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.
Top1. Introduction
The currently most interesting developments in the field of gaseous photodetectors are electron multipliers with a high sensitivity to the visible light. In order to emphasize the great importance of these efforts, it will be useful to start with a review of other types position-sensitive photomultipliers, vacuum and solid-state, which appeared in the market in the 1990s.
Non position-sensitive vacuum photomultipliers which are sensitive visible light has existed since the 1930ies. In the 1950ies came the first position-sensitive vacuum devices such as TV camera/ tubes and image intensifiers. Due to high levels of electronic noise they were not sensitive to single photons.
Vacuum photomultipliers use various compounds as highly efficient reflective and semitransparent photocathodes, e.g. SbCs, bialkali and many others. The spectral sensitivities as a function of wavelength for some of them are shown in Figure 1.
Figure 1.
Radiant sensitivity of some photocathodes used in vacuum photomultipliers; the dashed lines indicate levels of equal quantum efficiency.
Note that the sensitivity of any photodetector can be expressed either in units of the quantum efficiency as was used in the previous chapters, or in the units of the radiant sensitivity as used in Figure 1. The latter is defined as the photocurrent from the photocathode (in mA) per radiated power (in W) in a certain wavelength interval.
As can be seen, photocathodes optimized for the visible region of spectra have quantum efficiency between 25 and 50%. The active areas of these devices are typically between 3 and 15 cm2. Some special designs with a few cm thick entrance windows to withstand the atmospheric pressure may have active areas up to1200-1500 cm2. These gigantic photomultipliers were manufactured in the past in Russia and also by Hamamatsu photonics in Japan. To produce photomultipliers with larger active surfaces is difficult due to the serious mechanical constrains on the window size.
In the last decades position-sensitive PMs were introduced into the market having grid type dynodes combined with a multi-anode readout structure. The typical spatial resolution of these detectors is about 1 mm and their maximum effective area is about 30 cm2.
Another type of position-sensitive photomultipliers is vacuum microchannel plates (MCPs). These devices are usually combined with semi-transparent photocathodes emitting photoelectrons which are multiplied inside the capillary holes via a secondary-electron emission process. Some of these devices offer a position resolution about10 μm. However, the maximum sensitive area of this type of detector is around 100 cm2.
Other types of position sensitive photodetectors, which recently has started to compete with vacuum PMs, are the solid-state devices, e.g. avalanche photodiodes (APD) and silicon photomultipliers (SiPM).
APD is a highly sensitive electronic device which converts light to primary electrons via photoelectric effect. Its unique feature is that at high reverse bias voltage (typically 100-200 V), an avalanche multiplication process starts inside the semiconductor material and a current gain around 100 can be reached. Some advanced silicon APDs with alternative doping allows an even greater bias voltage (> 1500 V) without breakdown resulting in gains of >1000.
Much higher gains, up to 105-106, can be reached in certain APDs operating in Geiger mode. In these devices the reverse bias voltage is above the APD's breakdown limit. As in the case of gaseous Geiger counters, such APD needs to have means for quenching the breakdown rapidly. The Geiger mode is particularly useful for single photon detection provided that the dark current event rate is sufficiently low. An APD is not positions sensitive. A matrix of Geiger APDs can be used for rough position measurements.
The breakthrough happened after the introduction of silicon photomultipliers, or SiPMs, which is simply speaking an array of micro-APDs on a common carrier. The dimension of each single APD can vary from 20 to 100 μm, with a density up to 1000 APDs per mm2. Each micro-APD operates in Geiger mode and is coupled with the others via a quenching resistor.