Definition of cross sections 1 - Moodle

Definition of cross sections 5The ratio I(s)/I 0 is the probability for the neutron to be uncollided after a distance s. Theprobability of an interaction for this neutron in the following elemental width ds is Σds. TheprobabilityP(s)ds for a neutron in I 0 to collide in ds is therefore(8)P(s)ds = Σe −Σs ds .This equation is useful to define the mean free path λ of neutrons in an infinite slab, animportant quantity in reactor physics. The mean free path is the average trajectory length ofthe neutrons in an infinite and homogeneous material. This value is obtained from equation(9)λ =∫ ∞0dssP(s) = 1 Σ .ENE6101: Week 4 Cross sections – 7/37