D. Scigocki

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.

Investigation of operation of a parallel-plate avalanche chamber with a CsI photocathode under high gain conditions

Abstract We report results of a systematic study of the operational characteristics of a single-step parallel-plate avalanche chamber with CsI photocathode under high-gain conditions at room temperature and 1 atm pressure. Different mixtures of He and Ar with hydrocarbons were tested, as well as with ethylferrocene vapor which are known to form an adsorbed photosensitive layer on the CsI photocathode. The chamber can reach high gains, up to 10 6 , has a very good time resolution (500 ps FWHM), and an energy resolution of 8.2% FWHM for 3 × 10 3 primary photoelectrons with a quantum efficiency of the CsI photocathode of about 20% at 193 nm. Photon feedback, caused by avalanche emission with wavelength longer than 200 nm, was observed for large total charge and found to be nearly independent of the concentration of quencher in the range 7 to 70 Torr. Breakdown appears at a total charge of 10 10 electrons and is always of the slow type. There is good proportionality up to the breakdown limit.

Organometallic photocathodes for parallel-plate and wire chambers

Abstract We have investigated the possibility of using organometallic photocathodes (ferrocene, diethyl ferrocene, ethyl ferrocene, and cyclopentadienyl-iron-hexamethyl-benzene) in gaseous detectors. We found that condensed layers of these substances could increase the sensitivity of metallic cathodes to VUV radiation by several orders of magnitude. The best choice seems to be ethyl ferrocene, which can be used as the photosensitive element in the vapour phase as well as in condensed and absorbed layers. A parallel-plate chamber with this photocathode, operated at atmospheric pressure, combines a rather good time resolution (∼ 10 ns or better) with a sensitivity better than 1% for wavelengths shorter than 230 nm.

New photocathodes for fast gaseous detectors

Abstract Semitransparent and reflective liquid and solid photocathodes were studied. The highest quantum efficiency and the best timing properties were obtained with reflective photocathodes made from substrates having a small working function (CsI, Cul, differrocenyl mercury) and covered by an adsorbed layer of ethyl ferrocene (EF) or tetrakis (dimethylamine) ethylene (TMAE).

Some studies of the applications of CsI photocathodes in gaseous detectors

Abstract Cesium-iodide photocathodes have been investigated in combination with gaseous detectors. The quantum efficiency of a semi-transparent CsI photocathode has been measured in the UV range and found to be 10% at 140 nm. We report experimental results obtained with such photocathodes coupled to BaF2 and KMgF3 scintillators and read out by parallel-plate gaseous detectors.

Test of an electromagnetic calorimeter using BaF2 scintillators and photosensitive wire chambers between 1 and 9 GeV

Abstract We describe an electromagnetic calorimeter constructed from layers of BaF 2 crystals, coupled to low pressure MWPCs with hot TMAE gas as the photosensitive constituent. By making use of the fast component from the BaF 2 scintillation, this detector is well suited for a high rate, intense radiation environment. We present the results of a test performed with our prototype in a 1–9 GeV/c beam, which gives an energy resolution better than 4% E , a position resolution of 1 mm, and a time resolution better than 1 ns. The detector is highly segmented, with tracking capabilities and good e/ π rejection. We discuss the possible application to experiments with intense colliders.

Test of a BaF2-TMAE detector for positron-emission tomography

Abstract A detector consisting of BaF 2 scintillators and wire chambers has been tested for 511 keV gamma-rays. The wire chamber is filled with photosensitive tetrakis(dimethylamine)ethylene (TMAE) vapour and is operated at a pressure of a few Torr, using different gases at various temperatures. Energy, time, and position resolutions are given for BaF 2 crystals with sections of 10 × 10 mm 2 and 5 × 5 mm 2 . We discuss the potential of this gamma detector for positron-emission tomography (PET) and compare it with other systems.

Test of a BaF2-TMAE Detector for Positron-Emission Tomography

A solid-state proportional counter, consisting of the BaF2-TMAE combination, has been tested for the detection of 511 keV annihilation gamma-rays in a PET system. Energy, time, and position resolutions are given for 1 cm wide BaF2 crystals, using different temperatures and gases at low pressures.

Ethyl ferrocene in gas, condensed, or adsorbed phases: three types of photosensitive elements for use in gaseous detectors

Abstract We have investigated the properties of an organometallic compound, ethyl ferrocene (EF), which we propose to use as the photosensitive element in gaseous detectors, both in the gas (vapour) phase and, in condensed or adsorbed layers, as photocathodes. The big advantage of EF is that it is easy to handle, as it is not reactive to oxygen. The sensitivity for the detection of BaF2 fast emission was measured with EF vapour and was found to be lower by a factor of close to 1.5 compared with TMAE vapour measured under the condition of full light absorption. Adsorbed or condensed layers of EF used as photocathodes in a gaseous detector achieved an efficiency that was lower by a factor of 4 to 10, depending on the experimental conditions.

Liquid and solid photocathodes and their applications for fast gaseous particle detectors

Semitransparent and reflective liquid photocathodes were investigated. Experiments have led to the conclusion that, in the case of semitransparent photocathodes, the photoemission increases strongly at certain thicknesses, when the electron temperature of the photoelectrons obtained in the applied electric field is higher than the conduction-band energy of the liquid. In the case of reflective photocathodes, a thin intermediate layer of vapor between the liquid and the gaseous phases contributes to the creation of photoelectrons, deteriorating the timing properties of the photocathode. The best timing properties and the highest quantum efficiency were achieved with a reflective photocathode made from a substrate with a small workfunction, covered by an adsorbed layer of photosensitive vapors. >