| Περίληψη: | The understanding of the interactions between ionizing particles and
biomolecules, as well as the prediction of their effects, remains a key
challenge of radiobiology. When ionizing radiation interacts with biological
materials, it changes the electronic properties of the targeted biomolecules.
During this time, a great number of chemical reactions take place.
Each cell is primarily consisting of water, just like the human body. As a result,
there is a high probability of the radiation to interact with the water as it covers
most of the cell's volume. The interaction of ionizing radiation with atoms of
water molecules, which are common in biological systems, is known as water
radiolysis. The damages caused by water radiolysis products are termed
indirect effects of ionizing radiation. Water radiolysis is a procedure during
which molecules become unstable by ionizing radiation particles such as
electrons, or heavy ions, and leads to the formation of free radical species in
water (OH-·, H, e-aq, H2, H202). The number of free radicals that are
produced by ionizing radiation depends on the total dose. Molecules or
molecular fragments with one or more unpaired electrons in atomic or
molecular orbitals are referred to as free radicals. Free radicals are
uncharged, very reactive, and short-lived molecules. These new molecular
species may either interact with each other, producing other molecules or they
may interact chemically with tissues, cells, and the DNA causing molecular
structural damage. Free radicals may cause damage to DNA by stealing their
electrons through a process called oxidation, leading to the impairment of
function or cell death. Indirect biological damage due to reactive species and
radicals produced by water radiolysis reactions is responsible for most of the
biological effects (~2/3 of the total radiation damage), as water is the primary
constituent of most living organisms. However, this proportion varies,
depending on the type of the radiation as well as its energy.
Several Monte Carlo Track Structure (MCTS) models have been developed to
estimate biological damage caused by ionizing radiation at the DNA level.
Geant4-DNA simulates ionizing radiation interactions in liquid water, as well
as the pre-chemical and chemical stages of water radiolysis.
The purpose of the present study is to investigate the effect of radiation type
in water radiolysis by using Monte Carlo simulations. More specifically the
Geant4-DNA extension of Geant4 was used. The Geant4-DNA collaboration
provides a list of six examples for the simulation of water radiolysis. In this
work, an in-house version of the chem3 example of the Geant4-DNA package
was used (geant4 / examples / extended / medical / dna / chem3). This
example simulates and records the free radicals produced when a particle isallowed to interact with a water sphere. This thesis, investigated the number
of free radicals produced, depending on the energy and the type of the
radiation particle.
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