Neutron Radiation Shielding for the NIF Streaked X-Ray Detector (SXD) Diagnostic

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) is preparing for the National Ignition Campaign (NIC) scheduled in 2010. The NIC is comprised of several "tuning" physics sub-campaigns leading up to a demonstration of Inertial Confinement Fusion (ICF) ignition. Some of these experiments requires to use the NIF streak x-ray detector (SXD) to measure fuel capsule trajectory (shock timing) or x-ray "bang-time" from time-resolved x-ray imaging of the imploding capsule fuelled with pure tritium (T) instead of a deuterium-tritium (DT) mixture. The resulting prompt neutron fluence at the planned SXD location (~1.7 m from the target) would be ~ 1.4e9/cm². Previous measurements suggest the onset of significant background at a neutron fluence of ~ 1e8/cm² and the radiation damage and operational upsets which start at ~ 1e8 rad-Si/sec must be factored into an integrated experimental campaign plan. Monte Carlo analyses were performed to predict the neutron and gamma/x-ray fluences and radiation doses for the proposed diagnostic configuration. A possible shielding configuration is proposed to mitigate radiation effects. The primary component of this shielding is an 80 cm thickness of Polyethylene (PE) between target chamber center (TCC) and the SXD diagnostic. Additionally, 6-8 cm of PE around the detector reduces the large number of neutrons that scatter off the inside of the target chamber. This proposed shielding configuration reduces the high-energy neutron fluence at the SXD by approximately a factor of ~50.