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 detector's stability and the photocathode's lifetime. We report on the significant progress in ion-blocking and provide first results on bialkali-photocathode aging under gas multiplication.

We studied gain and position resolution of gaseous UV-photon detectors combining single- and cascaded- glass capillary-plate multipliers and CsI photocathodes. Two modes of operation were investigated: a conventional one, where the main amplification occurs within capillary holes and a parallel-plate amplification mode, where the main amplification occurs between the capillary plate and the readout anode. Results of these studies demonstrate that in the parallel-plate amplification mode one can reach both high gains (>105) and good position resolutions (∼100μm) even with a single-element multiplier. This offers a compact amplification structure, which can be used in many applications.

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 104, 106, and 105 were reached at respective pressures of 1 and 10 Torr isobutane and 740 Torr Ar/5%CH4, 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.

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 ioninduced 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.

We report on avalanche-ion back-flow measurements in the novel Micro-Hole and Strip-Plate (MHSP) multiplier and in gaseous photomultipliers comprising Gas Electron Multipliers (GEMs) followed by an MHSP. In a 3-GEMs/MHSP photomultiplier with reflective photocathode, avalanche-ion back-flow fraction of ∼7% and ∼2% were recorded for respective effective gains of 107 and 106, in Ar/CH4 (95/5) at 760 Torr. This is about one order of magnitude reduction in ion back-flow compared to the best values measured in 4-GEMs photomultiplier at the same gain. We describe the mode of operation of the MHSP and explain its ion back-flow reduction features.

In gaseous photomultipliers, avalanche generated ions back-flowing to the photocathode can drastically limit the detector operation and lifetime. This is especially the case for photocathodes with low electron emission threshold, where impinging ions induce ion feedback effects by secondary electron emission.We present ways of reducing ion-backflow to the photocathode, and thus suppress ion-feedback effects in multi-stage Gas Electron Multiplier detectors. We studied the effect of the various electric fields on the ion transport in the detector and present our results on active ion gating with a dedicated gating electrode. (C) 2003 Elsevier B.V. All rights reserved.

Gas Electron Multipliers with a reflective photocathode deposited on the surface of the first multiplying element are very attractive devices for photon detection and imaging over large area at moderate cost. They combine production and operation simplicity, high sensitivity to single photons, fast time response and accurate localization. In this work we present in detail the mechanisms governing the operation of these photon detectors. The results of electron extraction, transfer, multiplication and detection processes in this multi-element structure are presented and analyzed. We discuss the role of important elements and parameters influencing the detector's operation and performance: the GEM geometry, the choice of the different electric fields and the gas mixture.

Cascaded gas electron multipliers (GEMs) are used for the fast detection and imaging of single and multiple charges; they operate in a variety of gases, with very high gains attained due to an efficient suppression of avalanche-induced photon feedback. However, since they are highly transparent to the flow of charges it is not straightforward to restrict the ion backflow without losing charge gain. When the cascaded-GEM detector is coupled to a TPC it may benefit from favorable electric field configuration, resulting in an efficient trapping of a large fraction of the ions on the GEM electrodes and a reduced ion backflow to the TPC volume. We have recently shown that the ion backflow is further reduced by replacing the last GEM in the cascade with the newly proposed Micro-Hole-and-Strip Plate (MHSP). A new operation-mode arises, reducing the ion backflow by about an order of magnitude without losing speed or gain of the detector.

A study of a dual-GEM detector coupled to a strip readout anode is described. The effects of the induction electric field and GEM-to-anode gap are presented, for an operation in atmospheric pressure Ar/CO2 (70/30) and Ar/CH4 (95/5). Visible gain and anode signal pulse-shapes, measured with 5.9keV X-rays are presented for 1–6mm wide induction gaps and for induction fields ranging up to 6kVcm−1. The spatial distribution of the anode charge is provided for induction gaps of 2–12mm. The results are useful for matching the detector parameters to the position recording circuit requirements.

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 103 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.

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.

The Micro-Hole & Strip-Plate gas electron multiplier (MHSP) was studied as a stand-alone device or in combination with a cascade of Gas Electron Multipliers (GEMs), for x-ray and UV-photon detection. An MHSP operating in Ar/5%Xe yielded gains above 10/sup 4/ and energy resolutions of about 14% FWHM for 5.9-keV x-rays. Gains as high as 10/sup 7/ were reached in a 3-GEM/MHSP gaseous photomultiplier operating in an Ar/5%CH/sub 4/; the ion-backflow fraction to the top of the first GEM could be reduced down to ∼2% 2D-imaging performed using signals induced by avalanche ions on a Wedge-and-Strip readout cathode, placed at close proximity of the anode strips yielded position resolutions of the order of 200 to 250 μm FWHM for 5.9-keV x-rays.

We describe the preparation of a sealed, atmospheric pressure gaseous photomultiplier (GPMT) for the visible spectral range and present the properties of the first prototypes. They consist of a 50 mm diameter semitransparent bialkali photocathode coupled to a 30/spl times/30 mm Kapton-made multi-GEM electron multiplier. High gain of 2/spl times/10/sup 4/ in two-GEM mode and a quantum efficiency of 13% at 405 nm have been reached at atmospheric pressure of Ar/CH/sub 4/ (95:5). The detector structure and experimental setup are described; results are presented on the GPMT gain, ion-feedback and its suppression, stability, and other critical parameters in various gas mixtures. We examine also hot sealing techniques with In/Sn and In/Bi solders.

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 CF4 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.

The construction of a micro-pattern gas detector of dimensions 40×10cm2 is described. Two gas electron multiplier foils (GEM) provide the internal amplification stages. A two-layer readout structure was used, manufactured using the same technology as the GEM foils. The strips of each layer cross at an effective crossing angle of 6.7° and have a 406μm pitch. The performance of the detector has been evaluated in a muon beam at CERN using a silicon telescope as reference system. The position resolutions of two orthogonal coordinates are measured to be 50μm and 1mm, respectively. The muon detection efficiency for two-dimensional space points reaches 96%.

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.

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.

We report on the performance of a large micro-pattern detector with two gas electron multiplier foils and a two-layer readout structure at ground potential. The two readout layers each have a 406μm pitch and cross at an effective angle of 6.7°. This structure allows for two orthogonal coordinates to be determined. Using a muon beam at CERN together with a silicon tracking system, the position resolutions of the two coordinates are measured to be 50μm and 1 mm respectively (1 standard deviation). The muon detection efficiency for the two-dimensional space points reaches 96%. The detector was found to be well operational over a wide range in the settings of the different electrical fields.

We report on the efficient operation of a CsI-coated GEM photon detector. We describe its operation mode and the dependence of the single electron detection efficiency on the electric fields. Conditions for obtaining full efficiency of photoelectron extraction and their focusing into the GEM apertures, in 1atmCH4, are presented. The quantum efficiency of the CsI-coated GEM is 35% at 150nm.

We describe systematic measurements, carried out with single and double GEM detectors with printed circuit read-out. The maximum safe operating gain has been measured at increasing radiation flux, and under exposure to heavily ionizing tracks. Detection efficiency, localization accuracy and cluster size have been measured in a minimum ionizing particle beam. With a suitably configured readout electrode, fast two-dimensional localization of radiation is demonstrated.