Problem 6.10

A narrow beam of alpha particles with kinetic energy $T$ $=0.50 \mathrm{MeV}$ and intensity $I=5.0 \cdot 10^{5}$ particles per second falls normally on a golden foil. Find the thickness of the foil if at a distance $r=15 \mathrm{~cm}$ from a scattering section of that foil the flux density of scattered particles at the angle $\theta=60^{\circ}$ to the incident beam is equal to $J=40$ particles $/\left(\mathrm{cm}^{2} \cdot \mathrm{s}\right)$