Gamma rays refer to the high energy photons that are emitted by radioactive nuclei. They are a form of electromagnetic radiation, just like visible light or x rays, but with a much higher energy. They are only produced (artificially) in highly sophisticated devices called particle accelerators, and (naturally) in outer space. The gamma ray spectrum is usually defined as light having a frequency between 1018 and 1021 hertz (cycles per second) and wavelengths of about 10-11 meters.
Gamma rays are more energetic than x rays, but are less energetic than cosmic rays. Gamma ray photon with of frequency of 1019 Hz is approximately 10,000 times more energetic than x rays . Gamma rays are sufficiently energetic to cause nuclear transitions within atoms and, thus, also correspond to the energy release that accompanies the transition of an excited nucleus to its ground (most stable) state. Gamma rays interact with material by colliding with the electrons in the shells of atoms.
They lose their energy slowly in material, being able to travel significant distances before stopping. Depending on their initial energy, gamma rays can travel from 1 to hundreds of meters in air and can easily go right through people. Important sources of gamma radiation include natural sources are: medical uses, atmospheric nuclear weapons tests, nuclear accidents, and nuclear power generation. Ionizing radiation is present naturally in the environment from cosmic and terrestrial sources.
Cosmic radiation is a minor source of exposure to gamma radiation but stars that are moving fast (shooting stars) emit a lot of gamma rays that can affect and penetrate the earth’s atmosphere; most natural exposure is from terrestrial1 sources. Soil contains radioactivity derived from the rock from which it originated. However, the majority of radioactive elements is chemically bound in the earth’s crust and is not a source of radiation exposure unless released through natural forces (earthquake or volcanic activity) or human activities like mining or construction.
Generally, only the upper 25 cm of the earth’s crust is considered a significant source of gamma radiation. The Geiger counter is one of the oldest and simplest of the many particle detectors. The counter was developed in the early part of the twentieth century by Hans Geiger and Wilhelm Muller, shortly after the discovery of radioactivity. A wire electrode runs along the centerline of a cylinder with conducting walls. The tube is usually filled with a monatomic gas such as argon at a pressure of about 0. 1 atmospheres.
A high voltage, slightly less than that required to produce a discharge in the gas, is applied between the walls and the central electrode. A rapidly moving charged particle which gets into the tube will ionize some of the gas molecules in the tube, triggering a discharge. The result of each ionizing event is an electrical pulse that can be amplified to activate earphones or a loud speaker, making the counter useful in searches for radioactive minerals or in surveys to check for radioactive contamination. Scintillation counters are made from materials which emit light when charged particles move through them.
To detect these events and to gain information about the radiation, some means of detecting the light must be used. One of the first scintillation detectors was a glass screen coated with zinc sulfide. This sort of detector was used by Ernest Rutherford in the early versions of his classic experiment in which he discovered the nucleus of the atom by scattering alpha particles from heavy atoms such as gold. The scattered alpha particles hit the scintillating screen, and the small flashes produced were observed by experimenters using only the human eye.