Experimental Evidence of a Variant Neutron Spectrum from the $T(t,2n)\alpha$ Reaction at Center-of-Mass Energies in the Range of 16–50 keV

Full calculations of six-nucleon reactions with a three-body final state have been elusive and a long-standing issue. We present neutron spectra from the $T(t,2n)\alpha$ (TT) reaction measured in inertial confinement fusion experiments at the OMEGA laser facility at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies ($E_{\mathrm{c}.\mathrm{m}.}$) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two $E_{\mathrm{c}.\mathrm{m}.}$, with the ${}^5\mathrm{He}$ ground state resonant peak at 8.6 MeV being significantly stronger at the higher than at the lower energy. The data provide the first conclusive evidence of a variant TT-neutron spectrum in this $E_{\mathrm{c}.\mathrm{m}.}$ range. In contrast to earlier available data, this indicates a reaction mechanism that must involve resonances and/or higher angular momenta than $L=0$. This finding provides an important experimental constraint on theoretical efforts that explore this and complementary six-nucleon systems, such as the solar ${}^3\mathrm{He}({}^3\mathrm{He},2p)\alpha$ reaction.

Figure 1

Calculated center-of-mass energy distributions for the implosions with $T_{\text{ion}}=3.7\mathrm{keV}$ (solid black curve), $T_{\text{ion}}=11.1\mathrm{keV}$ (dashed blue curve), and $T_{\text{ion}}=18.3\mathrm{keV}$ (dash-dotted red curve).

Figure 2

NTOF-measured signal traces for shots 77951 (dash-dotted red curve, $T_{\text{ion}}=18.3\mathrm{keV}$) and 77963 (solid black curve, $T_{\text{ion}}=3.7\mathrm{keV}$). The time axis has been corrected for capsule burn time (0.8 ns for 77951 and 1.7 ns for 77963). The corresponding neutron energy scale is indicated at the top. The traces are normalized to match in the time interval 361–371 ns (6.8–7.2 MeV neutron energy). The inset shows an enlargement of the ${}^5\mathrm{He}$ gs peak at $E_n=8.6\mathrm{MeV}$.

Figure 3

Total $R$-matrix fit (solid red curve) to measured NTOF data for shot 77951 (black points) with $T_{\text{ion}}=18.3\mathrm{keV}$ using the Verbinski light-yield function. Also shown in the figure are the $R$-matrix components representing the $3/2^{-}$ (dash-dotted green curve), $1/2^{-}$ (short-dashed blue curve), and $1/2^{+}$ (dotted magenta curve) partial waves and the dineutron (nn) interaction (long-dashed cyan curve), an interference component (double-dot-dashed purple curve), and the contribution from the DT neutron background (solid orange curve). The DT background level is constrained by the small DT tail visible before the rise of the TT spectrum (compare Fig. 2).

Figure 4

Final neutron spectra inferred from $R$-matrix analysis of the measured NTOF-spectra for shot 77951 (red curve, $E_{\mathrm{c}.\mathrm{m}.}=50\mathrm{keV}$) and 77963 (black curve, $E_{\mathrm{c}.\mathrm{m}.}=16\mathrm{keV}$), normalized to each other in the 4–7 MeV range. The width of the lines represents the estimated systematic uncertainty. These spectra clearly demonstrate a difference in the shape of the neutron spectrum with the peak at 8.6 MeV corresponding to the ${}^5\mathrm{He}$ gs interaction being stronger at a higher energy.