Physics and Engineering of Radiation Detection

6.5. Photodetectors 381

Rn−1

Rl Rn

Vout

Cs

Anode

+HV

C

Figure 6.5.13: Typical output circuit of a PMT. The anode

current is converted into voltage, which is measured across

the load resistanceRl. Csrepresents the combined stray

capacitance due to the tube, output circuit, and the cables.

Rn−1
Dynodes

To +HV and Signal

C

C

Figure 6.5.14: Bypass capacitors installed at

the last two dynodes in a PMT act as a source

of current and do not let the interdynode volt-

age to change.

have different cutoff wavelengths below which their transparency decreases to unac-
ceptably low values. The definition of cutoff wavelength is somewhat arbitrary but
generally a value at which the transmission falls to 10% is used. Fig.6.5.15 shows the
typical transmission curves for fused silica, a material commonly used to construct
PMT windows. The good thing about fused silica is that it has a very wide range in
which the transmission remains almost constant. Another advantage is that it has
low cutoff wavelength (160nm).
Fused silica, though commonly used, is not the only material available with good
transmission properties. Other materials of choice are lime glass, uv-glass,LiF,and
sapphire.