Advanced Physics

Question

Suppose we consider a beam of neutrons incident upon a thin target with an intensity

of 1012 neutrons/cm2-sec. Suppose further that the total cross section for the nuclei in

this target is 4 b. Using this information, determine how long one would have to wait,

on the average, for a given nucleus in the target to suffer a neutron interaction

Answer 1

Assuming that 4 b means σ = 4 x 10^-24 cm^2, and the flux rate is R = 1012 /cm^2 s),

for a given nucleus, one would have to wait t = 1/(R σ) = 1/( 1012 x 4 x 10^-24) = 2.5 x 10 20 s - a very long time.

However for the target over all, this number would be divided by the product of the beam area and the areal density of nuclei,