# Stochastic Methods for the Neutron Transport Equation II: Almost sure growth

## TL;DR

This paper advances the understanding of stochastic growth in neutron transport equations by improving growth property results in the supercritical regime, aligning neutron transport theory with modern branching process theory.

## Contribution

It provides a significant improvement on existing growth results for the physical process in the supercritical regime of the neutron transport equation.

## Key findings

- Enhanced growth property characterization in supercritical regime
- Alignment of neutron transport theory with modern branching process results
- Improved probabilistic understanding of neutron flux growth

## Abstract

The neutron transport equation (NTE) describes the flux of neutrons across a planar cross-section in an inhomogeneous fissile medium when the process of nuclear fission is active. Classical work on the NTE emerges from the applied mathematics literature in the 1950s through the work of R. Dautray and collaborators, [7, 8, 19]. The NTE also has a probabilistic representation through the semigroup of the underlying physical process when envisaged as a stochastic process; cf. [7, 17, 18, 20]. More recently, [6] and [16] have continued the probabilistic analysis of the NTE, introducing more recent ideas from the theory of spatial branching processes and quasi-stationary distributions. In this paper, we continue in the same vein and look at a fundamental description of stochastic growth in the supercritical regime. Our main result provides a significant improvement on the last known contribution to growth properties of the physical process in [20], bringing neutron transport theory in line with modern branching process theory such as [14, 12].