J. Séguinot

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.

Theory of ring imaging Cherenkov counters

A short review of Cherenkov radiation and its use for particle identification with threshold and differential counters is presented. The focusing principles of Ring Imaging Cherenkov (RICH) counters are then developed and applied successively to gas and liquid radiators. An analysis of proximity focused images from liquid or solid radiators follows. A method to reconstruct the Cherenkov angle from the experimental data is developed for spherical as well as for flat detector surfaces. These formulas define the Cherenkov angle sensitivity due to intrinsic as well as extrinsic (measurement) errors. Particle identification limits are then given for each of the different imaging methods.

A historical survey of ring imaging Cherenkov counters

Abstract In this survey we discuss the development of Ring Imaging Cherenkov (RICH) counters for particle identification. Since 1977, it has been based on the use of gaseous photosensors in detectors with gaseous amplification (MWPC and MSAC) and sensitive to single UV photons. The detectors are classified into three groups, each developed over the last sixteen years, but based on different techniques and readout methods. The main developments, and the principal operating RICH detectors, are described and critically analyzed in order to compare their respective absolute performances. The newer detector designs and photosensors are also discussed. The aim of this historical survey is to give an overview of the technique, even if not completely exhaustive, in order to help in understanding RICH counters, their problems and their possibilities.

A fast-cathode pad-photon detector for Cherenkov ring imaging

Fast ring imaging Cherenkov counters, with pad readout for unambiguous image reconstruction, are considered for use in a high luminosity environment. The quantum efficiency of several new photosensitive gases (TMA and DMA) are measured and compared to the known standard gases (TEA and TMAE). A solid CsI/TMAE reflective photocathode is also considered as an alternative fast and efficient photosensor. A detailed experimental investigation has been made of a multiwire photon detector with TEA as the photosensitive converter. This detector, designed as part of a RICH counter for a B factory electron-positron collider, may also be used at hadron colliders as well as at fixed target facilities. These tests allowed the optimization of relevant construction parameters to fulfill the requirements of speed (σt 0.98) and pixel error (σx < 1.5 mm) over the full surface of the detector. Its performance was used to tune the parameters of a general Monte Carlo simulation program in order to predict the response of a photodetector with various photoconverters and pad sizes.

Liquid xenon ionization and scintillation studies for a totally active-vector electromagnetic calorimeter☆

Abstract In these experiments MeVs to GeVs energies were deposited in a liquid xenon test cell by an electron accelerator with kinetic energy ≤100 keV, intensity ≤10 6 e and pulse width ≤30 ns FWHM. Purification of the liquid xenon is effected by continuous circulation of boil-off gas through Oxisorb. This technique will allow operation of a liquid xenon calorimeter in a high radiation ..environment because the liquid can be continuously purified and replenished. An ionization signal was detected by collecting drift electrons onto an anode mesh, a scintillation signal by collecting photoelectons in a silicon photodiode immersed in the liquid. The energy to create an ionization electron was measured to be W = 9.76±0.70 eV. The corresponding energy to produce a 175 nm scintillation photon was found to be W s = 14.2 eV. The scintillation signal is observed to be fast ( σ t ≤14 ns). Anti-correlation of the ionization and scintillation signals was also seen. Intrinsic energy resolution σ E / E ≈ 0.07%/√( E /GeV) was determined from ionization and σ E / E ≈ 0.2%/√( E /GeV) from scintillation. Doping of xenon with up to 2% methane did not adversely affect the ionization/scintillation yields or resolutions but increased the drift velocity hence, the current signal by about 75%. The intrinsic energy resolution of a totally active xenon scintillation calorimeter will be limited by uniformity of light collection and the dead material of the calorimeter to ( σ E / E ≤ 0.5%). Detection of scintillation with a silicon (or CsI gas) photodiode permits a fast energy measurement which can be used in a first level trigger. The ionization signal will be sampled to determine the vector direction of the photon (or electron) which initiates the shower. Directional resolution σ θ = 5 mrad/√( E /GeV) achievable with this method will allow association of a detected photon to its true vertex point. At LHC ( ≈ 20 interactions per beam crossing) this calorimeter can give a Higgs mass resolution σ M / M H ≈ 0.6% via the 2γ decay mode compared to 3% for calorimeters with equal energy resolution ( σ E / E = 0.5%) but without this vertex capability. Sampling of ionization in the early part of the shower will allow discrimination against π 0 production and detect individual photons from π 0 decays thus enhancing discrimination against these important sources of hadronic background. The essential problem in measuring the shower profiles is the large amount of readout electronics needed. Cheap, current sensitive, digital VLSI electronics has already been developed and produced for the FAST RICH detector project. Studies to adapt this electronics for use in liquid xenon are underway.

Liquid and solid organic photocathodes

We have investigated the possibility of creating photocathodes for gaseous detectors with a high sensitivity in the photon spectral region between 105 and 300 nm. Metal cathodes covered with liquid or solid organic layers, such as tetrakis (dimethylamine)ethylene (TMAE) and tetramethyl-p-phenylenediamine (TMPD) and solutions of these substances, were studied using two different experimental setups: a proportional wire chamber and a single-wire counter. There effects were observed. First, a thin film of absorbed vapours led to the efficiency of the metallic cathode being increased by more than one order of magnitude at photon wavelengths up to λ ≈ 400 nm. Secondly, a thick layer of liquid strongly increased the cathode efficiency Q for radiations of λ < 270 nm: e.g. with TMAE we obtained a yield of about 0.5%, for λ = 235 nm. Thirdly, we found that some solid layers, such as neopentane+TMAE, give a maximum efficiency Q ≈ 3% at λ = 235 nm. The quantum yields of liquid and solid photocathodes increase with increasing applied electric field. Some applications of the liquid and solid photocathodes are discussed.

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.

Photosensitive gas detectors for the ring-imaging Cherenkov (RICH) technique and the delphi barrel rich prototype

Abstract After a short introduction to the ring-imaging technique, the principal types of photosensitive gas detectors are discussed. In the second part, status and results of the DELPHI barrel RICH prototype are presented. A short description of a possible very fast RICH for future hadron colliders is given.

Liquid xenon scintillation: photon yield and Fano factor measurements

Abstract This paper presents a new measurement of the photon yield and the first measurement of the Fano factor in liquid xenon with a photomultiplier as photodetector. The observed photoelectron yield is 3.2/MeV which, after correction for detector solid angle and efficiency, corresponds to 78 600 photons/MeV. The Fano factor observed, F s = 0.033 ± 0.045, agrees with the value measured in the gaseous state by other authors.

Beam tests of a Fast-RICH prototype with VLSI readout electronics

Abstract In this report we discuss the Fast Ring Imaging Cherenkov technique that we have developed for application to proximity-focused LiF (or CaF2) solid radiator and multiwire chamber photon detector with cathode-pad readout using TEA in CH4 as photosensor. We describe the full-scale Prototype of 12 000 pads (5.334 × 6.604 mm2) we have built, and briefly the dedicated VLSI readout electronics we have developed. We report in detail the investigations we have performed in a hadron test beam at the CERN PS, and compare the results obtained to the expected performances. The maximum momentum for π/K separation at 3σ achieved in these tests is 2.86 GeV/c for LiF (2.39 GeV/c for CaF2). The experimentally achieved Cherenkov merit factors, after correction for azimuthal angle acceptance, are N0 = 65.5 cm−1 (57.7 cm−1), to be compared with 53.8 cm−1 (50.2 cm−1) from Monte Carlo calculations. Operation of the detector over several months has proven the technique reliable and robust, and suitable for application in high-luminosity hadron colliders like LHC, as well as e+e− B-Factories like KEK (Japan), SLAC (USA), and Cornell (USA).