We all come into contact with radioactive substances on a day-to-day basis.
They are in the air we breathe, the water we drink and bathe in, and the food
Radioactivity has been around since the creation of the earth. It naturally
exists in the atmosphere, the soil, the seas and rivers. Human activity, including
energy production and military operations, also causes a certain amount of radioactivity
to be released into the environment.
Some of this radioactivity gets into the food you eat. It is invisible, tasteless
and not mentioned on food labels, so how can you be sure it is safe?
This booklet explains what radioactivity is, the ways it gets into the food
chain, and how the Ministry of Agriculture, Fisheries and Food (MAFF) monitors
Any substance which is radioactive has unstable atoms, and this instability
causes them to break down. To understand why this is, you need to look at the
structure of atoms:
- Atoms consist of a nucleus which contains positively charged protons and uncharged neutrons
- The nucleus is orbited by negatively charged electrons
- The Positively charged protons are balanced by a corresponding number of negatively charged electrons.
- As the protons in the nucleus are all positively charged they repel
each other (imagine the way two like poles of magnets repel each other,
and then increase the effect by millions of times).
So why don't they pull the nucleus apart?
The answer is binding energy - an energy which is strong enough to counteract
the strong repelling forces.
The stability of an atom depends upon the balance between forces -for example,
when there are too many neutrons compared to protons there is an imbalance of
forces holding the nucleus together and so the nucleus rearranges itself.
on the type of nucleus, radioactivity in the form of energy and particles is
released, and in some cases the nucleus breaks up.
In certain cases, significant amounts of heat accompany this radioactive rearrangement.
In nuclear power stations the process is controlled and the heat is used to
generate steam to drive turbines.
All atoms of the same element have the same number of protons and electrons.
However, different forms of the same element have different numbers of neutrons,
and these different forms are called isotopes.
For example carbon is an element and carbon isotopes include:
- carbon 12 - six protons and six neutrons (stable)
- carbon 14 - six protons and eight neutrons (radioactive)
A common term which refers to any isotope of any element is nuclide.
As the atoms of radioactive nuclides disintegrate (and emit radiation), new
nuclides are formed. For example:
- carbon 14 disintegrates to form stable nitrogen 14
- cobalt 60 disintegrates to form stable nickel 60
Both isotopes and nuclides can be stable.
Units of radioactivity
Radioactivity is measured as the rate at which the atoms disintegrate. It is
measured in becquerels (Bq), and one Bq is equal to one disintegration per second.
When measuring radioactivity in the environment it is usually measured relative
to weight, eg Bq per kilogram of substance.
As a substance disintegrates, there is obviously less of it left.
The time it takes for a radioactive nuclide to halve in quantity is called
the half-life (T5 ½), and each nuclide has its own particular half-life.
- nitrogen 16 has a half-life of 7.4 seconds
- carbon 14 has a half-life of 5,730 years