JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by Weizmann Institute, Israel, in 2005

Dirk M�rmann

Supervisor: Amos Breskin

Study of novel gaseous photomultipliers for UV and visible light

 Keywords:

• Photon detectors for UV, visible and IR photons (gas) (gas-photocathodes, solid-photocathodes)
• Electron multipliers (gas)
• Charge transport and multiplication in gas
• Cherenkov detectors

 Abstract:
The present thesis work was motivated by the growing demand for large-area, fast, visible- sensitive photon imaging detectors, with single-photon sensitivity, often capable of operation under high magnetic fields. Gaseous photomultipliers (GPM) unique features make them prime candidates in this field.

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.