Physics and Engineering of Radiation Detection

380 Chapter 6. Scintillation Detectors and Photodetectors

important factor to consider is that the geometry of the last dynode also plays an
important part in efficient collection of electrons by the anode. There is also a
potential gradient between the last dynode and the anode that directs the electrons
towards the anode.

Last Dynode

Dynodes Anode

Figure 6.5.12: Sketch showing the col-

lection of electrons by the gridded anode

in a linear focused type PMT. The shape

of the last dynode is different from the

other dynodes to ensure efficient collec-

tion of charges by the anode.

A.6 SignalReadout.........................

The output current at the anode of a PMT can be directly measured by a precision
current measuring device. However the more convenient and generally used method
is to convert this current into voltage and then pass this voltage through amplifica-
tion stages to achieve good signal-to-noise ratio. The current to voltage conversion
requires addition of a load resistance to the circuit as shown in Fig.6.5.14. In addi-
tion to this load resistance there is also some capacitance present at the output as
represented byCsin the figure. The value of the load resistance is generally chosen
to be very large specially for low level current output. However, as we will see later
when we discuss the frequency response of PMTs, its value can not be set arbitrarily
high.
One practical aspect of the PMT readout is that, specially in high rate applica-
tions, the signal current at the last few dynodes may become much larger than the
voltage divider current. This would force the last dynodes to draw the current from
the voltage divider causing the interdynode voltage to change. Consequently the
tube gain would change and the response of the tube would become nonlinear. The
easiest way to overcome this problem is to provide a secondary source of current
to the last one or two dynodes. Fig.6.5.14 shows such a circuit where the last two
dynodes have been bypassed with a capacitor to ground.

A.7 Enclosure

The electron multiplication structure of a PMT is kept in a good vacuum to mini-
mize the loss of electrons. The enclosure is generally made of glass but, in principle,
can be made of any material. The entrance window, on the other hand, has strict
requirements in terms of its transparency to the incident photons. The choice of
the window material, therefore, is dependent on the type of photomultiplier tube, or
more specifically, on the spectral range of detection of the tube. Different materials