Authors: G.A. de Nolfo (NASA Goddard Space Flight Center), J.M. Ryan (University of New Hampshire)

Solar eruptive events often unfold into distinct phases—a prompt impulsive phase sometimes followed by a delayed and prolonged, high-energy (>100 MeV) gradual phase, referred to as Long Duration Gamma Ray Flares (LDGRF).  Numerous secondaries are produced including γ rays, X-rays, and neutrons.  While γ rays have been detected for decades by various missions, the detection and measurement of solar neutrons has been much more elusive.  Yet, neutron measurements from 20-150 MeV complement high and low γ-ray measurements and fill the decade-wide energy gap (30-300 MeV) in the accelerated proton spectrum produced by a declining proton spectrum and declining cross sections.  A solar spectroscopic neutron measurement from above 20 MeV would be an important measurement when combined with high-and low-energy γ-ray measurements, such as those from Large Area Telescope (LAT) and Gamma-ray Burst Monitor (GBM) on Fermi.  This would provide continuous sensitivity in the progenitor proton spectrum from 1 MeV to beyond 1 GeV.  Thus, solar neutron observations will extend our knowledge of the dynamic parent proton spectrum into a hard-to-measure band and fill a critical gap in the accelerated ion spectrum.  Solar neutron observations, a game-changing measurement, will open up a relatively unexplored radiation channel and thus lead to a significantly improved understanding of particle acceleration at the Sun.  We discuss the science motivation for solar neutron observations, the technologic advancements to make progress, and highlight several neutron spectrometer developments underway.