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

Methods of detecting radioactivity

• A Geiger-Muller (GM) tube and counter.
  • This instrument 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
  • A photographic 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.

3b. What are the units and measurements of radioactivity? - measurements of ionising radiation

• 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 (3.7 x 10¹⁰ Bq/s).
  • 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.
  • The activity might be just simply quoted as counts per second (cps = Bq).
• Doses of radiation are measured in gray, sievert or roentgen.
  • The quantity of radiation you are exposed to is called the absorbed radiation dose.
  • The radiation dose you receive depends on where you live (local background radiation) and whether at work you are likely to be exposed to harmful radiation at work (e.g. radiographer, nuclear plant worker).
  • 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 be the most important when dealing with health and safety issues.
    • 1 sievert is quite a large dose of radiation, so doses often quoted in mill-sieverts (1 Sv = 1000 mSv).
    • Radiation dose is not a measure of the total amount of radiation your body absorbs, but it is a measure of the risk of harm due to your body absorbing that amount of radiation.
    • The risk depends on the total amount of radiation you absorb and how harmful that type of radiation is.
    • Another dose unit: 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)
• Measurement of radioactive contamination
  • This is radioactivity, due to the presence of radioisotopes in any material
  • It's 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

• 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 - your immune system has evolved to take care of a low dose of ionising radiation.
• This is called the background radiation, it is always around in the environment,  and there are two principal 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 laboratory results using radioisotopes - this is a 'fair test factor in investigations of radioactive materials.

• 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.

• 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 subsequent radioactive waste 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

Easier Foundation Tier Radioactivity multiple choice QUIZ

Harder Higher Tier Radioactivity multiple choice QUIZ

Worksheet QUIZ Question 1 on RADIOACTIVITY - absorption of alpha, beta and gamma radiation

Worksheet QUIZ Question 2 on RADIOACTIVITY - dangers & monitoring ionising radiation levels

Worksheet QUIZ Question 3 on RADIOACTIVITY - revision of atomic structure

Worksheet QUIZ Question 4 on RADIOACTIVITY - what happens to atoms in radioactive decay?

Worksheet QUIZ Question 5 on RADIOACTIVITY - uses of radioisotope and half-life data

ANSWERS to the WORD-FILL WORKSHEET QUIZZES

Crossword puzzle on radioactivity and ANSWERS!