Physics and Engineering of Radiation Detection

376 Chapter 6. Scintillation Detectors and Photodetectors

Incident

Photons

Electrons

Photocathode

Vacuumed Glass Enclosure

Dynode Chain

Focusing Structure

Anode

Figure 6.5.5: Box grid type PMT.

this structure demands high degree of precision in construction to achieve best

performance.

Incident

Photons

Electrons

Photocathode

Vacuumed Glass Enclosure

Dynode Chain

Anode

Structure

Focusing

Figure 6.5.6: Circular cage type PMT.

Venetian Blind Type:In the three types of PMTs we saw earlier the elec-

trons were guided through a single path. This, of course, is not necessary as

it is also possible to construct an electron multiplication structure where elec-

trons can follow several different paths. Such a structure can minimize the

loss of electrons and even simplify the design and construction. The venetian

blind type of PMT is an example of such a design. As shown in Fig.6.5.6, the

dynodes of this type of PMT are straight and make up an array that looks like

a blind. In each row the dynodes are stacked parallel to each other and slanted

at an appropriate angle with respect to the tube axis. A feature of this design is

the larger area available to the electrons coming out of the photocathode. This

simplifies the design considerably since it relieves the focusing requirements.

The electrons can follow different paths as they travel down the dynode chain

(see Fig.6.5.6). The biggest advantage of this design is the larger area it can

cover as compared to the single-path type PMTs while its main disadvantage is

the slow response time. The slow time response occurs due to the lower electric

potential applied across the subsequent dynodes.