When two deuterons fuse, I understand they form an alpha particle in an excited state. Based on mass differences the fusion releases 23.85MeV, which is enough to eject a proton (19.81MeV) or a neutron (20.58MeV) from the alpha particle. It seems to prefer neutron emission, as this fusion is used to make neutron beams. What mechanism is at work to make the alpha particle emit a neutron and not a proton? Does it always emit a neutron in this case, or is it a probabilistic mix of neutron and proton emission?