6.5. Photodetectors 383
of the incident photons. This implies that the quantum efficiency is also essentially
dependent on the wavelength. This is a problem for the PMT manufactures who
would very much like to assign a simple efficiency to the photocathode. Naturally
it is very difficult, if not impossible, to measure the efficiency of the photocathode
for each and every wavelength of interest. Therefore the practice is to either quote
the quantum efficiency at a few wavelengths or use another measure instead. This
other measure is the sensitivity of the PMT and will be discussed shortly.
Electron Collection Efficiency
The electron collection efficiencyCEis a measure of how efficiently the photo-
electrons are collected by the first dynode. Note that since the quantum efficiency
of typical photocathodes is not very good, therefore the collection of each and every
photoelectron is important. The collection efficiency is generally obtained by divid-
ing the average number of photoelectrons collected by the first dynodeNedyby the
average number of photoelectrons emitted by the photocathodeNepc,thatis
CE=
Nedy
Nepc. (6.5.8)
Collection efficiency depends on many factors, such as shape of the photocathode,
structure of the dynode, electric field profile in the space between the photocath-
ode and the first dynode, and the orientation of the dynode with respect to the
photocathode.
An interesting aspect ofCEis that it can be improved by changing the mechan-
ical structure and the electric field profile. In fact, the mechanical and electrical
designs of the PMTs are optimized to ensure the best possible collection efficiency.
Overall Detection Efficiency
We just mentioned that the overall efficiency of a PMT can be approximated
by multiplying the quantum efficiency with the collection efficiency. However some
developers prefer to use a direct measure of the overalldetectionefficiency instead.
The overall detection efficiency of a PMT is defined as the ratio of the average
number of pulses counted by the measuring device to the average rate of incident
photons.
=
Nc
Nγ, (6.5.9)
whereNerepresents the average count rate (average number of pulses counted per
unit time) andNγis the average rate of incident photons. Note that this is a very
useful quantity since it gives a direct measure of the efficiency of the whole system.
The difficult part, however, is again the wavelength dependence of the photoelectric
effect. Therefore, as with quantum efficiency, the overall detection efficiency is also
quoted only at certain photon wavelengths. Another thing worth noting is the ease
in determining this efficiency, since it can be done simply by illuminating the tube
with a source of known strength and measuring the output response.