1. The problem statement, all variables and given/known data 2. Relevant equations T=2*(m*M/(m+M)^2)*E(initial)*(1-cos(angle)), T is transfered energy from the neutron to the target. displacement energy for Zr=40eV 3. The attempt at a solution the question is strange for me it seems to me there is a trick since we dont have the initial energy of the incident particle! I cant start solving without knowing the energy of the incident particle! any help please!