9780521516358book.pdf

Ionisation counters are used for routine monitoring of the laboratory to check for

contamination. They are also useful in experimental situations where the presence or

absence of radioactivity needs to be known rather than the absolute quantity, for

example quick screening of radioactive gels prior to autoradiography, checking that

A

+ + +

+

• 
  

  – –

  
  

Fig. 14.2Detection based on ionisation.

(a)

Insulated base

Metallised cathode

surface

Anode wire

Glass bead

End-window

(b)

Count rate

Continuous discharge region

Threshold

potential

G-M plateau

(approx 300 V)

Operating

potential

Breakdown

potential

Starting potential

Applied voltage

Fig. 14.3(a) The Geiger–Mu ̈ller (G–M) tube and (b) the effect of applied voltage on count rate.

562 Radioisotope techniques