for the MS6 withan anode voltage of 240 V. The solid line represents a Gaussian fit to the data inthe 600–800 channel region.

Fig. 9. Detector relative amplitude as a function of anode voltageV for5.9-keV X-rays for the different CsI-coverd MSPs working as the photosensorof a driftless GPSC.

both primary electrons that come from the window region andphotoelectrons (a factor of about eight larger in amplitude[13]) that come from the microstrip cathodes. When the fieldin this region is low the ejected electrons from the CsI-filmare backscattered. The 55 : 320 and 25 : 140 curves cross overneighbor lines when the optical feedback occurs demonstratingthat the optical positive feedback factor is more importantfor the larger cathode geometries, as predicted by numericalsimulations [12]. On the other hand, from the experimentalresults it is not clear the dependence of the optical positivefeedback on the anode-to-cathode gap, which was predictedby the simulations [12].

2) Energy Resolution:Fig. 10 presents the energy resolu-tion as a function of for the 5.9-keV X-rays. A compila-

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Fig. 10. Detector energy resolution as a function of anode voltageV for5.9-keV X-rays for the different CsI-coverd MSPs, working as the photosensorof a driftless GPSC.

TABLE IIIBEST ENERGY RESOLUTION FOREACH MICROSTRIPGEOMETRY FOR5.9-KEV

X-RAY AND FOR THE TWO OPERATION MODES

tion of the best energy resolutions is presented in Table III thevalues are within a variation of 1% except for MS6 and MS8,which present the lowest and the highest energy resolutions,respectively.

Analyzing Table III, it becomes clear that the energy resolu-tion for the MSP operating as a proportional counter is larger(about 14%) than the energy resolution for the MSP operatingas the photosensor of a GPSC (about-12%).

From Table III it can be concluded that the MSP geometrieswith larger cathodes have worse energy resolutions.

IV. CONCLUSION

The behavior of a xenon-filled MSGC instrumented witheight different MSP geometries was reported. They eitheroperate as a standard MSGC or as a driftless GPSC when the

MSPs are without or with a CsI-coating. The CsI-coated MSPfunctions as the GPSC photosensor in substitution of the PMT.

For the MSGC characteristic operation, the measured en-ergy resolution variations are less than 0.5% and 1% for 22.1and 5.9 keV, respectively. Only MS5 presents a significa-tive degradation in the energy resolution. Generally, smalleranode-to-cathode gaps will produce higher gains. Also, highergains are achieved for geometries with larger cathodes, forthe same gap value and for the same voltage.

For the gas chamber operating as a GPSC, optical positivefeedback limits the maximum useful gain. Optical positivefeedback is more important for larger cathode geometries.Additionally, larger cathode widths lead to worse energy res-olution. On the other hand, it is not noticeable a dependenceof optical positive feedback on the anode-to-cathode gap.

Higher gains and improved energy resolutions are achievedfor the gas chamber operating as a GPSC. The best energy reso-lution achieved for 5.9-keV X-rays is 11.4% for the MS6, whichis better than that achieved with a standard proportional coun-ters, e.g., about 13% for argon-based fillings [14].

REFERENCES

[1] J. M. F. dos Santos, A. C. S. S. Bento, and C. A. N. Conde, “A simple,inexpensive gas proportional scintillation counter for X-ray fluorescenceanalysis,”X-ray Spectrometry, vol. 22, pp. 328–331, 1993.

[2] A. Peacock, R. D. Andresen, E. A. Leimman, G. Manzo, and B. G.Taylor, “Performance characteristics of a gas scintillation spectrometerfor X-ray astronomy,”Nucl. Instrum. Methods, vol. 169, pp. 613–625,1980.

[3] A. Smith and M. Bavdaz, “Gas scintillation proportional countersfor X-ray synchrotron applications,”Rev. Sci. Instrum., vol. 63, pp.689–692, 1992.

[4] J. F. C. A. Veloso, J. M. F. dos Santos, and C. A. N. Conde, “Large-window gas proportional scintillation counter with photosensor compen-sation,”IEEE Trans. Nucl. Sci., vol. 42, pp. 369–373, Aug. 1995.

[5] A. Oed, “A position sensitive detector with microstrip anode for elec-tron multiplication with gases,”Nucl. Instrum. Methods, vol. A263, pp.351–359, 1988.

[6] K. Zeitelhack, J. Friese, R. Gernhäuser, P. Kienkle, H.-J. Körner, P.Maier-Komor, and S. Winkler, “A microstrip gas counter for singleVUV photons,”Nucl. Instrum. Methods, vol. A351, pp. 585–587, 1994.

[7] J. F. C. A. Veloso, J. A. M. Lopes, J. M. F. dos Santos, and C. A. N.Conde, “A microstrip gas chamber as a VUV photosensor for a xenongas proportional scintillation counter,”IEEE Trans. Nucl. Sci., vol. 43,pp. 1232–1236, June 1996.

[8] J. F. C. A. Veloso, J. M. F. dos Santos, C. A. N. Conde, F. Mulhauser, P.Knowles, and C. Donche-Gayet al., “A driftless gas proportional scin-tillation counter for muonic hydrogen X-ray spectroscopy under strongmagnetic fields,”Nucl. Instrum. Methods, vol. A460, pp. 297–305, 2001.

[9] T. Beckers, R. Bouclier, C. Garabatos, G. Million, F. Sauli, and L. I.Shekhtman, “Optimization of microstrip gas chamber design and oper-ating conditions,”Nucl. Instrum. Methods, vol. A346, pp. 95–101, 1994.

[10] R. Bouclieret al., “Optimization of design and beam test of microstripgas chambers,”Nucl. Instrum. Methods, vol. A367, pp. 163–167, 1995.

[11] Z. Q. Yu, S. S. Wang, H. J. Wei, M. L. Chu, and P. K. Teng, “A study ofsmall gap microstrip gas chamber,”Nucl. Instrum. Methods, vol. A372,pp. 35–38, 1996.

[12] D. S. A. P. Freitas, J. F. C. A. Veloso, J. M. F. dos Santos, and C. A.N. Conde, “A comparative study of microstrip plate geometries as UVphotosensors with reflective photocathodes: Simulation,”IEEE Trans.Nucl. Sci., vol. 48, pp. 411–416, June 2001.

[13] J. F. C. A. Veloso, J. M. F. dos Santos, and C. A. N. Conde, “Gas propor-tional scintillation counters with CsI-covered microstrip plate UV photo-sensor for high resolution X-ray spectrometry,”Nucl. Instrum. Methods,vol. A457, pp. 253–261, 2001.

[14] Metrorex International Oy, Espoo, Finland.