Three isotropic neutron sources, each emitting $S$ neutrons$/$sec$,$ are located at the three corners

of an equilateral triangle of side $a.$ Find the flux $()$ and the current $(J)$ at the midpoint of one

side for:

a$.$ An infinite vacuum

b$.$ An infinite medium with the neutron flux given by:

$=\frac{S{e}^{-\frac{r}{L}}}{4Dr}$

where $D$ is the diffusion coefficient $($not a function of spatial variables$),$ and $L$ is a

constant