4. METHOD OF SOLUTION
Primary protons are assumed to travel in straight lines through the shield, losing energy by ionizing inter- actions at a rate given by tabulated range-energy data. Inelastic collisions remove protons from the beam and generate the following secondaries:
i) Cascade protons and neutrons, assumed to be emitted in the same direction as the primaries
ii) Evaporation neutrons, emitted isotropically, with a fission energy spectrum.
Cascade protons and neutrons are propagated through the system in the same way as the primaries, and they produce higher generations of secondary particles. Evaporation neutrons are assumed to produce no secondaries, their flux at the inside of the shield is computed by the numerical evaluation of the appropriate integral expression.
Elastic scattering between cascade neutrons and hydrogen is assumed to reduce the neutron energy without changing its direction. The recoil proton is assigned the energy lost by the neutron, and is assumed to be omitted in the same direction.