Modelling Matter Across Multiscales: A Meso-Bio-Nano Approach to Modelling Crystal-Based Gamma-Ray Light Sources

Multiscale modelling has become an increasingly important tool across the natural sciences, driven by the need to understand complex systems that span multiple spatial and temporal scales. By coupling models conducted over different scales, the multiscale approach enables insights that are inaccessible to any single modelling method. In this context, gamma-ray crystal-based light sources (CLSs) have emerged as a promising, novel technology for the production of high-brilliance, ultra-short wavelength (λ≪1Å) radiation, operating on the principle of channelling. The design and practical realisation of gamma-ray CLSs is the focus of the Horizon Europe EIC-Pathfinder Project TECHNO-CLS: Emerging Technologies for Crystal-Based Gamma-Ray Light Sources, for which multiscale modelling plays a key role.

This thesis explores, develops, and builds upon existing methodologies for the multiscale modelling of structural and radiation processes in gamma-ray CLSs, contributing to the theoretical framework of the TECHNO-CLS project. Multiscale simulations are essential not only for understanding the fundamental physics underpinning gamma-ray CLSs, but also for informing their design for future exploitation. The work presented here focuses on two central questions: (i) how do substitutional dopant atoms modify the atomic-scale structure of host crystals, and (ii) how might these structural changes affect the radiation characteristics of gamma-ray CLSs. These problems are intimately linked, as doping represents a viable route for fabricating periodically bent crystals: essential components of gamma-ray CLSs.

Using molecular dynamics simulations, we analyse the effects of germanium doping in silicon (Si₁₋ₓGeₓ), and boron doping in diamond (C₁₋ₓBₓ), observing a linear lattice expansion with dopant concentration. Notably, the magnitude of lattice expansion is smaller than that reported in previous literature. We attribute this discrepancy to the superior crystalline quality of our simulated systems, which are free from many defects associated with experimental sample preparation and introduced during crystal growth.

Using relativistic molecular dynamics simulations, we then model the positron channelling process in harmonic, periodically bent crystals. We analyse the effects of small variations in the amplitude and period of bending on the characteristics of the emitted radiation. A shift in the peak energy of emitted radiation to lower photon energies is observed upon increasing the bending amplitude and period, with larger amplitudes and shorter periods accompanied by an increase in radiation intensity. Both of these results support the development of high-quality crystal-based light sources, where control over atomic-scale structure is essential for achieving reliable radiation production.