Kiloelectronvolt Energy Range Research Articles

Experimental benchmarking of Monte Carlo simulations for radiotherapy dosimetry using monochromatic X-ray beams in the presence of metal-based compounds

The local dose deposition obtained in X-ray radiotherapy can be increased by the presence of metal-based compounds in the irradiated tissues. This finding is strongly enhanced if the radiation energy is chosen in the kiloelectronvolt energy range, due to the proximity to the absorption edge. In this study, we present a MC application developed with the toolkit Geant4 to investigate the dosimetric distribution of a uniform monochromatic X-ray beam, and benchmark it against experimental measurements. Two validation studies were performed, using a commercial PTW RW3 water-equivalent slab phantom for radiotherapy, and a custom-made PMMA phantom conceived to assess the influence of high atomic number compounds on the dose profile, such as iodine and gadolinium at different concentrations. An agreement within 9% among simulations and experimental data was found for the monochromatic energies considered, which were in the range of 30–140 keV; the agreement was better than 5% for depths <60 mm. A dose enhancement was observed in the calculations, corresponding to the regions containing the contrast agents. Dose enhancement factors (DEFs) were calculated, and the highest values were found for energies higher than the corresponding K-edges of iodine and gadolinium. The in-silico results are in line with the empirical findings, which suggest that Geant4 can be satisfactorily used as a tool for the calculation of the percentage depth dose (PDD) at the energies considered in this study in the presence of contrast agents.

An electrostatic in-line charge-state purification system for multicharged ions in the kiloelectronvolt energy range.

The performance of a newly built omega type electrostatic analyzer designed to act as an in-line charge-state purification system for ions in the kiloelectronvolt energy range is reported. The analyzer consists of a set of four consecutive electrostatic 140° concentric cylindrical electrodes enclosed by Matsuda electrodes. This setup was recently tested and validated using O5+, Ar9+, and Xe20+ ion beams at an energy of 14 qkeV at the ARIBE facility. A resolving power of 10.5 and a transmission of 100% of the desired charge state are measured allowing a good purification of incoming ion beams with charge states up to 10+ and a fairly good purification for charge states at least up to 20+. In comparison with other in-line solutions such as the Wien filter, our system has the advantage of being purely electrostatic and therefore lacking common drawbacks as, for example, hysteresis.

Single-electron charge transfer and excitations at collisions between Bi4+ ions in the kiloelectronvolt energy range

Pioneering theoretical data for single-electron charge transfer and excitations due to collisions between Bi 4+ ions in the ground (6 s ) and metastable (6 p ) states are gained in the collision energy interval 5- 75 keV in the center-of-mass frame. The cross sections of the processes are calculated in terms of the close- coupling method in the basis of two-electron quasi-molecular states for the Coulomb trajectory of nuclei. It is found that single-electron capture into the singlet 6 s 2 states of Bi 3+ ions makes a major contribution to the charge transfer total cross section for Bi 4+ (6 s ) + Bi 4+ (6 s ) collisions (reaction 1), whereas single-electron capture into the singlet 6 s 6 p states is the basic contributor to the total cross section in Bi 4+ (6 s ) + Bi 4+ (6 p ) collisions (reaction 2). In the collision energy interval mentioned above, the collision cross sections vary between 1.2 × 10 -17 and 1.9 × 10 -17 cm 2 for reaction 1 and between 3.8 × 10 -17 and 5.3 × 10 -17 cm 2 for reaction 2. In reaction 1, the 6 s 6 p excitation cross sections vary from 0.6 × 10 -16 to 0.8 × 10 -16 cm 2 for the singlet channel and from 2.2 × 10 -16 to 2.8 × 10 -16 cm 2 for the triplet channel. The calculation results are compared with the data obtained in experiments with crossed ion beams of kiloelectronvolt energy. The fraction of metastable ions in the beams is estimated by comparing the experimental data with the weighted average theoretical results for the cross sections of reactions 1 and 2. From the data for the charge transfer cross sections, one can estimate particle losses in relativistic beams due to a change in the charge state of the ions colliding with each other in the beam because of betatron oscillations.

Laser based synchrotron radiation

Beams of x rays in the kiloelectronvolt energy range have been produced from laser-matter interaction. Here, energetic electrons are accelerated by a laser wakefield, and experience betatron oscillations in an ion channel formed in the wake of the intense femtosecond laser pulse. Experiments using a 50 TW laser (30 fs duration) are described, as well as comparisons with numerical simulations. These results pave the way of a new generation of radiation in the x-ray spectral range, with a high collimation and an ultrafast pulse duration, produced by the use of compact laser system.

Effects of the surface structure and cluster bombardment on the self-sputtering ofmolybdenum

The understanding of different features of the sputtering of materials lies inunderstanding the factors contributing to the emission of atoms from a regularcrystal structure at the surface. Although the sputtering of fcc crystals hasreceived much attention, the database on bcc materials is still scarce. We usemolecular dynamics simulations to study the self-sputtering of the (100), (110),(111) and (112) surfaces of molybdenum. Single atoms as well as Mo2 andMo4clusters are used as the irradiation projectiles, in the cluster energy range of0.125–4 keV. Contrary to the usual assumption, enhanced (nonlinear) sputteringyields are observed for the cluster bombardments at both ends of the energyrange studied. The enhancements can be explained with lower thresholdenergies for sputtering at low energies and with a decreased fraction ofchannelled projectile atoms in the kiloelectronvolt energy range.

Real-Space Surface Crystallography from Ion Scattering

Ion beams in the kiloelectronvolt energy range impinging on crystal surfaces are atomic-size probes of the nanoscale magnifying and focusing formed by the periodic structures, These lenses disperse the scattered and recoiled atom trajectories into macroscopic projections according to their velocities as a function of projectile/target atom masses and deflection angles. The resulting patterns reflect the near-surface interatomic vectors. The spatially and temporally resolved images of these patterns can now be acquired by means of new developments in ion scattering spectrometry, providing direct, element-specific. real-space projections of surface structure. The resulting scattering and recoiling imaging spectrometry (SARIS) provides unique capabilities for analyzing surface composition and structure and for investigating the interactions of ions with surfaces. A classical ion trajectory program called scattering and recoiling imaging code (SARIC) that simulates the spatial and time intensity distributions of the SARIS images has been developed. A quantitative comparison of experimental and simulated images is achieved by means of a two-dimensional reliability (R) factor. Examples of applications include experimental stereographic projections of the Ni(110) and Pt(111) surfaces, the termination layer of the CdS(0001) surface, the chemisorption of Cl-2 on Ni(110) and H-2 On Pt(111), and the atomic lenses in the Pt(111) surface.

Carbon films of amorphous and oriented graphitic structure from fullerene ion beam deposition

Energetic fullerene deposition has been performed with an intense, highly ionized beam in the kiloelectron-volt energy range to form carbon thin films. The deposited material is amorphic and essentially free of hydrogen that is bonded to carbon. The films show graphitic nanocrystallites embedded in amorphous carbon which are preferentially oriented with the c-axis parallel to the substrate surface. With increasing substrate temperature, the size of the crystallites increases together with a decrease in the percentage of sp[sup 3]-bonded carbon. There is evidence that the primary deposition product is mainly sp[sup 3]-bonded carbon which then relaxes to form the oriented, graphitic nanocrystallites. The method of fullerene ion beam deposition has been used to form a dense amorphous carbon film on a single-crystalline fullerite film, where the crystalline quality of the fullerite film is only insignificantly affected by the deposition process. 38 refs., 8 figs.

Relative cross-sections and kinetic energy release values in dissociative negative ion formation by successive single-electron transfer from alkali metal targets

Positive ions formed from C 2H 2 were collided with Cs, K and Na targets in the kiloelectronvolt energy range. The dissociative negative ion formations of C − 2 and C 2H − ions from C 2H + 2 ions and C − 2 ions from C 2H + ions with an alkali metal target were determined to be double-collision processes using the target density dependence. By considering the reaction energies and comparing with reionization to positive ions, main processes of the dissociative negative ion formations were assigned to successive single-electron transfers with the dissociation of excited neutrals. The kinetic energy release values at FWHM of C 2H − ions from C 2H + 2 ions were 0.55eV for Cs, 0.28 eV for K and 0.17eV for Na. The relative cross-sections of C 2H − ions from C 2H + 2 ions to those of C 2H − ions from C 2H + 2 ions were 5.35 for Cs, 1.89 for K and 0.32 for Na. These dependences on target species are discussed in relation to the energy difference for the exothermic neutralization.

Absorptive Part of Charge Polarization Corrections for Electron Scattering in the Kiloelectron Volt Energy Range

The calculation of the effect of the absorptive part of charge polarization on elastic electron scattering is investigated in the second Born approximation using closure in an approximate manner. Errors in a previous study are corrected, and the angular dependence of the polarization correction is found to be strongly peaked in the forward direction with a long tail at large s values which makes a significant contribution to the scattering. The effects of the polarization correction on the scattered intensity and on the phase of the scattered amplitude are discussed in view of experimental observations. Results for the case of elastic electron scattering from Xe are found to be in good agreement with experiment. Corrections to the diffraction measurement of chemical binding energies of molecules containing first-row elements appear to be negligible.

Proportional Counter for Detection of Ions and Atoms in Kiloelectronvolt Energy Range

A proportional counter designed as a detector of ions and neutral atoms for use in collision experiments at kiloelectron volt energies is described. Response to 1–40 keV H+ ions and 5–40 keV Ar+ and N+ ions is investigated. Particles enter the detector from an attached evacuated chamber through a thin film covered narrow slit or through a differentially pumped open aperture. Construction of the counter and performance with both types of aperture are described. Performance is evaluated in terms of two statistical measures of the differential pulse height distribution—the most probable pulse height and the pulse height spread. Sufficient data are given to permit evaluation of the proportional counter as an ion energy spectrometer. Measurements of scattering and energy loss of protons in the thin film window are described.