Author: Michael Dingfelder 👨🔬
Affiliation: East Carolina University 🌍
Purpose:
To study and accurately evaluate the influence of the Fermi Density effect on stopping powers and cross sections of charged particles in liquid water used in Monte Carlo radiation transport calculations at high relativistic energies.
Methods:
The Fermi Density effect is a relativistic polarization effect in condensed matter. It is in general introduced within the dielectric response theory and requires knowledge of the optical dielectric response function or optical constants of the material. It is well understood and implemented in stopping power calculations within the Bethe theory. However, calculations for differential cross sections as used in detailed radiation transport codes are scares and not well understood. They are often neglected as radiation transport simulations at relativistic energies often use condensed history methods which only require stopping powers as input. Also, in case of protons in liquid water, the Fermi Density effect can only be seen in velocities above b > 0.55 or energies above approximately 450 MeV, which is outside of the therapeutic range.
Results:
In this communication we will review the literature and update older calculations to provide formulas and numbers for cross sections used for radiation transport simulations. Even if the main contribution of the Fermi Density effect is currently above the therapeutic range, there is interest in the field, and especially in space radiation applications.
Conclusion:
The Fermi Density effect on cross sections is currently of academic interest but with practical applications in space radiation and radiation protection for astronauts on future space missions.