It is found that a few naturally occurring substances consist of
atoms which are unstable.
-that is they undergo spontaneous transformation Into more stable product
Such substances are said to be radioactive
and the transformation process is known as radioactive decay.
Radioactive decay is usually accompanied by the emission of
charged particles and gamma rays .
As a result of that transformation process , these unstable nuclei
emit radiations of three main types , called alpha , beta and gamma radiation.
Fission The nucleus is divided into two parts, fission fragments. and 3-4 neutrons. Examples: Cf-252 (spontaneous), U-235 (induced) -decay The nucleus emits an - particle (He-4). Examples: Ra-226, Rn-222 -decay Too many neutrons results in -decay. n=>p + +e - + . Example:H-3, C-14, I-131. Too many protons results in -decay p + =>n+ e + + Examples: O-16, F-18 or electron capture (EC). p + + e - =>n+ Examples: I-125, Tl-201
Part 2: Radiation Physics It is impossible to know at what time a certain radioactive nucleus will decay. It is, however possible to determine the probability l of decay in a certain time. In a sample of N nuclei the number of decays per unit time is then:
Alpha decay is a common radioactive process encountered with heavier isotopes. The alpha particle is a helium nucleus having a mass of 4 and a charge of +2. Isotopes with mass numbers less than about 150 (Z 60) seldom yield alpha particles. Alpha particles progressively lose their energy as a result of collisions as they pass through matter and are ultimately converted into helium atoms through capture of two electrons from their surroundings .
Beta particles are just electrons from the nucleus, the term "beta particle" being an historical term used in the early description of radioactivity . The high-energy electrons have greater range of penetration than alpha particles , but still much less than gamma rays . The radiation hazard from betas is greatest if they are ingested
The emission of the electron's antiparticle, the positron , is also called beta decay. Beta decay can be seen as the decay of one of the neutrons to a proton via the weak interaction . The use of a weak interaction Feynman diagram can clarify the process
Gamma rays are produced by nuclear relaxations. Gamma-ray emission is the result of a nucleus in an excited state returning to the ground state in one or more quantized steps with the release of monoenergetic gamma rays. Gamma rays, except for their source, are indistinguishable from X-rays of the same energy.
X-Ray emission are formed from electronic transitions in which outer electrons fill the vacancies created by the nuclear process. One of the processes is electron capture. A second process which may lead to X-rays is internal conversion, a type of nuclear process that is an alternative to gamma-ray emission.