Measurement of the T plus T Neutron Spectrum Using the National Ignition Facility

Neutron time-of-flight spectra from inertial confinement fusion experiments with tritium-filled targets have been measured at the National Ignition Facility. These spectra represent a significant improvement in energy resolution and statistics over previous measurements, and afford the first definitive observation of a peak resulting from sequential decay through the ground state of He-5 at low reaction energies E-c.m. less than or similar to 100 keV. To describe the spectrum, we have developed an R-matrix model that accounts for interferences from fermion symmetry and intermediate states, and show these effects to be non-negligible. We also find the spectrum can be described by sequential decay through l = 1 states in He-5, which differs from previous interpretations.