JINST Instrumentation Theses Archive
Ph.D. degree thesis
Supervisor: Amos Breskin
Study of novel gaseous photomultipliers for UV and visible light
This work explores the concepts of novel cascaded hole-multipliers and the possibility of expanding the sensitivity of such cascaded GPMs to the visible spectral range. This new generation of GPMs, with single-photon sensitivity and imaging capability is based on the Gas Electron Multiplier (GEM), produced in thin metalized insulators by photochemical etching techniques.
The properties of multi-GEM GPMs with reflective, UV-sensitive photocathodes deposited on the first GEM element were studied; conditions are presented for their operation at high gains (>106), in a variety of gases, with efficient photoelectron collection and avalanche-electron transmission in cascaded GEM elements. Such photon detectors have superior properties, in many aspects (e.g. photon- and ion-feedback, maximum gain, time and localization resolution etc.) to that of conventional wire-chamber GPMs. Simple position-sensing methods, e.g. adequate for sealed detectors, were investigated; 2-D single-photon localization resolutions in the 150 micrometer range were reached with a patterned Wedge & Strip readout electrode. Time resolutions in the 2 to 0.3 ns range were measured with single- and 150 photons, respectively. These UV-sensitive GPMs already found applications in particle-physics experiments.
Extended studies were carried out towards the conception and assembly of sealed visible-sensitive GPMs and their operation with cascaded GEMs and bialkali photocathodes. Operation instabilities due to ion-feedback effects were found to be the main obstacle towards high-gain operation with bialkali photocathodes in this spectral range. Detailed investigations of the ion-feedback processes provided quantitative results on their probability in different gas mixtures. Methods to suppress the backflow of avalanche-generated ions were investigated, both by optimizing the detector's electric-field configurations and by introducing a pulsed ion-collection gate-electrode; the latter permitted reducing the ion backflow by factors >104. The ion-gate permitted, for the first time, to operate a visible-sensitive GPM with a bialkali photocathode at avalanche gains close to 106.
The results of this research constitute an important milestone in the field of gaseous photomultipliers: for the first time the spectral sensitivity of such detectors was expanded to the visible range, with single-photon sensitivity. This paves ways for further developments in this field, aiming at stable, sealed, large-area detectors operating in a DC mode.
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