Determining the uncertainty of Eγ

To determine the uncertainty in Eγ we pick an angle for the neutron within [[math]\displaystyle{ theta_n }[/math], [math]\displaystyle{ theta_n + delta theta_n }[/math]] and a momentum of the neutron between [[math]\displaystyle{ P_n }[/math], P_n + delta P_n]. What are reasonable delta theta_n and delta P_n?