Physics of solar neutron production: Questionable detection of neutrons from the 31 December 2007 flare

Spacecraft observations in the inner heliosphere offer the first opportunity to measure 1-10 MeV solar neutrons. We discuss the cross sections for neutron production in solar flares and calculate the escaping neutron spectra for monoenergetic and power law particle spectra at the Sun and at the distance (0.48 AU) and observation angle of MESSENGER at the time of its reported detection of low-energy solar neutrons associated with the 31 December 2007 solar flare. We detail solar physics concerns about this detection: (1) the inferred number of accelerated protons at the Sun for this modest M2 class flare would have been 10 times larger than any flare observed to date, and (2) the implied energy in accelerated ions would have been 50-10(4) times what we would expect based on the observed energy in nonthermal electrons and the energy in the thermal X-ray plasma. We find that there is no compelling evidence for a high electron/proton ratio in the solar energetic particle (SEP) event, raising concerns that the neutron counts came mostly from SEP ion interactions in the spacecraft; this concern is supported by the similarity of the SEP and neutron count rates. The MESSENGER team made detailed calculations of neutron production from SEP protons. However, if interactions < 30 MeV had been included in their calculations and the carbon spacecraft structure were a significant source of secondary neutrons, we estimate that SEP proton and a-particle interactions could account for the observed fast neutron rate. This is due to (13)C that has a 3 MeV proton threshold for neutron production and is exothermic for alpha-particle interactions.