Table of Contents
Advertisement
It is interesting to see how the GM tube detects radioactivity. A 500-volt potential is applied to the anode
(center electrode) through a ten mega-ohm current limiting resistor. To the cathode of the tube a 460-k ohm
resistor is connected.
In the initial state the GM tube has a very high resistance. When a particle passes through the GM tube, it
ionizes the gas molecules in its path. This is analogous to the vapor trail left in a cloud chamber by a parti-
cle. In the GM tube, the electron liberated from the atom by the radioactive particle and the positive ion-
ized atom both move rapidly towards the high potential electrodes of the GM tube. In doing so they collide
with and ionize other gas atoms. This creates a small conduction path allowing a momentary surge of elec-
tric current to pass through the tube.
This momentary pulse of current appears as a small voltage pulse across R2. The halogen gas quenches
the ionization and returns the GM tube to its high resistance state making it ready to detect radioactivity.
Each output pulse from the GM tube is a count. The counts per second give an approximation of the
strength of the radiation field. Below is the GM tube used in the DTG-01's response to a cesium-137
source.
DTG-01
C oun t Ra te vs Dos e R a te
13
Advertisement
Table of Contents
