If the nucleus contains either excess of neutrons or protons, the force between these constituents will be unbalanced leading to unstable nucleus. The atomic number of an element is the same, but different mass numbers are called isotope of an element. The number of proton present in the nucleus is called atomic number (Z), and total number of neutrons and protons present in the nucleus is called mass number (A). The nucleus and proton were discovered by Rutherford in 1911, and neutron was discovered by James Chadwick in 1932. The nucleus consists of positive-charged proton and neutral-charged neutron referred as nucleons. The electron had been discovered by J. J. The atom consists of positively charged nucleus and surrounded by a number of negatively charged electron, so that atom as a whole is electrically neutral. The concept of atoms and molecules was first introduced by John Dolton in 1811, and he proposed the atomic theory. The atom is the basic building block of matter. Therefore, the main objective is to focus on the applications and effects of radioisotopes and radiological protection. Therefore, it is necessary to require an urgent decision for protective actions. The release of radioisotopes from nuclear fuel cycles, naturally occurring radioactive materials (NORM) from mining activity, mishandling of radioisotopes in industries and laboratories, and accidental release of radioactive materials could enter into the atmosphere. The IAEA in cooperation with the WHO offers on intercomparison service to check and improve accuracy of radiation dosimetry due to increase in the effectiveness of the radiotherapy. The precise dose is a life-and-death matter therefore the IAEA has several program components to assist institutions in the members of the countries and aspect of radiation therapy and diagnosis. For this technique, Rosalyn Sussman Yalow was awarded Nobel Prize in 1977. There are numerous applications of radioisotopes in medical fields one of the revolutionized techniques is radioimmunoassay this is used to detect and quantify minute levels of tissues components such as hormones, enzymes, or serum proteins by measuring the components ability to bind to an antibody or other proteins in competition with a standard amount of the same component that had been radioactivity tagged in the laboratory. The World Health Organization (WHO) and International Atomic Energy Agency (IAEA) jointly coordinated a research program on radioactive tracers in cardiovascular diseases and searched for clues to this widespread health problem. Shorter half-lives of radioisotopes are used in medicine because they decay quickly and they are suitable for medical diagnosis and therapy. The radioisotopes have been a valuable gift to many braches of medicine and biology. The application of radioisotopes in medical, industry, and research field has served human civilization over a several decades. At present more than 200 radioisotopes were produced from nuclear reactors and accelerators. More than thousand natural radioisotopes are present in our nature. The radioactive decay law was also proposed by him. The name “isotope” was first suggested by Soddy in 1913. The discovery of isotope was one of the results of work on the radioactive elements. Further investigation by M. Curie and P. Curie (1898) and Rutherford (1911) showed that radioactivity is exhibited by heavy elements such as uranium, thorium, and radium. Scientific understanding of radiation and radioactivity and their benefits and effects on humans, that’s back almost century to the pioneering work of Roentgen (1895) and Becquerel (1896). Therefore, without radiation life does not exist. All organisms including human beings on the earth are getting benefits from radiation in a direct way without realizing it. Therefore we live in a natural radioactive world. The radiation coming from the sun is due to the nuclear fusion it is very essential for the existence of life on earth. In addition to these, some of the radionuclides such as polonium and radium are present in our bones our muscle contains radiocarbon and radiopotassium, radon, thoron, and their progeny in our lungs, and they emit ionizing radiation. We are continuously exposed to natural and artificial radiations. Radiation and radioactivity existed long before life evolved on the earth and are indispensable parts of the environment.
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