Radioactivity - The Changing Arctic Environment: The Arctic Messenger - page 45

The emission of two protons and two neutrons (a helium nucleus), which is called
an alpha ( α ) particle. We saw this in the decay of uranium-238 to thorium-234.
This form of radiation ( α ) does not have much penetrating power and is unlikely to
pass through the outer dead layers of the skin. However, biological damage can
occur (such as an increased risk of cancer) if alpha particle emitters get into the
body and expose tissues to alpha radiation.

· The emission of a beta ( β ) particle. These are physically equivalent to electrons

but originate from inside the nucleus of some radioactive atoms. Such emission
occurs, for example, when thorium-234 decays to protactinium-234. Some familiar
sources of β radiation are tritium, cobalt-60, strontium-90, technetium-99, iod-
ine-129 and iodine-13, phosphorus-32 and caesium-137. β radiation is able to pen-
etrate 1-2 centimetres of skin, but inhaled or ingested beta particle emitters carry
the greatest risk of causing biological damage at the molecular and cellular levels.

· A burst of energy called gamma ( γ ) radiation . This form of very high-energy ion-

izing radiation has no mass and no electrical charge. It can pass through many ma-
terials, including the human body. The high energy of gamma radiation can lead to
biological damage directly and indirectly by causing secondary ionization of
atoms with which they interact. A common naturally occurring source of gamma
radiation is potassium-40, which is found in soil, water and foods with a high po-
tassium content, such as bananas.

Nuclides that release energy are called radionuclides . The degree of instability of a radio-
nuclide is expressed as its half-life. Looking again at the first three nuclides in the natural
series - from uranium-238 to thorium-234 to protactinium-234 ( 234 Pa), we have half-lives
of 4.47 billion years, 24.1 days and 1.17 minutes, respectively.
There are three categories of natural radionuclides: primordial radionuclides, which
date from when Earth was formed (such as 238 U); those that are decay products of primor-
dial radionuclides (such as 234 Th); and cosmogenic radionuclides produced when high-en-
ergy cosmic radiation has kicked a nucleus into an unstable condition, such as carbon-14