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Study Notes: Introduction to Radiation

Radiation is something that we hear about in the news, usually in a negative sense. However, radiation is a natural phenomenon and one that surrounds us at all times in various forms. All soils and rocks, air and water contain natural radioactivity to a greater or lesser extent.

There are various forms of radiation. For example:

Electromagnetic Radiation
Particle Radiation
  • *Heat radiation from a warm fire
  • Alpha radiation
  • *Light radiation
  • Beta radiation
  • *Microwave radiation
 
  • *Radio waves
 
  • Gamma radiation and X-rays
 

The four with asterisks (*) are usually considered to be benign and useful and we use them daily in our lives. But these forms of radiation can harm, e.g. radiated heat in bushfires can burn the skin at a distance, light in the form of a laser can severely damage the eyes and microwaves leaking from a microwave oven can also damage the body. Powerful radio waves can also be harmful.

Alpha, beta and gamma radiation are often considered to be dangerous and harmful but when handled with proper care they are also useful tools. For instance:

  • Cobalt 60 (60Co) is used for cancer therapy
  • Technetium 99 (99Tc) is used for diagnostic tests
  • X-rays are used for diagnosis, security at airports and checking the integrity of welds in industry
  • Americium 241 (241Am) is used in domestic smoke detectors.

Alpha, beta and gamma radiation are quite distinct forms of radiation and may be classified as followed:

Radiation
Symbol
Composed of: HazardProcess Range in Air Range in Tissue Health Risk
Alpha
α
Helium nuclei (two proton and two neutrons). Collisions with bound electrons. 3 cm 0.04 mm Is very heavy, penetrates poorly but if ingested will cause great damage such as tissue damage, cancer or death.
Beta
β
High speed electrons or positrons. Collisions with atomic electrons; slowing down in field of nucleus. 3 m 5 mm Many systemic effects including tissue damage, cancer or death.
Gamma
γ
Electromagnetic radiation – composed of photons (as is light). Photoelectric effect; Compton effect, Pair Production. Very large Through body Many systemic effects including radiation sickness leading to tissue damage, cancer or death.

These forms of radiation are produced due to radioactive decay of the nucleus of an atom. Such a reaction is a natural and normal process in nature and occurs spontaneously according to the laws of radiophysics.

For example uranium (238U) will decay to lead (206Pb usually called just Pb as it is stable and does not decay further) spontaneously. The rate of decay is usually given as a half-life which is a constant. The half-life denotes the time taken for half of the material to decay. For instance if the half-life of a radionuclide (a species of atomic nucleus which undergoes radioactive decay) is one minute then 50% will have decayed after one minute, 75% after two minutes, 87.5% after three minutes etc. Note that a constant 50% of the remaining material decays after each half-life time interval.

In the 238U to 206Pb example given above the half-life is 4.50 X 109 years – a very long time! Other half-lives are much shorter, for instance 99Tc used for diagnostic tests has a half life of only six hours. This short half-life is what makes 99Tc so useful in medicine. It is not in the body long enough to cause damage.

In liquid scintillation counting β-radiation (beta-radiation) is usually detected. This kind of radiation is commonly associated with the following radionuclides with the exception of 125I which is a γ emitter:

  • 3H (Hydrogen also called tritium)
  • 35S (Sulphur)
  • 32P (Phosphorus)
  • 14C (Carbon)
  • 125I (Iodine)

Find out more information on radiation by going to the Resources and Training Room and choose the ANSTO's web site from the World Wide Web Catalogue.

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