Measuring radiation

 

Find answers to some commonly asked questions such as: How do we measure radiation? What is the difference between activity and exposure? How is exposure expressed? What are some of the average levels of medical radiation exposure per treatment? What about average background radiation and occupational radiation exposure?

 

What is radiation?


Radioactive materials consist of atoms that are unstable – that is they undergo spontaneous transformation into more stable atoms. This process (radioactive decay) is usually accompanied by the emission of charged particles and/or gamma rays. Find out more on our Radioactivity page.

 

How is radiation measured?


There are two things being measured when we discuss radiation: 'activity' and 'exposure'. Activity is how much radiation is coming out of something, while exposure measures the effects of that radiation on anything that absorbs it.

 

Measuring radiation activity


Radiation activity (i.e. radioactivity) is measured in an international (SI) unit called a Becquerel (Bq). The Becquerel counts how many particles or photons (in the case of wave radiation) are emitted per second by a source. 1Bq = 1 disintegration per second (a very small unit).

 

Measuring radiation exposure


The most important thing to understand about radiation exposure is that it is measured by what radiation does to substances, not anything particular about the radiation itself.

 

There are three ways to express radiation exposure:

 

  1. Absorbed dose: the energy 'deposited' in a kilogram of a substance by a radiation source; measured in an international (SI) unit called the Gray (Gy).
  2. Equivalent dose: relates the absorbed dose to the biological damage of the type of radiation. The absorbed dose is multiplied by radiation weighting factor (wR); measured in an international (SI) unit called the Sievert (Sv).
  3. Effective dose: the equivalent dose multiplied by tissue weighting factor (wT) for the different harmful effects of radiation on different types of tissue. If more than one organ has been exposed, the overall effective dose is the sum of all the effective doses of all the exposed organs. The unit of effective dose is the Sievert (Sv).

 

Dose rate is exposure over time
 

Radiation is often measured as a dose over a specific period of time, known as the ‘dose rate’. The actual exposure received depends upon the dose rate and the exposure time.

 

Please note: As the Sievert is a very large unit, effective radiation exposure is normally expressed in milliSieverts. A milliSievert (mSv) is one thousandth of a Sievert (1/1,000).

 

Examples of medical radiation exposure


Radioactive isotopes are produced to be used in vital medical scanning procedures. Here are some examples of the effective radiation doses received during some typical treatments:

 

  • Bone scan: 4.6 mSv per treatment.
  • Thyroid scan: 2.6 mSv per treatment.
  • Barium meal x-ray: 2.5 mSv per treatment.
  • Soft tumour scan: 40 mSv per treatment.


General background radiation exposure


We are all exposed to a certain level of natural background radiation each year from a variety of sources. In some parts of the world the background radiation levels are around ten times higher than those in Australia:

 

  • General Australian background radiation dose: 1.5 mSv per yr.
  • Cornwall, UK general background radiation dose: 7.8 mSv per yr.


Occupational radiation exposure


The federal and state regulators require that members of the public should receive no more than 1 mSv per year from radiation sources other than background radiation and medical procedures. The regulatory limit for radiation workers is 20 mSv per annum averaged over 5 years, with no more than 50 mSv received in any one year. These limits reflect recommendations made by the International Commission on Radiation Protection (ICRP).

 

  • Single year limit under ARPANSA regulations: 50 mSv.
  • ARPANSA regulations dose limit for radiation workers: 20 mSv per yr.
  • ANSTO self imposed annual effective dose constraint: 15 mSv per yr.
  • Average ANSTO radiation worker effective dose in 2010: 1.75 mSv per yr.


Reference:


Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). 2008. Units for measuring radiation.