Physics and Engineering of Radiation Detection

6.5. Photodetectors 377

Incident

Photons

Photocathode

Dynode Chain

Focusing Structure

Vacuumed Glass Enclosure

Anode

Electrons

Figure 6.5.7: Venetian blind type PMT.

Mesh Type:Just like the venetian blind type PMT, this structure also pro-

vides the electrons with different paths as they move down the dynode chain.

In this structure, however, the dynodes are made of meshes that are stacked

one after the other (see Fig.6.5.8). It has several advantages over other types

such as position sensitivity. less susceptibility to external magnetic field, and

good linearity.

Incident

Photons

Vacuumed Glass Enclosure

Electrons

Anode

Photocathode Dynode Mesh

Figure 6.5.8: Mesh type PMT.

Metal Channel Type: This structure is made of a large number of nar-

rowly spaced dynodes providing fast electron transit capabilities and position

sensitivity (see Fig.6.5.9).

Micro-Channel Plate (MCP) Type:This structure consists of several mil-

lion parallel electron multiplication structures in the form of micro glass tubes

(see Fig.6.5.10(a)). The typical diameter of each of the glass tubes ranges from

5 μmto 25μm. As shown in Fig.6.5.10(b), each of the channels in this struc-

ture is capable of electron multiplication. The overall gain obtainable from

a single micro-channel plate is on the order of 10^4. If higher gain is desired,

two plates can be used in succession. The main feature of a PMT constructed

with micro channel plates is its very fast time response and the possibility of

position sensitivity. Another positive aspect of this design is low susceptibility

to external magnetic fields.