Gaseous ionization detector

If a particle has enough energy to ionize a gas atom or molecule, the resulting electrons and ions cause a current flow which can be measured.

The accompanying plot shows the variation of ion pair generation with varying applied voltage for constant incident radiation.

[1] The strength of the electric field between the electrodes and the type and pressure of the fill gas determines the detector's response to ionizing radiation.

Each ion pair produces a single avalanche so that an output current pulse is generated which is proportional to the energy deposited by the radiation.

The main advantages of these microelectronic structures over traditional wire chambers include: count rate capability, time and position resolution, granularity, stability and radiation hardness.

The UK Health and Safety Executive has issued a guidance note on the correct portable instrument for the application concerned.

A small source of radioactive americium is placed so that it maintains a current between two plates that effectively form an ionisation chamber.

Plot of variation of ion pair current against applied voltage for a wire cylinder gaseous radiation detector.
Families of ionising radiation detectors
Schematic diagram of ion chamber, showing drift of ions. Electrons typically drift 1000 times faster than positive ions due to their much smaller mass. [ 2 ]
The generation of discrete Townsend avalanches in a proportional counter.
Visualisation of the spread of Townsend avalanches by means of UV photons