Cristian D. Denton

Energy Loss of Hydrogen- and Helium-Ion Beams in DNA: Calculations Based on a Realistic Energy-Loss Function of the Target

Abstract We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I  =  81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ≲100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells.

Target inner-shells contributions to the stopping power and straggling for H and He ions in gold

The electronic energy loss of swift H and He ions in gold is studied, paying special attention to the contribution to the projectile energy loss due to the ionization of the target inner-shells. We calculate the stopping power and the energy-loss straggling as a function of the projectile energy, taking into account the different charge states that the projectile can acquire inside the target and using the dielectric formalism. The electronic response of gold is described by the MELF-GOS model, where the excitation of valence and N-shell electrons is characterized by a linear combination of Mermin-type energy-loss functions, whereas the contribution to the projectile energy loss due to the target K-, L-, and M-shell ionization is included through hydrogenic generalized oscillator strengths. Stopping powers and straggling are, respectively, systematically above and below the data in our linear response formulation when H and He ions are below 0.1 MeV/u. Our calculations show that the total contribution of K-, L-, and M-shells to stopping power and straggling is less than 0.5% and 3.5% below 1 MeV/u, respectively. For stopping powers, the contribution of these inner-shells increases to 9% at 10 MeV/u and continues to rise to 20% at 100 MeV/u. For straggling, it increases to 23% at 10 MeV/u and rises to 35% at 100 MeV/u.

Electronic Stopping Power of Amorphous Carbon for H2+ and H3+ Beams

Partial support from the Spanish DGICYT (project PB96-1118) and from the Generalitat Valenciana (project GV99-54-1-01). C.D.D. thanks the Fundacion Seneca (Comunidad Autonoma de la Region de Murcia) for financial support.

Exit angle, energy loss and internuclear distance distributions of H2+ ions dissociated when traversing different materials

Abstract We have performed computer simulations of the trajectory followed by each proton resulting from the dissociation of H2+ molecules when traversing a thin solid target. We use the dielectric formalism to describe the forces due to electronic excitations in the medium, and we also consider the Coulomb repulsion between the pair of protons. Nuclear collisions with target nuclei are incorporated through a Monte Carlo code and the effect of the coherent scattering is taken into account by means of an effective force model. The distributions of exit angle, energy loss and internuclear separations of the protons fragments are discussed for the case of amorphous carbon and aluminum targets.

Velocity and orientational dependence of h3 + energy loss

Abstract The energy loss of H3 + molecular beams in amorphous carbon foils has been studied within the dielectric formalism and taking into account the Coulomb explosion among the three protons of the H3 + molecule; in this study we have incorporated the effects of the velocity-dependent effective charge. The stopping power was calculated as the time average, over the dwell time, of the instantaneous stopping power corresponding to each interproton distance, as the molecule explodes during its travel through the target. We have analysed the dependence of the stopping ratio as a function of the molecule velocity as well as of its orientation relative to the direction of motion, finding a reasonably good agreement with the available experimental data.