Effective atomic number of soft tissue, water and air for interaction of various hadrons, leptons and isotopes of hydrogen.

Purpose: Characterization of soft tissue, water and air in terms of effective atomic number (Z) with respect to the interactions of hadrons, leptons and isotopes of hydrogen.

Method: Mass collision stopping powers (MCSPs) were calculated first using Bethe formula. Then, these values were used to estimate Z using linear-logarithmic interpolation. A scale equation was also used to calculate MCSP.

Results: Variation in Z, over the 0.5-50 MeV energy range considered, is minimum for muon and pion (π meson) interactions (relative difference [RD] ≤ 7%), while maximum variation has been noticed in Zfor heavy charged particles, i.e. alpha particle (RD ≤ 26%). The highest values of Z were obtained for muon particle, the lightest particle while the minimum values of Z were obtained for alpha particle interaction. Except for very low kinetic energies, water equivalence of soft tissue is very satisfactory (RD ≤ 3%). The Z of water relative to air was found to be almost constant at high energies. The present results should be valid for especially high energies where the Bethe formula can be applied. This applies to relatively higher energies (>2 MeV) for heavier particles such as alpha particles and applies to relatively lower energies (>0.5 MeV) for lighter particles such as protons.

Conclusions: In view of the importance of water equivalence in particle therapy, new data on Z in soft tissue, water and air for fundamental particle interaction should be important. Results revealed that soft tissue could be considered as water equivalent for interaction of various fundamental particles.