Gamma background calculation for the HEXRI diagnostic at the National Ignition Facility

Abstract

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is nearing completion and the commencement of experiment campaigns leading to ignition. Different parameters of an implosion, such as the neutron yield and images of the target, will be inferred from the measurement of emitted neutrons, gammas, and x-rays using different diagnostics. The High-Energy X-Ray Imager (HEXRI) is one of these diagnostics and will be used on cryogenic Deuterium-Tritium implosions at NIF to provide data on the size and shape of the imploding inertial confinement fusion (ICF) capsules. These data are fundamental for understanding the reasons for ignition failure. Following an implosion, the background from neutrons and photons can be very large and can affect the quality of HEXRI images in terms of lowering the signal-to-background ratio below the threshold of 1. In this work we present Monte Carlo simulations of the n-gamma background from the components present inside the target chamber, i.e. the cryogenic target positioner, the Laser Plasma Interaction collimators, the neutron and HEXRI pinholes, and the hohlraum. We found that the ratio of the self-emission x-ray signal to n-gamma background signal stays well above 1 in all cases, allowing a good HEXRI image quality.