Stopping cross-section for protons across different phases of water

TL;DR

This study uses advanced computational methods to accurately compare proton energy loss in liquid water and ice across a wide energy range, revealing their equivalence and improving understanding for medical and astrophysical applications.

Contribution

The paper introduces a non-perturbative, computationally-efficient model that accurately predicts phase-dependent proton stopping power in water, including the liquid phase, which was previously elusive.

Findings

Proton stopping power in liquid water and ice are identical.

The model agrees well with experimental data across 0.001-10 MeV.

Phase differences are significant around the maximum of the stopping power curve.

Abstract

Accurately quantifying the energy loss rate of proton beams in liquid water is crucial for the precise application and improvement of proton therapy, whereas the slowing down of proton in water ices also plays an important role in astrophysics. However, precisely determining the electronic stopping power, particularly for the liquid phase, has been elusive so far. Experimental techniques are difficult to apply to volatile liquids, and the availability of sufficient reliable measurements has been limited to the solid and vapor phases. The accuracy of current models is typically limited to proton energies just above the energy-loss maximum, making it difficult to predict radiation effects at an energy range of special relevance. We elucidate the phase differences in proton energy loss in water in a wide energy range (0.001-10 MeV) by means of real-time time-dependent density functional theory combined with the Penn method. This non-perturbative model, more computationally-efficient than current approaches, describes the phase effects in water in excellent agreement with available experimental data, revealing clear deviations around the maximum of the stopping power curve and below. As an important outcome, our calculations reveal that proton stopping quantities of liquid water and amorphous ice are identical, in agreement with recent similar observations for low-energy electrons, pointing out to this equivalence for all charged particles. This could help to overcome the limitation in obtaining reliable experimental information for the biologically-relevant liquid water target.