Angular and Energy Distributions of Electrons Produced in Arbitrary Biomaterials by Proton Impact

We present a simple method for obtaining reliable angular and energy distributions of electrons ejected from arbitrary condensed biomaterials by proton impact. Relying on a suitable description of the electronic excitation spectrum and a physically motivated relation between the ion and electron scattering angles, it yields cross sections in rather good agreement with experimental data in a broad range of ejection angles and energies, by only using as input the target composition and density. The versatility and simplicity of the method, which can be also extended to other charged particles, make it especially suited for obtaining ionization data for any complex biomaterial present in realistic cellular environments.The authors recognize the financial support from the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Project No. FIS2010–17225). PdV acknowledges financial support from the European Union’s FP7-People Program (Marie Curie Actions) within the Initial Training Network No. 608163 "ARGENT". Support from the European COST Action MP1002 NanoIBCT is gratefully acknowledged

Excitation and ionisation cross-sections in condensed-phase biomaterials by electrons down to very low energy: application to liquid water and genetic building blocks

Electronic excitations and ionisations produced by electron impact are key processes in the radiation-induced damage mechanisms in materials of biological relevance, underlying important medical and technological applications, including radiotherapy, radiation protection in manned space missions and nanodevice fabrication techniques. However, experimentally measuring all the necessary electronic interaction cross-sections for every relevant material is an arduous task, so it is necessary having predictive models, sufficiently accurate yet easily implementable. In this work we present a model, based on the dielectric formalism, to provide reliable ionisation and excitation cross-sections for electron-impact on complex biomolecular media, considering their condensed-phase nature. We account for the indistinguishability and exchange between the primary beam and excited electrons, for the molecular electronic structure effects in the electron binding, as well as for low-energy corrections to the first Born approximation. The resulting approach yields total ionisation cross-sections, energy distributions of secondary electrons, and total electronic excitation cross-sections for condensed-phase biomaterials, once the electronic excitation spectrum is known, either from experiments or from a predictive model. The results of this methodology are compared with the available experimental data in water and DNA/RNA molecular building blocks, showing a very good agreement and a great predictive power in a wide range of electron incident energies, from the large values characteristic of electron beams down to excitation threshold. The proposed model constitutes a very useful procedure for computing the electronic interaction cross-sections for arbitrary biological materials in a wide range of electron incident energies.This work has received funding from the European Union's Horizon 2020 Research and Innovation programme under the Marie Sklodowska-Curie grant agreement no. 840752, from the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Project no. PGC2018-096788-B-I00), from the Fundación Séneca (Project no. 19907/GERM/15) and from the Conselleria d'Educació, Investigació, Cultura i Esport de la Generalitat Valenciana (Project no. AICO/2019/070). PdV acknowledges further financial support provided by the Spanish Ministerio de Economía y Competitividad by means of a Juan de la Cierva postdoctoral fellowship (FJCI-2017-32233)

Calculated energy loss of swift light ions in platinum and gold: importance of the target electronic excitation spectrum

Understanding and predicting the energy loss of swift ions in metals is important for many applications of charged particle beams, such as analysis and modification of materials, and recently for modelling metal nanoparticle radiosensitisation in ion beam cancer therapy. We have calculated the stopping power of the transition metals Pt and Au for protons and alpha particles in a wide energy range, using the dielectric formalism, which realistically accounts for the excitation spectrum of each metal through the Mermin Energy Loss Function - Generalised Oscillator Strength methodology. For each combination of projectile, energy and target, we have considered: (i) the equilibrium charge state of the projectile through the target, (ii) the energy-loss due to electron capture and loss processes, and (iii) the energy loss resulting from the polarisation of the projectile's electronic cloud due to the self-induced electric field. Our calculated stopping powers show a fairly good agreement with the available experimental data for platinum and gold, particularly the most recent ones around the stopping power maximum, which validates the methodology we have used to be further extended to other transition metals. For the materials studied (platinum and gold), two commonly used and different sources of the experimental excitation spectrum yield comparable calculated stopping powers and mean excitation energies, the latter being closer to the most recent data provided in a recent ICRU Report than to previous compilations. Despite the small differences in the sources of excitation spectra of these metals, they lead to practically the same stopping power results as far as they reproduce the main excitation features of the material and fulfil physically motivated sum rules.We thank financial support from the European Union's Horizon 2020 Research and Innovation programme under the Marie Sklodowska-Curie grant agreement no. 840752, the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Project no. PGC2018-096788-B-I00), and the Fundación Séneca (Project no. 19907/GERM/15)

Simulation of the energy spectra of swift light ion beams after traversing cylindrical targets: a consistent interpretation of experimental data relevant for hadron therapy

We have performed detailed simulations of the energy spectra, recorded at several angles, of proton and helium ion beams after traversing thin cylindrical targets of different nature (liquid water and ethanol jets, as well as a solid aluminium wire), in order to reproduce a series of measurements intended to assess the stopping power of 0.3–2 MeV ions. The authors of these experiments derived values of the stopping power of liquid water (a quantity essential for the evaluation of radiation effects in materials, particularly for radiotherapy purposes) that are ~10% lower than what is expected from other measurements and theories. In our simulations, instead of treating the stopping power as an unknown free parameter to be determined, we use as input the electronic stopping power accurately calculated within the dielectric formalism. We take into account in the simulations the different interactions that each projectile can experience when moving through the target, such as electronic stopping, nuclear scattering or electron charge-exchange processes. The detailed geometry of the target is also accounted for. We find that our simulated energy distributions are in excellent agreement with the published measurements when the diameter of the cylindrical targets is slightly reduced, what is compatible with the potential evaporation of the liquid jets. On the basis of such an excellent agreement, we validate the accuracy of the model we use to calculate electronic excitation cross sections for ions in condensed matter in its range of applicability (particularly the electronic stopping power) needed for charged particle transport models, and we offer a consistent, but alternative, interpretation for these experiments on ion irradiation of cylindrical targets.Financial support was provided by the Spanish Ministerio de Economía y Competitividad, the Spanish Ministerio de Ciencia, Innovación y Universidades and the European Regional Development Fund (Projects No. FIS2014-58849-P and PGC2018-096788-B-I00), as well as by the Fundación Séneca - Agencia de Ciencia y Tecnología de la Región de Murcia (Project No. 19907/GERM/15). PdV also acknowledges financial support provided by the Alexander von Humboldt Stiftung/Foundation through a postdoctoral fellowship

Energy deposition around swift proton tracks in polymethylmethacrylate: How much and how far

The use of proton beams in several modern technologies to probe or modify the properties of materials, such as proton beam lithography or ion beam cancer therapy, requires us to accurately know the extent to which the energy lost by the swift projectiles in the medium is redistributed radially around their tracks, since this determines several endpoints, such as the resolution of imaging or manufacturing techniques, or even the biological outcomes of radiotherapy. In this paper, the radial distribution of the energy deposited around swift-proton tracks in polymethylmethacrylate (PMMA) by the transport of secondary electrons is obtained by means of a detailed Monte Carlo simulation. The initial energy and angular distributions of the secondary electrons generated by proton impact, as well as the electronic cross sections for the ejection of these electrons, are reliably calculated in the framework of the dielectric formalism, where a realistic electronic excitation spectrum of PMMA is accounted for. The cascade of all secondary electrons generated in PMMA is simulated taking into account the main interactions that occur between these electrons and the condensed phase target. After analyzing the influence that several angular distributions of the electrons generated by the proton beam have on the resulting radial profiles of deposited energy, we conclude that the widely used Rudd and Kim formula should be replaced by the simpler isotropic angular distribution, which leads to radial energy distributions comparable to the ones obtained from more realistic angular distributions. By studying the dependence of the radial dose on the proton energy we recommend lower proton energies than previously published for reducing proximity effects around a proton track. The obtained results are of relevance for assessing the resolution limits of proton beam based imaging and manufacturing techniques.Financial support was provided by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund (Project No. FIS2014-58849-P), as well as by the Fundación Séneca (Project No. 19907/GERM/15). P.d.V. acknowledges financial support from the European Union’s FP7 People: Marie-Curie Actions program within the Initial Training Network No. 608163 “ARGENT”, Advanced Radiotherapy, Generated by Exploiting Nanoprocesses and Technologies

Energy deposition around swift proton and carbon ion tracks in biomaterials

Hadron therapy is a modern cancer treatment based on the interaction of proton or heavier ion beams with living tissue, whose purpose is the destruction of the malignant tumor cells producing minimal effects on the surrounding healthy tissue. To study the physical basis of the Relative Biological Effectiveness (RBE) of different projectiles, such as protons or carbon ions, we calculate the radial distribution of energy deposited by the secondary electrons generated in biomaterials by these ions at characteristic energies around the Bragg peak. This is done by means of the simulation code SEED, which follows in detail the motion and interactions of the secondary electrons as well as the subsequent electron cascade

Experiències senzilles de física recreativa: flascó de Mariotte, refracció de la llum i tira d’alumini ondulant

En aquest treball presentem una sèrie d’experiments senzills i estimulants de física que resulten útils per a reforçar els coneixements propis d’aquesta matèria, consolidar conceptes físics, captar l’atenció dels estudiants i fomentar-ne l’interès per la matèria de física. En aquesta ocasió hem dissenyat i dut a terme experiències de diversos camps de la Física, com ara fluids (flascó de Mariotte), òptica (refracció de la llum) i magnetisme (tira d’alumini ondulant a causa d’un camp magnètic). Amb aquest tipus d’experiments simples volem augmentar la motivació dels estudiants cap a la matèria de física, ja que l’objectiu d’aquestes experiències és tractar d’entendre el fenomen que s’està observant, en comptes d’obtenir i tractar dades. Aquests experiments estan adreçats als estudiants dels primers cursos dels graus de ciències i de les diverses enginyeries. Aquesta manera d’introduir conceptes de física mitjançant experiments sorprenents fa que l’aprenentatge resulte més significatiu, i constitueixen una eina pedagògica de gran ajuda en el procés d’ensenyament-aprenentatge de la física

Energy Deposition around Swift Carbon-Ion Tracks in Liquid Water

Energetic carbon ions are promising projectiles used for cancer radiotherapy. A thorough knowledge of how the energy of these ions is deposited in biological media (mainly composed of liquid water) is required. This can be attained by means of detailed computer simulations, both macroscopically (relevant for appropriately delivering the dose) and at the nanoscale (important for determining the inflicted radiobiological damage). The energy lost per unit path length (i.e., the so-called stopping power) of carbon ions is here theoretically calculated within the dielectric formalism from the excitation spectrum of liquid water obtained from two complementary approaches (one relying on an optical-data model and the other exclusively on ab initio calculations). In addition, the energy carried at the nanometre scale by the generated secondary electrons around the ion’s path is simulated by means of a detailed Monte Carlo code. For this purpose, we use the ion and electron cross sections calculated by means of state-of-the art approaches suited to take into account the condensed-phase nature of the liquid water target. As a result of these simulations, the radial dose around the ion’s path is obtained, as well as the distributions of clustered events in nanometric volumes similar to the dimensions of DNA convolutions, contributing to the biological damage for carbon ions in a wide energy range, covering from the plateau to the maximum of the Bragg peak.This research was funded by European Union’s Horizon 2020 Research and Innovation programme grant number 840752, by the Spanish Ministerio de Economía y Competitividad grant number PGC2018-096788-B-I00 funded by MCIN/AEI/10.13039/501100011033 and by FEDER “A way to make Europe”, and by the Fundación Séneca grant number 19907/GERM/15. The APC was funded by the Spanish Ministerio de Economía y Competitividad and the European Regional Development Fund grant number PGC2018-096788-B-I00

Simulating the nanometric track-structure of carbon ion beams in liquid water at energies relevant for hadrontherapy

The nanometric track-structure of energetic ion beams in biological media determines the direct physical damage to living cells, which is one of the main responsibles of their killing or inactivation during radiotherapy treatments or under cosmic radiation bombardment. In the present work, detailed track-structure Monte Carlo simulations, performed with the code SEED (Secondary Electron Energy Deposition), are presented for carbon ions in a wide energy range in liquid water. Liquid water is the main constituent of biological tissues, and carbon ions are one of the most promising projectiles currently available for ion beam cancer therapy. The simulations are based on accurate cross sections for the different elastic and inelastic events determining the interaction of charged particles with condensed-phase materials. The latter are derived from the ab initio calculation of the electronic excitation spectrum of liquid water by means of time-dependent density functional theory (TDDFT), which is then used within the dielectric formalism to obtain inelastic electronic cross sections for both carbon ions and secondary electrons. Both the ionisation cross sections of water by carbon ions and the excitation and ionisation cross sections for electron impact are obtained in very good agreement with known experimental data. The elastic scattering cross sections for electrons in condensed-phase water are also obtained from ab initio calculations by solving the Dirac-Hartree-Fock equation. The detailed simulations fed with reliable cross sections allow to assess the contribution of different physical mechanisms (electronic excitation, ionisation and dissociative electron attachment –DEA–) to the carbon ion-induced direct biodamage.This project received funding from the European Union's Horizon 2020 Research and Innovation programme under the Marie Sklodowska-Curie grant agreement no. 840752. This work was also supported by the Spanish Ministerio de Ciencia e Innovación and the European Regional Development Fund (Project PGC2018-096788-B-I00); the Fundación Séneca-Agencia de Ciencia y Tecnología de la Región de Murcia (Project 19907/GERM/15); and the Ministry of Science and Higher Education of the Russian Federation as part of World-class Research Center program: Advanced Digital Technologies (contract No. 075-15-2020-934 dated 17.11.2020)

Experiències senzilles de física recreativa com a recurs didàctic per a introduir i consolidar conceptes de física

En aquest projecte utilitzarem experiències senzilles de física recreativa per a introduir i consolidar conceptes de física general. Aquest recurs didàctic s’ha posat en marxa a la matèria de Complements per a la Formació Disciplinar en Física, corresponent al Màster Universitari en Formació del Professorat d’Educació Secundària, perquè molts estudiants no han cursat els estudis universitaris de Física. Amb la realització d’aquests experiments pretenem dotar al futur professorat de secundària d’una eina pedagògica que motive els estudiants cap a la física, fent que l’aprenentatge siga més significatiu i perdure en el temps. Per altra banda, es prestarà especial atenció a ressaltar la interdisciplinarietat de la física amb altres ciències i amb la vida quotidiana, ja que és un factor motivador que afavoreix l’aprenentatge. Durant la realització d’aquest projecte es dissenyarà i es posarà en marxa un recull d’experiències de diversos camps de la física. Cadascun dels experiments que es realitzaran, senzills i moltes vegades sorprenents, tindrà l’objectiu d’introduir i consolidar un concepte físic, a més de fomentar l’interès dels alumnes per la matèria de física