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 + Δ\theta_n }[/math]] and a momentum of the neutron between [[math]\displaystyle{ P_n }[/math], [math]\displaystyle{ P_n + ΔP_n }[/math]]. What are reasonable [math]\displaystyle{ Δ\theta_n and ΔP_n }[/math]?