Auger electron emission spectra from noble gas atoms and molecular-bound iodine: a Monte Carlo simulation

Nuclides decaying by electron capture (EC) and/or by internal conversion (IC) are of special interest in radiobiology and nuclear medicine. Incorporated into the DNA structure, they can cause severe molecular and cellular damage. These findings open up an unique opportunity for applying these nuclides in tumour therapy. Therefore, to utilize them effectively an understanding of their radiation action mechanism is essential, which requires first of all a precise knowledge of the nuclides' electron emission spectra. Due to a lack of experimental data for the particular nuclides of interest (e.g.123I and 125I), computer simulations have become necessary. Moreover, electron spectra for individual decays are needed as input for track structure calculations. A decay by EC and/or IC induces an inner electron shell vacancy, the starting point for a complex cascade of photon (radiative) and Auger (non-radiative) transitions within the atomic energy levels. Because of the stochastic nature of these transitions, the Monte Carlo technique is an appropriate tool for the study and simulation of these processes and the resulting emission spectra. In this paper, Auger cascades in noble gas atoms were simulated by a Monte Carlo computer code. For these atoms many available experimental data allow a comparison of the results and a validation of the simulation code. In view of the quite rough assumptions used here, a remarkably good agreement has been obtained e.g. with the experimentally found distributions of charges left on the atoms after the cascades have finished. On this reasonable basis, the program was used to calculate electron energy spectra for 125I. In addition, also implications following from a molecular bonding of Auger emitters will be discussed. Ion fragment distribution from small iodine-labelled molecules (CH3125I, C2H5125I) in the gaseous phase has been explained by a Coulomb explosion model. In a first approach the possibility of such a damage mechanism for an iodine-labelled DNA base was also confirmed. A semi-empirical quantum mechanical calculation of labelled thymine with different charges showed no stable molecule configuration with charges of greater than +5. (author)