[math]\frac {m}{e}[/math] = we'll have 2.5"
[math]\frac {2.5}{x}[/math] = tan(1.95)
radiator distance - collimator
x = 73.4" = 186.5 cm
radiator is 73.4" - 48" upstream of the wall
Something someone needs to do:
Look up which has the highest neutron production energy thresh hold: Cu, Zn, Ni
2 * p n cos θ = 15 MeV
2 * sqrt(pn 2 + m n 2) = 15 MeV = m d