Significance in Elastic Scatter
Recall we said that for any value of q, the energy transferred to the target nucleus, Et, isEt = En (4MaMn)cos2q / (Ma + Mn)2
So the average energy imparted to the target nucleus is
= {En (4MaMn) / (Ma + Mn)2}
We just spent three pages proving =1/2
Thus = 2EnMaMn / (Ma + Mn)2