True whole body dose rate from an x-ray cabinet apparent radiation "leak" widget
Published: May 19, 2025
Source: Design & implementation by Dr Chris Robbins (Grallator) / Ionactive radiation protection resource
Prelim
Users of this widget may gain some additional benefit from it by reading the following Ionactive blog article: A dose rate of 1 micro Sv/h. A magic line or a bit of a nonsense? (opens in a new tab).
In summary, the above linked blog article considers manufacturers who will often state their equipment meets a limit of < 1 micro Sv/h (on the surface or at 10 cm from the surface or at some other distance). If this level is exceeded, even by a small margin (say 1.3 micro Sv/h) the user or their RPA will often insist the unit is "repaired" (lead patched etc). Whilst ALARP (as low as reasonably practicable) will somewhat influence decisions to insist on a repair, the blog asks 'what is the actual potential radiation hazard'.
The blog article derives where < 1 micro Sv/h comes from and notes that the term 'ambient dose equivalent rate’ (an operational quantity often used as an approximation of effective whole body dose) is intended. This is pretty much in keeping with dose assessments that we have seen around cabinet x-ray systems - they compare potential exposures to whole body (effective) dose limits. In the blog we conclude in nearly all relevant cases you cannot receive a whole body dose from such "leaks". This latest widget demonstrates many of the findings from the blog and packages them into a neat interactional widget (developed for Ionactive by Dr Chris Robbins from Grallator).
True whole body dose rate from an x-ray cabinet apparent radiation "leak" widget
The widget follows below. The controls are reasonably self-explanatory, but we will in any case go over them further below and examine some of the outputs from the widget.
Widget controls (inputs)
The widget controls and purpose are as follows:
- Scatter surface distance to gap (S) - This control adjusts the distance between the source of scatter and the gap in the shielding (the shielding is assumed to be near the x-ray cabinet outer covers). For an actual x-ray machine you are unlikely to know this parameter (unless you have a suitable schematic available). However, it's nice to observe how this changes dose rate and area of coverage. This can be set between 0.5 and 10 cm.
- Gap diameter - the gap in the shielding (0.1 to 1.0 cm).
- Reference measure distance to surface (R) - This is the distance between the probe measurement point and the surface (1-10cm). The 'standard' (see blog reference earlier) often quotes the dose dose rate 10 cm from the surface - explore the effect of changing this value.
- Reference dose rate - This is the dose rate that the radiation monitor is detecting and / or you can set to a value between 0.1 and 100 micro Sv/h and explore whole body exposure potential.
- Ionactive man - Move Ionactive man nearer and further away from the x-ray unit (supplied by Spangling Pangolins X-ray Systems Inc) and investigate the resulting dose rate, and importantly the coverage. How near whole body (trunk) dose do you get?
Widget outputs / display
The widget outputs / display the following numerical information (which is enhanced by graphical representation):
- Distance to gap - This is the distance between the shielding gap and Ionactive man (0-3 m).
- Diameter of beam - This is the diameter of the beam projected on to Ionactive man (cm)
- Area of beam - This is the area of the beam projected on to Ionactive man (cm2). Assume that the front surface area of the trunk of the body (70kg, 170cm adult) is something like 3000–3600 cm².
- Dose rate - calculated dose rate (micro Sv/h) on Ionactive man, projected over the area given above.
Limitations of this widget compared to real life
The mathematics in this widget are precise (as you would expect). You should therefore consider the output carefully and interpret with this in mind. Here are a couple of examples of what we mean.
Consider the widget set up as shown in the picture below.
Dose rate to Ionactive man - significant distance from x-ray unit
Mathematically (i.e. using inverse square law and geometry considerations) the dose rate is correct at 0.001 micro Sv/h. However, this is below general background radiation and would not be distinguishable from it. In addition, likely monitors in use around the x-ray machine would not be capable of detecting down to this level.
Now consider the following set up - which is at the other extreme, very close to the x-ray (i.e. body up against the covers).
Dose rate to Ionactive man - on the surface of the x-ray unit
Again - maths is at play here. 441 micro Sv/h seems excessive, but would be over a tiny area of the body. Given the geometry, 441 micro Sv/h is correct mathematically, but a typical radiation detector would show considerably less due to the beam size compared to the detector volume. Note in this extreme example the reference dose rate is set at 1 micro Sv/h at 10cm from the surface, and the source of the radiation is right at the surface of the covers (Scatter surface distance to gap (S) = 0.5 cm). This would be a very unusual geometry - nothing we have seen in decades working around cabinet x-ray units. Despite this, move Ionactive man just 20cm away from the unit and the dose rate is down to under 0.26 micro Sv/h with a coverage area of < 14cm2 (so about 0.5% front area coverage of an average body).
Now we have dealt with these extremes, we can use the widget to demonstrate some more realistic scenarios.
Typical widget output for realistic scenarios
Here is a slider with a few pictures from the above referenced blog article (link reminder: A dose rate of 1 micro Sv/h. A magic line or a bit of a nonsense?) to provide more context. [Ionactive comment: why is there a Rankins Dragon in some of the pictures? We have no idea; probably something to do with whole body exposure size?!]
Consider the following setup:
- Scatter surface distance to gap (S) - Set to 10 cm (realistic based on Ionactive inspection of the inner workings of various cabinet x-ray systems).
- Gap diameter - 0.1 cm (realistic, rarely do we see gaping holes in lead shielding).
- Reference measure distance to surface (R) - Let's set this to zero for the moment, so this is a surface measurement.
- Reference dose rate - Let's set this to the magic 1 micro Sv/h.
- Ionactive man - Let's start with Ionactive man literally hugging the x-ray unit within the region of a potential shielding imperfection.
[Ionactive comment: if you have read the blog article and / or our introductory summary above, you will note the 1 micro Sv/h 'standard' is set at 10 cm from the surface - except in the US where a different criteria is used. We will look at 10 cm from the surface further down this page.]
The situation graphically is shown below, together with the widget output.
Not surprisingly the dose rate reported is 1 micro Sv/h. Note the area of coverage on the body - 0.031 cm2.
How about moving to 10cm from the surface? We will caculate this one manually as the Ionactive man distance points are in 20 cm increments (and 10 cm is really not that far away from the x-ray unit). Look carefully at the geometry - the result is 0.25 micro Sv/h at 10 cm and a beam coverage of 0.071 cm2.
[Ionactive comment: whilst the purpose of this widget and accompanying article is to look at potential whole body dose rate, let's briefly divert and consider equivalent dose (dose rate) to a part of the body - the skin. IRR17 - schedule 3 (7) on dose limits says the following in respect of other persons (not working with ionising radiation) '...the limit on equivalent dose for the skin is 50 mSv in a calendar year as applied to the dose averaged over any area of 1 cm2 regardless of the area exposed..'. Note that 'regardless of the area exposed' is strictly for equivalent dose, and does not apply to effective dose. So even at these low dose rates, the area of coverage of equivalent dose to the skin would need to be averaged over 1cm2, meaning that the actual exposure to the skin, with reference to IRR17, will be even less. As the beam coverage expands beyond 1cm2 any potential dose rate drops to background and is then irrelevant. As noted earlier, you can force some extreme values form the widget if you try - be wary of that if you are having a play!
Whilst we have diverted, let's also look at exposures to the hands and feet. IRR17 - schedule 3 (7) says (for those not involved in work with ionising radiation 'other persons') the following '...the limit on equivalent dose for the extremities is 50 mSv in a calendar year...'. Note that whereas whole body effective dose is by definition whole body, and equivalent dose to the skin is averaged over 1cm2, nothing is defined in the same way for the extremities. It is therefore reasonable to think about the area of the extremity exposed, as we did for whole body exposure. Consider that the area of an average hand palm (adult male and female data combined) is 76 cm2, and this is placed over a surface "hot spot" of 1 micro Sv/h over an area of 0.031 cm2 (from earlier), then this represents 0.041 % of the hand in total! At this coverage even much higher dose rates could never be recorded on an extremity dosimeter (not that anyone would be wearing one anyway), unless you aligned the dosimeter exactly with the leak.
We will now return to looking at whole body exposures - the above diversion was for those that want to delve deeper. ]
Let's now move Ionactive man back to a more realistic (but a still unusually close) 20 cm. The situation graphically is shown below, together with the widget output.
The dose rate is reported as just over 0.1 micro Sv/h and the coverage over the body is - 0.126 cm2. This is a negligible dose rate (within expected background) over a negligible area of the body.
Let's now move to 1 m from the x-ray unit. The situation graphically is shown below, together with the widget output.
The dose rate reported is truly indistinguishable from background and the coverage over the body is - 1.131cm2.
So how might you receive a whole body exposure?
For realistic scenarios, the answer is - you can't (unless you take a hacksaw and drill bit to the cabinet lead shielding) Try setting the widget up as follows:
- Scatter surface distance to gap (S) - Set to 5 cm (perhaps unlikely but possible with compact x-ray cabinet).
- Gap diameter - 1 cm (assume a small circular lead shielding port plate is missing!).
- Reference measure distance to surface (R) - Set this to 10cm.
- Reference dose rate - Let's set this to 1 micro Sv/h for the moment (but would likely be more than this with a lead shielding fault - we will look at this shortly)
- Ionactive man - Set this to 300 cm to force the area of the whole body.
The widget output is as shown in the graphic below.
Forcing an apparent whole body exposure with 1 micro Sv/h at 10cm (with 3 m separation)
We have achieved an area over the body geometrically of > 3000 cm2 (the surface area of the forward facing trunk of an average adult). However, the dose rate is calculated to be 0.002 micro Sv/h which is indistinguishable from background - there is no exposure taking place.
Move Ionactive man to 20cm from the unit. What do you see? You will find 0.36 micro Sv/h and an area of just over 28 cm2 - so no whole body exposure happening here either as you get closer (as expected).
In our final example we will consider a dose rate at 10 cm from the surface which is beyond the 'standard' specified earlier. For this example we are not endorsing the higher dose rate, just looking at its significance. If you want to follow on with us, set the widget up as noted below:
- Scatter surface distance to gap (S) - Set to 10 cm (more likely based on our observations).
- Gap diameter - 0.5 cm (likely excessive, but why not).
- Reference measure distance to surface (R) - Set this to 10cm (as expected by the 'standard').
- Reference dose rate - Let's set this to 2 micro Sv/h (so twice the expected dose rate of the 'standard' and ordinarily would be treated as a fail).
- Ionactive man - Set this to 20 cm for the moment - employees don't consistently hug their x-ray units.
This scenario is shown in the widget graphic below.
Exploring 2 micro Sv/h at 10cm - whole body dose at 20cm separation
Note here we have 0.889 micro Sv/h where Ionactive man is located at 20cm from the side of the x-ray unit, the area coverage is 3.142 cm2 (absolutely not whole body exposure). Can we get any where near whole body exposure geometry - not really, but let's finish by moving Ionactive man to 3 m from the unit as shown below.
Exploring 2 micro Sv/h at 10 cm - whole body dose at 3 m separation
Note we have 0.008 micro Sv/h where Ionactive man is located 3 m from the side of the x-ray unit, this will not be distinguishable from background. Even at this distance, and with the geometry set as shown above, we have only achieved just over 200 cm2 of coverage to the front of the body (so around 7%).
A word about cumulative exposure and occupancy
Nothing in this widget and discussion discusses cumulative exposure or occupancy. This is deliberate and due to the wording of the '< 1 micro Sv/g at 10 cm from the surface' standard referenced earlier, and also considered in detail in the mentioned Ionactive blog article. What we (and the reader- we hope) have seen is that the potential real world instantaneous dose rates (IDR) are low, and are not even whole body exposures (so are not effective dose as defined). Briefly we have also discussed equivalent dose to the skin and the extremities (hands) and shown these also to be trivial in the example cases tested.
Furthermore, we can also note that:
- We have not accounted for residence time at the worst possible dose rate / geometry.
- General occupancy in the area around the x-ray unit in the worse potential configuration.
- Accessibility (is the leak readily accessible by anyone, or inaccessible - needing a step ladder for example).
In truth we don't really need to - you have observed the figures above and they reveal trivial exposure. But if you are still not convinced, then factor in cumulative exposure and occupancy and you will no doubt agree that weekly, monthly and annual exposures in the worst cases specified earlier are so near background exposure, as to be indistinguishable from it.
A word about ALARP
ALARP (as low as reasonably practicable) does not mean 'as low as possible'. Ionactive agrees that the '<1 micro Sv/h at 10 cm from the surface' is a satisfactory ALARP criteria and could also be treated as a quality standard by suppliers of x-ray cabinet equipment in the UK and beyond. However, in most likely cases the widget shows that real world exposures at this level are trivial, and would still be so at larger IDRs at 10 cm from the surface. So the least we (the radiation protection community) should do is have some pragmatism if the 'standard' appears to have been exceeded (for whatever reason). A dose assessment will usually show that exposures are trivial.
A word about compliance and the Ionactive RPA service
This widget article does not interfere or conflict with formal Ionactive RPA advice via our commercial services. The free radiation protection resources on our website allow us to argue and debate radiation protection theory and practice. The facts are:
- We are RPA to numerous suppliers and installers of X-ray cabinet equipment which are designed to meet the 'standard' specified in this article.
- We are RPA to numerous customers and users of such equipment who expect the standards noted to be met absolutely (and we support what they expect).