Definition of cross sections 1 - Moodle

Definition of cross sections 4I 0 (cm-2 s-1)sLet’s consider now a one-speed and parallel beam of neutrons of intensity I 0 neutrons perunit surface and unit time. The beam hits perpendicularly a slab of finite width. The collisionor total reaction rate dR is the number of collisions of neutrons in an elemental width dslocated at distance s, per unit time and unit surface of the slab. Each time a neutron collidein ds, it is removed from the uncollided beam. The reaction rate is writtends(6)dR = −dI(s) = I(s)Σdswhere I(s) is the intensity of the uncollided beam after a distance s in the slab. Integratingthis equation between 0 and s, we obtain(7)I(s) = I 0 e −Σs .ENE6101: Week 4 Cross sections – 6/37