Estudio de la energía requerida para generar un par ionico por impacto de radiación ionizante. Aplicaciones en dosimetría para radioterapia y hadronterapia / Study of the energy required to generate an ion pair by impact of ionizing radiation. Dosimet

M. E. GALASSI; V. TESSARO

Villa Carlos Paz

Congreso; 7º Congreso Latinoamericano de Física Médica ? 13º Congreso Argentino de Física Médica; 2016

Asociación Latinoamericana de Física Médica (ALFIM) y Sociedad Argentina de Física Médica (SAFIM)

INTRODUCTION. Heavy charged particles slow down (i.e., lose their initial energy) as they travel through a material as a result of collisions with the molecules of the media. The products of these interactions are either excited atoms or ion pairs, i.e., pairs consisting of a free (primary) electron and the corresponding positive ion (ionization). Secondary free electrons produced in further reactions of primary electrons are used as a basis for the detection of ionizing radiation by ionization chambers. Reference dosimetry in proton therapy and hadron therapy is performed in liquid water by ionization chambers filled with air. The procedure of absorbed dose determination based on ionization chamber dosimetry is recommended by IAEA TRS 398. In order to determine the dose (energy deposited by unit mass in liquid water) from the measured number of electrons produced by the ionization of the air in the chamber, conversion factors are necessary, such as the W-values or mean energy spent by the incident particle for the formation of an electron-ion pair after complete dissipation of its initial energy in air. Available data on W are often fragmented, deficient, dispersed, and missing systematic, nevertheless several compilations and reviews, such as ICRU Report 31 and Chapter 8 in the IAEA TECDOC 799, provide a basis for assessing the present knowledge of W values for different charged particles in various gases. However, for ion impact the W-values are a very important source of uncertainties (estimated between 1.5 and 5%) that directly influences the overall uncertainties in dose determination. Therefore, research in this field is required.OBJECTIVE. The aim of this work was to study the W-values of charged particles on different gases to improve the current uncertainties and to provide values when experimental data are not available. MATERIALS AND METHODS. W-values were calculated using the continuous-slowing-down approximation (CSDA). In this model, the rate of energy loss at every point along the track is assumed to be equal to the same as the total stopping power (energy lost by unit of length traversed in the medium). The cross sections of all the physical processes that produce the spent of energy of the primary and secondary particles and the stopping power are required. To study the dependence of W-values on the incident particle type and energy and on the medium, we have considered the impact of protons, antiprotons and electrons on water vapour and on the gases that compose the air. The cross sections of the most important processes (ionization and electronic excitation) were calculated using ab-initio and semiempirical models (usually used in codes of particle transport). RESULTS AND CONCLUSIONS. The stopping powers for the different particles and mediums were calculated in the CSDA with these cross sections and compared with data from ICRU and NIST, obtaining a very good agreement. CSDA calculation of W-values for electron impact on water vapour shows the same trend than the experimental data but some differences in the absolute values are observed. This effect could be related to the cross sections used but also with the CSDA approximation, that does not take into account the stochastic nature of the interactions of the ionizing radiation with the molecules of the medium. We are working in the employ of more accurate cross sections and in the development of a Monte Carlo code for transport of ions and electrons in gases.