Atomic Radiation in Nuclear Decay

Abstract

Auger electrons emitted in nuclear decay offer a unique tool to kill cancer cells at the scale of a DNA molecule. Over the last forty years many aspects of this promising therapeutic tool have been explored, however it is still not in the phase of large scale clinical trials. In this paper we review the physical processes of Auger emission in nuclear decay and present a new model being developed to evaluate the energy spectrum of Auger electrons, and hence overcome the limitations of existing computations. Unstable atomic nuclei release excess energy through var- ious radioactive decay processes by emitting radiation in the form of particles (neutrons, alpha, beta particles) or electromagnetic radiation (gamma-ray photons). Most of the applications using nuclear isotopes are based on the fact that the interaction of the radiations passing through material will depend on their type (photons, neutral or charged particles) and the transferred energy. Most ra- dioisotopes used in clinical therapy emitparticles, which are ionizing radiations. The biological effect is often char- acterized by Relative Biological Effectiveness (RBE) - which is related to Linear Energy Transfer, LET. LET is expressed in units ofkeV/µm,which isameasureofthe en- ergy deposited along the particle track. A new class of ra- dionuclides (1), including 149 Tb, 211 At, 211 Po, 213 Bi, 223 Ra, 225 Ac, 226 Th, 227 Ac, and 230 U, which emit® particles have been considered for therapy. The LET for most therapeu- tic® emitters ranges from 25 to 230 keV/µm. On the other hand, electrons and positrons emitted in nucleardecay, and in the internal conversion processes, referred to here asparticles, have kinetic energies ranging from tens of keV to several MeV and their LET is much lower, typi- cally» 0.2 keV/µm. A third type of ionizing radiation is Auger electrons (2), named after the French physicist Pierre Victor Auger. When an inner-shell electron is removed from an atom, the vacancy will be filled by an electron from the outer shells and the excess energy will be released as an X-ray photon, or by the emission of an Auger electron. Referred to as atomic radiations, X-ray and Auger electron emission are competing processes. The atomic transition rate and the energy of emitted X-rays and/or Auger electrons depend on the atomic number, the electron shells involved, and the electron configuration of the atom. The full relaxation of the inner-shell vacancy is a multi step process, resulting in a cascade of atomic radiations. The energy of atomic radi- ation is typically in the range from a few eV to 100 keV. Due to their short range (nm toµm), Auger electrons with relatively low energies can have a much higher LET. For