A neutron travelling with a velocity v and kinetic energy E has a perfectly elastic head-on collision with a nucleus of an atom of mass number A at rest. The fraction of total energy retained by the neutron is approximately
[A−1A+1]2
v′1=(m1−m2)v1+2m2v2m1+m2
As v2 is zero. m2>m1,v′1 is in the opposite direction.
m1=1,m2=A.
∴|v′1|=(A−1)2(A+1)2V1
The fraction of total energy retained is
12mv′212mv21=(A−1)2(A+1)2