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 [H-3(d,gamma)He-5/H-3(d,n)He-4] value of (4.2 +/- 2.0) x 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 gamma-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 gamma-ray and neutron emissions using absolutely calibrated detectors and (2) a separate cross-calibration against the better known D-He-3 gamma-to-proton branching ratio [He-3(d,gamma)Li-5/He-3(d,p)He-4]. Here we include a detailed explanation of these results, and introduce as a corroborative method an in-situ gamma-ray detector calibration using neutron-induced gamma-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 He-5 and Li-5 systems resulting from D-T and D-He-3 fusion, respectively, and result in improved ICF gamma-ray reaction history diagnosis at the National Ignition Facility. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4718291]