Moderating a Neutron In a nuclear reactor, neutrons released by nuclear fission

Question

Moderating a Neutron In a nuclear reactor, neutrons released by nuclear fission must be slowed down before they can trigger additional reactions in other nuclei. To see what sort of material is most effective in slowing (or moderating) a neutron, calculate the ratio of a neutron's final kinetic energy to its initial kinetic energy, Kf/Ki, for a head-on elastic collision with each of the following stationary target particles. (Note: The mass of a neutron is m=1.009u, where the atomic mass unit, u, is defined as follows: 1u=1.66×1027kg.)

Explanation / Answer

from the pricniple of conservation of momentum

Vf = (m1-m2)Vi/(m1+m2)

Vf/Vi = (m1-m2)/m1+m2)

so the ratio of speed of neutron = (1.67 e -27 - 9.11 e-31)/(1.67 e-27 + 9.11 e-31)

Vf/Vi = 0.9989

thud the ratio of Eenrgy = 0.5 m Vf^2/0.5 mvi^2

Ratio = 0.9989^2

Ratio of KE = 0.9978

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