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More on the odd geometry

Dear Reader,

The saga of the odd geometry continues, I was going to try to work out a correction for different distances between the radionuclide (photon emitter) and the detector. But then I decided that a better way is to use the fact that the special geometry chosen allows us to ignore the absolute distance between the source and the detector.

I then did some calculations based on the idea of having an infinite cylinder (infinite in length) of water which is surrounded by a coaxial detector (a super long NaI well detector). Again we are using our special isotope which emits photons only in limited angles. The angle which I have chosen is perpendicular to the axis of the cylinder.

By the way the word perpendicular is a word with some funny cultural baggage. In the delightful film “Kind Hearts and Coronets” the antihero of the film is visiting the clergyman (Reverend Lord Henry D’Ascoyne). The antihero (Louis disguised as a clergyman) makes a comment about church architecture about the “early perpendicular era”. At the time I thought it was Louis making a joke about a totally made up movement in church architecture, but then I discovered that the perpendicular style was a part of the gothic time. So while you might have come to this blog to read about my thoughts on chemistry, nuclear and radiological matters you can also read here about english black comedy films and church architecture.

But for those of you who came here only for my thoughts on counting of cesium-137 in humans, here is the graph of the fraction of the photons which can escape from the 14.5 cm radius cylinder as a function of the distance of the point source from the axis of the cylinder.

The graph you have been waiting for !

As you should be able to see as the source is moved towards the surface of the cylinder more and more photons are able to escape from the cylinder. As the cesium in a human is spread out over the whole of the torso it will be more easy to count the cesium in a human than it would have been if the cesium was concentrated at the centre of the torso in a point source.

I will start the 3D calculations, but in the words of “deep thought”, this will take me some time.


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