Ionization ranges of protons in water vapour in the energy range 1-100 keV

Projected ranges of protons in water vapour were experimentally determined for proton energies from 1 to 100 keV by counting the total number of ionizations produced by protons during their slow down. Using these experimental ranges and semiemprical detour factors, the stopping powers of water vapour for protons were derived and compared with semiempirical data.     

Characterisation of a monolithic active pixel sensor for electron detection in the energy range 10–20 keV

As part of a feasibility study into the use of novel electron detectors for X-ray photoelectron emission microscopes (XPEEM), we have characterised the imaging performance of a back-illuminated monolithic active pixel sensor (MAPS) operating under both integrating and counting modes for electrons in the energy range 10–20 keV. For integrating mode, we present the detective quantum efficiency (DQE), which shows marked improvements over conventional indirect detectors based on microchannel plates. We also present the modulation transfer function (MTF) and noise power spectrum (NPS), again demonstrating significantly improved performance. For counting mode, we present the quantum efficiency (QE) as a function of incident electron energy. We have evaluated the charge collection efficiency (CCE) and we thereby demonstrate the presence of a ~200 nm thick dead layer that is linked with reduced CCE at low electron energies. Based on our findings, we believe that the MAPS technology is well matched to future XPEEM instruments using aberration correction.     

L-shell X-ray production ratios for 100–150 keV protons

Typical relative intensity ratios for X-ray production induced by 100–150 keV protons incident on thick targets of Gd (Z₂ = 64) and Dy (Z₂ = 66) are reported. Experimental subshell ionization cross sections for thick Dy targets bombarded by protons are also reported. The results are compared to predictions of the ECPSSR and RPWBABC theories and to other reported work in this proton energy range. Substantial discrepancies are observed and possible explanations for them are discussed.     

Efficiency calibration of a HPGe detector in the [46.54–2000] keV energy range for the measurement of environmental samples

Here we describe a general method of calibrating the efficiency of a Ge γ-ray spectrometer. The method, which is based on the work of Quintana and Fernández (Appl. Radiat. Isot. 47 (1996) 911), can now be applied to many different experimental set-ups including both liquid and solid environmental samples. The method requires two different types of experimental inputs. Firstly, it requires measurements with radioactive sources emitting cascades of γ rays covering the energy range of interest. Secondly, it requires measurements with sources emitting isolated γ rays in order to provide coincidence-summing corrections. On this basis, we establish a general function to describe the energy dependence of the efficiency for the particular geometry and source matrix. The method has been applied to 11 different experimental arrangements to provide efficiency calibrations over the range 46.54–2000 keV with associated uncertainties ranging from 0.1% to 1.8%. This allows high precision measurements with environmental samples, which often have very low activities.     

Analytical differential cross section of electron elastically scattered by solid targets in the energy range up to 100 keV

In this paper, we propose a new approach to determine the adjustable parameters of differential elastic cross-section of Bentabet et al. approximation. The resultant approximation is an analytical expression that can be used to study the interaction of electron beams with solid targets. This approximation is applied for the energy range up to 100 keV, to calculate some quantities such as: the mean penetration depths, the mean number of wide angle collisions and the backscattering coefficient (BSC) of Al, Cu, Ag and Au semi-infinite solid targets. BSC was calculated by using both the Monte Carlo method and the Vicanek and Urbassek analytical model. The obtained results are compared with the experiment and good agreement is remarked.     

Piezoresistive response of thin film manganin gauges in the 50–100-GPa range

To extend the measurement range to 100 GPa, a new type of manganin gauge was prepared by thin film techniques. The manganin sensing elements were deposited by magnetron sputtering, and then encapsulated in an alumina matrix by electron beam evaporation. Shock loading in the 50–107-GPa range by a light gas gun resulted in flat-topped stress–time profiles with approximately 1-μs record history. The initial calibration presented a linear curve having a piezoresistance coefficient of 0.0198±0.0002 GPa⁻¹.     

W values of protons in alkane-based TE gases and dimethylether in the energy range 1-100 keV: measurements and additivity models

W values of tissue-equivalent gases based on propane and butane were measured for protons in the energy range 1-100 keV and compared with those of dimethylether and water vapour. The experimental values of the tissue-equivalent gas mixtures were compared with data calculated from the W values of the mixture components using five existing additivity models and two new models derived within the scope of the present work. Apart from one model--the simplest one--all models show a good agreement with the experimental data.     

Evaluation of silicon neutron resonance parameters in the thermal to 1800 keV energy range

Because silicon is a major constituent of concrete and soil, neutron and gamma ray information on silicon is important for reactor shielding and criticality safety calculations. Therefore, much effort was put into the ENDF/B-VI evaluation for the three stable isotopes of silicon. The neutron capture cross section of natural silicon was recently measured at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range 1-700 keV. Using the ENDF/B-VI evaluation for initial values, a new evaluation of the resonance parameters was performed by adding the results of the ORELA capture measurements to the experimental database. The computer code SAMMY was used for the analysis of the experimental data; the new version of SAMMY allows accurate calculations of the self-shielding and multiple scattering effects in the capture measurements. The accuracy of the radiative capture widths of the resonances was improved by this analysis. Accurate values of the s-, p- and d-wave neutron strength functions were also obtained. Although the resonance capture component of the present evaluation is 2-3 times smaller than that in ENDF/B-VI, the total capture cross section is much larger, at least for energies >250 keV, because the direct capture component contributes values of the same order of magnitude as the resonance component. The direct component was not taken into account in the ENDF/B-VI evaluation and was calculated for the first time in the present evaluation.     

Development of a momentum determined electron beam in the 1–45 GeV range

A beam line for electrons with energies in the range of 1–45 GeV, low contamination of hadrons and muons and high intensity up to 10⁶ per accelerator spill at 27 GeV was setup at U70 accelerator in Protvino, Russia. A beam tagging system based on drift chambers with 160 μm resolution was able to measure relative electron beam momentum precisely. The resolution σ_p/p was 0.13% at 45 GeV where multiple scattering is negligible. This test beam setup provided a possibility to study properties of lead tungstate crystals (PbWO₄) for the BTeV experiment at Fermilab.     

Molecular dynamics computer simulations of 5 keV C60 bombardment of benzene crystal

Coarse-grained molecular dynamics computer simulations have been used to investigate the damage of a benzene crystal induced by 5 keV C₆₀ projectile bombardment. The sputtering yield, mass distributions and the depth distributions of ejected organic molecules are analyzed. The temporal evolution of the system reveals that impinging C₆₀ cluster leads to creation of almost hemispherical crater. Most of the molecules damaged by the projectile impact are ejected into the vacuum during cluster irradiation. This “cleaning up” effect may explain why secondary ion mass spectrometry (SIMS) analysis of some organic samples with cluster projectiles can produce significantly less accumulated damage compared to analysis performed with atomic ion beams.     

Response functions of Si detectors to monoenergetic electrons and positrons in the energy range 0.8–3.5 MeV

The response functions of Si surface barrier detectors (depletion depth: 2 mm, active area: 200 mm²) to monoenergetic electrons and positrons have been measured in the energy range 0.8–3.5 MeV at the Giessen electron linear accelerator. Lower peak-to-total ratios were observed in the positron response functions compared to electrons. The measured response functions were compared with Monte Carlo simulations enabling a separation of the individual contributions to the response function such as: total energy loss, backscattering, transmission, bremsstrahlung emission. A parametrization of the response function for electrons is given, which allows a reliable approximation of the response function in the investigated energy range.     

Studies of the knee of the cosmic rays primary spectrum through measurements in the 10–10 eV energy range

In this contribution I review the most recent results obtained by experiments studying the primary cosmic rays spectrum at energies around the knee, i.e. the change of slope observed at .The main results are: the change of slope has been observed in the spectra of all secondary components of extensive air showers. The primary chemical composition gets heavier above the knee. Almost all experiments, with the exception of TIBET ASγ, attribute the change of slope to the light component of cosmic rays. The arrival directions of cosmic rays are highly isotropic.These results favor an astrophysical explanation of the knee either as the maximum energy achievable in galactic sources or of end of the containment inside galactic magnetic fields. Nevertheless the scenario is neither complete nor univocally accepted and relevant informations can be obtained studying the 10¹⁶–10¹⁸ eV energy range.     

100 keV nitrogen ion beam implanted polycarbonate: A possibility for UV blocking devices

Polycarbonate samples were implanted with 100 keV N⁺ ions at fluences 10¹⁵, 10¹⁶ and 5 × 10¹⁶ ions cm⁻². Drastic alterations in UV-Visible transmittance spectra were observed which are interrelated with change in surface color and optical absorption of the implanted samples. UV-Visible transmission studies show that at ion fluence of 10¹⁶ ions cm⁻², transmission approaches to zero at about λ = 427 nm and below up to 200 nm. Optical band gap (E_OPT) reduces with increase in fluence and at maximum ion fluence of 5 × 10¹⁶ N⁺ cm⁻², E_OPT was determined to be 1.56 eV whereas for pristine its value was 3.00 eV. Raman analysis indicates the formation of amorphous carbon on the surface of polycarbonate at an ion fluence of 10¹⁶ N⁺ cm⁻². Rise in fluence to 5 × 10¹⁶ N⁺ cm⁻² results in enhancement in disorder on the surface of the host polymer. Modifications in the structural arrangements were found to be in strong association with changes in optical properties with increase in ion fluence and the same is discussed.     

Molecular dynamics simulation of graphite melting

Questions on the behavior of the graphite melting curve have remained open during the last fifty years. The process of graphite melting in the pressure range of 2–14 GPa is investigated by the method of molecular dynamics using the model of reactive interatomic potential; the dynamics of melting-front propagation upon crystal superheating is considered, and the melting curve is plotted. The self-diffusion coefficient in the liquid phase is determined for the aforementioned pressure range, and the question of the existence of the liquid-liquid phase transition in carbon is considered.     

Molecular dynamics simulation of silicon oxidization

We compare here the different oxidization protocols that can be used to generate an SiO₂/Si interface. All these protocols are based on molecular dynamics at high temperature but differ by the way the oxygen atoms are incorporated one-by-one. When they are inserted between two neighbouring Si atoms, forming one of the Si-Si pair closest to the surface, the silicon oxide grows layer-by-layer and it is structured as a random network of SiO₄ entities connected by vertices with only a small amount of SiO₃ and SiO₅ defects. On the contrary, when they are incorporated into the longest Si-Si bond instead of the highest one, a dendritic-like SiO₂ oxide spreads into the substrate in contradiction with experimental observations, which definitely rules out such protocols as realistic ones.     

Molecular dynamics simulations of ion bombardment processes

Abstract

An improved molecular dynamics technique that allows reduction of the computation time required in ion bombardment simulations is presented. This technique has been used to study the influence of the target temperature and structure on the argon sputtering of silicon. Molecular dynamics simulations of l keV Ar+ ion bombardment of silicon were carried out for several types of sample: (100) crystalline at 0 K, (100) crystalline at 300 K, and amorphous at 300 K. The yield of the sputtering process and the energy distribution of the sputtered atoms have been obtained. These results show that the sputtering process depends on the target surface binding energy which, in turn, is very sensitive to the structure of the sample surface.     

Molecular dynamics simulation of mechanical behaviour

A molecular dynamics (MD) simulation of strain and failure of a crystal, the effect of environment on these processes, the interaction between the adsorption-active atoms and the environment walls, the effect of stress on mobility of interstitial admixtures, the formation and the failure of a contact between two crystals has been performed using a two-dimensional system consisting of Lennard-Jones atoms. The basic features of strain observed by means of MD included generation and motion of dislocations, various mechanisms of shear and brittle-to-ductile transition at low temperature. Environmentsensitive mechanical behaviour has been studied for the first time on an atomic scale. It is shown that rapid local processes whose unit act takes about 10^–10 sec and involves several tens or a few hundred atoms may provide for environment-induced embrittlement. The features common to these microscopic processes are (1) pronounced interaction between the foreign atoms and the atoms of the solid, i.e. a sharp decrease in the surface energy of the solid in contact with the environment, and (2) direct participation of thermal fluctuations in the failure and rearrangement of interatomic bonds. By interacting with the crack walls, the environment atoms create a force compatible with the interatomic bond strength, which promotes crack propagation. Tensile stress causes appreciable acceleration of diffusion of interstitial admixtures in the direction normal to the strain axis and hinders diffusion along the axis. Under constant load the failure of interatomic bonds and sintering involve a thermal fluctuation mechanism.     

Optimizing the design of a moderator-based neutron detector for a flat response curve in the 2–14 MeV energy range

The efficiency of a neutron detector with boron trifluoride proportional tubes embedded in a polyethylene moderator was simulated with a Monte Carlo program. A moderator structure where the detector had uniform sensitivity for neutrons from 2 to 14 MeV was determined by simulation. A counter was built based on the simulation results. The counter's efficiencies were calibrated with an Am–Be source and an accelerator that served as a D–D and D–T neutron source. Experimental neutron efficiencies of these sources are approximately uniform. The simulated model was validated by the consistent results between the calculated and experimental data.