Measurement of theT+TNeutron 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 $^{5}\mathrm{He}$ at low reaction energies ${E}_{\mathrm{c}.\mathrm{m}.}$\lesssim${}100\text{ }\text{ }\mathrm{keV}$. 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 $$\ell${}=1$ states in $^{5}\mathrm{He}$, which differs from previous interpretations.