The removal cross section RR(En) is the probability that a fast
neutron undergoes its first collision which removes it from the
group of penetrating uncollided neutrons. Therefore the effective
removal cross section RR(En) is less than the total neutron cross
section. Fast neutrons are slowed down principally by elastic and
inelastic collisions with few captures occurring at high energies.
The moderated neutrons are effectively removed by capture since
the capture cross section increases significantly with the decrease
in neutron energy.