R. Chechik

Advances in thick GEM-like gaseous electron multipliers. Part I: Atmospheric pressure operation

Thick GEM-like (THGEM) gaseous electron multipliers are made of standard printed-circuit board perforated with sub-millimeter diameter holes, etched at their rims. Effective gas multiplication factors of 10 5 and 10 7 and fast pulses in the few nanosecond rise-time scale were reached in single- and cascaded double-THGEM elements, in atmospheric-pressure standard gas mixtures with single photoelectrons. High single-electron detection efficiency is obtained in photon detectors combining THGEMs and semitransparent UVsensitive CsI photocathodes or reflective ones deposited on the top THGEM face; the latter benefits of a reduced sensitivity to ionizing background radiation. Stable operation was recorded with photoelectron fluxes exceeding MHz/mm 2 . The properties and some potential applications of these simple and robust multipliers are discussed.

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 gaseous photomultipliers

Abstract We review latest progress in gaseous photomultipliers (GPMs) combining solid photocathodes and various types of novel electron multipliers. Cascaded gaseous electron multipliers (GEMs) coated with CsI photocathodes can efficiently replace UV-sensitive wire chambers for single-photon recording in Cherenkov and other detectors. Other hole-multipliers with patterned electrodes (Micro-Hole and Strip Plates) and improved ion-blocking properties are discussed; these permit reducing considerably photon- and ion-induced secondary effects. Photon detectors with other electron-multiplier techniques are briefly described, among them GPMs are based on Micromegas, capillary plates, Thick-GEMs and resistive Thick-GEMs. The two latter techniques, robust and economically produced, are particularly suited for large-area GPM applications, e.g. in RICH. Cascaded hole-multipliers with very high ion-blocking performance permitted the development and the first demonstration of visible-sensitive GPMs operated in continuous mode, with bialkali photocathodes and single-photon sensitivity. Recent progress is described in GPMs operating at cryogenic temperatures for rare-event noble-liquid detectors and medical imaging.

A gas-filled UV-photon detector with CsI photocathode for the detection of Xe light

Abstract Xe-scintillation UV photons are detected with a CsI photocathode coupled to a double-stage low-pressure wire chamber. At 20 Torr of Ch4 the quantum efficiency of the photocathode is 9%. The combination of a gas scintillation chamber with the solid photocathode avalanche chamber (SPAC) yields an energy resolution of 4.1% (FWHM) for 60 keV X-rays. The stability of the photocathode is discussed.

Thick GEM-like multipliers: A Simple solution for large area UV-RICH detectors

We report on the properties of thick GEM-like (THGEM) electron multipliers made of 0.4 mm thick double-sided Cu-clad G-10 plates, perforated with a dense hexagonal array of 0.3 mm diameter drilled holes. Photon detectors comprising THGEMs coupled to semi-transparent CsI photocathodes or reflective ones deposited on the THGEM surface were studied with Ar=CO2 (70:30), Ar=CH4 (95:5), CH4 and CF4. Gains of � 10 5 or exceeding 10 6 were reached with single- or double-THGEM, respectively; the signals have 5–10 ns rise times. The electric field configurations at the THGEM electrodes result in an efficient extraction of photoelectrons and their focusing into the holes; this occurs already at rather low gains, below 100. These detectors, with single-photon sensitivity and with expected sub-millimeter localization, can operate at MHz=mm 2 rates. We discuss their prospects for large-area UV-photon imaging for RICH.

Photoelectron backscattering effects in photoemission from CsI into gas media

The photoemission from solid surfaces into gas is important in view of the application of solid photocathodes in fast-RICH devices. The photoemission from CsI into gas has been investigated in He-, Ar- and CH,-based gas mixtures as a function of the electric field at the photocathode surface. Measurements were made both in laboratory, with a UV source, and in a beam with a RICH detector. The results are interpreted in terms of current models of electron transport in gas. The electron collection efficiency, below gas ionization threshold, is reduced by backscattering. This phenomenon is particularly important in He-based gas mixtures. In CH, and Ar/CH, mixtures the backscattering is very low. At high electric field, under charge multiplication, a full collection efficiency, similar to that in vacuum, is obtained in all gases investigated. We discuss the parameters governing the choice of the gas mixture in this kind of photon detectors.

The scintillation of CF4 and its relevance to detection science

Abstract The scintillation properties of CF 4 are presented in comparison with those of Xe and CH 4 . Alpha-particle induced photon emission was measured with vacuum phototubes and with a CsI-based gaseous photomultiplier. The latter method provides an absolute sensitivity of such devices to particle-induced UV-photon background in CF4 and CH4 gaseous Cherenkov radiators. Integrated CF 4 scintillation yields over the range of 150–220 nm are, on the average, 315±95 to 242±60 photons/MeV, in the respective pressure range of 0.063 to 0.75 atm, compared to CH 4 which emits 0.06±0.01 photons/MeV at 1 atm. The total photon yield, integrated over the full emission spectrum of CF 4 (150–500 nm), is of the order of 1200 photons/MeV × 4 π. The primary scintillation photon yield of CF 4 is about 16(±5)% of that of Xe. No proportional secondary scintillation was observed in CF 4 . The avalanche-induced photon yield was measured to be of the order of 0.3 photons per electron. The implications of this considerable photon emission, are discussed.

The performance of a novel ion-counting nanodosimeter

Abstract We present the performance of a novel device conceived for measuring minute energy deposits in a low-density gas, capable of operating in various radiation fields, including in an accelerator environment. The ion-counting nanodosimeter provides a precise measurement of the ionization distribution deposited within a small wall-less gas volume, modeling nanometer-scales of condensed matter, e.g. the DNA molecule. We describe the instrument and its data acquisition system. The results of systematic studies with low-energy alpha particles, protons and carbon ions are compared to model simulations; they demonstrate the capabilities and indicate the limitations of this novel technique.

Progress in ultrafast CsI-photocathode gaseous imaging photomultipliers

Abstract A large area, low-pressure gas-filled UV-imaging photomultiplier with CsI-photocathode is presented. The double-step electron multiplier with a 10 Torr CH 4 gas-filling enables stable high gain operation. The detection efficiency of photons in the wavelength range λ ∼ 170 nm (Xe scintillation light) is about 10% for 200 to 2000 nm thick photocathodes. We investigated the influence of various substrate materials, the thickness of the CsI-layer, the gas pressure and gas composition on the performance of the photocathode. Furthermore we studied the stability of the photocathode under different operating conditions and its sensitivity to air. Measurements of the timing characteristics of the device yielded an ultimate time resolution of 350 ps (FWHM).

Sealed GEM photomultiplier with a CsI photocathode: ion feedback and ageing

Abstract We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH 4 (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.