G. Charpak

MICROMEGAS: a high-granularity position-sensitive gaseous detector for high particle-flux environments

Abstract We describe a novel structure for a gaseous detector that is under development at Saclay. It consists of a two-stage parallet-plate avalanche chamber of small amplification gap (100 μm) combined with a conversion-drift space. It follows a fast removal of positive ions produced during the avalanche development. Fast signals (≤1 ns) are obtained during the collection of the electron avalanche on the anode microstrip plane. The positive ion signal has a duration of 100 ns. The fast evacuation of positive ions combined with the high granularity of the detector provide a high rate capability. Gas gains of up to 10 5 have been achieved.

The use of multiwire proportional counters to select and localize charged particles

Abstract Properties of chambers made of planes of independent wires placed between two plane electrodes have been investigated. A direct voltage is applied to the wires. It has been checked that each wire works as an independent proportional counter down to separations of 0.1 cm between wires. Counting rates of 105/wire are easily reached; time resolutions of the order of 100 nsec have been obtained in some gases; it is possible to measure the position of the tracks between the wires using the time delay of the pulses; energy resolution comparable to the one obtained with the best cylindrical chambers is observed; the chambers operate in strong magnetic fields.

Some developments in the operation of multiwire proportional chambers

Abstract Discussion of the limits of proportional amplification in multiwire proportional chambers. Chambers with 2 mm wire spacing can give pulses of more than 200 mV on 20 pF loads with 25 nsec maximum time jitter. Some examples of operation beyond the proportional region are given. Multigrid chambers permit an increased gain for smaller wire separation and wave multiple applications. Better time and space resolution can be obtained from the time-position correlation. Resolutions of 7 nsec (fwhm) and 0.2 mm (fwhm) have been observed. The use of the positive pulse permits the two-dimensional readout from single gaps. The effects of magnetic fields up to 45 kG have been measured. Some new lines of research are discussed.

Reflective UV photocathodes with gas-phase electron extraction: solid, liquid, and adsorbed thin films

Abstract The photoemission quantum efficiency of reflective photocathodes in methane gas has been investigated in the spectral range between 140 and 250 nm. The spectral response of solid metals and CsI, as well as of liquid and solid TMAE film, have been measured. The high quantum efficiency of CsI (35% at 170 nm) makes it attractive for BaF 2 or xenon scintillation detection. A BaF 2 crystal coupled to an ionization chamber with a reflective CsI photocathode has been successfully tested. Adsorbed TMAE films can significantly increase the quantum yields of metal and CsI (to 46% at 170 nm), making them suitable for fast RICH and other applications.

Investigation of some properties of multiwire proportional chambers

Abstract This article describes a systematic study of the efficiency, space resolution and multiparticle separation in multiwire proportional chambers. For a variety of gas mixtures, results were obtained as functions of high voltage, time resolution and incidence angle of particles. A new gas, argon+isobutane+freon-13 B1 permits a considerable gain in amplification without entering into the Geiger or spark region.

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.

Progress in high-accuracy proportional chambers

Abstract For ionizing events punctually localized in the volume of a proportional chamber, the mean position of the corresponding avalanches are well defined. By reading the position of the ion cloud, using pulses induced in cathode strips arising from absorption of low-energy X-rays, the position accuracies obtained are: along the anode wire σ ∼ 35 μ m, and on the direction orthogonal σ ∼ 150 μ m with 2 mm wire spacing. The intrinsic position accuracy of the method of measurement is much better.

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.

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.

Coupling of a BaF2 scintillator to a TMAE photocathode and a low-pressure wire chamber

Abstract The short wavelength component of a BaF2 scintillator has been successfully coupled to a tetrakis(dimethylamino)ethylene (TMAE) photocathode and a low-pressure wire chamber. An energy resolution of 28.5% fwhm has been measured for protons losing about 18 MeV in the crystal. A timing resolution of 540 ps fwhm has been measured for 350 MeV α-particles. We foresee a new generation of calorimeters with good spatial, temporal, and energy resolution, able to work at high rates. Other possible applications are also discussed.