Self characterization of a coded aperture array for neutron source imaging

Abstract

The neutron imaging system at the National Ignition Facility (NIF) is an important diagnostic tool for measuring the two-dimensional size and shape of the neutrons produced in the burning deuterium-tritium plasma during the stagnation stage of inertial confinement fusion implosions. Since the neutron source is small (∼100 μm) and neutrons are deeply penetrating (>3 cm) in all materials, the apertures used to achieve the desired 10-μm resolution are 20-cm long, triangular tapers machined in gold foils. These gold foils are stacked to form an array of 20 apertures for pinhole imaging and three apertures for penumbral imaging. These apertures must be precisely aligned to accurately place the field of view of each aperture at the design location, or the location of the field of view for each aperture must be measured. In this paper we present a new technique that has been developed for the measurement and characterization of the precise location of each aperture in the array. We present the detailed algorithms used for this characterization and the results of reconstructed sources from inertial confinement fusion implosion experiments at NIF.

Source localization for neutron imaging systems using convolutional neural networks

“…where m is the number of pixels in the aperture image, I k is the kth pixel of the recorded image, Ĩk is the kth pixel of the model image produced by Eq. ( 1), and σ k is the standard deviation of the kth pixel computed according to the noise model described in Volegov et al 7 At each iteration, SrcLocOpt minimizes Eq. ( 2) by adjusting the source location and shape parameters.…”

“…Typically, the noise in environments encountered by the neutron imaging system can be described by either a Gaussian or Poisson distribution. 7 In order to confirm the reliability of our models, we require that they perform well under noisy conditions. In this section, we test the performance of the CNN by adding noise to the aperture images and computing the change in prediction error.…”

Simultaneous neutron and x-ray imaging of inertial confinement fusion experiments along a single line of sight at Omega

“…8 However, characterization of the aperture array currently in use at NIF has required extensive effort, and modifying or replacing it would require months of work and dozens of NIF shots to repeat this characterization process. 9 The current concept for NXI focuses on a solution which can be rapidly deployed without modifying the neutron imaging characteristics of the aperture array. Merrill et al 1 proposed to attach an array of thin, tantalum x-ray pinholes, similar to those used on the existing x-ray imagers, to the neutron imaging aperture array, covering one of the penumbral apertures.…”

Dynamic high energy density plasma environments at the National Ignition Facility for nuclear science research

“…Considerable work has been done to demonstrate the viability of a gamma imaging system with prototype detector systems already developed and tested at HED facilities, such as Omega, and at the High Intensity Gamma Source at the Duke University FEL [203]. The necessary image reconstruction algorithms are available to invert the coded aperture source [204][205][206].…”