Quantum Mechanisms of Polarization and Relaxation in Solid-State Polarized Targets

Irradiated ammonia (NH₃) and deuterated ammonia (ND₃) crystals have served for years as polarized targets in nuclear and high-energy physics experiments to probe nucleonic spin structures. They are ideal for this application due to their mechanical robustness, radiation resistance, and favorable spin-lattice relaxation times. Despite their extensive use, several fundamental mechanisms—the spin-lattice and spin–spin relaxation, the quantum origins of polarization loss, and the sudden polarization enhancement observed in ND₃ during beam exposure—remain inadequately understood.

In this work, we employ both theoretical and computational approaches to study the quantum processes governing polarization and relaxation in solid-state polarized targets. We begin by calculating g-tensors and hyperfine coupling tensors for both ṄH₂ and ṄD₂ radicals, generating electron spin resonance (ESR) spectra that we validate against experimental results. We further explore nitrogen and hydrogen-based radicals that can potentially be formed during beam exposure of ND3. Additionally, we perform density functional theory (DFT) and neural-network quantum molecular dynamics (NNQMD) calculations to investigate vibrational modes and nuclear quantum effects leading to polarization and relaxation mechanisms in NH₃ and ND₃. Finally, we construct spin-lattice relaxation times from first principles, providing a microscopic understanding of polarization dynamics in these systems.