Study and development of UV photodetectors envisaging COMPASS RICH-1 upgrade and applications

COMPASS is an experiment at CERN’s SPS whose goal is to study hadron structure and spectroscopy. The experiment includes a wide acceptance RICH detector, operating since 2001 and subject to a major upgrade of the central region of its photodetectors in 2006. The remaining 75% of the photodetection area are still using MWPCs from the original design, who suffer from limitations in gain due to aging of the photocathodes from ion bombardment and due to ion-induced instabilities. Besides the mentioned limitations, the increased luminosity conditions expected for the upcoming years of the experiment make an upgrade to the remaining detectors pertinent. This upgrade should be accomplished in 2016, using hybrid detectors composed of ThGEMs and MICROMEGAS. This work presents the study, development and characterization of gaseous photon detectors envisaging the foreseen upgrade, and the progress in production and evaluation techniques necessary to reach increasingly larger area detectors with the performances required. It includes reports on the studies performed under particle beam environment of such detectors. MPGD structures can also be used in a variety of other applications, of which nuclear medical imaging is a notorious example. This work includes, additionally, the initial steps in simulating, assembling and characterizing a prototype of a gaseous detector for application as a Compton Camera.

Thick-microstructures for MPGDs: simulations and experimental studies

Cherenkov Imaging counters require large photosensitive areas, capable of single photon detection, operating at stable high gains under radioactive backgrounds while standing high rates, providing a fast response and a good time resolution, and being insensitive to magnetic fields. The development of photon detectors based in Micro Pattern Gaseous detectors (MPGDs), represent a new generation of gaseous photon detectors. In particular, gaseous detectors based on stacked Thick-Gaseous Electron Multipliers (THGEMs), or THGEM based structures, coupled to a CsI photoconverter coating, seem to fulfil the requirements imposed by Cherenkov imaging counters. This work focus on the study of the THGEM-based detectors response as function of its geometrical parameters and applied voltages and electric fields, aiming a future upgrade of the Cherenkov Imaging counter RICH-1 of the COMPASS experiment at CERN SPS. Further studies to decrease the fraction of ions that reach the photocathode (Ion Back Flow – IBF) to minimize the ageing and maximize the photoelectron extraction are performed. Experimental studies are complemented with simulation results, also perfomed in this work.