2662 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 55, NO. 5, OCTOBER 2008 Dual-Cathode CsI Covered Microstrip Plate as VUV High Efficiency Photosensor Diogo S. A. P. Freitas, Rui M. Curado da Silva, Nuno F. C. Mendes, and Carlos A. N. Conde, Life Member, IEEE Abstract—A Gas Proportional Scintillation Counter based on a dual-cathode Microstrip Plate covered with a CsI film is described. This new dual-cathode technique has the advantage of increasing the VUV sensitive area of the Microstrip Plate. A detailed descrip- tion of the technique is presented together with a discussion of the performance. The results obtained for a xenon filled Gas Propor- tional Scintillation Counter show an improvement of the energy resolution for 5.9 keV X-rays from 12.7%, for a single cathode de- vice, to 11.1% for the dual-cathode device. Index Terms—Gaseous radiation detectors, microstrip detectors, photocathodes, X-ray spectroscopy detectors, scintillation detec- tors. I. INTRODUCTION S INGLE Cathode Microstrip Plates (MSP) covered with a CsI film have been used for some time [1] as VUV photosensors for xenon filled Gas Proportional Scintillation Counters (GPSC) yielding energy resolutions of 12.7% for 5.9 keV X-rays [2], [3] for equivalent geometry devices as the one presented herein, although 11.4% resolution has been achieved with optimised geometry devices [4], which is a resolution not as good as that for GPSCs instrumented with photomultipliers (8.0%), but better than that for standard Proportional Counters (14%). CsI covered MSP based GPSCs have the advantage of being much more compact than the photomultiplier based ones. The schematic of a MSP based GPSC is shown in Fig. 1. X-rays are absorbed in an absorption region where they produce primary electrons. These electrons drift towards a fairly strong electric field region delimited by a grid and the CsI covered MSP; in this region, the so-called scin- tillation region, the primary electrons excite the xenon atoms which in the decay process produce VUV photons in quite large numbers (typically 500 photons per primary electron). These photons when absorbed in the CsI film covering the cathodes of Manuscript received April 11, 2008; revised June 25, 2008. Current version published December 04, 2008. This work was supported by FEDER through Project POCTI/FP/FNU/49561/2002 from Fundação para a Ciência e para a Tecnologia-Portugal, and carried out in Centro de Instrumentação (Unit 217/94), Departamento de Física da Universidade de Coimbra, Portugal. C.A.N. Conde acknowledges travel support from Fundação Luso-Americana para o Desen- volvimento. D. S. A. P. Freitas was supported by a grant from Fundação para a Ciência e para a Tecnologia-Portugal: SFRH/BD/3244/2000. R. M. Curado da Silva was supported by a post doctoral grant from Fundação para a Ciência e para a Tecnologia-Portugal: SFRH/BPD/11670/2002. The authors are with the Departamento de Física da Universidade de Coimbra, P-3004-516 Coimbra, Portugal (e-mail: diogo@gian.fis.uc.pt; ruisilva@saturno.fis.uc.pt; nmendes@gian.fis.uc.pt; canconde@gian.fis.uc.pt). Digital Object Identifier 10.1109/TNS.2008.2002325 Fig. 1. Schematic of a MSP based gas proportional scintillation counter. the MSP release photoelectrons which drift towards the anodes of the MSP where they are charge multiplied by an avalanche process, leading to a fairly large signal, approximately propor- tional to the energy of the X-ray photon absorbed in the xenon gas. However, as soon as the photo-electrons leave the pho- tocathode they collide with the xenon atoms and may be backscattered towards the CsI photocathode itself, and so lost. This effect reduces the efficiency of the photocathode for the VUV scintillation. The study of VUV photoelectric emission of a CsI film in a gaseous atmosphere has been the subject of both experimental [5] and theoretical [6] treatments. It was shown that the electric field intensity at the surface of the CsI film has to be stronger than about 3 for proper photoelectron extrac- tion. However, we must point out that this extracting electric field must have a pointing direction opposed to that of the field in the scintillation region, which means that if we want to have improved photoelectron extraction, the electric field intensity in the scintillation region must be reduced, so reducing the number of VUV photons and thus the number of photoelectrons. Fig. 2 shows the electric field lines obtained with Ansoft Maxwell 2D Field Simulator in the case of a standard MSP with 80 wide cathodes, 10 wide anodes and 55 anode-cathode gap. For this example, the anodes were biased at 200 V and the MSP placed in a xenon (800 Torr) scintillation region with the electric field set at 2500 electric field 0018-9499/$25.00 © 2008 IEEE Authorized licensed use limited to: Universidade de Coimbra. Downloaded on March 19,2010 at 11:38:22 EDT from IEEE Xplore. Restrictions apply.