Very early in the last century, it was discovered by a New Zealand-born British physicist, Ernest Rutherford that three different kinds of radiation are emitted in the decay of radioactive substances. The alpha particles, which were found to be identical with the nuclei of helium atoms and hence deflected towards the negatively charged plate in an electric field, and the beta particles, which were identified as electrons, and therefore deflected towards and the positively charged plate. However, a third kind of radiation, gamma rays which was not deflected at all was also emmited. In 1912 it was shown that these gamma rays have all the properties of electromagnetic radiation.
Gamma rays have very short wavelengths (10-9m to 10-11cm), and consist of highly energetic photons with energies between 1 MeV - 10 GeV. They have extremely high frequencies, and a very high penetrating power, which results from the fact that they have no electric charge and thus do not interact with matter as strongly as charged particles do. They carry large amounts of energy and can penetrate almost everything, including metals and concrete, and our bodies.
Gamma rays are released by some radioactive atomic nuclei (Cobalt-60 and Caesium-137), during radioactive decay and nuclear fission, where an unstable nucleus breaks into stable complex nuclei along with other atomic particles. These stable nuclei are usually in excited state and tend to release energy to come back to their ground state by emitting gamma rays.
The sun and the other stars are sources of gamma rays. Distant galaxies are also prodigious sources of gamma rays, where they are thought to be produced by very hot matter falling onto a massive black hole in the center of the host galaxy. Some gamma rays are produced in the atmosphere by cosmic ray interactions. Gamma rays also form a fraction of the cosmic rays in the space.
Gamma rays are detected by their ability to ionize gas atoms or to create electron-hole pairs in semiconductors or insulators, with the help of ionisation detectors and scintillating counters. They however cannot penetrate the layer of our atmosphere because of the high interaction between gamma photons and the atmospheric particles. Hence, special sensors aboard satellites have to be set up, to record gamma radiation from the space.
One of astronomy's most exciting branches, gamma ray astronomy provides a tool to observe the most dynamic, highest energy processes in the universe. Images of our universe taken with the help of gamma rays have revealed important information of the life and death of stars, and other numerous violent processes in the universe.
Doctors however, use the gamma rays for treatment purposes, mainly for killing cancer cells, tumors and other malignant cells in the human body. A radioactive substance, known as a tracer is put inside the human body, and its path (trace) inside the body is followed. A special gamma camera uses those rays to build up a picture. The patient however, gets only a small dose of the radiation and does not suffer.
They are also used for impurity analysis of metallurgical specimens (neutron-activation analysis), or to determine the effects of chemicals on biological systems and plants. Gamma rays can also be used for sterilising medical products.
Engineers make use of the gamma rays, to look for cracks in pipes and aircraft parts, since they can penetrate better than X-rays.
Gamma rays, have also revealed huge information about the structure of the atomic nucleus, as they interact with matter by various discrete elementary processes. Much of what is known about the internal structure and energies of nuclei has been obtained from the emission or resonant absorption of gamma rays by nuclei. Absorption of gamma rays by nuclei can cause them to eject neutrons or alpha particles. It can even split a nucleus like a bursting bubble in what is called photodisintegration. Gamma photons while passing through atomic nuclei can also lead to pair production (e.g. electron-electron pair, neutron-positive pi-meson pair, neutral pi-meson-proton pair etc.).
Gamma rays, being much rarer than many lower energy photons, are however one of the least known. We still are not aware about the origin of some of the most energetic gamma rays.