Intense {\gamma}-photon and high-energy electron production by neutron irradiation: effects of nuclear excitations on reactor materials

TL;DR

This paper investigates how neutron irradiation produces intense gamma photons and high-energy electrons in materials, revealing a two-step process that impacts material properties and could influence reactor material design.

Contribution

It introduces validated equations to evaluate photon and electron fluxes from neutron interactions, highlighting a two-step n-γ-e process affecting material behavior under irradiation.

Findings

Gamma photon and electron fluxes extend into MeV range.

The process causes vacancy diffusion and impurity dissociation.

W tungsten converts neutron energy into gamma flux with 99% efficiency.

Abstract

The effects of neutron irradiation on materials are often interpreted in terms of atomic recoils, initiated by neutron impacts and producing crystal lattice defects. In addition, there is a remarkable two-step process, strongly pronounced in the medium-weight and heavy elements. This process involves the generation of energetic {\gamma} photons in nonelastic collisions of neutrons with atomic nuclei, achieved via capture and inelastic reactions. Subsequently, high-energy electrons are excited through the scattering of {\gamma} photons by the atomic electrons. We derive and validate equations enabling a fast and robust evaluation of photon and electron fluxes produced by the neutrons in the bulk of materials. The two-step n-{\gamma}-e scattering creates a nonequilibrium dynamically fluctuating steady-state population of high-energy electrons, with the spectra of photon and electron energies extending well into the mega-electron-volt range. This stimulates vacancy diffusion through electron-triggered atomic recoils, primarily involving vacancy-impurity dissociation, even if thermal activation is ineffective. Tungsten converts the energy of fusion or fission neutrons into a flux of {\gamma} radiation at the conversion efficiency approaching 99%, with implications for structural materials, superconductors, and insulators, as well as phenomena like corrosion, and helium and hydrogen isotope retention.