1. The problem statement, all variables and given/known data A beam of particles strike a wall containing 2 x 10^29 atoms per m^3. each atom behaves like a sphere of radius 3 x 10^-15 m. Find the thickness of the wall that exactly half the particles will penetrate without scattering. What thickness would be needed to stop all but one particle in 10^6 2. Relevant equations the mean free path (λ) = 1/nσ 3. The attempt at a solution I'm not sure how to start the solution. I can't find an equation that will bring the thickness of the wall into the problem Cheers.