VUV EMISSION AND BREAKDOWN IN PARALLEL-PLATE CHAMBERS

The emission of vacuum ultraviolet (VUV) light by the electron avalanches is responsible for the photon feedback in gaseous detectors, worsening their performance at high gains. It is also generally assumed that there is a connection between breakdown and photon feedback in these detectors. We measured the VUV light emission from the avalanches in a parallel-plate avalanche chamber (PPAC) in the spectral region 120-170 nm. The radiation was detected by a photosensitive wire chamber coupled to the PPAC through an UV-transparent window. By choosing the gas filling of the wire chamber several spectral bands could be selected, suggesting an identification of the emission origin. The breakdown limit of the PPAC was simultaneously measured and found to be fairly independent of the measured VUV emission. The results obtained provide a guide to the selection of gaseous mixtures for practical use in photon-counting detectors, minimizing the photon feedback.