GEM-based gaseous photomultipliers for UV and visible photon imaging

GEM-based gaseous photomultipliers for UV and visible photon imaging D. Mörmann, M. Balcerzyk 1, A. Breskin, R. Chechik, B.K. Singh 2 A. Buzulutskov 3 Department of Particle Physics, The Weizmann Institute of Science, 76100 Rehovot, Israel Abstract We present the current status of our research on GEM-based gaseous photomultipliers. Detectors combining multi-gem electron multipliers with semi-transparent and reflective photocathodes are discussed. We present recent progress in extending the sensitivity of these detectors into the visible range. We demonstrate the long-term stability of an Argon-sealed bi-alkali photo-diode and provide preliminary results of a gas-sealed Kapton-GEM detector with a bi-alkali photocathode. The problem of ion-induced secondary electron emission is addressed. Key words: GEM, CsI photocathodes, bi-alkali photocathodes, single electron detection, sealed gaseous photomultiplier PACS: 29.40.Cs, 29.40.Gx, 29.40.Ka, 85.60.Gt, 85.60.Ha 1 Introduction Gaseous photomultipliers (GPMTs) with a solid photocathode [1] are an interesting alternative to vacuum or solid-state detectors, particularly when large area and insensitivity to magnetic fields are required. Their high charge gain makes them sensitive to single photons, and they are successfully used in the UV spectral range; however, efforts are continuously made to extend their sensitivity towards the visible [1 3]. Incorporating state-of-the-art electron Corresponding author. Email: moermann@wicc.weizmann.ac.il 1 On leave from Soltan Inst. for Nucl. Studies, 05-400 Otwock-Swierk, Poland 2 Presently at INFN, Bari, Italy 3 Presently at BINP, Novosibirsk, Russia Preprint submitted to Elsevier Science 13 August 2002

a) b) Fig. 1. Schematic view of a 3-GEM photodetector coupled to a semi-transparent photocathode (a) and with a reflective photocathode deposited on the top face of GEM1 (b). multipliers, GPMTs can reach sub-nanosecond time resolutions [4] and submilimeter position accuracies. Large area UV-sensitive GPMTs are currently employed in Ring Imaging Cherenkov (RICH) detectors for relativistic particle identification in high energy physics experiments. In this application, mainly CsI photocathodes coupled to wire chambers are recently used [5]. A drawback of the electron multiplication process in gas, is the generation of secondary ions and photons. These, upon hitting the photocathode surface, do not only accelerate the photocathode aging but can also result in extensive secondary electron emission, limiting the operation of these devices in terms of gain and localization resolution [1]. Therefore, efforts have been directed towards replacing the usual open-geometry of wire-chamber GPMTs by closed-geometry devices [6], based on the Gaseous Electron Multiplier (GEM) [7]. The GEM is a thin ( 50µm) insulator foil (generally Kaptonmade), metal-coated on both sides and perforated by a dense matrix of holes. Upon the application of a potential difference between the two sides (typically a few hundred volts), a strong electric field is created in the GEM apertures. Electrons guided by the electric field lines into the holes, experience gas amplification, of up to 10 4 in a single GEM foil [8]. Due to the GEM s low optical transparency ( 20% per GEM), multi-gem GPMTs totally suppress photon feedback. Moreover, the confinement of the avalanche process within the GEM holes strongly attenuates photon-mediated gas processes which typically lead to divergence of the avalanche in poorly quenching gases. This allows operation at high gains even in noble gas mixtures [9] and in CF 4 [4]. 2

2 GEM-based GPMTs Fig.1 shows a schematic view of a GPMT consisting of 3 GEMs coupled to: a) a semi-transparent photocathode [6] and b)a photocathode deposited on the first GEM [10]. In both cases photoelectrons emitted from the photocathode are transfered into the apertures of GEM1 and undergo a first amplification. The electron cloud than drifts to the subsequent GEM stages where it is further amplified and finally collected on the anode strips connected to the read-out electronics. Both detector schemes have been subject to detailed studies [10 12], showing that conditions can be found whereby a 100% efficient collection of photoelectrons from the photocathode into the GEM apertures is obtained; this allows to fully exploit the photocathode s quantum efficiency (QE). It should be noted that contrary to vacuum, the QE in gas media strongly depends on the choice of the gas mixture and the electric field at the photocathode surface. Discussion on this important subject are provided elsewhere [4]. As the main source of avalanche-generated photons and ions is the avalanche generated in the last GEM, the detector geometry of Fig.1 a) leads to a strong suppression of the photon feedback [9] and to a considerable reduction in ion feedback [13]. The second scheme shown in Fig.1 b) has the advantage of an even better suppression of the photon feedback. In addition, the production of the thicker reflective photocathodes is simpler than that of thin semi-transparent ones. Although the active area of the photocathode in this configuration is only 80% due to the holes in the GEM, one still has higher sensitivity due to the higher QE of reflective photocathodes. Fig.2 shows the gain characteristics of a detector with 4 GEMs and a reflective CsI photocathode, operated in selected gases. Typical gains are in excess of 10 6, which is more than sufficient for most applications, including single photon imaging. The excellent time resolution for single photons of σ= 1.6ns for the same detector operated in atmospheric CF 4 is shown in Fig.3. 3 GPMTs for visible light The extension of the sensitivity to the near-uv-to-visible light range (350-700nm) is the logical next step in the development of gaseous photomultipliers. Compared to UV-photocathodes, all known visible-sensitive photocathodes are chemically very reactive and thus vulnerable to gas impurities or outgasing of detector components, even at the sub-ppm level. Such detectors can operate in a stable way only when sealed within a vacuum-tight vessel. 3

Fig. 2. Gain vs. the potential across the GEM, V GEM for a 4-GEM GPMT with reflective CsI photocathode evaporated on top face of GEM1 in different atmospheric pressure gases. Fig. 3. Time resolution measured in atmospheric CF 4 for a 4-GEM multiplier with reflective CsI photocathode. Coating of the photocathode with chemically stable thin alkali-halide dielectric films, having good electron transport properties, was demonstrated to allow for photocathode protection and reaching stability, but at the cost of about a five-fold reduction in QE [3]. Therefore, GPMTs with protected photocathodes could be of a practical use only in applications with copious photon yields [14]. We are currently investigating on the use of bare bi-alkali photocathodes (K- Cs-Sb) sealed in gas to a multi-gem multiplier. Such multipliers were successfully sealed in gas in combination with semi-transparent CsI photocathodes [13]. In this sealed mode of operation one has to carefully choose the detector materials and the cleaning procedures prior to sealing. We are mounting the 4

Fig. 4. Quantum efficiency evolution of a bi-alkali photocathode sealed as a photodiode under 680torr of Ar. multiplier within Au-coated Kovar-made packages; these are sealed with In-Sn alloys at 130-150 C, to glass substrates carrying the photocathode. A description of the package, detector components, mounting and the sealing technique is provided in [13]. In a first study of the photocathode stability in a sealed gas-filled package, we successfully sealed a photodiode in atmospheric pressure Ar to a bi-alkali photocathode. Fig.4 shows that there is no decay in QE over than half a year. The low QE values of this particular photodiode, are due to the high photoelectron backscattering to the photocathode in Ar ( 65%) [15] and a non-optimized sealing temperature; the latter leads to considerable loss of QE. Following the successful gas-photodiode sealing, we have proceeded with sealing regular Kapton-made multi-gem multipliers with semi-transparent bialkali photocathodes. The sealing of such GPMTs in atmospheric pressure Ar was, for technical reasons, so far only partly successful. After being stable for almost a month, the QE rapidly decreased within a few days as shown in Fig.5. The decay of the photocathode started at one spot close to the sealing rim of the detector. This gradually expanding decolorated spot (bi-alkali photocathodes typically have a light-brown aspect), is a clear indication of a micro-leak in the In-Sn sealing. Though the photocathode was stable for a month, we can not exclude at this stage, a longer-term degradation of the photocathode due to the possible contamination by the Kapton GEMs. Although Kapton is known to be ultra-high vacuum compatible and was properly baked prior to the sealing, long-term stability studies must be carried out. We measured single-gem gain characteristics in the Ar-sealed bi-alkali GPMT. At rather low gains ( 30) we observed the appearance of secondary pulses due 5

Fig. 5. Quantum efficiency evolution of a bi-alkali photocathode sealed under 680torr of Ar to a 3-GEM GPMT. The decay is attributed to a visible micro-leak in the In-Sn seal. Fig. 6. Gain curves in pure Argon of a single GEM coupled to CsI and bi-alkali photocathodes. The deviation from the exponential, in the bi-alkali photocathode case, is due to ion-induced secondary emission that leads to higher currents and finally to breakdown. to avalanche-generated ions hitting the photocathode. In gain measurements that rely on current measurements, this ion feedback mechanism leads to a deviation from the expected exponential dependence of the gain on the GEM voltage, as can be seen in Fig.6. It should be noted that, prior to sealing, the same detector operated in Ar with a CsI photocathode did not show any ion feedback and could even be operated in 3-GEM mode up to a gain of 2 10 4. We attribute this very different behaviour to the known lower electron emission threshold and thus higher secondary emission efficiency of K-Cs-Sb. The issue of ion feedback essentially prevents the operation of multi-gem 6

Fig. 7. Absolute quantum efficiency of a semi-transparent K-Cs-Sb photocathode sealed under 700torr of Ar/CH 4 (95:5). detectors in pure argon, unless solutions are found to dramatically reduce the ion feedback to levels of 10 3 to 10 5. According to our recent studies [16], with multi-gem detectors one can reach ion feedback reduction levels at best close to 10 1, with electric fields and GEM geometries optimized for single photon detection. This is not sufficient and further investigations are on the way. On the other hand, we have strong indications from some preliminary measurements, that operation in Ar/CH 4 mixtures allows for reaching gains of at least 10 3, in single GEM coupled to a bi-alkali photocathode, before the onset of ion feedback. The phenomenon is not fully understood and the gas-mixture selection criteria is subject of further studies. In addition, Ar/CH 4 mixtures have considerably lower electron backscattering ; therefore, they allow for reaching higher effective QE, as can be seen in Fig.7 showing the relatively high QE of a semi-transparent K-Cs-Sb photocathode measured shortly after sealing in 700torr Ar/CH 4 (95:5). Another solution investigated is ion-gating after the appearance of the electron avalanche; such methods are currently used in TPCs [17]. First results indicate that we are able to reach an ion feedback suppression level of 10 4 or better. Further investigation on this is currently under way. 4 Conclusions GEM-based gaseous photomultipliers are a mature technique for UV photodetection. Detectors with reflective photocathodes deposited on the first GEM surface have the advantages of fully suppressing photon feedback and a higher QE. A large variety of photocathodes sensitive in the UV spectral range ex- 7

ist and can operate under gas multiplication, e.g. CsI, CsBr [18] and CVD diamond films [19]. Extension of the sensitivity of GPMTs towards the visible spectral range is in process. The stability of a bi-alkali photocathode sealed in Argon and preliminary results with GPMTs having Kapton-made GEMs, give us confidence that sealed multi-gem GPMTs have good chances to operate in the visible range. Application of this detector for the readout of scintialltion light in neutron detection is already being considered [20]. Nevertheless, several issues still have to be addressed, particularly the compatibility of Kapton GEMs with the photocathode, search for other inert GEM substrate materials, long term stability of the QE and ion feedback suppression. Glass [21] or ceramic substrates could be good candidates for GEM; it should be remarked that similarly to GEM, glass made MCP-like gas multipliers have shown successful operation [22]. These important matters are subject to an intensive research carried out at our laboratories. Acknowledgments The work was partly supported by the Israel Science Foundation. D. Mörmann acknowledges the Fellowship provided by the MINERVA Foundation for his stay at the Weizmann Institute of Science. M. Balcerzyk is grateful to the Feinberg Graduate School of the Weizmann Institute of Science for his fellowship. A. Breskin is the W.P. Reuther Professor of Research In The Peaceful Use Of Atomic Energy. References [1] A. Breskin et al., Nucl. Instr. and Meth. A 442 (2000) 58 and references therein. [2] V. Peskov et al., Nucl. Instr. and Meth. A 353 (1994) 184 and references therein. [3] E. Shefer et al., Nucl. Instr. and Meth. A 433 (1999) 502. [4] A. Breskin et al., Nucl. Instr. and Meth. A 483 (2002) 670. [5] F. Piuz et al., Nucl. Instr. and Meth. A 433 (1999) 222. [6] A. Buzulutskov et al., Nucl. Instr. and Meth. A 442 (2000) 68. [7] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531. [8] J. Benlloch et al., Nucl. Instr. and Meth. A 419 (1998) 410. 8

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