382 Chapter 6. Scintillation Detectors and Photodetectors
Wavelength ( )nm100 200 2000050100Transmission(%)Figure 6.5.15: Typical transmission curve of a fewmmthick
fused silica.A.8 Efficiency
There are two distinct processes that occur in a PMT before the electron multiplica-
tion starts. The first is the production of photoelectrons followed by their collection
by the first dynode. The photoelectron production is independent of the transit of
electrons towards the first dynode. Similarly the collection of charges by the dynode
is independent of the physical processes taking place inside the photocathode. This
implies that the efficiencies of these two processes are independent of each other. It
was previously mentioned that the efficiency of the photocathode is generally termed
as quantum efficiency orQE. The efficiency of collection of these electrons is called
collection efficiencyor simplyCE. Although there are other processes involved
that may decrease the overall efficiency of a PMT (such as collection of secondary
electrons by subsequent dynodes and by the anode) but their effect is generally too
small and the overall efficiency can be characterized by the product ofQEandCE.
These two efficiencies will be briefly discussed below.
Quantum Efficiency
Quantum efficiency defines the probability of production of photoelectrons in
the photocathode. Practically it is measured by dividing the average number of
photoelectrons emittedNeby the average number of photonsNγincident on the
photocathode, that is
QE=Ne
Nγ. (6.5.7)
The reader would recall that the number of photoelectrons produced by the incident
photons depends on the energy of the photons. Or in other words the intensity or the
flux of photoelectrons coming out of the photocathode depends on the wavelength