What is radiation? 

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Radioactivity is all around us

Radioactivity is a naturally occurring phenomenon that manifests everywhere around, and even inside, us. Plants, water, rocks, the air we breathe and the potassium in our bones all contain low levels of radioactive substances. This has been so since the creation of the universe. Radiation even reaches us from outer space and is known as cosmic radiation.

The structure of atoms and nuclei

To understand radioactivity, we have to make a journey into the heart of matter. All matter is made up of atoms, which are the extremely small, invisible building blocks of the elements. Each atom consists of a positively charged nucleus around which negatively charged particles, called electrons, move in orbits, roughly analogous to planets moving around a star. The nucleus is made up of particles called protons and neutrons. The protons carry positive electric charge while the neutrons, as the name suggests, are neutral. There is a very strong attractive force, called the nuclear force, that binds the protons and neutrons together and prevents the mutual repulsive electric force between positively charged protons from making it fly apart. Neutrons contribute to the attractive nuclear force without adding repulsive forces.  For a nucleus to be stable the attractive and repulsive forces must be balanced in such a way that the total energy of the nucleus becomes a minimum. There are many different atoms, but they all consist of protons, neutrons and electrons (apart from hydrogen, which has no neutrons). Atoms differ according to the number of protons, neutrons and electrons they have.

Stable and unstable atomic nuclei

To understand radioactivity, we have to make a journey into the heart of matter. All matter is made up of atoms, which are the extremely small, invisible building blocks of the elements. Each atom consists of a positively charged nucleus around which negatively charged particles, called electrons, move in orbits, roughly analogous to planets moving around a star. The nucleus is made up of particles called protons and neutrons. The protons carry positive electric charge while the neutrons, as the name suggests, are neutral. There is a very strong attractive force, called the nuclear force, that binds the protons and neutrons together and prevents the mutual repulsive electric force between positively charged protons from making it fly apart. Neutrons contribute to the attractive nuclear force without adding repulsive forces.  For a nucleus to be stable the attractive and repulsive forces must be balanced in such a way that the total energy of the nucleus becomes a minimum. Figure 1: There are many different atoms, but they all consist of protons, neutrons and electrons (apart from hydrogen, which has no neutrons). Atoms differ according to the number of protons, neutrons and electrons they have.

Stable and unstable atomic nuclei

Some atomic nuclei are stable - they stay as they are. To be stable, the number of protons and neutrons in the nucleus must be balanced to achieve a minimum energy state, though this does not mean the numbers must be equal. Heavy nuclei contain many protons and require more neutrons than protons for stability. If this is not achieved -  as is the case in some naturally occurring  and man-made nuclei - the nucleus is unstable and will decay to a more stable state through emission of radiation. The time-scale of the decay is characteristic of the particular unstable nucleus. Such nuclei are called radioactive and the radiation they emit is called radioactive radiation. 

Radioactive radiation

The radiation emitted can be in the form of rays of charged particles or highly energetic electromagnetic radiation. The particle rays are known as alpha rays (He nuclei) and beta rays (electrons) respectively, while the highly energetic electromagnetic radiation is known as gamma rays.

Radioactivity is a natural phenomenon through which unstable atomic nuclei achieve a better balance by emitting radiation.

Ionising radiation

When radiation passes through matter it transfers energy to atoms and molecules. When the radiation is sufficiently energetic, it will knock electrons out of atoms to form charged ions and free electrons – a process known as ionisation. When radiation is energetic enough to cause ionisation it is called ionizing radiation. The energetic shorter wavelength ultraviolet radiation in the sun’s spectrum is a well-known form of ionizing radiation and can cause skin damage through disruption of atomic and molecular bonds. Radioactive radiation is an even more energetic form of ionizing radiation.