Studies on thick-GEM UV Photon Detector

Abstract

Large area photodetectors with single photon counting ability motivates to develop gaseous photomultiplier (GPM), that combine thin film photocathode with electron multipliers for efficient detection of UV photon. This involves extensive research on photocathode preparation, characterization and photoemission properties of UV photocathodes; detector’s operating parameters which influence avalanche process, electron and ion transport properties and hence performance of the detector. Thick Gas Electron Multiplier (THGEM) based photomultipliers offer high gain, fast response, high rate capability and affordable costs with large detection area and efficient detection of light at single photon level and is therefore motivation for studying such detector and related technologies. In this thesis, a detailed study that concerns the development and performance of UV photon detector, realized by coupling THGEM with semi-transparent CsI photocathode is presented. The initial part of the thesis contains simulation studies carried out to understand the effect of gas properties and geometrical parameters of THGEM on detector’s performance. The simulation tools, ANSYS and Garfield++ allow a detailed calculation of electron avalanche developed across the THGEM hole. The CsI photocathode, used to convert UV photons into electrons is prepared by thermal evaporation technique and characterized to analyze its morphological, structural and optical properties. The challenges such as humid air exposure and damages due to electron bombardment faced during characterization are discussed here. The effect of substrate heating, during CsI film deposition, in enhancing the efficiency and increased surface area coverage, and their optical and photoemission properties are discussed in the work presented here. An improvement of 5%-20% in surface area coverage is observed with the heat treatment during deposition. A major portion of the thesis focuses on the single electron spectra obtained from the detector realized by coupling THGEM in double-stage mode with semi-transparent photocathode. The dependence of electron spectrum and performance parameters on operating parameters such as drift field, multiplication voltage and transfer field is studied in detail. Detectors are operated in the gain range of 104-105 without any spark or discharge. Simulations are carried out to interpret the experimental observations. In the next section of the thesis, position sensing capability of the THGEM UV detector has been explored using simulation. Detailed simulation study enables to optimize the parameters affecting spatial resolution for a given THGEM based imaging detector achieving better performance. Last section of the thesis deals with electroluminescence light produced during avalanche in THGEM UV detector. The light, produced in gas medium by excitation and de-excitation of gas during the electron multiplication across the THGEM holes is simulated using Garfield++. The performance of the detector is evaluated in terms of electroluminescence (EL) yield. It is shown that the EL yield depends on the gas pressure, type of gas and the applied voltage. At atmospheric gas pressure, the EL yield is found to be 105 at a gain of 104. These studies are important to analyse the scintillation capability in order to build position sensitive and large area UV detector with optical readout. The study reported here enhances understanding of THGEM based UV detectors enabling us to develop highly efficient/sensitive UV detectors for variety of applications in particle physics and astronomy.