Photon scattering between source and detector widget

What's the "real" dose rate

This widget poses an interesting question:  What's the "real" dose rate? Some users, but by no means all, may tend to treat their radiation monitor of choice as an 'all seeing, all knowing' device, where they take a measurement and record the reading - as fact. At Ionactive we have written about this along several different themes (in blog articles, or technical guidance on this website). Examples include the following:

• Use Counts per Second (CPS) to find it, and Dose Rate to define it - includes comments on the labelling of monitors (such as "contamination monitor" - and yes, you can often measure x-rays from an x-ray machine using such a labelled device).
• Using a "CPS monitor" to find a small dropped radioactive source on the floor. Often a "contamination" labelled monitor (such as a sensitive NaI detector), used in conjunction with a dose rate monitor, is better at locating the source, than using a dose rate monitor alone.
• In the Ionactive blog article 'A dose rate of 1 micro Sv/h. A magic line or a bit of a nonsense?' we look at what it means to measure a dose rate from a very small x-ray short path through a gap in shielding, and how this cannot yield a whole body dose rate.
• Measuring a dose rate from a linear accelerator (linac) pulsed x-ray source using a Geiger–Müller tube based detector can yield some interesting measurement results (!) - which will differ depending on the PRF (Pulse Repetition Frequency) of the linac. If in doubt use an ion chamber based detector instead.
• Skyshine – radiation scattering around and over shielding. In this Ionactive article we looked at sky-shine and how it can catch you out. Take a dose rate very close to a shielding wall of an open-topped shielded enclosure containing an exposed source of radiation. Then move backwards away from the shield and (potentially) watch the dose rate increase. Photon scattering in action!
• The Ionactive resource  - Radioactive source holders - Dose rate through an aperture, yielded its own widget  - Dose rate through a small aperture widget. This resource demonstrates that the headline dose rate measured (or calculated) does not provide the full radiation exposure potential (i.e. the highest headline dose rates cannot yield a whole body exposure). 

The above examples are somewhat more advanced compared with this particular photon scattering widget, but illustrate that things are not always what they first seem. Later widgets will explore more practical examples such as 'how does a maze / labyrinth work when used as a means of radiation protection during the entry / exit from a shielded bunker'. The maze example is all about photon scattering, with the added complications of photon energy, scattering material, area of scatter, inverse square and so on.  Let's start simple! 

For this particular widget we are not concerned with photon back scatter off the source, or self absorption within the source. 

The  photon scattering widget is not an animation of concepts - play around and you will not get exactly the same result each time. You should give the widget a good 30 seconds averaging time before noting the dose rate, which is in relative units (so could also indicate counts per second  - CPS etc). You should leave the widget to settle each time you make a change or reset, and like an actual radiation monitor, the values will fluctuate (so try averaging by eye if you wish). 

The widget controls are very simple:

• No Collimator / Collimator. Select if you wish to place a collimator around the photon emission source.
• No Shield / Shielding present. You can choose to shield the detector such that photons arriving towards the sides of the detector are not recorded (measured).
• Scatters are random / Scatters go to the tube. A magic button for a bit of fun! In later widgets we will explore how photons might be scattered towards a detector (e.g. down a shielded labyrinth towards an exit, or perhaps over a shielding wall of an open-topped shielded bunker as sky-shine).