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Dynodes absorb fast-moving electrons and

create larger number of slower-moving electrons

etc.

Light-sensitive surface

emits photoelectrons Pulse height

analyser

Scaler

High voltage accelerates

electrons down the tube

Fluor

(a)

% of total particles at each energy level

Energy (MeV) of radiation or pulse height from photomultiplier

Emax

(b)

Fig. 14.5(a) The mode of action of a photomultiplier and (b) the energy spectrum of a typicalb-emitter.

Crystal phosphor

e.g. Nal (g)

PM tube

High

voltage

supply

Pulse

height

analyser

Scaler

(a)

Sample added to

liquid scintillator

PM tube

High

voltage

supply

Pulse

height

analyser

Scaler

(b)

Fig. 14.6Diagrammatic illustration of (a) solid and (b) liquid scintillation counting methods.

565 14.3 Detection and measurement of radioactivity