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.