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3. Detection of radioactivity and its measurement, units and radiation sources & 'Background Radiation'

Doc Brown's Chemistry

KS4 science GCSE Physics Revision Notes

How do we detect radioactivity? How do we measure radioactive emissions? What are the units of radioactivity? What do we mean by background radiation? Why is there radioactivity all around us? Does it do us any harm? What is a Geiger counter? What are the natural and man-made sources of radioactive materials? These revision notes on detecting radioactive emissions, measurements and sources of background radiation should help with GCSE/IGCSE physics courses and A/AS level physics courses



3. Detection of Radioactivity and its measurement, units and ionising radiation sources

3a. The radiation can be detected and measured in several ways

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  • By use of a Geiger-Muller (GM) tube and counter.
    • This electronically amplifies the ionising effect of the radiation and is used for very accurate measurements of radioactivity and it can detect a single radioactive event.
  • A Geiger-Muller (GM) tube and counter set up in the laboratory may record a background radiation of 25 counts per second.
    • That means 25 individual, mainly gamma rays, and some beta particles (probably no alpha particles) are 'hitting' the approximately 1cm2 detector area every second.
    • So, think how many must hit your body!, but don't worry, we seem to have survived millions of years of evolution so far, and the body's repair system can deal with a few hits!
    • Just out of curiosity, look up how many neutrino's we survive from passing through our body from the Sun every second! its scary!!!!!
  • Photographic film reacts to radiation in the same way as it does to light. It is used in film badges by workers in the nuclear industry and hospitals to monitor how much radiation people are exposed to in their potentially harmful environment. The film is developed after specified time interval, and the amount of 'exposure' or darkening of the film is a measure of how much radiation has 'hit' the person.
  • The activity of a radioactive source is measured in ...
    • Becquerel units (Bq, s-1), 1 Becquerel = 1 disintegration of an unstable nucleus per second.
    • or in curie, 1 curie = 3.7 x 1010 disintegrations per second (Bq).
    • A disintegration means the decay or breakdown of an individual unstable nucleus,
    • so 1 curie = 3.7 x 1010 Becquerel of unstable nuclei decaying per second.
  • Doses of radiation are measured in gray, sievert or roentgen.
    • Gray units (Gy, J kg-1) are based on the absorbed dose of ionising radiation energy in joules per kilogram of absorbing material.
      • 1 Rad = 10-2 Gy
    • Sievert units (Sv, J kg-1) are based on the dose equivalent of ionising radiation and these units seem to the most important when dealing with health and safety issues.
      • 1 Rem = 10-2 Sv
    • Röentgen units are based on the ionising effect of the radiation.
      • 1 Röentgen = 2.58 x 10-4 C kg-1 (charge produced in coulombs per kilogram of material)
  • Radioactive contamination in a material e.g. its activity in food or fluids, might be measured in. Bq/Kg for solids or Bq/litre for liquids.
  • Biologically significant levels of radiation:
    • Maximum dose allowed for general public: 5 mSv/year (mSv = millisievert = Sv/1000, 1 mSv = 100 mRem)
    • Maximum dose allowed for radiation workers (medical, industrial, nuclear power): 50 mSv/year
    • Natural background dose rate: 1.25 mSv/year
    • Maximum dose due to atmospheric atomic weapon testing 1954-61: 12µSv/year (µ = micro = 10-6)
    • Maximum dose due to medical and industrial use: 120µSv/year
    • Average dose due to nuclear reactors: 2µSv/year
    • Threshold for nausea ('radiation sickness'): 1 Sv in a few hours
    • Threshold for death: 1.5-2.0 Sv in a few hours (not 100%, but fatalities start to occur in the days or weeks after exposure to the radiation)
  • Other examples of radiation doses
    • In the UK your background radiation dosage is around 2.2 mSv/year (2.2 millisieverts/year), though this can vary and is greater in areas of granite rocks containing isotopes of uranium.
    • Radiation doses are an important factor in designing and applying radiotherapy for cancer treatment or diagnostic techniques using radioactive tracers e.g. the radiation dose of a single PET scan (see uses of radioisotopes) is ~7 mSv (seven millisieverts), over three times what you receive naturally from the environment.
    • One dental X-ray 0.20 mSv, 1 chest X-ray 0.30 mSv, 1 C-T scan 4 mSv
  • Dangers of ionising radiation, precautions when dealing with radioactive materials are dealt with in section 4b

Advanced Chemistry Page Index and Links

3b. Sources of ionising radiation - emissions from radioactive sources

Background Radiation - sources

Typical relative % of background radiation sources - typical values

radon gas from rock minerals 42-51%
building materials, rocks and soil 14-18%
cosmic rays from the sun 10-14%
radioisotopes used in medicine 12-14%
food and water 11-12%
nuclear power industry 1%
  • Dangers of ionising radiation, precautions when dealing with radioactive materials are dealt with in section 4b
  • If a Geiger counter (a radioactive emission detector) is set up anywhere in the world it will register (hopefully!) a very low level of radioactivity.
  • Every second of the day you will be hit by some particle from a radioactive source or the sun, but don't worry, under normal circumstances, the does is far to low to cause you any harm.
    • Low-level radiation is all around us and passing through us!
    • Your body can take care of a little radioactive emission.
  • This is called the background radiation, it is always around in the environment,  and there are two sets of sources.
  • When doing accurate experiments this background radiation must be taken into account.
  • The background radiation is measured and subtracted from any experimental results using radioisotopes.

Natural sources of radiation

  • Radiation from outer space e.g. cosmic rays from the Sun.
    • Fortunately, the Earth's upper atmosphere absorbs some of the Sun's high energy radiation and the Earth's magnetic field deflects cosmic rays from us.
  • Radioactivity from naturally occurring unstable radioisotopes in rocks at the surface e.g. there are traces of radioisotopes of uranium in granite rocks.
    • There is geological factor involved here.
    • The background radiation from soil and rocks is quite variable, depending on their chemical composition.
  • The radioactive gas radon is formed in the process, and can build up to harmful levels in cellars, which in certain areas of granite rocks e.g. Cornwall and Scotland., such cellars should be well ventilated, or the radon gas will build up.
  • Uranium miners are exposed to far more than the average background radiation and should wear protective clothing.
  • Radioactivity from naturally occurring radioisotopes deep in the Earth's core, the energy released keeps the core very hot and heats the magma in the Earth's mantle.

Radiation sources due to human activity

  • Emissions from nuclear power stations are governed by health and safety legislation, and the nuclear industry is allowed to emit tiny amounts of radioactive material into the environment).
  • Safe storage of nuclear waste from power stations is a current problem that is yet to be solved for the long-term future. It is very contentious issue for obvious health, safety and environmental reasons and no satisfactory solution has been found to the problem of safe waste disposal.
    • The used radioisotopes and nuclear fuel most be processed into a safer form e.g. a glass solid. This solid waste is stored in long-term and leak-proof containers which could be buried in a deep and well shielded storage area underground.
    • BUT even before this long-term process, nuclear reactor/weapon waste is particularly and exceptionally dangerously radioactive due to radioisotopes with short half-lives. So initially it is stored in containers under water until it has 'cooled off' and safer to handle.
    • Some idea of the dangers and problems in handling radioactive materials are mentioned in section 4. and the long-term considerations in the notes on half-life data in section 6.
  • Radioisotope tracers are used in industry and hospitals (see later) and so their use and disposal must be carefully controlled.
  • Nuclear accidents, the worst being at Chernobyl power station in the Ukraine. Parts of the Lake District in England are still slightly contaminated from the 'fallout' in the rain.
  • Atomic weapons testing in the 40's, 50's and 60's. The 'super powers' were testing their latest nuclear bombs in the air or on the surface, producing contaminated dust in the atmosphere. Some of the radioisotopes formed in the explosions, like strontium-90, are still around in the environment.
  • Dangers of ionising radiation, precautions when dealing with radioactive materials are dealt with in section 4b

Advanced Chemistry Page Index and Links


1. Atomic structure, fundamental particles and radioactivity

2. What is radioactivity? Why does it happen? What radiations are emitted?

3. Detection of radioactivity, measurement, dose units, ionising radiation sources, background radiation

4. The properties and dangers of alpha, beta & gamma radioactive emission

 5. The uses of radioactive Isotopes emitting alpha, beta or gamma radiation

6. Half–life of radioisotopes, how long does material remain radioactive? Uses of decay data & half–life values

7. Nucleus changes in radioactive decay? how to write nuclear equations? Production of Radioisotopes

 8. Nuclear fusion reactions and the formation of 'heavy elements'

 9. Nuclear Fission Reactions, nuclear power energy resources

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Easier-Foundation Radioactivity Quiz

or Harder-Higher Radioactivity Quiz

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crossword puzzle on radioactivity and ANSWERS!

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