Angular distribution of neutron inverse beta decay , n̄

The reaction n̄e1p ̃e 1n is very important for low-energy (En&60 MeV) antineutrino experiments. In this paper we calculate the positron angular distribution, which at low energies is slightly backward. We show that weak magnetism and recoil corrections have a large effect on the angular distribution, making it isotropic at about 15 MeV and slightly forward at higher energies. We also show that the behavior of the cross section and the angular distribution can be well understood analytically for En&60 MeV by calculating to O(1/M ), where M is the nucleon mass. The correct angular distribution is useful for separating n̄e1p ̃e 1n events from other reactions and detector backgrounds, as well as for possible localization of the source ~e.g., a supernova! direction. We comment on how similar corrections appear for the lepton angular distributions in the deuteron breakup reactions n̄e1d ̃e 1n1n and ne1d ̃e 1p1p . Finally, in the reaction n̄e1p ̃e 1 1n , the angular distribution of the outgoing neutrons is strongly forward peaked, leading to a measurable separation in positron and neutron detection points, also potentially useful for rejecting backgrounds or locating the source direction. @S0556-2821~99!04015-1#