D-T gamma-to-neutron branching ratio determined from inertial confinement fusion plasmas

A new deuterium-tritium (D-T) fusion gamma-to-neutron branching ratio [3H(d,γ)5He/3H(d,n)4He] value of (4.2 ± 2.0) × 10−5 was recently reported by this group [Y. Kim et al. Phys. Rev. C (submitted)]. This measurement, conducted at the OMEGA laser facility located at the University of Rochester, was made for the first time using inertial confinement fusion (ICF) plasmas. Neutron-induced backgrounds are significantly reduced in these experiments as compared to traditional beam-target accelerator-based experiments due to the short pulse nature of ICF implosions and the use of gas Cherenkov γ-ray detectors with fast temporal responses and inherent energy thresholds. It is expected that this ICF-based measurement will help resolve the large and long-standing inconsistencies in previously reported accelerator-based values, which vary by a factor of approximately 30. The reported value at ICF conditions was determined by averaging the results of two methods: (1) a direct measurement of ICF D-T γ-ray and neutron emissions using absolutely calibrated detectors and (2) a separate cross-calibration against the better known D-3He gamma-to-proton branching ratio [3He(d, γ)5Li/3He(d,p)4He]. Here we include a detailed explanation of these results, and introduce as a corroborative method an in-situγ-ray detector calibration using neutron-induced γ-rays. Also, by extending the established techniques to two additional series of implosions with significantly different ion temperatures, we test the branching ratio dependence on ion temperature. The data show a D-T branching ratio is nearly constant over the temperature range 2–9 keV. These studies motivate further investigation into the 5He and 5Li systems resulting from D-T and D-3He fusion, respectively, and result in improved ICF γ-ray reaction history diagnosis at the National Ignition Facility.