An Attempt to Modify the MOCA Water-Vapor-Ion Code to Simulate Liquid Phase

Abstract

An attempt was made to apply to liquid water the Monte Carlo code, MOCA14, which calculates track-structure (i.e., three-dimensional distribution of ionizations) for heavy charged-particle tracks in water-vapor. The criterion used was that the stopping powers for four energies of protons (1, 2, 5 and 10 MeV) should correspond to the stopping powers in liquid water found in ICRU Report #49. To adjust phenomenologically the vapor source data, two modifications were made: (1) several vapor-phase excitation interactions were assumed to result instead in ionizations in liquid; and (2) the optical oscillator strength along the energy-loss scale was shifted by adding (or subtracting) a constant while simultaneously and independently subtracting a constant energy from the inelastic ionization energy thresholds. The total energy-loss was used to overcome the binding energy (which was decreased by an arbitrary amount) and to provide energy to the ejected secondary electron. To be consistent, a similar adjustment was made to the secondary electron energy-loss processes. It was found that no adjustment of the two constants brought the values of the average stopping powers into agreement with the ICRU stopping power values. It is concluded that no simple and physically meaningful manipulation of the water vapor code for protons in the energy range between 1 and 10 MeV could bring the stopping powers into agreement with currently accepted values.