Neutron Flux in Infinite Vaccum

[chriskay301]

I have to figure out how to prove that the neutron flux for a point source is given by ø=$\frac{S}{4πr^2}$.

I can get this type of solution, but I have an e^(-r/L) in the numerator. I'm assuming I'm missing some theory somewhere as apparently this is the solution for a point source in an infinite medium, not vacuum.

does anyone have some insight they could offer?

 

[chriskay301]

I realized there's no diffusion in a vacuum, therefore no diffusion coefficient. Please delete this post!

 

[mathman]

The exponential term comes from neutron interactions with the medium. For a vacuum, L is infinite.