The Effect of Areal Density Asymmetries on Scattered Neutron Spectra in ICF Implosions

TL;DR

This paper introduces a new analysis method for interpreting scattered neutron spectra in ICF experiments, enabling direct measurement of areal density asymmetries with reduced confounding influences.

Contribution

A novel model is developed to fit the shape change in scattered neutron spectra, allowing direct inference of areal density asymmetries and their modes in ICF.

Findings

Model successfully fits spectra from neutron transport calculations.

Multiple lines of sight improve areal density distribution inference.

Analysis shows optimal detector arrangements for accurate measurements.

Abstract

Scattered neutron spectroscopy is a diagnostic technique commonly used to measure areal density in ICF experiments. Deleterious areal density asymmetries modify the shape of the scattered neutron spectrum. In this work a novel analysis is developed which can be used to fit the shape change. This allows scattered neutron spectroscopy to directly infer the amplitude and mode of the areal density asymmetries, with little sensitivity to confounding factors which affect other diagnostics for areal density. The model is tested on spectra produced by a neutron transport calculation with both isotropic and anisotropic primary fusion neutron sources. Multiple lines of sight are required to infer the areal density distribution over the whole sphere -- we investigate the error propagation and optimal detector arrangement associated with this inference.