Units of Radioactivity

Geiger-Müller radiation detector

Geiger-Müller radiation detector: Picture from Wikipedia

At last! A rational article on the BBC about radiation and its dangers. Sadly, but predictably, the article is not written by a BBC journalist. However, in the context of the Fukushima incident, what is needed is not just rationality but reports of actual measurements. I work in a National Measurement Institute and tend to take measurement for granted. But at times like this measurements are critical and liberating. In the context of this incident:

  • If a government official says radiation levels are ‘high’ then one is scared.
  • If a government official says radiation levels are ‘low’ then one mistrusts them.
  • If a government official says radiation levels are 4.2 micro sieverts per hour, then people can check if they are right. And people can look up the numbers and make up their own mind.

I am not attempting to downplay this incident. It is very serious. But quantifying the actual hazard to which people are exposed is the only way to gauge the severity of the incident against other more familiar hazards.

What do you want to measure? The measurement of radiation uses several units that measure different effects of radiation.

  • The amount of radioactivity is measured as the number of nuclear events per second. The unit for this is the becquerel (Bq), which corresponds to one event per second.
  • Often one is interested not in the amount of radioactivity in a source, but the effect of radiation from a source upon an object. A unit called the gray (Gy) measures the amount of energy absorbed per unit mass. If an object absorbs one joule of energy for each kilogram of material in the object we say it absorbs one gray. If an object weighs 7 kilograms and each kilogram  absorbs one thousandth of a joule (0.001 J) then
    • the absorbed energy is 7 x 0.001 = 0.007 joule.
    • the absorbed dose is 0.001  gray (Gy) = 1 milligray (mGy).
  • Most importantly we are concerned when the object absorbing the radiation is a human being. The effect of the absorbed dose – the number of grays absorbed –  depends on the type of radiation and the type of tissue in which the energy is absorbed. We use a unit called the sievert to take this into account.
  • Normally we ignore the effect of the tissue because we can’t determine in advance what type of tissue will be exposed and just assume the worst case which has a weighting factor of 1. We then measure the number of the grays which would be absorbed if a person were there, and then factor in the type of radiation that was absorbed. The actual weighting factor is quite complex, but simplifying for clarity:
    • If one gray of alpha particles is absorbed, we say that the dose is 20 sieverts.
    • If one gray of beta particles is absorbed, we say that the dose is 1 sievert.
    • If one gray of gamma rays is absorbed, we say that the dose is 1 sievert.
    • If one gray of neutrons is absorbed, the dose depends on the energy of the neutrons but varies between 2 sieverts and 20 sieverts.

How do you know you are safe? One uses a simple meter to measure the dose rate: i.e. how many sieverts would be absorbed in a given period of time. One then works out a total dose by multiplying the dose rate by the time of exposure. So for example,

If the radiation level (dose rate) were 1 milli sievert (mSv) per hour, then exposure to that environment for half an hour would lead to an absorbed dose of 0.5 milli sieverts.

This dose can then be compared to relevant safety limits. If you live in the UK then you can reasonably expect to be exposed to 2 mSv (milli Sieverts) of radiation each year, or 0.006 mSv per day from entirely natural sources. Wikipedia has an extensive list of the doses acquired in various activities, but here is an abbreviated list. Please see Wikipedia for the sources of the information

Single dose examples

Yearly dose examples

  • Average individual background radiation dose: 2 mSv/year in the UK; 1.5 mSv/year for Australians, 3.0 mSv/year for Americans. This comprises typically:
    • Cosmic radiation (from sky) at sea level: 0.24 mSv/year
    • Terrestrial radiation (from ground): 0.28 mSv/year
    • Natural radiation in the human body: 0.40 mSv/year
  • Living near a nuclear power station: an additional 0.0001–0.01 mSv/year
  • Living near a coal power station: an additional 0.0003 mSv/year
  • Smoking 1.5 packs/day: an additional 13-60 mSv/year
  • Current average limit for nuclear workers (including dentists): 20 mSv/year
  • Lowest clearly carcinogenic level: 100 mSv/year
  • Elevated limit for workers during Fukushima emergency: 250 mSv/year

So what the BBC need to do is simply get a person to measure the radiation levels (dose rates) in various places at various distances from the Fukashima plant, and then we would know exactly the nature of the hazard to which people have been exposed.


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6 Responses to “Units of Radioactivity”

  1. iamamro Says:

    Another great post, Michael!

  2. Nestor Patrikios Says:

    Very illuminating (as opposed to irradiating) – thanks.
    Delighted to see Google putting in ads for lead blankets.

  3. Will Says:

    An excellent piece.

    (just tiny error with “If an object weighs 7 kilograms and each kilogram absorbs one thousandths of a joule (0.001 J) then the absorbed dose is 7 x 0.001 = 0.007 milligray (mGy)” < I believe you mean, "…then the absorbed dose is 7 x 0.001 = 0.007 Grays = 7 milligrays (mGy). )

  4. Peter Says:

    …. Another good reason not to smoke…. Radioactive too! Could you post the plot of background radiation over the past 60yrs…. Showing that we have had it much worse in the past.

  5. xkcd « Protons for Breakfast Blog Says:

    […] week I struggled to express as clearly as I could the key quantities relating to measurements of radiation dose. I felt it would help to make sense of some of the very few numbers emerging from Japan with […]

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