Measurement of the $T+T$ 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 ${}^5\mathrm{He}$ at low reaction energies $E_{\mathrm{c}.\mathrm{m}.}\lesssim 100\mathrm{keV}$. 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.

Figure 1

$R$-matrix spectra resulting from the fitted feeding factors are shown above. Decays through partial waves of the ground and first-excited states of ${}^5\mathrm{He}$ are shown in the top and middle panels, respectively. Each partial wave contains an interference term from antisymmetrization, which is proportional to the overlap between primary and secondary distributions. For the $J=1/2^{-}$ partial wave, the interference term makes a $\approx 30\%$ contribution. The bottom panel displays the coherent sum of both partial waves. An interference created within the neutron angular correlation manifests itself in the spectrum. The feeding factors determine the sign of this interference.

Figure 2

Experimental corrections applied to the modeled spectrum, as discussed in the text, are displayed in successive order (top panel). The combined fit is plotted over energy-binned data from the 22 m detector (bottom panel), in the region of interest and over the full range of neutron energies. Statistical errors are approximately the size of the data points. Transitions through the ground state of ${}^5\mathrm{He}$ produce a peak at $\approx 8.7\mathrm{MeV}$; the broad distribution peaked around 6 MeV can largely be described by the first-excited state in ${}^5\mathrm{He}$. For reference, the short-dashed (cyan) curve shows the effect of not including antisymmetry. The background from deuterium contamination, which is illustrated better in the inset, has a minimal effect in the region of interest.