Gamma dose rate (D=AE/6r2) - or is it? A widget to calculate the validity of this formula.
Published: Apr 02, 2024
Source: Design & implementation by Dr Chris Robbins (Grallator) / Facilitated by Ionactive radiation protection resource
Prelim
A rigorous analysis of D=AE/6r2 (also often quoted as D=ME/6r2 where M is the activity in MBq) has already been presented on our website: Formula for calculating dose rates from gamma emitting radioactive materials. The latest addition of this linked resource also includes the widget presented below. However, whereas some might want all the physics and maths (the recommended link will be of interest), others may just want to use the widget as is. To avoid doubt the terms in the widget are as follows:
D - Dose rate in µGy/h (also broadly equivalent to µSv/h)
A - Activity (expressed in MBq)
E - Energy (expressed in MeV)
r - distance (expressed in m)
The widget can be used to demonstrate the validity of this typical 'rules of thumb' formula. For example, using Co-60 will show that D=AE/7r2 is more appropriate, where as using the same methodology for I-125 reveals that D=AE/1r2 is more appropriate. The widget demonstrates where there are differences in the formula, the linked resource explains why there are differences (if you want to know).
We have worked with Dr Chris Robbins (Grallator) to create this resource exclusively for Ionactive. Chris is a whizz at mathematics, physics, nuclear physics and coding. We have worked with Chris on other similar resource (explore this section of the website) and many more are planned! Visit Grallator !.
Take a look.
Features to consider in this widget
Do look at all four radionuclides. They have significantly different energy distributions and probability of gamma ray emission.
Inspect Ir-192 carefully - the very lowest gamma ray photons provide over 10% of the total dose available.
Look carefully at the rule of thumb equation and see how the denominator changes with selected radionuclides.
Look at the energy adsorption graph for each of the radionuclides - note the differences.
Look at the dose distribution graph and note how it differs, particularly for Ir-192.