What is a Geiger counter?
A Geiger counter is simply a radiation detector. It detects the presence of ionizing radiation. Ionizing radiation is an energetic alpha or beta particle, or gamma quanta that has enough energy to rip an electron from an atom, thereby creating an ion. Your typical Geiger counter uses a Geiger-Muller tube to detect the presence of ionizing radiation. The tube is a chamber filled with a gas, most commonly a noble gas, whose atom’s electrons interact with alpha, beta particles and gamma radiation passing through the tube.
As an aside we’ll also state that fast moving neutrons and protons are also capable of ionizing (by interacting with an atom’s nucleus) but they do not readily interact with the gases in the typical Geiger-Muller tube. Special neutron detector tubes can be built with walls that interact with neutrons to release alpha/beta particles that then emanate from the chamber walls into the gas-filled chamber where they can be detected.
What is a Geiger-Muller tube?
Hans Geiger in 1908 first discovered a method of detecting ionizing radiation in 1908. In 1928, he refined his design with suggestions from Walther Muller to create the Geiger-Muller tube. Modern Geiger counters still use the same general principles of this first design.
The Geiger-Muller tube is a gas-filled tube with a really thin metal wire in the center about 0.1mm in diameter. The wire has a large charge or potential of 500-2000V relative to the tube wall. An energetic alpha or beta particle that enters the tube, via a thin mica end, can collide with an electron bound to one of the gas atoms in the chamber and release an electron from the atom. Alternatively for gamma radiation, a free electron can be created by the interaction of the gamma quanta with the atoms of the tube wall. The high positive voltage on the central wire (anode) attracts the free electron and causes the electron to accelerate towards it. In the process it can collide with other electrons creating more free elections that travel towards the wire that can in turn collide with even more electrons creating a cascade or avalanche effect. This shower of electrons hit the wire and create a measurable spike in the current.
There are 3 types of Geiger-Muller tubes:
- Side-wall detectors: The side wall tube is an aluminum/stainless steel chamber. The tubes are used to detect gamma radiation, however if the wall of the tube is thin enough, high energy beta particles can also be detected. Side window tubes can have sliding windows for greater exposure of the anode and to increase efficiency.
- End-window detectors: The end-window tube is also an aluminum/stainless steel gas-filled chamber. It has a long cylindrical shape. The end-window detector has a thin mica or glass window at one end to allow alpha and beta radiation to enter the detector with minimal attenuation.
- Pancake detectors: These also have an end-window but have a larger diameter than the other types of detectors. The pancake detector looks like a truncated cylinder or a pill box. This is advantageous because the number of cascade events in the tube for beta radiation detection is proportional the surface area of the end window. The pancake detector has a larger end-window surface area than the end-window detectors and therefore is superior in measuring beta radiation. For gamma radiation the number of cascade events in the tube is proportional to the whole surface area of the detector (not just the end window surface).