Radiation skyshine (photon scattering) over a shielding wall widget

Where you have the potential for skyshine there are several matters to establish which could include:

• Is the level of skyshine (instantaneous dose rate - IDR) acceptable at ground level where employees or other persons may be working / have access?
• How does the dose rate vary with distance away from the source of the skyshine?
• How does the dose rate vary with height above ground level (this might be a critical radiation safety issue, requiring restriction to working above a certain height within a certain distance from the facility).
• Could occupancy or workload be taken into account to mitigate the significance of skyshine? 

We will consider some of the above in more detail later in this article. 

Skyshine. Skyshine is the scattering of x-ray or gamma photons above a facility such that they may impact dose rate (i.e. exposure above background) on the apparent safe side of the shielding. The photons can scatter off the air above the facility and other objects (such as an overhead crane or building roof). Whilst some mitigation may be achieved via restriction of working height outside the enclosure,  mitigation usually starts inside the facility, such as using collimation, and restricting the placement of the radiation source within the facility etc - more of that later. 

For now lets check out the widget.  

Radiation skyshine (photon scattering) over a shielding wall widget

This widget is designed to demonstrate skyshine. More detailed considerations will follow below, but for now note the following:

• The source of radiation is uncollimated (height can be varied).

• The shielding wall is perfect (any dose rate above background detectable to the right of the shielding wall is from skyshine alone). 

• Dose rates are in relative units (shielded and unshielded dose rates are given). 

Have a play and we will consider the output of the widget further. 

Some interesting observations

The following observations can in some cases be treated as general observations of similar (real) setups (obviously with more than just a single shielding wall!).  However, some caution is required because skyshine is sensitive to:

• Use of a collimator (or not).
• The horizontal placement of the source (not adjustable in this version of the widget).
• The vertical placement of the source (adjustable in this widget).
• The material of the shielding walls.
• The space above the facility (air, denser objects such as a overhead crane, building roof material and its elevation above the top of the enclosure).
• The shape of the enclosure (e.g. relative height of the shielding walls to plan view area).
• The energy of the ionising radiation source (including energy spectrum -  continuous for x-ray radiation generators and discrete for gamma source).

So quite a few variables there. Despite this, let's look at the observations taken from this particular widget. 

• At ground level (say 0.9 m) the dose rate at 1 m from the wall is only about double that at 10 m from the wall. What has happened to the inverse square law? [Clue: nothing, it does not play a part here].
• As Ionactive man is raised higher, the skyshine dose rate increases.
• The dose rate at a given height beyond the shielding increases, as the height of the source above ground level increases.
• The maximum skyshine dose rate, for a given height of Ionactive man is about 4.5m from the shielding wall.

The actual monitoring data used in the widget does not show huge variation over horizontal or vertical distance, but the trends are clear enough. 

[Ionactive comment: As will be noted further below, this is a good set of data for the enclosure on which it is based. The dose rate data in the widget is uncollimated, and was originally used to demonstrate the effect of not using a collimator during industrial radiography. For those interested, if you add a collimator (not currently supported in the widget), all dose rates at 0.9 m off the ground are then substantially < 0.5 micro Sv/h, with the exception of data for the maximum vertical source height (2.6 m), where the maximum dose rate (measured at 0.9 m from the floor) is <0.7 micro Sv/h (collimated), compare with 1.9 micro Sv/h (uncollimated as reported by the widget). The message here is clear - use a collimator!]

What the widget does reveal is the result (unexpected ?) that peak skyshine dose rates occur some distance from the shielding wall. This is an important radiation protection finding and demonstrates that critically examining a new open top shielded facility, and only monitoring close to the perimeter, could lead to some embarrassment later on (or worse). This could be exacerbated further if there are persons working above ground level (perhaps on a mezzanine floor), where it might be assumed the area is unaffected by radiation from the open top facility. 

Visualising skyshine

Skyshine can be a little difficult to get your head around - especially when dose rates seem to increase as you move away from the source of radiation, and then decrease again (as shown in the widget). Let's take a completely non physics look at this, and as the great Richard Feynman might say, 'use some imagination'. 

Consider replacing the radiation source used by the widget, with an intense visible light - which illuminates the interior of a room which is enclosed on all sides, but has no roof.  The light is from a single source, not collimated and is 0.9 m off the ground.

Further imagine that the environment outside the room is dark. The room is located on flat ground which turns into a slight incline further away. 

The graphics which follow are deliberately exaggerated in colour and perspective to make their point. 

Let's first imagine you are inside - you might observe something like the following.

Now let's consider what you might see from the outside. 

Now imagine we are right up close to the enclosure - touching the concrete wall. What might we see? Without any physics, but thinking about life experiences, we are likely to "see" not a lot. The hands touching the wall are hardly visible. It's as if they are in a 'shadow' and are not being illuminated from the light scatter above the room. 

Now walk backwards from the exterior of the room  - looking at your hands as you do so, just as you moved back from the shielding wall using the widget. As you move out of the 'shadow' you hit a 'sweet spot' where your hands are illuminated (as shown below, somewhat exaggerated!).  This is probably intuitive, based on life experience, and you accept that climbing up a ladder at this point might illuminate your hands even more, just as you move up the slight inclining ground away from the room. The perspective is highly exaggerated here to make the point clear.

Now move back - further and further  - recalling that the only illumination in the immediate vicinity is those light photons being emitted from above the room (and perhaps the night sky). Now look at your hands - can you now see them?  

That is skyshine - using electromagnetic radiation in the visible region, as a surrogate for ionising radiation. It is not a perfect comparison, but hopefully helps visualise what is going on when using the widget (somewhat exaggerated!)  Remember that the findings of the widget are only applicable to the situation it is based on (described earlier). A different radiation source with different emission energies, overlying objects, height of walls of the enclosure relative to the cross-sectional area, and the relative placement of the source (e.g. away from the centre, right up against a wall) are just some of the variables which will give a different result.