On the absolute calibration of a DT fusion neutron yield diagnostic

Abstract

Recent advances in Inertial Confinement Fusion (ICF) experiments at Lawrence Livermore National Laboratory's National Ignition Facility (NIF) have underscored the need for accurate total yield measurements of DT neutrons because yield measurements provide a measure of the predicted performance of the experiments. Future gas-puff DT experiments at Sandia National Laboratory's Z facility will also require similar measurements. For ICF DT experiments, the standard technique for measuring the neutron (14.1 MeV) yield, counts the activity (counts/minute) induced in irradiated copper samples. This activity occurs by the 63Cu(n,2n)62Cu reaction where 62Cu decays by positrons (β+) with a half-life of 9.67 minutes. The calibrations discussed here employ the associated-particle method (APM), where the α (4He) particles from the T(d,n)4He reaction are measured to infer neutron fluxes on a copper sample. The flux induces 62Cu activity, measured in a coincidence counting system. The method leads to a relationship between a DT neutron yield and copper activity known as the F-factor. The goal in future experiments is to apply this calibration to measure the yield at NIF with a combined uncertainty approaching 5%.