A. Breskin

Electron field emission for ultrananocrystalline diamond films

Ultrananocrystalline diamond (UNCD) films 0.1–2.4 μm thick were conformally deposited on sharp single Si microtip emitters, using microwave CH4–Ar plasma-enhanced chemical vapor deposition in combination with a dielectrophoretic seeding process. Field-emission studies exhibited stable, extremely high (60–100 μA/tip) emission current, with little variation in threshold fields as a function of film thickness or Si tip radius. The electron emission properties of high aspect ratio Si microtips, coated with diamond using the hot filament chemical vapor deposition (HFCVD) process were found to be very different from those of the UNCD-coated tips. For the HFCVD process, there is a strong dependence of the emission threshold on both the diamond coating thickness and Si tip radius. Quantum photoyield measurements of the UNCD films revealed that these films have an enhanced density of states within the bulk diamond band gap that is correlated with a reduction in the threshold field for electron emission. In additio...

CsI UV photocathodes: History and mystery

Abstract CsI films are known to be efficient photoconvertors, intensively investigated for UV-photon imaging devices. The article reviews the production and characterization techniques of CsI photocathodes and their photoemission properties in vacuum and gas media, in charge collection and multiplication modes. The important roles of surface phenomena, gas and electric fields are described. The stability of the films is discussed.

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.

The multistep avalanche chamber: A new family of fast, high-rate particle detectors

Abstract We have investigated an unusual charge-multiplication mechanism in gases, mainly photon-mediated, that allows a controlled avalanche spread in a parallel-plate chamber. Exploiting this mode of operation, electrons can be multiplied and transferred through a succession of amplifying elements, thus constituting a multistep avalanche chamber. Several detection schemes are analysed, which provide remarkable energy, time, and position resolution both for soft X-rays and for charged particles. A gated operation of the device is described, which should allow efficient detection of particles fluxes some orders of magnitude larger than the conventional multiwire proportional chambers. Applications in several domains, such as Cherenkov ring imaging, detection of thermal neutrons, and radio-chromatography, are also discussed.

Recent Observations and Measurements with High Accuracy Drift Chambers

Abstract We describe several recent measurements in high-accuracy drift chambers, in particular on the detailed behaviour of the drift in strong magnetic fields, on the multitrack separation, and on the rate dependence of efficiency and pulse height.

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.

Further results on the operation of high-accuracy drift chambers

Abstract Optimization of the working parameters in the drift chambers with adjustable electric fields permits stable operation and high accuracies. Full saturation of the drift velocity leads to remarkable improvements, namely a very linear space-time correlation for perpendicular tracks, and simple geometrical distortion of linearity for inclined tracks. The same results can be obtained when properly tilting the electric field equipotentials in a wide range of external magnetic fields. This simple behaviour should allow a practical use, even for large systems, of the intrinsic high accuracy of the drift chambers (100–200 μm). They appear then as a very promising high-resolution fast detector for high-energy particle physics.

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.

Progress in low-pressure gaseous detectors

Abstract Some outstanding properties of low-pressure MWPCs open new prospects in the domain of heavy as well as light particle detection. In the present mode of operation, the mechanism of avalanche formation is divided into two steps: it combines an amplification process in a high, constant, electric field region with a second one, occurring in the vicinity of the proportional wires. Large gains associated with fast charge-collection times are reached at pressures as low as a fraction of a Torr, leading to excellent timing properties and to a high counting-rate capability. Transmission timing detectors, having a total thickness in the 100 μg/mm 2 range, have been designed for heavy-ion detection and offer time resolutions of the order of 100 ps (fwhm) at rates as high as 10 5 c/cm 2 s. The properties of this type of detectors are summarized and localization and multitrack detection capabilities are discussed. Present and future heavy-ion detection systems based on this simple, efficient and low cost technique are described. Possible applications in nuclear and particle physics are also discussed.