A neutron travelling with a velocity v and kinetic energy E collides head-on elastically with a nucleus of mass number A at rest. Calculate the fraction of total energy retained by the neutron.

Explanation

`v_1 = ((m_1 - m_2)u_1)/((m_1 + m_2))` while `u_2 = 0`
`:. v_1 = ((1 - A)u_1)/((1 + A))`
Neutron is `""_(0)n^(1)`. Its mass number = 1.
Fraction of energy retained by neutron
`= ((1/2 mv_1^2)/(1/2 m u_1^2)) = ((v_1)/(u_1))^(2) = ((1 - A)/(1 + A))^(2) = ((A -1)/(A + 1))^(2)` .