In this chapter the main applications of gaseous photomultipliers beyond RICH detectors will described. They include applications in spectroscopy, plasma diagnostic, astrophysics, flame detection, readout gaseous and solid scintillators, and cryogenic detectors. Their advantages will be described and compared with alternative techniques.
Top1. Introduction
Currently the main application of position-sensitive photomultipliers is in RICH detectors. In this Chapter review of the most promising applications beyond the RICH detectors, e.g.:
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Plasma diagnostics,
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Spectroscopy,
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Readout of UV scintillators,
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UV visualization in daylight condition,
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Forest fire detection,
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The use of photosensitive substances in X-ray imaging.
Top2. Plasma Diagnostics
The first prototypes of MWPCs filled with photosensitive gases were developed at the same time by two independent groups, one located at CERN (Séguinot, 1977) and the other one in P.L. Kapitza laboratory in Moscow (Bogomolov, 1978).
The work of (Séguinot, 1977) is well known and led to development various RICH detectors. The application of photosensitive MWPCs for plasma diagnostic is much less known, since most of the publications were in Russian, and this is why we dedicate this paragraph to this important subject (Bogomolov, 1978).
A stationary plasma is induced by ultrahigh frequency microwaves (λmw= 25 cm or 50 cm) in a cylindrical resonator filled with H2, D2 or noble gases at pressure of 1-30 atmospheres (see Figure 1). An interesting feature of this ultra-high frequency (UHF) plasma is sporadic appearance of ionization instability leading to the creation of dense short-lived (a few μs) filaments inside the plasma volume. The typical dimensions of the stationary plasma column depend on the microwave wavelength λmv and the generator power. The length can be between 12 to 30 cm and the diameter between 1.6 and 6 cm. The typical diameter of the dense filaments is 2-4 mm. The initial dense granularity is created in the center of the plasma. Due to the positive feedback between the resonator and the UHF generator, they immediately start to grow in both directions along the resonator axis until they reach the region of low electric field where they finally decay.
Figure 1. Illustration of a UHF plasma generator (called “Nigotron”) coupled to a cylindrical resonator and diagnostic instruments used for the study of the ionization instability of the plasma
The stationary plasma column has an electron temperature about 6000°C and in H2 and D2 it emits a strong molecular spectra (H2/D2) as well as a so-called H- continuum (Diatroptov, 1972), making it difficult to observe the instability in the UV or in the visible region of spectra (λ>220 nm). In contrast, the electron temperature of the short living dense filaments in the head regions was about few eV and they emitted bursts of bremsstrahlung radiation in the VUV region (λ<160 nm).
The most appropriate technique to visualize this instability and to detect the spectrum emitted from the filaments is the use a photosensitive MWPC, which practically is not sensitive to the UV and visible part of the bright H- continuum. This allowed instability investigation practically without interference from the background (Peskov, 1980).