Novel MPGD based detectors of single photons in COMPASS RICH-1

Highlights: • For the first time, single photons are detected by MPGDbased photon detectors in the upgrade of COMPASS RICH1. • The novel photon detectors are hybrid ones including two THGEM layers and a MicroMegas multiplication stage. The first THGEM also acts as photoconverting layer (CsI). • In 2017 the novel MPGDbased photon detectors have operated stably at gains above $10^4$ for the first time in an experimental environment. • $m^2$ The detector deadtime due to current sparks is kept minimum; spark rate 10/day/$m^2$ with recovery time 10 s. COMPASS is a fixed target experiment at CERN SPS aimed to study Hadron Structure and Spectroscopy. Hadron Identification in the momentum range between 3 and 55 GeV/c is provided by a large gaseous Ring Imaging Cherenkov Counter (RICH-1). To cope with the challenges imposed by the new physics program of COMPASS, RICH-1 has been upgraded by replacing four MWPCs based photon detectors with newly developed MPGD based photon detectors. The architecture of the novel detectors is a hybrid combination of two layers of THGEMs and a MicroMegas. The top of the first THGEM is coated with CsI acting as a reflective photo-cathode. The anode is segmented in pads capacitively coupled to the APV-25 based readout. The new hybrid detectors have been commissioned during 2016 COMPASS data taking and stably operated during 2017 run. In this paper all aspects of the novel photon detectors for COMPASS RICH-1 are discussed.