A narrow beam of alpha particles with kinetic energy T = 0.50

A narrow beam of alpha particles with kinetic energy T = 0.50 MeV and intensity I = 5.0 路 105 particles per second falls normally on a golden foil. Find the thickness of the foil if at a distance r = 15 cm from a scattering section of that foil the flux density of scattered particles at the angle 0 = 60 掳 to the incident beam is equal to J = 40 particles/(cm2-s).