Study of scattered background neutron in NIF and time-of-flight (TOF) to measure neutron

Some of the planned core diagnostics for National Ignition Facility (NIF) will use neutron time-of-flight (TOF) spectroscopy techniques to gather information for primary neutron yield measurement or neutron imaging. TOF methods will be used to observe target fuel areal density $\langle\rho {\rm R}\rangle$ (radial integral of density) via measuring the number of primary 14.1 MeV neutrons that are down-scattered to lower energies by nuclear collisions inside the compressed target core. The large number of primary neutrons scattered by the target chamber and structures at NIF will contribute a significant background signal during $\langle\rho {\rm R}\rangle$ measurements. The optimum detector locations outside the target chamber or target bay wall are proposed. Appropriate collimators at the chamber port and the bay wall (between the neutron source at target chamber center (TCC) and detector) that maximize detection of signal neutrons while minimizing the background from scattered neutrons and neutron induced gamma rays are presented