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Gamma rays inverse law 3M equation?

Q1. At what distance would a dose of 6uSv(microsieverts) be obtained from a 35MBq source emitting y(gamma) rays with an energy of 60MeV?.. Can anyone show me the equation & answer to this mock study questions?

Thank you
:smile:
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Original post by Ayr123
Q1. At what distance would a dose of 6uSv(microsieverts) be obtained from a 35MBq source emitting y(gamma) rays with an energy of 60MeV?.. Can anyone show me the equation & answer to this mock study questions?

Thank you
:smile:


1. There's a useful empirical relationship in Health Physics for gamma dose rate

Dose rate at 1 m = 1.46 x 10-13 A E Sv/hr

where A = activity (Bq) and E = gamma energy (MeV)

2. Doing this from first principles is hard. You have to assume the gamma field is uniformly distributed around the surface of a sphere otherwise the problem is insoluble. So you can get the photon (and hence energy) flux per unit solid angle. However, the Sv is energy absorbed per unit mass and you know nothing about the mass per unit area of the absorber (a quantity know in this game as the thickness).

3. The Q also doesn't make sense. It asks for a dose, which is of course dose rate x time, so you could get 6 uSv from 1 hour at 6 uSv/hr, or 6 minutes at 60 uSv/hr, or....... Are you sure the Q isn't asking for a dose rate?
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Ayr123
OP
Thank you for replying👍😀
I've since answered(7.64metres)and got the equation worked out,I agree that the question doesn't make sense and I've another one I'll be posting tomorrow which is in-depth!
These 2questions I had to move on from last week, but realise I have to understand them.
I've enrolled on a distance learning course and do not get as much help as I would like as the tutors are very busy!
Well I hope I was some help.

Oh, by the way, 60 keV sounds a much more realistic gamma energy (Am-241). Worth checking that bit of the question. The empirical equation I gave you is only valid up to 3 MeV

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