The development of CsI and Cs2Te solid photocathodes presented the breakthrough in photosensitive gaseous photodetectors we all had been waiting for, and put so much effort in finding. It was actually going back to basic. CsI had for a long time been used in vacuum photomultipliers. It proved to also work well in gas floes, and can even be exposed to air for shorter times. We will describe the development work that led to the current state of the art in solid photocathodes.
Top1. Introduction
The tests of photocathode materials used since a long time in vacuum photomultipliers, to see if they can also operate in gas flows and even be air-proof gave positive results eventually. A first successful test of a CsI photocathode combined with a parallel plate avalanche chamber and flushed with CH4 was developed by Charpak (Charpak, 1989). This photocathode, manufactured on a stainless steel cathode by a vacuum deposition technique, showed excellent stability and the highest quantum efficiency in the UV region among all solid photocathode tested earlier in gaseous detectors. Surprisingly enough, the quantum efficiency of this photocathode remained high after transfer from the evaporation system into gaseous detector in air without any special precaution.
CsI had been tested before, but not with successful results (Peskov, 1988). The problem turned out to be that the CsI was evaporated on a copper cathode and then during storage and transportation in air to the test facility it was exposed to sunlight. Both these factors (Cu cathode and sunlight) damaged the CsI coating.
The high and stable quantum efficiency of CsI opened for a wide range of new applications for photosensitive gaseous detectors. The possible applications ranged from readout of crystal scintillators to detection of Cherenkov light. This is why the report by Charpak made at a conference at Fermilab (Fermi National accelerator laboratory) in 1989 triggered a great interest in the high energy physics community to this approach (Charpak, 1989). It was also at this conference that the two authors (T. Francke and V. Peskov) met and formed a lifelong friendship and close collaboration.
Shortly after that a wire chamber with a CsI photocathode was developed (Dangendorf, 1990), and systematic studies were performed of CsI photocathodes and ways of enhancing their quantum efficiency further (Séguinot, 1990). These three publications initiated a new direction in the development of photosensitive gaseous detectors. Inspired by these initial results other groups started to actively investigate CsI photocathodes. At CERN the RD-26 collaboration was established with focus on production and characterization of large area photocathodes, susceptible to equip fast UV-photon imaging devices (Nappi, 1992; Paic, 1996).
A true fun story, originally written by one of us (V. Peskov) and recited in Sauli´s book (Sauli, 2011), reflects well the enthusiasm the spirit of these times: