Radiation time, distance & shielding interactive ALARP widget

Source: Design & implementation by Dr Chris Robbins (Grallator) / Facilitated by Ionactive radiation protection resource


The concept of ALARP (as low as reasonably practicable), or ALARA (as low as reasonably achievable) is one of the cornerstones of radiation protection. The protection principles (for an external gamma / x-ray) source are well understood and practiced in all radiation safety training courses - time (minimise), distance (maximise) and shielding (use if you can). In modern radiation protection, shielding is often the first choice, but the distance principle (inverse square law) is often used daily in certain workplace situations (e.g. nuclear medicine). The concept of protection by minimising time is still valid but often frowned upon (where there is often an expectation that engineered protection will be available). Minimising time is still a significant protective feature used by cyclotron engineers during machine maintenance or repair. There will always be circumstances where minimisation of dose uptake will require optimisation of all three principles , and this is where the ALARA widget can be helpful.

Often the above principles are taught individually and then put together in the form of a question / problem or delegate discussion / exercise. Not everyone has the chance to experience the concepts for real - i.e. really practice exposure control and ALARP. This is where the new ALARP widget is useful - interact with it and do your own ALARP optimisation! This resource and our other widgets are used in our online radiation protection training courses, and during live face to face training events.

Ionactive has worked with Dr Chris Robbins (Grallator) to create this resource exclusively for Ionactive. Chris is a whizz at mathematics, physics, nuclear physics and coding and it's a pleasure to be working with him on this widget and many others which will be featured in this resource section.

Tip! - Set a dose rate and then decide on your dose / dose rate constraint. Then see how many ways you can achieve your constraint using the controls in the widget.

The ALARP widget

Go ahead and have a play.

Additional information

The widget is fairly self explanatory. Here are some general notes / links etc that might help you make best use of this resource.

Distance attenuation factor - This is simply \( 1/d^2 \) where d is the distance from the source (i.e. the inverse square law). This factor shows you the calculation taking place and the resulting numerical attenuation factor. Have a play - doubling the distance for the first time will yield 1/4 attenuation factor (as expected). Going from 1m to 10m will yield 0.01 (100 fold reduction) - as expected.

Shielding attenuation factor - This combines the concepts of TVT and HVT (follow the links for a quick reminder). A more rigorous description of TVT can be found here: How do I convert TVT (10th value thickness) values to attenuation for Gamma or X-ray sources of radiation? and here: How do I convert TVT to HVT (or the other way around)? Essentially a single TVT provides a 1/10 reduction whereas an HVT provides a 1/2 reduction. Whilst you add TVT (or HVT) material thicknesses together, you multiply their attenuation. So this factor will show that TVT+TVT+HVT = 1/10 x 1/10 x 1/2 = 1/200 (0.005).

For this widget we are not looking at actual shielding type or thickness - there is no need. If you need to think in terms of actual shielding then imagine for example that the ALARP widget is considering Cs-137 and you have lead as a shield. In this specific case the TVT would be 22mm lead and the HVT would be about 6.6mm of lead. So with this data you could investigate (for training purposes) the effective of adding physical shielding and noted the thickness of lead required.

In the beginning, there was nothing, which exploded

– Terry Pratchett -