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.
Top1. Photoionization Of Gases
Photons passing through a gas volume may experience absorption if their energy Eν > Ethr where Ethr is a characteristic energy which depends on the particular gas. When absorption occurs, the intensity of the light beam along its propagation line will be attenuated. For example, if Nph monochromatic photons enter the gas volume having a thickness of l then the number of photon exiting this volume, Nphex, will be:Nphex = Nph exp (-σabs Ngas l), (1) where σabs is the absorption cross section and Ngas is the number of the light absorbing atoms or molecules per cm3.
As will be shown later, absorption plays an important role in operation of gaseous detectors as it affects their efficiency.
If the photon energy Ev is higher than the ionization potential of an atom or a molecule, Ei, (typically Ei > Etr) it can photoionize the gas. In a general case this a complicated phenomenon which has many energy transfer channels and even may end up in the creation of few photoelectrons per absorbed photon.
In this chapter, for practical reasons, we will mainly be interested in the process leading to the emission of photoelectrons by photons with an energy Eγ in the range Eshell > Ev > Ei, where Eshell is the energy required to liberate an electron from the inner shell of a given atom.
Schematically this process can be represented in the following way:hν + M = M+ + e-, (2) where hν is a photon with energy Eγ = hν, (h is the Plank constant and ν is the frequency of the light), M and M+ are the molecule and the molecular ion, respectively, and e- is a free electron.
Even in the relatively narrow energy interval Eshell > Ev > Ei the photon energy dissipation may occur via several mechanisms, leading to the fact that the ionization efficiency even in the case of full light absorption is often below 100%.
In applications such as gaseous photodetectors two parameters are especially important, Ei and the quantum efficiency (QE) defined as the number of created photoelectrons per incident photon. For historical reasons some academic authors prefer to use terms photoionization efficiency or photoionization yield, which are in fact the same as quantum efficiency.
Benzene was the first photosensitive vapor successfully used by Séguinot in his first photosensitive MWPC (Séguinot, 1977). One of the early measurements of the quantum efficiency of benzene vapors C6H6 and its isotope C6D6 (ηH and ηD, respectively) are presented in Figure 1. In the same figure are also shown the photo absorption cross sections for the same vapors. As can be seen, in the case of benzene vapors the photoionization starts at Eγ > 9 eV corresponding to wavelengths shorter than 138 nm. The quantum efficiency at high enough photon energies reaches 60%. The deuterium isotope has slightly higher quantum efficiency at all wavelengths than normal benzene. Please also note that the absorption cross section has no correlation with the quantum efficiency.
Figure 1. The quantum efficiency of benzene C6H6 (ηH) and its isotope C6D6 (ηD), and the absorption cross sections, σH and σD, measured for C6H6 and C6D6, respectively as a function of photon wavelength (Person, 1965). As can be seen, in the case of benzene vapors the photoionization starts at Eγ > 9 eV corresponding to wavelengths shorter than 138 nm. The quantum efficiency at high enough photon energies reaches 60%. The deuterium isotope has slightly higher quantum efficiency at all wavelengths than normal benzene. Please also note that the absorption cross section has no correlation with the quantum efficiency.