The nuclei of some atoms are unstable and that these atoms are radioactive
Many very heavy element, such as the transuranics (elements with an atomic mass higher than that of
Uranium) are unstable because their nucleus is simply too large! These atomic nuclei emit radiation and so
lose mass and eventually end-up in a less massive and more stable (lower energy and higher entropy)
Less massive nuclei may also be unstable if they have an excess of neutrons (or too few neutrons, though
usually the former) since the ratio of protons to neutrons cannot deviate too far from 1. If an isotope has too
many neutrons then it must shed them to become stable.
The different properties of α-, β- and γ-radiations
There are a number of decay modes by which unstable nuclei can lose excess mass or energy. Protons or
neutrons may be emitted directly or the nucleus may undergo binary fission (splitting into two) or multiple
fission (splitting into more than two fragments) or it might emit a lighter nucleus (a cluster of neutrons and
protons in so-called cluster-decay).
However, in naturally occurring radioistopes the most common modes of decay are:
Other radioisotopes can also be viewed as excited states in which the transitions involves the emission of
radiation other than electromagnetic radiation, such as a and b particles, and in which the element changes as
the atomic number changes.
This equation is sometimes written (ignoring the neutrino which barely interacts with anything) as:
Where the superscript (top number) is the mass number (A = number of protons (Z) + number of neutrons (N),
A = Z + N) and the subscript (lower number) refers to the number of protons (the atomic number, Z). See the
introduction to atoms for more about these symbols and terms. Other particles can be written using this
Those particles carrying a net electric charge, i.e. the alpha-particle and the electron will be deflected by
magnetic fields and the degree of deflection is proportional to the charge/mass ratio, which is highest for
the very light electron.
Alpha, beta and gamma radiations are all capable of knocking electrons out of atoms, i.e. ionising them,
and so are called ionising radiation. Ultra-violet (UV) and X-rays are also ionising.
Visible light, radio waves, microwaves and heat radiation are non-ionising radiations.
Americium-241 is an alpha-emitter used in smoke detectors.
Strontium-90 is a beta-emitter used in forensic analysis of bones.
The table below gives the characteristics of the three most common types of ionising radiation:
|Introduction to Nuclear Reactions and Radiation
Radioactive half-Life (t½)
The term half-life refers to the time taken for half the radioactive nuclei in a sample to decay and is fixed for
any given isotope. Radioactive decay is a statistical process which is almost entirely independent of external
physical factors (such as temperature and pressure). As the particles decay there are fewer radioactive
atoms left in a sample - the radioactivity of a given sample diminishes with time.
Half-life (t½): the time taken for half of the radioactive nuclei to decay.
The half-life is a characteristic of the particular radioisotope, for example the half-life of iodine-131 is 8.1
days, whilst that of uranium-238 is 4.5 billion years.
- Notice that starting with an initial sample, say 100 g, the radioactivity falls to half after one half-life,
then it halves again (to a quarter) after two half-lives, then after three half-lives we have an eight of
the original sample left, and so on.
- As time progresses, the rate of decline in radioactivity diminishes (the curve becomes less steep).
- Exponential decay: the mass of radioisotope declines exponentially since the number of atoms
decaying depends upon the number of radioactive atoms present.
Radioisotopes have important applications as radiotracers in biosciences and medicine.
E.g. the radioactive metastable isotope of Tc, technetium-99m (m for metastable) is a g-ray emitter and can
be bound to phosphate and used as a tracer to study bone function. A ‘hot-spot’ will occur wherever bone is
being rapidly deposited, such as at the site of a healing fracture.
Iodine-131 can be used to treat thyroid cancer. Iodine is used by the thyroid gland to make the hormone
thyroxine. Rapidly growing thyroid cancer cells will accumulate large amounts of iodine, and if a radioisotope
of iodine is used it can kill these cancer cells.
Positron emission tomography (PET) uses positron-emitting radioisotopes introduced into the body to
reconstruct 3D images of the body’s systems. The emitted positron (e+) annihilates with an electron in the
body to produce a pair of gamma-ray photons which are detected. Since the photons travel away from their
source in opposite directions the source can be located in 3D.
Radiotracers can also be used for non-medical biological purposes, e.g. to study the fate of radioactive
carbon taken up by plants as CO2(g) during photosynthesis.
The half-life of tracers must be of an appropriate length to allow detection but not cause undue damage.
Isotopes that decay too rapidly will cause too much damage to tissues and may decay before the scanning
procedure is complete. Isotopes that decay too slowly may be hard to detect and may cause unnecessary
long-term exposure of the tissues to radiation.
PET uses short-lived radioisotopes as radiotracers, such as carbon-11 (half-life about 20 minutes) which
can be incorporated into molecules such as glucose. The metabolism of the molecule can then be traced.
Did you know?
The Oklo mine in Gabon was once the site of a depleted natural prehistoric nuclear reactor in which the
large quantities of uranium ore reached critical mass having been deposited by a river delta with the aid of
bacteria which caused the precipitation of uranium salts in the water. The reactions occurred almost 2 billion
years ago and lasted for about one million years.