Radioactivity is a natural phenomenon. It occurs when overly excited atoms seek stability by
emitting energy in the form of radiation. The amount of energy and the forms of radiation emitted
vary tremendously among the radioactive elements. It is due to this variation that the uses of
radiation range from powerful tracers of biological, physiological, and geological cycles; to
healing medicine; to weapons of mass destruction. In this introductory material you will learn
about what happens following exposure to radiation. The latter is fundamental in determining
when radiation has medicinal characteristics versus lethal ones. Understanding such fundamentals
Radiation protection of the environment: providing knowledge and skills to the user
community
Tom Hinton
French Institute for Radiation Protection and Nuclear Safety Page 2 of 7
18-Mar-14
https://wiki.ceh.ac.uk/x/hI9BBw
is required if we want to confidently evaluate the human and environmental risks from
radiological exposures.
Radioactive decay is accompanied by the emission of high energy radiation. Radioactive decay is
associated with the transition of the nucleus from a higher to a lower energy state, and occurs at a
rate which is described by a decay constant (λ), which is a property of the nucleus and totally
independent of its surroundings. The process of radioactive decay transforms one element into
another. There are long chains of naturally occurring transformations that occur within most
ecosystems. For example, uranium-238 undergoes radioactive decay and transforms into thorium-
234, thorium-234 changes into protactinium, and eventually (approximately 10
10
years later and
having undergone 14 different transformations) the original radioactive U atom is ultimately
transformed into stable lead. At each step the resulting product loses all the characteristics of the
parent element and acquires the characteristics of the newly formed daughter element.
Characteristics such as colour, melting point, hardness, even physical state change with each
transformation. For example, within the U-decay series, radium, a solid, is transformed by
radioactive decay into radon, a gas. Radioactive decay is nature’s alchemist (Hinton, 1998). The
web-based table of isotopes has decay schemes for all known isotopes linked to a periodic chart
of the elements at
http://ie.lbl.gov/toi/perchart.htm
.
Units of energy:
Einstein’s famous equation showed us that energy can be expressed in units of
mass, and vice versa. In nuclear and radiation science, energy is normally expressed as changes in
atomic mass units, μ, or as electron volts, eV. One eV is equivalent to 1.783 x 10
-36
kg. The
energy released during radioactive decay is measurable and can reach several million electron
volts (MeV). Radiation in the form of alpha particles is often in the MeV range (
e.g.
, plutonium-
239 emits an alpha particle with an energy of 5.2 MeV); whereas gamma emissions are generally
less energetic, some thousand of ev (
e.g
., cesium-137 emits gamma radiation with energy of 662
keV).
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