Radiation Protection Glossary

A radiation protection glossary for Radiation Protection Supervisors (RPS), Radiation Protection Advisers (RPA) and anyone else interesting in radiation safety terms and definitions. The glossary is a mixture of health physics , phrases related to radiation protection legislation, transport, practical safety, technical terms and similar.

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For formal advice, see our Radiation Protection Adviser pages. 

DME6r2 resource

Inverse Square law

The inverse square law applies to any process which radiates out from a point in space (see Point Source and Fluence Rate to expand on this concept). With respect to Radiation Protection, the law says if you double your distance from a source of Ionising Radiation you will reduce your exposure by 4. It follows that if you triple your distance from the source, the exposure will reduce to 1/9 of the original value. This concept is one of the cornerstones of radiation protection and is used with the Distance Rule. It is not always reliable where the distance between source and measurement position is small compared to the size of the source. Generally, the rule will work mathematically where the distance between the source and the calculation point, is at least 10 times the largest dimension of the source.

If r is the distance from the source, the dose dose (or dose rate) D is proportional to 1/r2 as shown below.

\[\text{ D (Dose or Dose rate)}\propto \frac{1}{r^{2}}\]

This can be expanded to consider two dose rates at two different distances in the following form:

\[\frac{D_{1}}{D_{2}}=\frac{r_{2}^{2}}{r_{1}^{2}}\]

Here D1 is dose rate at distance r1 and D2 is dose rate at r2 (some greater distance from the source). This can be rearranged in several ways, the example below allows us to find D2 if we known D1 and both distances r1 and r2 .

\[ D_{2}=D_{1}\frac{(r_{1})^{2}}{(r_{2})^{2}}\]

The graphic below is from our radiation protection widget resource where you can see this process interactively.

Inverse square law radiation protection widget

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[Updated March 2024]

Atoms are very special: they like certain particular partners, certain particular directions, and so on. It is the job of physics to analyze why each one wants what it wants.

– Richard P. Feynman -