D. Mörmann

Thick GEM-like hole multipliers: properties and possible applications

Abstract The properties of thick GEM-like (TGEM) gaseous electron multipliers, operated at 1–740 Torr are presented. They are made of a G-10 plate, perforated with millimeter-scale diameter holes. In single-multiplier elements, effective gains of about 10 4 , 10 6 , and 10 5 were reached at respective pressures of 1 and 10 Torr isobutane and 740 Torr Ar/5%CH 4 , with pulse rise-times in the few nanosecond range. The high effective gain at atmospheric pressure was measured with a TGEM coated with a CsI photocathode. The detector was operated in single and cascaded modes. Potential applications in ion and photon detection are discussed.

Advances in the Micro-Hole & Strip Plate gaseous detector

We report on the performance of a new gaseous electron multiplier: the Micro-Hole & Strip Plate (MHSP). It consists of two independent charge-amplification stages in a single, double-sided micro-structured electrode, deposited on a thin insulating substrate. Charge gains in excess of 10 3 were obtained in a MHSP operated with soft X-rays in Ar/CO2 (70/ 30). We present the results of a systematic study of the MHSP properties and those of a double-stage GEM+MHSP multiplier. Applications to gaseous photomultipliers are discussed. r 2003 Elsevier Science B.V. All rights reserved. PACS: 29.40.� n; 29.40.Cs; 85.60.Gz

A gas avalanche photomultiplier with a CsI-coated GEM

We describe the operation principle and properties of a CsI-coated GEM photodetector. This type of detector performs photon detection with reflective photocathodes, which are easy to produce and have high quantum efficiency. In the proposed configuration, the detector is practically free of avalanche-induced photon feedback effects. The influence of the GEM voltage and the electric fields close to the CsI-GEM electrode on the photon detection efficiency is studied. The conditions for obtaining full extraction of photoelectrons from the photocathode and their transfer through the GEM apertures are presented.

Ion-induced effects in GEM and GEM/MHSP gaseous photomultipliers for the UV and the visible spectral range

We report on the progress in the study of cascaded GEM and GEM/MHSP gas avalanche photomultipliers operating at atmospheric pressure, with CsI and bialkali photocathodes. They have single-photon sensitivity, ns time resolution and good localization properties. We summarize operational aspects and results, with the highlight of a high-gain stable gated operation of a visible-light device. Of particular importance are the results of a recent ion-backflow reduction study in different cascaded multipliers, affecting the detectors stability and the photocathodes lifetime. We report on the significant progress in ion-blocking and provide first results on bialkali-photocathode aging under gas multiplication.

GEM-based gaseous photomultipliers for UV and visible photon imaging

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.

On the efficient electron transfer through GEM

The absolute electron transfer efficiency of a gas electron multiplier (GEM) was systematically measured in several gas types and pressures and over a broad range of electric-field configurations, using a single-electron pulse-counting method. A complete understanding of the role played by the relevant variables was obtained; particularly, the critical part of electron transport in the gap preceding the GEM was demonstrated. A small electron multiplication in this gap was shown to result in a full detection efficiency of single-electron events, under proper gas diffusion and multiplication conditions. The relevance to single electron and single photon detection is discussed. The experimental results are in good agreement with simulation calculations.

Progress in GEM-based gaseous photomultipliers

Abstract We discuss recent progress in gaseous photomultipliers (GPMTs) comprising UV-to-visible spectral range photocathodes (PCs) coupled to multiple Gas Electron Multipliers (GEM). The PCs may be either semitransparent or reflective ones directly deposited on the first-GEM surface. These detectors provide high gain, even in noble gases, are sensitive to single photons, have nanosecond time resolution, and offer good localization. The operation of CsI-based GPMTs in CF 4 opens new applications in Cherenkov detectors, where both the radiator and the photosensor operate in the same gas. The latest results on sealed visible-light detectors, combining bialkali PCs and Kapton-made GEMs are presented.

On the efficient operation of a CsI-coated GEM photon detector

We report on the ecient operation of a CsI-coated GEM photon detector. We describe its operation mode and the dependence of the single electron detection e�- ciency on the electric Þelds. Conditions for obtaining full eciency of photoelectron extraction and their focusing into the GEM apertures, in 1 atmosphere CH4, are presented. The quantum eciency of the CsI-coated GEM is 35% at 150nm.

Recent investigations of cascaded GEM and MHSP detectors

We present results from our recent investigations on detectors comprising cascaded gas electron multipliers (GEM) and cascaded GEMS with micro-hole and strip (MHSP) electrodes as a final amplification stage. We discuss the factors governing the operation of these fast radiation-imaging detectors, which have single-charge sensitivity. The issue of ion-backflow and ion-induced secondary effects is discussed in some detail, presenting ways for its suppression. Applications are presented in the fields of photon imaging in the UV-to-visible spectral range as well as x-ray and neutron imaging.

2-D imaging with cascaded GEM/MHSP multipliers

The imaging properties of cascaded-GEM (Gas Electron Multiplier) and cascaded-GEM/MHSP (Micro-Hole and Strip Plate) gaseous electron multipliers, equipped with 2-D Wedge & Strip (W&S) readout electrodes, are presented. The W&S electrode was capacitively coupled to the electron multiplier through a resistive electrode, to adapt the charge-cloud size to the 1.6 mm readout pitch. The studies were carried out with soft X-rays and single UV-photons in Ar/5%CH4 gas mixture at atmospheric pressure. Spatial resolutions of ~225μm and ~250μm (FWHM) were reached, with 5.9 keV X-rays, in 3-GEM and 2-GEM/MHSP based detectors, respectively. Single photoelectrons, emitted from a CsI photocathode deposited on the top electrode of the first GEM in the cascade, were localized with resolution of ~ 170μm (FWHM) in a 4-GEM gaseous photomultiplier.