F. Piuz

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.

CsI-photocathode and RICH detector

Abstract The development of fast RICH detectors using solid photoconverters is reviewed in the context of basic research and applications to high-energy physics experiments. The novel layout of the CsI fast RICH and the associated technologies are discussed in view of design optimization. Experimental results are presented and discussed in relation to photoelectric yield, detector efficiency and stability, Cherenkov resolutions, background, ageing and pattern recognition.

Final tests of the CsI-based Ring Imaging Detector for the ALICE experiment

We report on the final tests performed on a CsI-based RICH detector equipped with 2 C6F14 radiator trays and 4 photocathodes, each of 6438 cm 2 area. The overall performance of the detector is described, using different gas mixtures, in view of optimizing the photoelectron yield and the pad occupancy. Test results under magnetic field up to 0.9 T, photocathode homogeneity and stability are presented.

Technology of photocathode production

Abstract Thin film photocathodes have been developed both for gas and vacuum based photodetectors. Over the last years we have gained experience in fabricating large area reflective CsI and semi-transparent mono- and bialkali-cathodes, i.e. Rb 2 Te and K 2 CsSb. Today we are producing stable cathodes with excellent quantum efficiency for large MWPC based pad detectors and Hybrid Photodiodes on a routine basis. We give an overview of the technologies involved in the preparation of the substrates, the cathode processing and the encapsulation. The dedicated processing plants and instrumentation, required for monitoring and characterisation, is discussed.

Direct observation of the decay of beauty particles into charm particles

Abstract The associated production of a pair of beauty particles B− and B 0 by a 350 GeV π− interaction has been observed in an emulsion target inserted in an array of silicon microstrip detectors. Both beauty particles decay into charm particles, both of which are also observed to decay in the emulsion. Two negative muons were identified and their momenta measured in a large muon spectrometer. One muon has a pT of 1.9 GeV/c and is associated with a beauty particle decay. The other, with a pT of 0.45 GeV/c is associated with a charm particle decay. The flight times of the two beauty particles are respectively (0.8 ± 0.1) × 10−13 s and (5+2−1) × 10−13 s. Alternative interpretations of this event have negligible probability.

Ring Imaging CHerenkov systems based on gaseous photo-detectors: trends and limits around particle accelerators

Since the last RICH Workshop held in 1998, a number of large RICH systems have been built and, some, already operated at different HEP experiments. The main features governing the operation and the performance of these systems are reviewed and discussed in view of possible improvements towards higher rates and ageing performance. As most of the new systems are based on CsI-photocathodes, a review is given on the progress done in the processing of efficient and reproducible large photocathodes. An alternative multi-GEM based photo-detector is discussed.

The CsI based Ring Imaging Detector for the ALICE experiment: Technical description of a large prototype

We report on the design and construction of a CsI-RICH detector composed of four CsI photocathodes of 64]40 cm2 each, and two C 6 F 14 radiator trays of 133]41 cm2. A detailed description of the novel elements is given and the performance of the detector is illustrated with some basic results obtained during the tests at the beam. ( 1999 Elsevier Science B.V. All rights reserved.

Performance of large area CsI-RICH prototypes for ALICE at LHC

Abstract We present the performances of large area CsI-RICH prototypes obtained in single-particle events. The differential quantum efficiency of the photocathodes has been deduced from Cherenkov rings by means of two different procedures: a direct measurement with a thin NaF radiator and a Monte Carlo-based estimation for a C 6 F 14 radiator. A factor of merit of 45 cm −1 has been found for the typical detector configuration. Two angle reconstruction algorithms have been used and the different errors affecting the Cherenkov angle resolution have been estimated combining the analytical treatment and the Monte Carlo simulation. Also the dependence on radiator thickness, Cherenkov ring radius, chamber voltage and particle incidence angle has been studied.

Identified pion interferometry in heavy-ion collisions at CERN

Abstract π + π + correlations from S + Pb collisions at 200 GeV/ c per nucleon, measured by the focusing spectrometer of the NA44 experiment at CERN, are presented. The large data set, particularly at small values of Q inv , allows determination of the detailed shape of the correlation function. We construct the background by event mixing, and correct for resolution, Coulomb interaction, and distortion of single-particle spectra which produce effects large compared to the statistical errors at small Q inv . An exponential, rather than gaussian, source distribution is slightly favoured, but both are statistically acceptable.

Performance of large area CsI RICH prototypes for ALICE at LHC

Abstract We present the performances of large area CsI-RICH prototypes obtained in single-particle events. The differential quantum efficiency of the photocathodes has been deduced from Cherenkov rings by means of two different procedures: a direct measurement with a thin NaF radiator and a Monte Carlo-based estimation for a C 6 F 14 radiator. A factor of merit of 45 cm −1 has been found for the typical detector configuration. Two angle reconstruction algorithms have been used and the different errors affecting the Cherenkov angle resolution have been estimated combining the analytical treatment and the Monte Carlo simulation. Also the dependence on radiator thickness, Cherenkov ring radius, chamber voltage and particle incidence angle has been studied.