Physics and Engineering of Radiation Detection

6.5. Photodetectors 375

issues the performance of a PMT is susceptible to mechanical jitters, which may
lower the ability of one or more dynodes to focus or direct the electrons. Loss of
electrons leads to signal deterioration and possibly non-linearity and therefore the
sensitive mechanical structure of a PMT is one of its major disadvantages. However,
as we will see later, some PMT designs are less prone to mechanical instabilities and
should therefore be preferred if the environment is mechanically unstable.
The electron multiplication structure shown in Fig.6.5.1 is by no means a standard
for PMTs. In fact, PMTs are manufactured in many different geometries. The
choice of the geometry is mainly application dependent. In the following we will
look at the most common of the PMT geometries and discuss their advantages and
disadvantages.

Linear Focused Type: This is perhaps the most famous type of electron

multiplication structure (see Fig.6.5.4). Generally it is used in the head-on

type of PMT. The distinguishing feature of this design is its extremely fast

time response, which is achieved by positioning the dynodes to minimize the

electron transit times. At each multiplication stage the electrons are focused

by high electric field between the carefully placed dynodes, leading to a very

fast response time. This design is susceptible to external magnetic field.

Incident

Photons

Photocathode

Dynode Chain

Vacuumed Glass Enclosure

Focussing Structure

Electrons

Anode

Figure 6.5.4: Linear focused type PMT.

BoxandGridType:This is a somewhat simplified version of the linear

focused type PMT. Such a structure is made of several quarter cylindrical

dynodes installed in succession as shown in Fig.6.5.5. Since the collection area

of the first dynode is larger than in the linear focused type therefore its focusing

requirements are a bit relaxed. Although this simple design looks identical to

the linear focused type but it lacks the progressive focusing of electrons between

the dynodes. Hence it has slower response time and is therefore not a popular

choice for most applications. Like linear focused type, this design also suffers

from magnetic field dependent sensitivity.

Circular Cage Type:This is a very compact structure with dynodes arranged

in a circular fashion (see Fig.6.5.6). This type of PMT has a fast response time

and is therefore suitable for use in timing applications. Although most of

the circular cage type PMTs have side-on type structure but this geometry

can be used to construct head-on type PMTs as well. The compactness of