Understanding Neutron Radiography Post Exam Reading VIII-Part 2a of 2A

Townsend Discharges (Avalanches)
The Townsend discharge or Townsend avalanche is a gas ionisation process
where free electrons are accelerated by an electric field, collide with gas
molecules, and consequently free additional electrons. Those electrons are in
turn accelerated and free additional electrons. The result is an avalanche
multiplication that permits electrical conduction through the gas. The
discharge requires a source of free electrons and a significant electric field;
without both, the phenomenon does not occur.
The Townsend discharge is named after John Sealy Townsend, who
discovered the fundamental ionisation mechanism by his work between 1897
and 1901.
Charlie Chong/ Fion Zhang
https://en.wikipedia.org/wiki/Townsend_avalanche