ABSTRACT
Nuclear radiations are ubiquitous. They are present in the atmosphere as cosmic rays originating in outer space, sources of which are yet unknown. They are also present in vegetation as carbon contains a small but finite radioactive isotope,1 a fact used advantageously to determine the ages of archaeological samples. Most living beings including humans contain calcium, which consists of a tiny amount of long-lived radioactive potassium (40K isotope). The list goes on. In the 20th century, we made significant progress in harnessing energy from nuclear fission, employing nuclear techniques for non-destructive testing of materials, medical diagnostics and therapy. Whether we handle radioactive materials for applications or we are concerned about health and safety, we need to have a good grasp of some basic terminology and be able to do simple calculations to make quantitative estimates of radiation phenomena. When one is concerned about radiation effects, one has to consider the species and energies of the radiations emitted and the activity levels and characteristic lifetimes of the radiation emitting sources. This chapter is devoted to radioactive levels and characteristic times.
