Survey of 0.5-8 MeV Neutron Events Detected by the MESSENGER Neutron Spectrometer between December 2007 and June 2013

The Neutron Spectrometer (NS) on the MESSENGER spacecraft has been operating nearly continuously from February 2007 to the present. The sensor element of the NS consists of three scintillator sensors, each having 100 cm2 area. The two outside plates are 4-mm-thick lithium glass (LG), and the middle sensor is a cubic block of borated plastic (BP) scintillator, 10 cm on a side. This detector can sense and identify neutrons between energies of 0.5 and 8 MeV. Energetic charged particles can be detected in three separate energy bands. The lowest band is identified by a singles event in any one of the sensor elements, denoting energetic electrons having energies > 1 MeV or protons with energies > 10 MeV (LG) and between 15 and 30 MeV (BP). Double coincidences between the LG and BP require proton energies > 30 to 50 MeV and electrons > 4 to 10 MeV. The highest energy band identifies triple coincidences (LG-BP-LG), which require proton energies > 100 to 120 MeV and electron energies > 30 MeV. To date, 61 energetic particle events have been detected when MESSENGER was more than 8000 km from Mercury. Of these 61 events, 33 contained fluxes of neutrons that were significantly higher than the background neutron counting rate from the interaction of galactic cosmic rays with spacecraft material. One of these 33 neutron events occurred on 4 June 2011 in the absence of enhanced energetic charged particles having energies above the double-coincidence threshold. The neutrons of this event were therefore interpreted as originating from the Sun. The purpose of this study is to build on that observation by surveying all neutron events to assemble a set of conditions common to all that can help identify the physical conditions that produced these neutrons at their origin. Two possibilities exist, they are generated at the Sun by collisions between ions accelerated in solar transient events, or they can be generated in the spacecraft by collisions between energetic ions and spacecraft material. Except for the 4 June 2011 event, the remaining 32 events were accompanied by enhanced energetic charged particles. For these remaining events, we are developing criteria to determine if the detected neutrons originate from the Sun or from local spacecraft material. An important parameter that can be calculated is the ratio, R, of measured neutrons to an estimate of locally produced neutrons. This ratio is determined using an assumed energetic ion spectrum and a neutron production and transport model of the MESSENGER spacecraft. We have determined that for six of the 32 neutron events, R was greater than 10. These large ratios suggest that for this subset of events, the neutrons may be dominantly produced at the Sun. Additional parameters we will investigate include the power-law slope and alpha to proton ratio of the assumed input ion fluences and the relative time profiles of the neutron and charged particle count rates.