Suppose a beam of particles with flux is incident on a uniform target of thickness T

Suppose a beam of particles with flux Φ is incident on a uniform target of thickness T and density ρ(mass/volume). Assuming that the atoms in the target do not block one another (the thin-target approximation),show that the reaction rate dN/dt is given by dN/dt = IσρN_AT/m, where I is the beam current (i.e.,the integral of the flux over the area of the beam), mis the molecular mass of the target, N_A is Avogadro’s number, and σ is the reaction cross section.