What Are Radioactivity and Radiation?

Abstract

We and everything around us are made of atoms . The idea of the atom came out of ancient Greek philosophy: some philosophers (notably Aristotle ) thought that matter could be divided into ever smaller parts, and others (notably Democritus ) thought that at some point, there was a minimum size of object, which they called “atomos”, meaning “unsplittable”. With the techniques of modern science, it has become clear that these atoms do exist, and each one is about a ten-billionth of a metre across: the full stop at the end of this sentence is about a million atoms across.

All composed things tend to decay.

Buddha 563–483 B.C.

Summary

Ionising radiation is released in most nuclear transformations. $\alpha$ rays are helium nuclei. $\beta$ sources emit high-energy electrons ($\beta ^-$) or their antiparticles ($\beta ^+$, called positrons). $\alpha$ and $\beta$ decays alter the chemical nature of an element. $\gamma$ rays, which are high-energy photons, are often emitted by a nucleus after $\alpha$ or $\beta$ decay. Fission (spontaneous or induced) is also possible, and the neutron excess in fission products can be reduced by prompt or delayed neutron emission, or by $\beta ^-$ decays. The essential units of radiation protection are the Becquerel (Bq) for the activity and the Sievert (Sv) for the effective dose (which includes weightings for the biological effectiveness). This is fundamentally a per-kilogram measure, because a larger person will be able to absorb a larger absolute amount of radiation for the same amount of damage. Each radioactive isotope has a particular half-life. Radioisotopes with large half-lives are associated with a low activity, and those with short half-lives with a high one. The dose from a source can be reduced by keeping a distance from it, reducing the exposure time, and using shielding.