OPTICAL DISPERSIVE FUNCTION STUDY FOR DETERMINATION OF MEAN IONIZATION ENERGY IN BIOLOGICAL MATERIALS

FEDERICO GESER; MAURO VALENTE

Merlo

Congreso; 100 Reunion Nacional de Fisica de AFA; 2015

AFA

Mean ionization energy value I = ~ ̄ ω is a fundamental quantity in radiation physics concerning energy depositionin matter and dose determinations[1,2]. The stopping power of heavy ions in different materials depends entirely on thisparameter[2,3]. If a closed theoretical form was available, it must show a dependence in the atomic number Z of themedium as well as other physical properties. A good approach for dense material is[4]:R ∞   1   ln(ω)dωln( ̄ ω ) = 0 RIm ∞ (ω,q)  1 ω  0 Im (ω,q) ωdωHere, Im(z) denotes imaginary part of z. The dielectric response (ω, q) contains all the dynamic properties of the materialrelated to its response to an external perturbation. From a general point of view, the framework for the dielectric functiondepends not only on the frequency (energy) but also on the transferred moment q (in case of ion projectiles). So, thefunction (q, E) is known as Bethe surface[5] of the material, with the limit q → 0 called the ?optical limit?.In this limit, exact integration is made for the normalization of the average calculation to obtain I. On the other hand,it will be shown that a closed form for the natural logarithm integral is not reachable by simple theoretical means becauseof the functional dependence of the integrals involved in the average calculations. An extensive study of available dielectricmodels[5,6] was carried out aimed to provide numerical integration methods capable of assessing new I values. The highand low frequency limits are compared between models to know which is better in each case.The main motivation for this study regards the requirements of reliable and accurate models to calculated radiation-matter interaction parameters of photons in biological tissues for medical physics applications.[1] Bohr N, j-PHIL-MAGAZINE 25-6 (1913)[2] Bethe H, j-ANN-PHYS-1900 397-3 (1930)[3] Fano U, Annual Review of Nuclear Science 13-1 (1963)[4] Ritchie H, Nuclear Instruments and Methods in Physics Research 198 (1982)[5] Inotuki M, Rev. Mod. Phys. 43-3 (1971)[6] Debye P, Journal of the Society of Chemical Industry 48-48 (1929)