When alpha, beta or gamma particles collides with a target, some of the energy in the particle is transferred to the target, typically resulting in the promotion of an electron to an “excited state”. In many “targets”, especially gasses, this results in ionization, and alpha, beta and gamma radiation is broadly referred to as ionizing radiation. A Geiger counter (or Geiger-Müller counter) takes advantage of this in order to detect these particles. In a Geiger tube, the electron produced by ionization of a captive gas travels to the anode and the change in voltage is detected by the attached circuitry. Most counters of this type are designed to emit an audible “click” in response to the change in voltage, and to also show it on a digital or analog meter. A simple schematic of a Geiger counter is shown in Figure 1.

Figure 1 : Schematic of a Geiger counter using an "end window" tube for low penetration radiation. A loudspeaker is also used for indication. from Wikipedia (CC-BY-SA-3.0 Svjo-2).

Although these scientists did not know it at the time, all of us are subjected to a certain amount of radiation every day. This radiation is called background radiation and comes from a variety of natural and artificial radiation sources. Approximately 82 % of background radiation comes from natural sources. These include

1. sources in the earth, including naturally occurring radioactive elements which are incorporated in building materials and also in the human body,
2. sources from space in the form of cosmic rays, and
3. sources in the atmosphere such as radioactive radon gas released from the earth and radioactive atoms like carbon-14 produced in the atmosphere by bombardment from high-energy cosmic rays.