Reflective CsI Photocathode Research Articles

Photoelectron extraction efficiency into Ar-CF4 and Xe-CF4 gas mixtures

Experimental measurements of the extraction efficiency f of the photoelectrons emitted from a reflective CsI photocathode into Ar-CF4 and Xe-CF4 gaseous mixtures are presented, as a function of reduced electric field E/p in the range 0.1 to 3 V cm−1 Torr−1 and as a function of CF4 concentration. The experimental data is compared to Monte Carlo simulation results. The investigation confirms that backdiffusion of low-energy photoelectrons emitted from a photocathode into a gas medium depends strongly on the nature of the gas. The effect is especially significant in pure noble gases, but it can be considerably reduced by the admixture of certain molecular gases, like CF4.

Design and construction of a new ultraviolet sensor using CsI deposition in the ionization chamber

Abstract In this article, a UV sensor that is an appropriate tool for fire detection has been designed and constructed. The structure of this UV sensor is an air-filled single-wire detector that is able to operate under normal air condition. A reflective CsI photocathode is installed at the end of the sensor chamber to generate photoelectrons in the ion chamber. An electric current is produced by accelerating photoelectrons to the anode in the electric field. The detector is able to measure the intensity of the incident UV rays whenever the current is sufficiently high. Therefore, the sensitivity coefficient of this sensor is found to be 7.67 × 10−6 V/photons/sec.

Study of the THGEM detector with a reflective CsI photocathode

The THGEM detector without and with a CsI has been tested successfully. The optimal parameters of THGEM have been determined from eight samples. The UV photoelectric effect of the CsI photocathode is observed. The changing tendency related to the extraction efficiency (εextr) versus the extraction electric field is measured, and several electric fields influencing the anode current are adjusted to adapt to the THGEM detector with a reflective CsI photocathode.

On the operation of a micropattern gaseous UV-photomultiplier in liquid-Xenon

Operation results are presented of a UV-sensitive gaseous photomultiplier (GPM) coupled through a MgF2 window to a liquid-xenon scintillator. It consisted of a reflective CsI photocathode deposited on top of a THick Gaseous Electron Multiplier (THGEM); further multiplication stages were either a second THGEM or a Parallel Ionization Multiplier (PIM) followed by a MICROMEsh GAseous Structure (MICROMEGAS). The GPM operated in gas-flow mode with non-condensable gas mixtures. Gains of 104 were measured with a CsI-coated double-THGEM detector in Ne/CH4(95:5), Ne/CF4(95:5) and Ne/CH4/CF4 (90:5:5), with soft X-rays at 173 K. Scintillation signals induced by alpha particles in liquid xenon were measured here for the first time with a double-THGEM GPM in He/CH4(92.5:7.5) and a triple-structure THGEM/PIM/MICROMEGAS GPM in Ne/CH4(90:10) with a fast-current preamplifier.

Further evaluation of a THGEM UV-photon detector for RICH – comparison with MWPC

The operation of single-, double- and triple-THGEM UV-detectors with reflective CsI photocathodes (CsI-THGEM) in Ne/CH4 and Ne/CF4 mixtures was investigated in view of their potential applications in RICH. The studies were carried out with UV, x-rays and β-electrons and focused on the maximum achievable gain, discharge probability, cathode excitation effects and long-term gain stability. Comparative studies under similar conditions were made in CH4, CF4 and Ne/CF4, with a MWPC coupled to a reflective CsI photocathode (CsI-MWPC). It was found that at counting rates ⩽ 10 Hz/mm2 the maximum achievable gain of CsI-THGEMs is determined by the Raether limit; at counting rates > 10 Hz/mm2 it dropped with rate. In all cases investigated the attainable CsI-THGEM gain was significantly higher than that of the CsI-MWPC, under similar conditions. Furthermore, the CsI-THGEM UV-detector suffered fewer cathode-excitation induced effects as compared to CsI-MWPC and had better stability at high counting rates.

Single-UV-photon 2-D imaging with multi-GEM detectors

The main goal being the construction of large-area, low-cost position-sensitive micropattern gaseous photomultipliers (GPMs) based on gas electron multipliers (GEM) cascades, we have studied the single-photon imaging capabilities of a 4-GEM GPM with a reflective CsI–photocathode deposited on the top surface of the 1st GEM. The charge readout was performed with a simple/economic Wedge and Strip (W&S) readout electrode, using pulses induced through a resistive electrode. We have demonstrated the feasibility of single-photon imaging GPM with sub-millimeter resolution; single-photon spatial resolutions as small as ∼150 μm (FWHM), i.e. ∼65 μm (RMS), were measured in Ar/5%CH 4 gas mixture at atmospheric pressure.

Photosensitive gaseous detectors for cryogenic temperature applications

There are several proposals and projects today for building LXe time projection chambers (TPCs) for dark matter search. Important elements of these TPCs are the photomultipliers operating either inside LXe or in vapours above the liquid. We have recently demonstrated that photosensitive gaseous detectors (wire type and hole type) can operate perfectly well, until temperatures of LN 2. In this paper, results of systematic studies of operation of the photosensitive version of these detectors (combined with reflective or semi-transparent CsI photocathodes) in the temperature interval of 300–150 K are presented. In particular, it was demonstrated that both sealed and flushed by a gas detectors could operate at a quite stable fashion in a year/time scale. Obtained results strongly indicate that they can be cheap and simple alternatives to photomultipliers or avalanche solid-state detectors in LXe TPC applications.

Development of a triple GEM UV-photon detector operated in pure CF 4 for the PHENIX experiment

Results obtained with a triple Gas Electron Multiplier (GEM) detector operated in pure CF 4 with and without a reflective CsI photocathode are presented. The detector operates in a stable mode at gains up to 10 4. A deviation from exponential growth starts to develop when the total charge exceeds ∼4×10 6 e leading to gain saturation when the total charge is ∼2×10 7 e and making the structure relatively robust against discharges. No aging effects are observed in the GEM foils after a total accumulated charge of ∼10 mC/cm 2 at the anode. The ion back-flow current to the reflective photocathode is comparable to the electron current to the anode. However, no significant degradation of the CsI photocathode is observed for a total ion back-flow charge of ∼7 mC/cm 2 .

Status of large area CsI photocathode developments

The development of large area reflective CsI photocathodes for RICH detectors is reviewed in the context of their production process and their performance. The quantum efficiencies of six large photocathodes recently produced by ALICE are compared with results available from the large photocathodes at HADES and STAR and with earlier R&D results obtained on small area samples. The CsI coating parameters and new developments in the domain of the production and the preparation of substrates are discussed in relation with the photocathode performance.

Evolution of the RICH technique

This paper presents recent advances in development of Ring Imaging Cherenkov counters. Work on bare and protected reflective CsI photocathodes is reviewed and compared with gaseous UV photosensors. Development of multipixel-visible light-vacuum photosensors allows much improved Cherenkov angular resolution (by factor 10) relative to the available gaseous UV photosensors. The Hybrid PhotoDiode and MultiAnode PMT concepts are reviewed with prototype realizations described and test results compared with expectations. The use of the Gas Electron Multiplier (GEM) with gaseous RICH counters is presented and discussed. Its advantages for single electron detection are shown by new experimental data.

An integrated photosensor readout for gas proportional scintillation counters

A xenon gas proportional scintillation counter has been instrumented with a novel photosensor that replaces the photomultiplier tube normally used to detect the VUV secondary scintillation light. In this implementation, the collection grid of a planar gas proportional scintillation counter also functions as a multiwire proportional chamber to amplify and detect the photoelectrons emitted by a reflective CsI photocathode in direct contact with the xenon gas. This integrated concept combines greater simplicity, compactness, and ruggedness (no optical window is used) with low power consumption. An energy resolution of 12% was obtained for 59.6 keV X-rays.

A UV light photo-detector based on a MicroGap Chamber with single electron response

Abstract A MicroGap Chamber (MGC) used as a fast UV light detector with single electron sensitivity is presented. The detector has a semitransparent or reflective CsI photocathode, 100–200 μm anode pitch and 2D capability. It works with gas mixtures based on DME and light noble gases at atmospheric pressure. A gas gain >106 has been achieved with a two-stage gas amplification (G > 102 in the drift region and G ≈ 104 in the strip region). The use of light noble gases, like neon, has brought to a sensible reduction of VUV light output (hence photon feedback) during electron multiplication in gas. Due to the very thin gap (5 μm) a very large fraction of the avalanche charge is quickly delivered to the fast amplifier (20 ns peaking time) thus improving the single electron detection capability of the MGC. Because of its higher gain the MGC with a semitransparent CsI photocathode provides a better single electron detection efficiency (>90%) than the MGC with a reflective photocathode. This latter, vice versa, has a higher quantum efficiency and a faster response. Experimental results on gas gain, drift properties, positional sensitivity and single electron detection efficiency are reported.

Measurements of the quantum efficiency of CsI photocathodes in a multiwire proportional chamber

The quantum efficiency of reflective CsI photocathodes in a multiwire proportional chamber (MWPC) has been measured. The MWPC is operated with methane at room temperature and atmospheric pressure and a gas gain of about 10 5. Two different methods are used to determine the quantum efficiency (QE). The first uses Cherenkov radiation of β-particles in a 1 cm thick NaF window of the CsI-MWPC and measures the average ratio of the CsI quantum efficiency to a standard QE s (λ) used in simulation. The second method uses a monochromator and compares the count rates of the CsI-MWPC with the count rates of a Philips XP2020Q photomultiplier. Results are presented for the dependence of the quantum efficiency on the photocathode production procedure, on the substrate material, on the thickness of the CsI layer, on exposure time to ambient air, on healing of the photocathodes, on flushing them with methane and on the cumulative charge collected.

Preparation of reflective CsI photocathodes with reproducible high quantum efficiency

CsI as a solid UV-photocathode material has many promising applications in fast gaseous photon detectors. They are proposed in large area Ring Imaging CHerenkov (RICH) devices in forthcoming experiments at various high-energy particle accelerators. A high photon-to-electron conversion efficiency is a basic requirement for the successful operation of these devices. High reproducible quantum efficiencies could be achieved with CsI layers prepared by electron beam evaporation from a water-cooled copper crucible. CsI films were deposited in the thickness range of 30 to 500 μg/cm 2. Absorption coefficients and quantum efficiencies were measured in the wavelength region of 150 nm to 250 nm. The influence of various evaporation parameters on the quantum efficiency were investigated.

Properties of reflective and semitransparent CsI photocathodes

Various aspects related to the quantum efficiency of the reflective and semitransparent CsI photocathodes have been investigated experimentally. The investigation explored the dependence of the quantum efficiency on the following: the thickness of the photocathode film, the gas used in the parallel-plate avalanche chamber, exposure to water vapor, and the density of the CsI film. With reflective CsI photocathodes quantum efficiencies of more than 25% at 180 nm have achieved in three different gases: CH 4, C 2H 6, and C 4H 10. Our study reveals that high quantum efficiency is also achievable for the semitransparent CsI photocathodes. For a 95-Å semitransparent photocathode the quantum efficiency at 185 nm can be as high as 20%. A simple model to explain the dependence of quantum efficiency on the thickness of CsI photocathodes is suggested, which may be a useful guide for further improvement of the performance of CsI photocathodes. Also reported are UV-transmittance measurements for various materials used in our studies, such as methane, ethane, isobutane, CF 4, CO 2, helium, neon, air, TMAE vapor, thin CsI films, and quartz windows, all in the wavelength range 115 nm to 230 nm.

Performance of CsI photocathodes in liquid Xe, Kr, and Ar

The successful detection of the UV scintillation light in liquid Xe, Kr and Ar by a reflective CsI photocathode with liquid-phase electron extraction is reported. High quantum efficiencies of 31% in liquid Xe, 17% in liquid Kr, and 10% in liquid Ar have been achieved. Excellent intrinsic energy resolutions of 2.8% FWHM for liquid Xe, 5.2% FWHM for liquid Kr, and 8.4% FWHM for liquid Ar have been measured for 5.5 MMeV alpha particles. The enhancement of the quantum efficiency due to the applied electric field and the favorable effect of the negative electron affinity of liquid rare gases on electron extraction from the photocathode are discussed.

Quantum efficiency and radiation resistance of bulk and porous CsI photocathodes in vacuum and methane

We present the result of a systematic investigation of the quantum efficiency of reflective CsI photocathodes in the spectral range 140–220 nm. Powder and crystal forms of CsI were used for evaporation of bulk and porous photocathodes. The measurements were made without gaseous amplification in vacuum and in methane. The results of radiation resistance measurements at doses reaching 4500 Gy are also reported.

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

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 BaF2 or xenon scintillation detection. A BaF2 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.