Physics and Engineering of Radiation Detection

6.5. Photodetectors 379

A.4 VoltageDividerCircuit

Proper focusing and acceleration of electrons at each stage of multiplication requires
establishment of high potential gradient between the subsequent dynodes. We will
see shortly that the electrons released by the last dynode are collected by the readout
electrode called anode, which is kept at a higher potential than the last dynode. All
the potentials to the dynodes and the anode can be provided from a single high
voltage supply through a voltage divider circuit as shown in Fig.6.5.11.

Rn−1

R 3

R 2

R 1

Ra Rn

Photocathode

Dynodes

Output

Anode

+HV

C

C

Figure 6.5.11: Typical voltage di-

vider circuit of a PMT.

A.5 ElectronCollection.......................

The electrode that is used to measure the current is called anode. It is generally
made up of a metallic grid and installed near the last dynode (see Fig.6.5.12). The
spaces in the grid ensures that the electrons reach the last dynode without any
appreciable collection by the anode. Since the secondary electrons from the last
dynode are not accelerated towards another dynode therefore they form a space
charge near the surface of the dynode. The gridded anode quickly collects these
charges so that the linearity of the PMT response does not get compromised. One