Other Applications of Photo-Sensitive Detectors

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.

A NaF RICH Counter

RICH detectors are an important type of photosensitive detectors often used in particle and astroparticle physics for particle identification. They are quite complicated to construct and operate. The following chapters will describe in more detail examples of such detectors and their performance.

Cherenkov Light

Consider an electron approaching a sample of glass with a velocity close to the speed of light, c. As the electron moves through the glass light will instantly be emitted along its track, if its velocity is high enough. Even more, the electron will leave the glass sample before the light since the velocity of the particle inside the sample is larger than the speed of the light. At first sight, this seems to be in contradiction to Einstein's theory of special relativity, which states that nothing can travel faster than the speed of light, but one often forgets the important condition: in vacuum. This is the story of light emitted at particle travelling faster than the speed of light.

Alternative Position Sensitive Photomultipliers

In this chapter, alternative position sensitive vacuum and solid state detectors are described. We start with a description of the vacuum photomultiplier tubes and how they have evolved into solid state detectors. These devices are promising, but still fall short compared to gaseous detectors.

Electron Multiplication and Electron Multipliers

This chapter describes the process of electron avalanche multiplication, which is the only method known to detect the smallest possible electrical signal, a single electron at rest. Electron avalanche multiplication can advantageously be used in position sensitive photomultipliers to with high efficiency detect single photons with respect to position, time, direction and energy.

Conversion of UV and Visible Photons to Photoelectrons

The operation of most of gaseous photomultipliers is based either on gas photoionization or on photoelectric effect from solid photocathodes. There have also been attempts to use liquid photocathodes which offer lower ionization thresholds compared to the corresponding vapors. A great success has been achieved with solid photocathodes covered with adsorbed layers of some photosensitive vapors which reduce the cathode work function and as a result extend the photosensitivity threshold towards long wavelengths. It also enhances their quantum efficiencies sometime on a factor of two. The main physic mechanisms of interactions of UV photons with gases as well as with liquid and solid photocathodes are described in detail in this chapter. This basic knowledge is important when designing and using gaseous photodetectors.

Gaseous Detectors Sensitive to Visible Light

In this Chapter the latest generation of gaseous photomultipliers, sensitive up to visible region of spectra are described. So far we have described the successful development of gaseous detectors with high sensitive to ultraviolet light, capable of detecting single photoelectrons with 100% efficiency and which can be operated stable for many years in high flux environments. The next step is to develop such detectors sensitive to visible light.

Early Work on UV Sensitive Solid Photocathodes for Gaseous Detectors

Ultimately, a solid photocathode is advantageous over gaseous and liquid photocathodes. Gaseous detectors with solid photocathodes are simpler to manufacture and have better time resolution than detectors filled with photosensitive gases. They should be able to operate at high gas gains and detect single photoelectrons. Finally, compared to vacuum photomultipliers they have little sensitivity to magnetic fields and can measure the coordinate of photon conversion and can be made with large areas and arbitrary shapes. All this proves that gaseous photomultipliers with solid photocathodes open a new page in photosensitive detection technique. We will here review the early development of solid photocathodes, and the road towards the modern photocathodes which have revolutionized the photosensitive detector technology.

Position-Sensitive Gaseous Photomultipliers Filled with Photosensitive Vapours

The ultimate goal of all development of photosensitive detectors is to find a detector capable of detecting single photons with high efficiency. Furthermore, the photon shall not only be detected as a photon somewhere. We want to know where it was with high precision in space, often down to a few micrometers. We want to know when it was there, preferably with a precision of less than a nanosecond. We want to know where and when for each individual photon in a high flux of photons. Sometimes we even want to know the polarization of each photon. Position-sensitive gaseous photomultipliers filled with photosensitive vapours are capable of all of this. It is a challenging task. A single photon is the weakest light there is. For UV and visible light the energy in the photon is so low that it can barely emit a single electron through photoelectric effect with a gas. This photoelectron has practically no kinetic energy when it is released. A single electron at rest is the weakest possible electrical signal there is, so the detector must be able to amplify this extremely weak signal without any noise. We will here describe the history of photosensitive gaseous detectors, their applications and what the state of the art technology is today.

Liquid Photocathodes

Liquid photocathodes were studied intensely for a number of years around 1988. Initially it was observed that the gaseous compounds used as photocathodes were absorbed on surfaces, making them slightly photosensitive. This opened up the dream of a well-defined photosensitive layer. In a photosensitive detector with a gaseous photoelectric converter it is difficult to know where each individual photoelectron is actually emitted. The conversion volume has to be made thick enough to allow efficient conversion of the incoming photons. This smears the position resolution, and reduces the time resolution. A thin layer photocathode would eliminate this smearing in space and time. Furthermore, the gas system might be simplified, or even removed with such a liquid cathode. We summarize the results of these studies, which led to the important development of solid photocathodes in gaseous detectors.