390 Chapter 6. Scintillation Detectors and Photodetectors
050100−h 0 +h(a)Side−on Type PMTHead−on Type PMT010050−r 0 +r(b)PMT Sensitivity (%)PMT Sensitivity (%)Distance from CenterDistance from CenterFigure 6.5.18: (a) Typical an-
ode sensitivity curves depicting
the response of PMTs to light
incident on different photocath-
ode positions normal to their
entrance windows. (a) Side-on
type PMT. (b) Head-on type
PMT. The abrupt decrease of
anode sensitivity at the edges
requires care in tube position-
ing.by the first dynode is almost independent of time. As the electrons move down
the multiplication chain, their time distribution widens due to statistical nature of
the process of electron emission from the dynodes. This implies that even if the
initial photon pulse is a delta function, the output pulse at the anode will have a
finite spread. The rise time of the output pulse is one of the two parameters used
to characterize the time response of the PMT. The other parameter is the transit
time of the electrons. The transit time is simply defined as the time between the
peak of the anode pulse and the time of incidence of the photons (see Fig.6.5.19).
The definition of this rise time is, however, somewhat arbitrary. Conventionally it
is defined as the time it takes the anode pulse to rise from 10% to 90%.
If a train of very narrow width pulses are incident on a PMT, the transit time, rise
time, and the decay time will show fluctuations about their mean values. For a well
designed and constructed PMT, these fluctuations are due mostly to the statistical
nature of the underlying physical processes. The spread in the transit time is what is
used to characterize the time response of a PMT. The larger the spread the less time
resolution the PMT has. The time spread can be obtained by taking the Fourier
transform of the pulse transit times obtained by illuminating the PMT with very
narrow width photon pulses. The FWHM of the frequency spectrum is generally
taken as the transit time spread. The spread in the rise and decay times is, in
most cases, very small. For the applications where the reproduction of the incident
photon pulses is desired, these two times and their spread become very important.
One of the main reasons for the popularity of the linear focused type PMT is
its superior time response characteristics compared to most other types of PMTs.
The reason lies in the optimized dynode structure, placements, as well as the high