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Radiation, radioactivity, uranium and fission
What is radiation?
Radiation is a broad term and includes light and radio waves. Ionising radiation from radioactivity occurs when atoms disintegrate. This radiation from radioactivity occurs naturally in the earth, and surrounds us all the time. It is in building materials, food, water, the air we breathe and our own bodies. This is called background radiation and includes cosmic radiation from outer space.
On average a person will receive about 2.4 milliseiverts (mSv) of natural ionising radiation a year, although this can vary, depending on geographical location. The average Australian would receive about 1.5 to 2.0 mSv per year of this background radiation yet someone living at a higher altitude might receive more than 2.5 mSv per year.
What is radioactivity?
Radioactivity refers to particles and or gamma rays which are spontaneously emitted from the nucleus of unstable atoms as they change to more stable atoms. This change or radioactive decay is normally stated in terms of half-life.
What is a half-life?
This is the rate of decay or the time required for the radioactivity to be reduced by one-half. In other words, the half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay.
After two half-lives, there will be one fourth of the original sample remaining; after three half-lives one eighth the original sample would remain, and so forth. Half-lives can vary from fractions of seconds to billions of years.
What is ionising radiation?
There are three different types of ionising radiation: alpha, beta and gamma.
Alpha radiation can be absorbed by thin materials like paper, but can be harmful if ingested. A naturally occuring alpha emitter is the uranium-238 isotope, the most common uranium isotope.
Beta radiation can be natural or man-made. It passes through paper but can be absorbed by thicker materials such as plastic, aluminium or lead. An isotope of common carbon is Carbon-14 which is an example of a beta emitter, which is very useful for dating materials.
Gamma radiation is the most penetrating kind of radiation, similar to X-rays. Technetium-99m, which ANSTO distributes for diagnostic medicine, is a weak gamma ray emitter.
Why is uranium used as nuclear fuel?
Uranium is the basic raw material of nuclear energy. It is a radioactive element with two isotopes that are fissile, i.e., undergo fission, (uranium-235 and uranium-233) and two that are known as fertile (uranium-238 and uranium-234).
Fertile isotopes are not themselves fissionable by thermal neutrons however, they may be converted directly or indirectly into fissile material by neutron capture in nuclear reactors.
What is fission?
This is the process by which thermal neutrons cause fissile isotopes to undergo fission, or division, of the heavy nucleus into two unequal masses or fission fragments, further neutrons, gamma radiation and a great deal of energy.
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