Measuring radioactivity

Custom Search

A Geiger counter will record "counts per minute", but this doesn't tell us what the radioactive substance is actually doing, merely what is reaching the detector.

It also tells us nothing about the amount of damage being done to you.

So we need several different units in order to measure radioactivity.

Units of Radioactivity
  • The activity of a source is measured in Becquerels (Bq),
    One Becquerel is one decay per second.

  • The amount of radiation that your cells absorb is measured in grays (Gy),
    One gray is one Joule of energy absorbed by 1kg of your body. This is the dose you receive.

  • To measure the harm done to you, we need to remember that particles ionise very strongly, and cause 20 times more cell damage than the same dose of particles, rays or X-rays.

    We measure the "dose equivalent" in sieverts (Sv).

    A dose of 1 gray of particles, rays or X-rays will give you a dose equivalent of 1 sievert.
    A dose of 1 gray of particles will give you a dose equivalent of 20 sieverts

    The sievert is quite a large unit, so we usually work in millisieverts (mSv)
    or microsieverts (µSv).

Here's a good way to think of it:
        imagine you're out in a rainstorm.

  Then the amount of rain falling is measured in Bequerels,

  the amount of rain hitting you is measured in grays,

  and how wet you get is measured in sieverts.


Our bodies have an activity of around 4000 Bq.

The average annual dose in the UK is around 2.5 millisieverts.

At GCSE level, you won't be expected to know about these units in detail. However, it's useful to know what they mean when you're answering questions.

Custom Search


            Can't see the menu? Try here:
                Home | Types BG info Alpha Beta Gamma | Sources | Uses | Dangers | Ionisation | Detecting |
                 Measuring | Half life | Famous people | Main Quiz | Exam questions |