The study of radioactive material started with Becqueral
in 1896, when he discovered that some of his films became
unuseable, as if they had been exposed to light, even though
they were kept in a sealed opaque container. Further investigation
revealed a substance that gave a green glow in the dark.
This was uranium, and Becqueral discovered the X-ray.
Marie Curie then studied radioactive material, discovering
several radioactive substances. This research led to a better
understanding of radiation, and peopls started bombarding
elements with particles emitted from radiation. Rutherford's
experimentations, for example, led him to his atomic model.
These bombardments also led to the discovery of many
particles, such as the proton.
The nature of radiation was also better understood as
the identity of the radiated particles came to be known,
and theories of radioactive decay were devised.
These experiments did not reveal a complete picture of radiation,
until Fermi's systematic bombardment of all elements.
The results led him to envision a chain reaction, where
the bombardment of an atom led to its splitting
and emitting more particles that bombarded the other atoms.
This was carried out in 1942, and led to the development
of the A-bomb shortly after.
Alpha radiation (or alpha rays) was distinguished and named by E. R. Rutherford in 1909, who found by measuring the charge and mass of alpha particles that they are the nuclei of ordinary helium atoms. Alpha particles consist of two protons and two neutrons.
Beta radiation (or beta rays) was identified and named by E. Rutherford too. He found that beta rays consists of high-speed electrons. Unlike alpha and gamma particles, whose energy can be explained as the difference of the energies of the radioactive nucleus before and after emission, beta particles emerge with a variable energy. This apparent violation of the law of conservation of energy led to the hypothesis that a second undetected particle, the neutrino, is emitted along with the electron and shares the total available energy. In some forms of induced, or artificial, radioactivity, the electron's antiparticle, the positron, is emitted from the excited nucleus; the positron in this case is also called a beta particle and denoted by b+ (the ordinary beta particle is b- ).
Gamma radiation is high-energy photons emitted as one of the three types of radiation resulting from natural radioactivity. It is the most energetic form of electromagnetic radiation, with a very short wavelength (high frequency). Wavelengths of the longest gamma radiation are less than 10-10 m, with frequencies greater than 1018 hertz (cycles per sec). Gamma rays are essentially very energetic X rays; the distinction between the two is not based on their intrinsic nature but rather on their origins. X rays are emitted during atomic processes involving energetic electrons. Gamma radiation is emitted by excited nuclei or other processes involving subatomic particles; it often accompanies alpha or beta radiation, as a nucleus emitting those particles may be left in an excited (higher-energy) state. The applications of gamma radiation are much the same as those of X rays, both in medicine and in industry. In medicine, gamma ray sources are used for cancer treatment and for diagnostic purposes. Some gamma-emitting radioisotopes are also used as tracers. In industry, principal applications include inspection of castings and welds. Data from artificial satellites and high-altitude balloons have indicated that a flux of gamma radiation is reaching the earth from outer space, thus opening up the field of research known as gamma-ray astronomy.