Fundamental knowledge about radiation Physics plays a significant role in discussing radiation and the health impacts it is linked to. Atoms, the fundamental constituents of matter, are made up of protons, neutrons, and electrons. Based on its atomic number, each chemical element has an equal number of protons. Nonetheless, atomic mass of the element can vary based on the quantity of neutrons in the nucleus. An element's isotope is produced by this discrepancy. Electrons are grouped in layers known as shells and orbit the nucleus. Energy is released when an atom's outer electron is removed; typically, this energy takes the form of gamma rays. Ionizing radiation is the term used to describe this phenomena. Isotope and nuclide stability is a fundamental idea in ionizing radiation. An isotope of an element becomes unstable when its nucleus contains either too few or too many neutrons. When protons and neutrons sum up to specific numbers, such as 2, 8, 20, 82, and 126, nuclei are most stable. Physics has demonstrated that odd numbers of protons and neutrons are less stable than even quantities. These radioactive and unstable nuclides have the ability to decay radioactively into a more stable nuclide that eventually releases beta particles, alpha particles, or photons, which include gamma and X-rays. Radiation is produced in the form of the particles. Since prolonged exposure to radiation can be harmful, it's critical to measure. Because radioactive decay is an unpredictable process, it is challenging to forecast when a certain atom will begin to decay. On the other hand, the number of isotopes in a group that decays over time can be measured. This decay rate, known as radioactivity, is proportional to the number of atoms in a particular radioactive element during the period of its half-life. The half-life of an isotope is defined as the duration required for half of the nuclides in that group to decay. The international unit of measurement for this rate is Becquerels (Bq), while the US uses Curies (Ci). An element's radioactivity can produce radiation by particle emission. The quantity of energy deposited in a particular area is equal to the absorbed dose, which is used in clinical settings to assess radiation. It is computed by dividing the average energy absorbed by the target material's mass. In radiation oncology, doses are expressed as fractions over a time period, and the absorbed dose is quantified in "Gray" (Gy).