JINST Instrumentation Theses Archive
2000 JINST TH 001
Ph.D. degree thesis
Supervisor: Amos Breskin
Supervisor: Amos Breskin
Study of novel stable photocathode materials for gaseous photon detectors in the near-UV to the visible spectral range
A search for materials with high photoemission yield, viable under gaseous electron multiplication is of prime importance for further development of efficient and stable gas avalanche photomultipliers. There is a strong motivation for achieving sensitivity of theses new photon imaging devices in the near-UV to visible range where numerous applications exist. It is a very difficult task, because unlike the far-UV spectral range, where photocathode materials are chemically stable, typically employed photocathodes in the visible spectral range are alkali-antimonides, which are highly reactive. Even minute levels of oxygen, water and CO2, commonly present in counting gases, result in total loss of the photocathodes sensitivity.
In this work we primarily investigated the possibility of modifying the surface of alkali- antimonide photocathodes by coating them with thin dielectric protective films. The protective coating film allows for the transport of photoelectrons through it to the gas, while preventing contact between the gas impurities and the photocathode. Its thickness, typically a few hundred Å, is a compromise between the need for high photo-yield and for chemical stability. The study of photoemission from coated photocathodes requires the investigation of low-energy photoelectron transport in the photocathode and in the coating layers, as well as in the interface between the two materials. It is of a more general interest, providing the important information regarding the electronic states of the coating material.
We therefore defined the following prime goals for this work:
In order to achieve these goals, methods were found for the preparation and coating of alkali-antimonide photocathodes in the laboratory. We studied in detail the photoemission properties of coated photocathodes by measuring quantum efficiency spectra for different coating films and photoelectron energy distributions were investigated in detail using photoelectron spectroscopy. We developed a model of low-energy electron transport through the photocathode coating film interface and the coating film. Monte Carlo calculations, based on the model, were performed providing the photoelectron energy distributions and the quantum efficiency attenuation due to the coating film. There is good agreement between the calculated and measured results.
The stability of the novel composite photocathodes
has been systematically investigated, under exposure to counting gases
and impurities and under gas multiplication conditions. The effect of
high photon flux on the photocathodes was also studied. In addition
to the prime goals and scope of this work the following related
subjects were studied:
This research work has paved the way towards imaging of visible light with gas avalanche detectors, which have numerous advantages. The novel coated photocathodes have other applications besides the field of photon detectors; they are already being investigated as possible stable laser-triggered electron sources in accelerators. From the results of our photoemission and electron transport studies we gained understanding of the low-energy electron transport mechanism in the coating film and new information regarding the photocathode coating film interface.
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