handbook of modern sensors

446 15 Radiation Detectors
convert incident light photons into low-energy electrons. Therefore, the photocathode
produces about 2000 electrons per pulse. The PM tube is a linear device; that is, its
gain is almost independent of the number of multiplied electrons.
Because all dynodes are at positive potentials (V 1 to V 10 ), an electron released
from the photocathode is attracted to the first dynode, liberating several very low
energy electrons at impact with its surface. Thus, a multiplication effect takes place
at the dynode. These electrons will be easily guided by the electrostatic field from the
first to the second dynode. They strike the second dynode and produce more electrons
which travel to the third dynode, and so on. The process results in an increasing
number of available electrons (avalanche effect). An overall multiplication ability of
a PM tube is in the order of 10 6 . As a result, about 2 × 10 9 electrons will be available
at a high voltage anode (V a ) for the production of electric current. This is a very
strong electric current which can be easily processed by an electronic circuit. A gain
of a PM tube is defined as
G = αδ N , (15.3)
where N is the number of dynodes, α is the fraction of electrons collected by the
PM tube, and δ is the efficiency of the dynode material (i.e., the number of electrons
liberated at impact). Its value ranges from 5 to 55 for a high yield dynode. The gain
is sensitive to the applied high voltage, because δ is almost a linear function of the
interdynode voltage.
A new design of a photomultiplier is called the channel photomultiplier or CPM
for short. It is the evolution of the classical photomultiplier tube. The modern CPM
technology preserves the advantages of the classical PM while avoiding its disadvantages.
Figure 15.2A shows the face plate with a photocathode, the bent channel
amplification structure, and the anode. As in the PM of Fig. 15.1, photons in the
CPM are converted inside the photocathode into photoelectrons and accelerated in a
vacuum toward the anode by an electrical field. Instead of the complicated dynode
structure, there is a bent, thin semiconductive channel which the electrons have to
pass. Each time the electrons hit the wall of the channel, secondary electrons are
(A)
(B)
Fig. 15.2. Channel photomultiplier: cross-sectional view (A) and external view with potted
encapsulation at left and without encapsulation at right (B). (Courtesy of Perkin-Elmer, Inc.)