Study of the water-to-air stopping power ratio for swift hadron impact

GALASSI, MARIEL; TESSARO, VERONICA; QUIROGA, FLAVIA; MIRANDOLA; MAIRANI, ANDREA; MOLINELLI; VILCHES-FREIXAS, GLORIA; CIOCCA, MARIO

Darmstadt

Congreso; Swift Heavy Ions in Matter; 2015

GSI (Alemania) - GANIL (Francia)

In particle therapy, the knowledge of the water-to-air stopping power ratio (STPR) is necessary for accurate ionization chamber dosimetry. According to the protocol provided by the International Atomic Energy Agency, Technical Report TRS-398, the STPR must be determined taking into account the entire particle-energy spectrum at the point of interest. For proton beams, it can be calculated using a semi-empirical law (obtained by fitting the results from a Monte Carlo code) as a particle range function. However, due to the complexity of the particle-energy spectrum for heavier ions, the protocol recommends to take a constant value of 1.13 with an estimated uncertainty of 2%.In a recent work, a parametrization of the water-to-air stopping power ratio is proposed, but it depends strongly on the ionization potentials of water and air used [1]. Other authors propose an approach based on experimental measurements of water-equivalent thicknesses of air gaps [2].In the present work, stopping power of antiprotons, protons and carbon ions in liquid water and air are calculated using a distorted-wave model to approximate the ionization cross sections required [4]. Taking into account only the primary particle spectrum (at the entrance channel), the water-to-air STPR can be approximated by the ratio of the stopping powers of water and air. The ratio of theoretical stopping powers as a function of the incident energy is compared with the obtained using the values recommended by ICRU Reports 49 (for protons) and 73 (for carbon ions). Bragg peaks for mono-energetic proton and carbon ionbeams, measured at the National Center of Oncological Hadrontherapy of Italy (CNAO), were used to determine the practical range of these particles as a function of the incident energy. These values were employed to obtain the STPR from the semi-empirical laws proposed in the literature and compared with the theoretical results.[1] A. Lühr et al, Phys. Med. Biol. 56, 2515-2533 (2011).[2] D. Sánchez-Parcerisa et al, Phys. Med. Biol. 57, 3629-3641 (2012).[3] M. E. Galassi et al, Phys. Rev. A 62,02270 (2000)01 - Atomic physics + Electronic excitation