The photocathode is usually an alloy of cesium and antimony that releases
electrons after absorption of light photons. The PM tube is fixed on to the
NaI(Tl) crystal with the photocathode facing the crystal by a special optical
grease or connected to the crystal using light pipes.
A high voltage of ~1000 V is applied between the photocathode and the
anode of the PM tube in steps of 50–150 V between dynodes (see Fig. 8.1).
When light photons from the NaI(Tl) crystal strike the photocathode,
photoelectrons are emitted, which are accelerated toward the next closest
(i.e., first) dynode by the voltage difference between the electrodes.
Approximately one to three photoelectrons are produced from the
photocathode per 7 to 10 light photons. Each of these photoelectrons is
accelerated to the second dynode and emits two to four electrons upon
impingement. The accelerated electrons strike the successive dynodes, and
more electrons are emitted. The process of multiplication continues until
the last dynode is reached, where a pulse of 10^5 to 10^8 electrons is produced.
The pulse is then attracted to the anode and finally delivered to the pre-
amplifier. The amplitude of the pulse is proportional to the number of light
photons received by the photocathode and in turn to the energy of the g-
ray photon absorbed in the detector. The applied voltage must be very
stable, because slight changes in dynode voltage cause a great variation in
electron multiplication factor.
Preamplifier
The pulse from the PM tube is small in amplitude and is initially amplified
by a preamplifier. The preamplifier adjusts the voltage of the pulse (pulse
shaping) and matches impedance level between the detector and the sub-
sequent circuits so that the pulse is appropriately processed by the system.
Linear Amplifier
A linear amplifier amplifies further the signal from the preamplifier and
delivers it to the pulse height analyzer for analysis of its amplitude. The
amplification of the pulse is given by the amplifier gain expressed as the
ratio of the amplitude of the outgoing pulse to that of the initial pulse from
the PM tube. The amplifier gains are given in the range of 1 to 1000 by gain
control knobs provided on the amplifier. The output pulses normally have
amplitudes of up to 10 V.
Pulse-Height Analyzer
g-rays of different energies can arise from a source of the same or different
radionuclides or can be due to scattering of g-rays in the source and the
detector. Thus, in counting a radioactive source, the pulses coming out of
the amplifier may differ in magnitude. A PHA is a device that selects for
Solid Scintillation Counters 87