Energy loss of swift protons in liquid water: role of optical data input and extension algorithms

Abstract

A short review of the dielectric approach used to describe the energy deposited in liquid water by swift proton beams is presented. Due to the essential role played by the electronic excitation spectrum of the target, characterized by its energy loss function (ELF), we discuss in detail the procedure to obtain a reliable ELF from experimental optical data, which corresponds to zero momentum transfer. We also analyse the influence of the different methods used to extend this optical ELF to non-zero momentum transfers. From these different methods we calculate the stopping power and energy loss straggling of liquid water for proton beams, comparing them with other data available in the literature. In general, a good agreement is found at high projectile incident energy, but differences appear at energies around and lower than the maximum in the stopping power. Finally, the energy delivered to the target as a function of the depth (i.e., the depth-dose distribution) is obtained by means of a simulation code that takes into account the main interactions of the projectile with the target.