Ion Energy Range Research Articles

Modeling swift heavy ion irradiation of substrate-supported two-dimensional material via two-temperature molecular dynamics simulations

This study employs two-temperature molecular dynamics simulations to investigate swift heavy ion irradiation of SiO2 substrate-supported two-dimensional material graphene. Material-dependent electronic and thermal properties are integrated into each region to model the energy transfer between the electronic and atomic subsystems of the studied materials. Simulations of interactions with Xe heavy ions are performed with ion kinetic energies ranging from 0.5 to 25 GeV with electronic stopping powers from ∼2.6 to 17.7 keV/nm. With the studied ion energy range, nanopores with a diameter of up to 5 nm can be formed in graphene due to the thermally driven sputtering effect, while amorphization occurs along the ion track in the SiO2 substrate. The coupling between the substrate and two-dimensional material significantly impacts the structural change due to heat transfer and atomic interactions among different layers of materials. The method applied in this work provides a valuable tool for modeling and understanding the structural modifications induced by ion irradiation in layered structures.

Ionization of small atmospheric acid–base clusters and its prospective role in seeding the growth of aqueous clusters

In the present study, we have examined the ionization of a set of small atmospheric acid–base clusters. The calculated ionization energy (IE) ranges from ∼660 kJ mol−1 to ∼1120 kJ mol−1, with clusters consisting of just the bases yielding the smallest IEs, while those of the acids yield the largest. For neutral clusters that form most readily, the IEs range from ∼790 kJ mol−1 to ∼880 kJ mol−1. These values are generally smaller than those of other chemicals with significant atmospheric presence. Thus, the radical cations of the acid–base clusters are not likely to ionize other atmospheric species. Charged atmospheric species have been hypothesized to facilitate the growth of large clusters. We have carried out a case study of the sequential hydration of an acid–base cluster and compared our results with previous studies of forming pure water clusters. The neutral acid–base cluster binds water more strongly than pure water clusters. Further strengthening of the binding can be seen for the radical cationic acid–base cluster. The resulting acid–base–water clusters may further represent a source of H3O+, which may proceed to facilitate cloud formation.

The dependence on displacement rate and temperature of near-surface void-denuding in self-ion irradiated pure polycrystalline and single-crystal iron

Pure iron has been irradiated with Fe2+ ions in a series of studies to identify neutron-atypical physical processes that influence the depth dependence of void swelling, focusing especially on suppression effects arising from injected interstitials and surface proximity. One paper in this series examined the surface influence in single-crystal Fe irradiated to 50 and 100 peak dpa over a range of temperature (425–525 °C) and ion energy (1.0, 2.5, 3.5, 5.0 MeV) while keeping the peak damage rate at 1.2 × 10–3 dpa/s, although the surface dpa rate was lower but increasing with decreasing ion energy, providing a small range of surface dpa rate. The observed denuded width Δx was modified to incorporate sputtering loss. The activation energy governing the denuding process was found to be EΔx4=1.65±0.03eV, higher than the vacancy migration energy EVm known to be 0.67 eV. This difference was attributed to the effect of dissolved carbon (103 appm) which reduces the effective vacancy mobility and thereby increases the effective migration energy.Since the previous study involved a factor of only 2.83 in near-surface dpa rate it is important to confirm that the dependence of EΔx4 on dpa rate is maintained over a larger range of damage rates. In this study polycrystalline Fe with 140 appm carbon was irradiated with 5 MeV Fe2+ ions to 50, 100 and 150 dpa over a range of peak dpa rates (2.0 × 10–4, 1.2 × 10–3, 6.0 × 10–3 dpa/sec) and temperatures (425, 475, 525 °C). At very high dpa levels sputtering and void growth lead to loss of voids via shrinkage, limiting the upper dose level where this technique can be applied. It was found that the single-crystal and polycrystal specimens yielded essentially identical behavior with EΔx4 = 1.65 eV, validating the application of this activation energy over a wider range of dpa, dpa rate, temperature, and crystal form.

NO3− loss from nitrate adducts of explosives by thermal decomposition in tandem Ion mobility spectrometry and by collision induced dissociation in tandem mass spectrometry

Nitrate adducts of nitroglycerin (NG) and 1,3-dinitroglycerin (1,3-DNG) were produced from atmospheric pressure chemical ionization with chloride reagent ions and in-source decomposition of M·Cl−. The nitrate adducts subsequently dissociated in the drift region with enthalpies of 109 ± 9 kJ mol −1 at 142–150 °C for NG·NO3− and 101 ± 8 kJ mol−1 at 161–173 °C for 1,3-DNG·NO3−. Similar behavior was not observed generally for other explosives although nitrate adducts of each explosive could be formed using electrospray ionization with a nitrate salt solution. Ion abundances were measured over a range of ion energies with collision induced dissociation in tandem mass spectrometry and models from Density Functional Theory were used to correlate the experimental findings to structural motifs and other adduct properties. The computational modeling showed that adduct stability is dominated by the electrostatic interaction between the nitrate ion and the dipole moment of the neutral explosive. Specifically, explosives having the ability to adapt a conformer with a large dipole moment showed the most stable adducts. Other binding contributions are possible yet were found to be minor in the explosive adducts studied here.

Deposition processes of superhard diamond-like carbon films co-adjusted by ion energy and ion flux in arc discharges

As a kind of protective material, Diamond-Like Carbon (DLC) film can effectively improve the mechanical and tribological properties of the substrate surface. In DLC family, tetrahedral amorphous carbon (ta-C) with ultra-high hardness and excellent protective properties has been widely concerned in the field of hard protective materials. It is well known that the structure and properties of ta-C are strongly dependent on carbon plasma characteristics in the plasma environment. Therefore, it is extremely crucial to obtain the optimal process window of ta-C described by plasma micro-parameters with general significance. This paper proposed the Langmuir probe was used to diagnose the carbon plasma of the afterglow generated by arc evaporation, aiming to investigate the characteristics of carbon plasma and sheath. The effect of ion energy and ion flux adjusted by the pulse bias voltage and arc current on the sp3 hybrid bonds and hardness of DLC films was studied, aiming to find out which conditions satisfied the formation of ta-C. The results showed the microstructure and properties of DLC films were co-adjusted by ion energy and ion flux. When the carbon ions met the appropriate ion energy range (50–300 eV) and low ion flux (<8.99 × 1010 cm−2 s−1), the films tended to form ta-C structures with high sp3 bonds. However, under the condition of high ion energy and high ion flux, the structure of the films changed from sp3 bonds to sp2 bonds, resulting in the graphitization and formation of DLC films with high sp2 bonds. This work will provide new solutions to guide the design and acquisition of high-performance DLC films.

Mass spectrometric study of low energy Cs+ ion-induced sputtered fragmentation of PADC polymer.

In this study, low-energy cesium (Cs+ ) ion-induced sputtered fragmentation of poly allyl diglycol carbonate (PADC) was investigated using mass spectrometry. The collision-induced dissociation mechanism revealed emission of various fragments, including monoatomic (H- , C1 - , O1 - ), diatomic (C2 - ), and multiatomic (C3 - , CO2 - , C2 O2 - , C3 O2 - ) species within the Cs+ ion energy range of 1-5keV. The anion current of these fragments exhibited a linear increase with rising incident Cs+ ion energy, indicating a corresponding rise in fragment abundance. Analysis of normalized yield indicated that at 1keV incident energy, the dominant fragment was monoatomic hydrogen (H- ), followed by diatomic carbon (C2 - ), monoatomic carbon (C1 - ), and monoatomic oxygen (O1 - ). Although C2 - remained dominant up to 5keV, other fragments exhibited varying normalized yields at different ion energy steps. The sputter yield estimation revealed that monoatomic hydrogen (H- ) and diatomic carbon (C2 - ) exhibited the highest yields, increasing exponentially beyond 3keV, while multiatomic fragments like C3 - , CO2 - , C2 O2 - , and C3 O2 - displayed the lowest yields. The sputter dissociation mechanism pointed to dehydrogenation, chain scission, and bond breakage as the primary processes during low-energy Cs+ ion impact. Postsputtering Scanning Electron Mircoscope (SEM) micrographs show craters, pits, and micropores on the PADC surface, indicating significant surface degradation. X-ray Diffraction (XRD) spectra exhibited reduced diffraction intensity, while Fourier Transform Infrared Spectroscopy (FTIR) analysis indicated the absence of molecular bands in the IR spectrum, confirming extensive surface damage due to Cs+ ion-induced sputtering.

Parametrization of Energetic Ion and Electron Fluxes in the Near‐Earth Magnetotail

AbstractThe near‐Earth plasma sheet region is the main source of energetic (tens to hundreds keV) ion and electron populations transported by convection and injections into the inner magnetosphere. Energetic ions from the plasma sheet contribute to the ring current, whereas energetic electrons contribute to the radiation belt seed population for further acceleration to relativistic energies. Near‐Earth plasma sheet energetic fluxes have been traditionally used to set boundary conditions for radiation belt and ring current models. This study provides an empirical parametrization for ∼75 keV flux intensity as a function of the geomagnetic activity index auroral electrojet and the equatorial magnetic field Bz. Such parametrization includes the dynamic magnetic field configuration in the near‐Earth plasma sheet and may be merged with empirical magnetic field models. We also provide models extending this parametrization to the [20, 300] keV of electron energy range and [75, 300] keV of ion energy range. The parametrization is developed based on THEMIS and Geostationary Operational Environmental Satellite measurements, and verified by comparison with MMS measurements in the near‐Earth plasma sheet. This parametrization incorporates meso‐scale transient flux variations associated with Bz perturbations into ring current and radiation belt simulations.

Energy Distribution of Sputtered Atoms Explored by SRIM Simulations

The energy of the sputtered atoms is important to control the microstructure and physical properties of thin films. In this work, we used the SRIM program to simulate the energy of sputtered atoms. We analyzed the energy distribution functions (EDFs) and the average energies of the atoms in different spatial directions for a range of target materials and Ar ion energies. The results were compared to the analytical equations for EDFs derived by Sigmund and Thompson and with experimental data from the literature. The SRIM simulations give realistic EDFs for transition metals, but not for elements lighter than Si. All EDFs show a low-energy peak positioned close to one-half of the surface binding energy and a high-energy tail decreasing as approximately E−2. We analyzed the characteristics of EDFs, specifically, the position of low- and high-energy peaks, FWHM, and the energy tail, with respect to the ion energy and position of the element in the periodic table. The low-energy peak increases with atomic number for elements within each group in the periodic table. Similar changes were observed for FWHM. For the period 5 and 6 elements, additional broad high-energy peaks were observed at emission angles above 45° when sputtered by Ar ions with 300 eV and also in some heavier elements when bombarded by 600 eV and 1200 eV ions. The transition metals in groups 4, 5, and 6 in periods 5 and 6 have the highest average energies, while the lowest average energies have elements in group 11. The results of simulations show that the average energies of sputtered atoms were inversely proportional to the sputtering yield, i.e., the higher the sputtering yield, the lower the average energy of sputtered atoms. We established an empirical equation for transition metals to estimate the average energy of sputtered atoms from the sputtering yield. The angular distribution of the average atom energy depends on the atomic number. Transition metals with 22 &lt; Z &lt; 72 have an anisotropic energy distribution, with the highest average energies in the 40°–70° range. For the elements in group 11, the angular distribution of the average energies is more isotropic.

Predicting electronic stopping powers using stacking ensemble machine learning method

PurposeAccurate electronic stopping power data is crucial for calculations of radiation-induced effects in a wide range of applications, from dosimetry and radiotherapy to particle physics. The data is dependent on the parameters of both the incident charged particle and the stopping medium. The existent Bethe theory can be used to calculate the stopping power of high-energy ions, but fails at lower energies, leaving incomplete and even contradictory experimental data, often expanded through extrapolations with fitting formula, as the only accessible resource. Moreover, the majority of the experimental data is available for elements only, further limiting the validity of fitting approaches for complex material compositions. A relatively novel machine learning methodology has been proven to be effective for exactly these types of problems. In this study, Stacking Ensemble Machine Learning (EML) algorithm was developedto predict electronic stopping power for any incident ion and target combination over a wide range of ion energies. For this purpose, five ML models, namely Bagging Regressor (BR), eXtreme Gradient Boosting (XGB), Adaptive Boosting (AdB), Gradient Boosting (GB), and Random Forest (RF), were selected as base and meta learners to construct the final Stacking EML. Methods40,044 experimental measurements, from 1928 to the present, available on the International Atomic Energy Agency (IAEA) website were used to train machine learning (ML) algorithms. This database consists of 593 various ion-target combinations across the energy range of 0.037 to 985 MeV. For model training, the eleven most important features were selected. The model evaluation was performed using several error metrics, including R-squared (R2), root-mean-squared-error (RMSE), mean-absolute-error (MAE), and mean-absolute-percentage-error (MAPE), on both the training and test datasets. ResultsBased on model performance evaluation tests, a stack of XGB and RF via BR meta-learner had the lowest error margin. The value of R2 = 0.9985 indicated a near-ideal fit to all samples in the training data across the entire range of stopping powers. R2 = 0.9955 for predictions made by the model on the unseen test data suggested that the model accurately predicted the test data. ConclusionsThe developed model can serve as a universal tool to generate the eSP data in a wide range of cases, regardless of the availability of experimental data or reliable theoretical equations. Overall, the results of the developed tool testified to the power of machine learning approaches, and the suitability of the chosen models for solutions to practically important physics problems.

Surface morphology and carbon structure effects on sputtering: Bridging scales between molecular dynamics simulations and experiments

Experiments report large scatter in the sputtering yield of carbon materials under the bombardment of low-energy noble gas ions. Here, we conduct scale-bridging molecular dynamics (MD) simulations on the xenon bombardment of carbon substrates across ion energies of 75 eV to 2 keV, and at ion incidence angles of 0°–75°, to resolve uncertainties in the sputtering data. Results show rapid amorphization of the carbon subsurface with ion bombardment, but the structural characteristics (sp/sp2/sp3 bond proportion, atomic density) eventually plateau once steady-state sputtering is achieved. In addition, we obtain virtually indistinguishable “steady-state” amorphous structures across the range of ion energies and ion incidence angles, as well as for several different carbon structures, which suggests that the steady-state sputtering yield data is independent of the initial carbon structure and prior sputtering history. By accounting for changes in the local incidence angle, surface shielding, and redeposition of the sputterants associated with surface morphology effects, we demonstrate that the sputtering predictions from MD are in perfect agreement with prior experiments across the range of ion energies and incidence angles. Using a Bayesian approach based on the MD data, we calibrate the parameters of a semi-empirical sputtering model for different surface morphologies.

The Ionizing Spectra of Extremely Metal-poor O Stars: Constraints from the Only H ii Region in Leo P

Metal-poor, star-forming dwarf galaxies produce extreme nebular emission and likely played a major role in cosmic reionization. Yet, determining their contribution to the high-redshift ionizing photon budget is hampered by the lack of observations constraining the ionizing spectra of individual massive stars more metal-poor than the Magellanic Clouds (20%–50% Z ⊙). We present new Keck Cosmic Web Imager (KCWI) optical integral field unit spectroscopy of the only H ii region in Leo P (3% Z ⊙), which is powered by a single O star. We calculate the required production rate of photons capable of ionizing hydrogen and helium from the observed Hβ and He i λ4471 emission-line fluxes. Remarkably, we find that the ionizing photon production rate and spectral hardness predicted by a tlusty model fit to the stellar spectral energy distribution agrees with our observational measurements within the uncertainties. We then fit Cloudy photoionization models to the full suite of optical emission lines in the KCWI data and show that the shape of the same tlusty ionizing continuum simultaneously matches lines across a wide range of ionization energies. Finally, we detect O iii] and N iii] nebular emission in the Hubble Space Telescope far-ultraviolet spectrum of the Leo P H ii region, and highlight that the rarely observed N iii] emission cannot be explained by our Cloudy models. These results provide the first observational evidence that widely used, yet purely theoretical, model spectra accurately predict the ionizing photon production rate from late-O stars at very low metallicity, validating their use to model metal-poor galaxies both locally and at high redshift.

Ion-induced electron emission by keV-range energy indium ions: Influence of material and geometry

This work provides measurements of the ion-induced electron emission yield caused by keV-range energy indium ions (3–8 keV). In our experiment, the ion sources, fueled with indium, are based on the working principle of field-emission electric propulsion (FEEP) technologies. The measured yields are given for different material properties (plain structure vs foam) and geometries (flat surface vs conical). Study outcomes show that electrons induced by keV-range indium ions can lead to overestimating the measured ion current by 30%–200% depending on the collector materials and geometry. We spotlighted that molybdenum, tungsten, and stainless steel 316L have relatively low yields compared to aluminum 2017A. Moreover, it is possible to lower down a specific material yield by two-thirds when using a foam structure. Finally, it is shown that off-axis ion influence on a collector increases the resulting electron emission yield. In the case of an indium-FEEP thruster, ion-induced electron emissions are a major perturbation when measuring ion current. Mainly, due to the energy range of ions studied, yields retrieved are orders of magnitudes larger than what is usually measured when studying the plume of an electrostatic electric propulsion system. It is, therefore, strongly recommended to use a so-called Faraday cup to study the ion beam from an indium-FEEP thruster as the probe’s close architecture allows us to greatly mitigate these perturbations.

Spontaneous Formation of Hydrogen Peroxide in Water Microdroplets.

There is accumulating evidence that many chemical reactions are accelerated by several orders of magnitude in micrometer-sized aqueous or organic liquid droplets compared to their corresponding bulk liquid phase. However, the molecular origin of the enhanced rates remains unclear as in the case of spontaneous appearance of 1 μM hydrogen peroxide in water microdroplets. In this Letter, we consider the range of ionization energies and whether interfacial electric fields of a microdroplet can feasibly overcome the high energy step from hydroxide ions (OH-) to hydroxyl radicals (OH•) in a primary H2O2 mechanism. We find that the vertical ionization energies (VIEs) of partially solvated OH- ions are greatly lowered relative to the average VIE in the bulk liquid, unlike the case of the Cl- anion which shows no reduction in the VIEs regardless of solvation environment. Overall reduced hydrogen-bonding and undercoordination of OH- are structural features that are more readily present at the air-water interface, where the energy scale for ionization can be matched by statistically probable electric field values.

Contribution of f -electron excitation to electronic stopping power of platinum for protons

The electronic stopping power of protons traveling along the channeling and off-channeling trajectories in platinum (Pt) is reported based on time-dependent density functional theory calculations for electrons combined with Ehrenfest molecular dynamics simulations for ions in real time and real space. We provided an intuitive description of the electronic stopping power for a wide range of ion energies, and revealed the excitation mechanism for the inner $4f$ electrons of Pt. The comparison of calculation results with experimental data showed that conduction electrons are sufficient to describe the electronic stopping power of Pt accurately in the low-energy range. The excitation of $4f$ electrons contributes substantially to the electronic stopping power of Pt as the proton velocity exceeds 1.0 a.u. Our calculated stopping power is in quantitative agreement with the experimental data up to the stopping maximum, and showed that the stopping power obtained from the off-channeling geometry is greatly improved in comparison with the channeling results. Finally, the velocity dependence of the mean steady-state charge of protons was quantified, and the results showed that the instantaneous charge state of protons exhibits periodic oscillations in the low-energy region, while the periodic oscillations disappears in the high-energy region due to more ionization of protons.

Experimental study of transition from electron beam to rf-power-controlled plasma in DFCCP in argon with additional ionization by an electron beam

Radio-frequency (rf) plasma with additional ionization by an electron beam (EB) is considered as a possible method for the independent control of plasma density, mean electron energy and mean ion energy. In this study, spatial transition from EB to rf-power-controlled dual-frequency capacitively coupled plasma (DFCCP) was studied using the following movable diagnostics: Langmuir and hairpin probes, a retarding field energy analyzer and optical emission spectroscopy. The beam (1.1–1.4 keV) is generated by a runaway EB module placed near the plasma chamber wall, while the plasma transition is caused by EB degradation with the distance from the EB module. The study was conducted in Ar at 200 and 400 mTorr gas pressures in 81 and 12 MHz DFCCP. When the EB is on, a significant decrease in the mean electron energy is observed, from 6 eV in the rf plasma down to 0.2–0.8 eV in the EB plasma. The EB also changes the shape of the electron energy probability function, from Druyvesteyn-like in rf plasma to Maxwellian-like. When both EB and rf power are applied, the mean electron energy increase and the electron density decrease, with the distance from the EB module, are observed due to the beam degradation. The ion energy distribution at the bottom electrode in rf plasma peaks at 25–30 eV and shifts down to a few eV in EB plasma. As in conventional DFCCP, the ion energy distribution can be fine-tuned by the application of a low-frequency rf bias. However, the use of an EB allows us to reduce the range of ion energies down to a few eV, which cannot be achieved in conventional rf discharges.

Energy-Loss Straggling and Delta-Ray Escape in Solid-State Microdosimeters Used in Ion-Beam Therapy

Microdosimetry is increasingly adopted in the characterization of proton and carbon ion beams used in cancer therapy. Spectra and mean values of lineal energy calculated in frequency and dose are seen by many as the tools which, by complementing dosimetric measurements, allow for the most complete characterization of the therapeutic radiation fields. The urgency is now to consolidate the experience and converge to commonly accepted methodologies. In this context, the purpose of this work is to study the effects of the energy-loss straggling and the delta-ray escape, considering slab-sensitive volumes; these are, in fact, the typical shapes of solid-state microdosimeters, which are widely used in investigating light ion therapy beams. The method considers the energy distribution of delta rays resulting from the collision of the impinging ion and, taking into account the escape, convolutes it with itself as many times as the expected number of collisions in the sensitive volume thickness. The resulting distribution is compared to the experimental microdosimetric spectrum showing a substantially good agreement. The extension of the methodology to a wider range of ion energy and detector characteristics is instrumental for a detector-independent microdosimetric assessment of the radiation fields.

Dynamics and reactivity of thermalized ions in a radiofrequency quadrupole gas-reaction cell used for the production of radioactive molecular ions

Two-dimensional radiofrequency quadrupole (RFQ) ion guides are versatile tools that are used over a wide range of ion energies. Properly tuned, they can provide an ideal environment for ion-molecule chemistry to proceed by allowing thermalized ions to react on-line with gaseous reactants. We exploit this capability in the Ion Reaction Cell (IRC), an RFQ-based system designed to form radioactive molecules (RM) from radioactive ion beams (RIB). Functionally, the IRC accepts RIB produced upstream by the isotope separation on-line (ISOL) method, decelerates it to eV energies for production of the RM and then re-accelerates RM to their incident energy. This system promises to form a highly versatile source of complex inorganic or organic RM that can be used in fundamental symmetry studies or as precursors to new radiopharmaceuticals. We discuss the challenges of transferring ions from a RIB into a gas cell at room temperature while constraining ions within the RFQ energy well, and of reforming a beam from synthesized RM. Various factors influencing ion dynamics and reactivity in thermal and non-thermal zones inside the IRC are considered, using results obtained recently with the Isobar Separator for Anions, an analogous RFQ technique used in Accelerator Mass Spectrometry, to illustrate key points.

Effect of the Magnetospheric Plasma Interaction and Solar Illumination on Ion Sputtering of Europa’s Surface Ice

AbstractFor the entire ion energy range observed at Europa, we calculate spatially resolved maps of the surface sputtering rates of H2O, O2, and H2 from impacts by magnetospheric ions. We use the perturbed electromagnetic fields from a hybrid model of Europa’s plasma interaction, along with a particle‐tracing tool, to calculate the trajectories of magnetospheric ions impinging onto the surface and their resultant sputtering yields. We examine how the distribution of the sputtering rates depends on the electromagnetic field perturbations, the angle between the solar radiation and the corotating plasma flow, and the thickness of the oxygen‐bearing layer within Europa’s surface. Our major findings are: (a) Magnetic field‐line draping partially diverts the impinging ions around Europa, reducing the sputtering rates on the upstream hemisphere, but allowing for substantial sputtering from the downstream hemisphere. In contrast, zero sputtering occurs in much of the downstream hemisphere with uniform electromagnetic fields. (b) If the oxygen‐bearing surface layer is thin compared to the penetration depth of magnetospheric ions, thermal ions dominate the O2 sputtering rates, and the region of strongest sputtering is persistently located near the upstream apex. However, if the oxygen‐bearing layer is thick compared to the penetration depth, energetic ions sputter the most O2, and the location of maximum sputtering follows the sub‐solar point as Europa orbits Jupiter. (c) The global production rate of O2 from Europa’s surface varies by a factor of 3 depending upon the Moon’s orbital position, with the maximum particle release occurring when Europa’s Sun‐lit and upstream hemispheres coincide.

First-principles study of the electronic stopping power of indium for protons and He ions

The electronic stopping power of protons and He ions traveling along the channeling and off-channeling trajectories in indium is reported based on time-dependent density functional theory combined with Ehrenfest molecular dynamics simulations. We provided an intuitive description of the electronic stopping power for a wide range of ion energies, and revealed the microcosmic excitation mechanism of the semicore $4d$ electrons of In. The velocity-proportional electronic stopping power and the kink velocity which is due to $4d$-electron excitation are reproduced in the low-energy regime. Because the $5s5p$ valence electrons are uniformly distributed in indium, the electronic excitation of valence electrons via Coulomb scattering is independent of the impact parameter in the investigated velocity range. On the contrary, due to the highly localized nature of semicore electrons, the excitation of $4d$ electrons increases significantly with decreasing of the impact parameter, which suggests that it is triggered by direct ion-electron collision. Our calculated stopping power is in quantitative agreement with the experimental data up to the stopping maximum, and showed that the stopping power obtained from the off-channeling geometry is greatly improved in comparison with the channeling results. Finally, we examined the extent to which the linear response theory is applicable to describe the electronic stopping power by quantifying the velocity dependence of the mean steady-state charge of protons and $\ensuremath{\alpha}$ particles and the effective charge state for $\ensuremath{\alpha}$ particles, and it is found that the linear response theory can be used to predict the stopping power in a wider energy range if the mean steady-state charge is used instead of assuming fully ionized charges.

Comprehensive comparison of local effect model IV predictions with the particle irradiation data ensemble.

The increased relative biological effectiveness (RBE) of ions is one of the key benefits of ion radiotherapy compared to conventional radiotherapy with photons. To account for the increased RBE of ions during the process of ion radiotherapy treatment planning, a robust model for RBE predictions is indispensable. Currently, at several ion therapy centers the local effect model I (LEM I) is applied to predict the RBE, which varies with biological and physical impacting factors. After the introduction of LEM I, several model improvements were implemented, leading to the current version, LEM IV, which is systematically tested in this study. As a comprehensive RBE model should give consistent results for a large variety of ion species and energies, the particle irradiation data ensemble (PIDE) is used to systematically validate the LEM IV. The database covers over 1100 photon and ion survival experiments in form of their linear-quadratic parameters for a wide range of ion types and energies. This makes the database an optimal tool to challenge the systematic dependencies of the RBE model. After appropriate filtering of the database, 571 experiments were identified and used as test data. The study confirms that the LEM IV reflects the RBE systematics observed in measurements well. It is able to reproduce the dependence of RBE on the linear energy transfer (LET) as well as on the αγ /βγ ratio for several ion species in a wide energy range. Additionally, the systematic quantitative analysis revealed precision capabilities and limits of the model. At lower LET values, the LEM IV tends to underestimate the RBE with an increasing underestimation with increasing atomic number of the ion. At higher LET values, the LEM IV overestimates the RBE for protons or helium ions, whereas the predictions for heavier ions match experimental data well. The LEM IV is able to predict general RBE characteristics for several ion species in a broad energy range. The accuracy of the predictions is reasonable considering the small number of input parameters needed by the model. The detailed quantification of possible systematic deviations, however, enables to identify not only strengths but also limitations of the model. The gained knowledge can be used to develop model adjustments to further improve the model accuracy, which is on the way.