6.5. Photodetectors 373
electron focusing structure to guide the photoelectrons to the dynode. The reason
is that the photoelectrons are produced in all directions with varying energies and
can easily go astray if they are not directed properly. Anidealelectron focusing
structure has the following two distinct characteristics.
It directs the electrons from the photocathode to the first dynode such that
all of them have the same transit time regardless of their initial energy. This
is a very important requirement specially for tubes that are to be used in fast
timing applications.It is able to focus all the photoelectrons produced in the photocathode.Of course such strict requirements can be fulfilled only by an ideal focusing structure.
However with a carefully designed structure one can achieve an electron collection
efficiency of 80% or better. One thing that should be pointed out here is that not
all PMTs have very stringent focusing requirements. Some PMT structures, as we
will see later, have large first dynodes and are therefore able to collect most of the
photoelectrons even without a focusing structure.
Focusing of electrons on to the first dynode depends on two factors: the geometry
of the focusing structure and the electric field intensity in the space between the
photocathode and the first dynode. The field intensity is generally made non-uniform
and has a higher value near the photocathode to minimize the dependence of electron
transit time on the initial electron velocity.
A.3 ElectronMultiplicationStructure...............
Earlier we saw that the quantum efficiency of a typical photocathode ranges between
10% and 30%. An efficiency of 10% means that for every 10 photons only one photo-
electron is getting out of the photocathode. For low to moderate photon fluxes this is
a problem since the resulting electron flux may not be sufficient to constitute a mea-
surable current (see Example below). Hence to obtain a measurable signal and good
signal-to-noise ratio the electron yield must somehow be increased. Now since the
photocathode materials have their own physical and engineering limitations and can
not be made more efficient, therefore the electron yield must be increased by some
other process. This is accomplished in a PMT through the electron multiplication
structure. The basic idea is to utilize the process of electron ejection from certain
metals when they are bombarded by electrons. Such metals eject more electrons
than the incident electrons provided the incident electrons have high enough energy.
This implies that the electrons must first be accelerated as well. This may sound
complicated but in actuality it can be done through a simple structure as shown in
Fig.6.5.1. This PMT has 5 dynodes and a readout electrode. Each of these dynodes
is kept at a higher potential than the preceding dynode to direct and accelerate the
electrons towards itself. The result of these successive accelerations and secondary
emissions is a cascade of electrons flowing down the dynode chain. Consequently
the initial small number of photelectrons gets multiplied into a very large number
at the final electrode. To collect these electrons another metallic structure called
anodeis placed near the final dynode. The electron current at the anode is passed
on to the associated electronics for further processing.