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.
Top1. Introduction
The threshold of sensitivity of photosensitive detectors is determined by the ionization potential of the gases, Ei, or the ionization threshold, Eliq, in the case of liquids. Eliq is related to Ei as described in Formula (3) in Chapter 1. The gases with the lowest ionization potential, which are used in physics experiments, are TMA, TEA, EF and TMAE. They were discovered a long time ago and afterwards there has actually been no real progress in finding suitable photosensitive substances, i.e. materials with lower ionization potentials. This is mainly because the substances with low ionization potential are usually chemically aggressive and/or unstable in air. This is why there has always been an interest to combine gaseous detectors with solid photocathodes.
The potential advantages of such an approach are:
- 1.
Very low thresholds of spectral sensitivity, i.e. they are sensitive to light of long wavelengths. This is given by the work function of the material.
- 2.
A high quantum efficiency.
- 3.
A high time resolution (better than 1 ns) since there are no space jitters in creating photoelectrons (they all come from the well-defined cathode).
- 4.
They can be manufactured with a large sensitive area since there is mechanical constrains on the window size.
What solid photocathodes can be use and what has been tested?
We already discussed earlier that metals, even those with a low work function, φ, are not efficient photocathodes for two main reasons. They have high reflectivity for UV and visible light, and the electron quickly loses their kinetic energy in collisions with other electrons inside the metal. As a result, only electrons created very close to the surface (typically a few atomic layers) can escape from the metal. Therefore, other types of photocathodes should be considered.
One should note that for vacuum photomultipliers there have been a lot of developments of various solid photocathodes. The main focus of these developments was on photocathodes sensitive to visible light (Sommer, 1968). It was shown, both experimentally and theoretically, that among various photocathode materials the most efficient are the semiconductors. In simple words this is because in a semiconductive material the electrons interact with phonons and the corresponding energy losses are small. Therefore, photoelectrons from deeper regions can reach the surface with energies above the mean thermal energy. Moreover, most semiconductors have a much greater absorption coefficient than metals.
There were early attempts to use some of the photocathodes developed for vacuum photomultipliers in gas filled photodiodes operating at small gas gains: 10-100 (see for example Sauli, 1982; Charpak, 1983 and references therein). Such devices were not only studied in the laboratories, but were even commercially available from various photonic companies. Examples are RCA Phototubes and photocells, EMI Industrial Electronics, ITL Instrument technology, Hamamatsu etc., (see for example RCA Technical manual PT-60, 1963). They are now replaced by solid-state detectors, and they have virtually disappeared from the market.
Studies show that the maximum achievable gains of these photodiodes are limited. It depends on gas type, and is mainly limited either by ion feedback or both by ion and photon feedback. High gains sufficient to detect single photoelectrons, are achieved only in quenched gases and with photocathodes having modest sensitivity in the UV region (although superior to liquid TMAE photocathodes) (Peskov, 1988). However, high stability with time is achieved only with very clean gases and this is why these photodiodes can be used only in sealed mode. Attempts to use them in gas-flush mode have been unsuccessful (Charpak, 1992).