JINST Instrumentation Theses Archive

Ph.D. degree thesis
accepted by Weizmann Institute, Israel, in 2000

Efrat Shefer

Supervisor: Amos Breskin

Study of novel stable photocathode materials for gaseous photon detectors in the near-UV to the visible spectral range

 Keywords:

• Photoemission
• Photon detectors for UV, visible and IR photons (gas) (gas-photocathodes, solid-photocathodes)
• Detector modelling and simulations II (electric fields, charge transport, multiplication and induction, pulse formation, electron emission, etc)
• Photocathodes and their production

 Abstract:
This work involves the search for novel photocathode materials sensitive over the UV-to-visible spectral range and compatible with an operation within advanced gas avalanche imaging photomultipliers. Such photon detectors, based on the conversion of photons in a thin solid photocathode, followed by the emission and multiplication of the photoelectrons in gas, have numerous advantages compared to other state of the art techniques. They are parallax-free, fast and have very good time and localization resolutions. Unlike vacuum-based devices they have small sensitivity to magnetic fields and can be made very large. Detectors having sensitive area of the order of a square meter, equipped with CsI photocathodes, sensitive in the UV spectral range, are successfully employed in Cherenkov detectors in numerous particle- and nuclear physics experiments.

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 �, 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:

• Development of thin-film-protected solid photocathodes, sensitive in the near-UV to visible spectral range and viable under gaseous electron multiplication.
• Characterization of the coated photocathodes, including absolute quantum efficiency spectra, photoelectron energy distributions and chemical composition.
• Acquiring of a better understanding of low-energy (<4eV) electron transport through thin dielectric protective films and through the photocathode coating film interface.
• Coupling of a photocathode to a gaseous electron multiplication element to form a visible-photon gas avalanche detector and investigating its performance.

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:

• In the UV spectral range we investigated diamond films produced by chemical vapor deposition (CVD). These films are radiation-hard and chemically inert. Some attention was given to solar-blind CsBr photocathodes.
• We have investigated the properties of some novel gas avalanche multipliers, and operation conditions which suit their application within gas avalanche photomultipliers.

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.