Nuclear quantum dynamics of boric acid as probed by a thermal-to-epithermal neutron station

Abstract

Boric acid (, BA), due to its neutron absorption and scattering properties, is used in nuclear medicine and nuclear engineering either as an absorber of thermal neutrons or a moderator of epithermal ones. BA was one of the earliest boron carriers historically evaluated for boron neutron capture therapy (BNCT). Recent advances in neutron spectroscopy and imaging, isotope labelling, and ab initio simulation have renewed interest in BA as a model system for pilot studies. Also in terms of BNCT treatment, BA emerges again as an efficient boron compound in the case of selected tumour types. Nuclear quantum dynamics and nuclear quantum effects, such as zero-point energy (ZPE), significantly influence the structure and vibrational properties of BA, and consequently, via Doppler-broadening, alter the way it scatters and absorbs neutrons. In this work, we systematically characterise the nuclear dynamics of boric acid in an isotope-resolved manner across a broad energy range using VESUVIO, a thermal-to-epithermal neutron station at the ISIS facility. We validate a series of thermodynamic and neutron observables against comprehensive ab initio calculations, providing direct insight into isotope-dependent neutron scattering and absorption in BA. Moreover, we establish limits of detection and quantification of boron and boric acid by concurrently employing neutron Compton scattering, the incident neutron energy-dependent transmission and prompt-gamma activation analysis.