| A Geiger counter (named after its inventor, Hans Geiger) is a tool for detecting the fast electrons and ions emitted by radioactive materials as they decay.
The device itself is typically a metal tube with a thin wire running down the axis. The tube is filled with a noble gas and sealed. The wire then carries a +1000 volt charge relative to the tube.
/================================\ glass
/ -voltage tube \
--------+---------------------------+ \
/ | (cathode) | \
/ | | \
/ +voltage wire \
< -------------------------------------------
\ (anode) /
\ | | /
\ | | /
\ +---------------------------+ /
\ /
\===============================/
Once an ion or electron enters the tube (or an electron is knocked out of the wall by a high energy photon), it rips electrons off the atoms in the gas. The electrons are then attracted to the positive charge of the wire and move to it. While moving they gain energy and collide with other atoms which release more electrons which rapidly snowballs into a pulse of current that is detectable.
Each particle that enters the counter produces an identical pulse. The only thing known is that whatever set off the snowball had sufficient energy to enter the walls of the canister - no information about the identity or initial energy of the event can be determined. | 
