Measured Stopping Cross Sections Research Articles

Energy deposition by H and He ions at keV energies in self-supporting, single crystalline SiC foils

The specific energy deposition of H and He ions in SiC has been studied for both random and channeling orientations. The experiments were carried out in transmission geometry using the Time-of-Flight Medium Energy Ion Scattering System at the 350 keV Danfysik Implanter at Uppsala University. The target was a self-supporting, single crystalline cubic 3C–SiC (100) foil with nominal thickness of 200 nm. The measured stopping cross sections are compared with data available from the literature and theoretical predictions. The results for random geometries reveal slightly lower values than predicted by SRIM for H projectiles whereas for He projectiles good agreement was observed over the whole energy range studied. Higher specific energy loss is observed along random trajectories in comparison to channeling geometry, for all measured energies and for both H and He ions. For H ions, however, differences are minor, whereas for He ions, they are found generally more pronounced.

Threshold behavior and isotope effect in low-energy electronic stopping of light ions

Electronic excitation of an atom by an ion in the energy regime below ∼20 keV/u can be strongly coupled with elastic scattering. This feature may cause noticeable deviations in the energy loss of light ions such as protons and deuterons from velocity-proportionality and is found to be an essential ingredient in the understanding of the stopping behavior near the threshold. We have incorporated this effect into the PASS stopping code and, in this way, expanded the range of validity of the code down to around 1 keV/u. Comparison of calculated with measured stopping cross sections for protons in Ag, Au, Cu, and Ni shows good agreement with recent data that deviate dramatically from the commonly assumed velocity proportionality. Most of these data were determined by the analysis of reflected-ion spectra. Computation of reflected-ion spectra with energy-loss functions from PASS leads to good agreement with measured spectra. A direct consequence of the coupling with elastic collisions is isotope dependence of the electronic-stopping cross section. Conventional stopping theory denies the existence of a significant isotope effect. Present calculations suggest an effect in the 1–10% range for hydrogen in Si at energies around and below 10 keV/u. This is found to be compatible with experimental data.

Note on measuring electronic stopping of slow ions

Abstract Extracting stopping cross sections from energy-loss measurements requires careful consideration of the experimental geometry. Standard procedures for separating nuclear from electronic stopping treat electronic energy loss as a friction force, ignoring its dependence on impact parameter. In the present study we find that incorporating this dependence has a major effect on measured stopping cross sections, in particular for light ions at low beam energies. Calculations have been made for transmission geometry, nuclear interactions being quantified by Bohr-Williams theory of multiple scattering on the basis of a Thomas-Fermi-Moliere potential, whereas electronic interactions are characterized by Firsov theory or PASS code. Differences between the full and the restricted stopping cross section depend on target thickness and opening angle of the detector and need to be taken into account in comparisons with theory as well as in applications of stopping data. It follows that the reciprocity principle can be violated when checked on restricted instead of full electronic stopping cross sections. Finally, we assert that a seeming gas-solid difference in stopping of low-energy ions is actually a metal-insulator difference. In comparisons with experimental results we mostly consider proton data, where nuclear stopping is only a minor perturbation.

Measurements of stopping cross sections of liquid water for fast protons and helium ions

Stopping cross sections of liquid water for MeV energy protons and helium ions have been measured by using a liquid jet target in vacuum. Energy loss spectra after passing through the target were measured at scattering angles from 5.0 to 50 mrad. The energy loss spectra have been successfully reproduced by Monte Carlo simulation calculations (GEANT4 v9.1.p03 toolkit) by taking account of multiple scattering of projectile particles inside the target. For protons, the stopping cross sections were deduced in an energy range from 0.3 to 2.0 MeV. It was found that the present data are in fairy good agreement with recent theoretical calculations.

Stopping of 0.3–1.2 MeV/u protons and alpha particles in Si

The stopping cross sections ε(E) of silicon for protons and alpha particles have been measured over the velocity range 0.3–1.2MeV/u from a Si//SiO2//Si (SIMOX) target using the Rutherford backscattering spectrometry (RBS) with special emphasis put on experimental aspects. A detection geometry coupling simultaneously two solid-state Si detectors placed at 165° and 150° relative to each side of the incident beam direction was used to measure the energies of the scattered ions and determine their energy losses within the stopping medium. In this way, the basic energy parameter, Ex, at the Si/SiO2 interface for a given incident energy E0 is the same for ions backscattered in the two directions off both the Si and O target elements, and systematic uncertainties in the ε(E) data mainly originating from the target thickness are significantly minimized. A powerful computer code has been elaborated for extracting the relevant ε(E) experimental data and the associated overall uncertainty that amounts to less than 3%. The measured ε(E) data sets were found to be in fair agreement with Paul’s compilation and with values calculated by the SRIM 06 computer code. In the case of 4He+ ions, experimental data for the γ effective charge parameter have been deduced by scaling the measured stopping cross sections to those of protons crossing the same target with the same velocity, and compared to the predictions of the SRIM 06 computer code. It is found that the γ-parameter values generated by the latter code slightly deviate from experiment over the velocity region around the stopping cross section maximum where strong charge exchanges usually occur.

ENERGY LOSS AND ENERGY STRAGGLING OF LIGHT IONS IN FULLERITE

To determinate the stopping cross section in fullerite a feasible approach, taking into account the high radiation sensitivity and mechanical fragility of fullerite films, should be employed. In this work, the stopping cross sections of 1H, 3H, 4He and 7Li ions for several selected energies were measured by Rutherford backscattering, neutron depth profiling and alpha spectroscopy using sandwich structures of samples composed of fullerite deposited on a firm substrate (Si, steel) with an intermediate marker (Au, N, Li, B, Pu). In addition, ion transmission through a thin C film supporting a fullerite layer was also utilized. The measured stopping cross sections follow the theoretical predictions calculated for carbon, but are systematically (10–35%) higher than the theoretical ones (with the exception of 5 — 5.5 MeV 4He). The observed deviation of the experimental data can partly be explained by the chemical state effects in fullerite, which accounts for about 20–50% of the difference. The measured energy straggling exceeds Bohr's value by a factor of about 2 for alpha spectroscopy and ion transmission, and 2.5 or 9.5 for Rutherford backscattering and neutron depth profiling, respectively. The discrepancy can be explained by a thickness variation, such as surface roughness of the fullerite films.

Effective charge and related/unrelated quantities in heavy-ion stopping

Stopping cross sections for swift heavy ions are commonly parameterized in terms of a velocity-dependent effective charge defined with reference to proton or helium stopping. This assumes projectile screening by core electrons to be a dominating factor governing the velocity dependence of the stopping cross section. In this note we demonstrate, on the basis of improved theoretical estimates of stopping cross sections, that projectile screening only affects the quantitative details of the velocity dependence of the effective charge while the general behavior reflects the transition from the Born to the classical regime. This is consistent with the experience that effective charges derived from measured stopping cross sections show dependencies on projectile and target atomic number which differ distinctly from those of the equilibrium projectile charge. It follows that there is no theoretical basis for the effective-charge concept as it is commonly used in heavy-ion stopping.

Stopping cross-section measurements of 4He in TiN 1.1O 0.27

The stopping cross-section for 4He projectiles in TiN x compounds has been measured using the backscattering method. A multi-compound marker layer deposited between the test film and the substrate was used to obtain the stopping cross-section at several energies with one energy of the incident beam. Two RBS spectra at definite tilt angles of the sample are taken for each beam energy. The assistance of computer codes to synthesize RBS spectra is very useful to obtain the pertinent information from the displacements of the peaks of the marker layers. Stopping cross-section values are obtained with an estimated uncertainty of about 6%.

Stopping cross section measurements of Ti thin films

The stopping cross section for 4He projectiles of Ti thin films have been measured with the backscattering method using a multi-compound marker layer deposited between the test film and the substrate. Problems derived from the non-uniformity of the marker layer as well as corrugations of the films under test have been solved with the help of a computer code to synthesize Rutherford Backscattering Spectrometry (RBS) spectra. Stopping cross section values are obtained with an estimated uncertainty of 4%.

Measured stopping cross sections for hydrogen in a-Si xC 1− x:H ternary compounds near their maximum values

Rutherford backscattering at two different angles on multilayer targets is used to measure the stopping cross sections for hydrogen in five amorphous hydrogenated silicon carbide compounds (a-Si x C 1− x :H) in the energy range from 30 keV/amu to 320 keV/amu. The measured stopping cross sections are compared to the values calculated by means of Bragg's rule. It is found that SiC chemical bonding effects limit the validity of this additivity law at energies near and below the stopping cross section maximum while CH bonding effects are important throughout the whole energy range.

Stopping power obtained from complex backscattering spectra: the isotopes of magnesium

The simple electronic structure of alkali and earth-alkali metals — tightly bound core electrons and almost free plasma electrons — makes these elements favorites of stopping theory. However, measurements of stopping cross sections ϵ will be afflicted with the high reactivity of these materials: targets generally have to be produced on massive backings in situ under UHV conditions and have to be measured in backscattering geometry. In that way, ϵ of Li, Rb Ca, and Sr has been determined before. The measurement of Mg suffers from the fact, that due to the three isotopes of Mg each backscattering spectrum is composed of three individual spectra. We have developed a computer code to determine by a least squares fit the widths of spectra of up to three components. From this we deduce absolute stopping cross sections of Mg for protons (from 15 to 600 keV) and helium ions (from 50 to 740 keV). Deviations from standard stopping power tables of more than 30% are found.

Measured stopping cross sections for helium in a-Si xC 1−x:H compounds near their maximum values

Rutherford backscattering at two different angles on multilayer targets is used to measure the stopping cross sections of helium in four amorphous hydrogenated silicon carbide compounds (a-Si xC 1−x:H) in the energy range from 200 keV to 1600 keV. The measured stopping cross sections are compared to the values calculated by means of Bragg's rule. It is found that CH and SiC chemical bonding effects limit the validity of this additivity law whereas the influence of SiH bonds is small in these ternary compounds.

Determination of aluminum oxide stopping cross sections for protons and deuterons by backscattering from thin targets

Self-supporting foils of aluminum oxide in the thickness range 100 to 300 nm were prepared by anodic oxidation of aluminum foil and subsequent dissolution of the unreacted aluminum. The foil thicknesses were measured by scanning electron microscopy. Using these foils, stopping cross sections of aluminum oxide for protons and deuterons were measured for particles in the energy range 0.90 to 2.50 MeV. These measured stopping cross sections are compared to other literature results and to the theoretical values as predicted by Bragg's Rule.

The influence of target composition on the specific energy loss measured in transmission geometry

At low projectile velocities, stopping cross section measurements by transmission methods with high directional correlation between incoming beam and detected projectiles and with a detector acceptance appreciably smaller than 4 π, may suffer from experimental artifacts. Thickness inhomogeneities make the measured specific energy loss appear angle dependent. The influence of the target areal density may be traced to the impact parameter dependence of both, the elastic and the inelastic stopping cross section. In this contribution we deal with a mixture of gases. A “true” deviation from Bragg's rule may be due to charge changing processes. Here, we discuss an additional but only apparent violation of Bragg's rule in gas mixtures: due to impact parameter selection by the experimental geometry, target atoms with different atomic numbers may contribute differently to the measured specific energy loss. Our arguments are based on experimental results and on Monte Carlo simulations.

Stopping cross sections of various carbon materials for 4He ions

We present a simple model of 0.3–2 MeV 4He stopping cross sections for graphite and diamond. The stopping cross sections can be calculated directly from measured stopping cross sections of methane, ethane and benzene. The effect of delocalized π electrons in graphite on stopping cross section is taken into account by using benzene stopping. Our simplified model agrees better than 4% with experimental values for carbon foils and reproduces the stopping ratio of diamond to graphite. RBS measurements with 1–2 MeV 4He ions on graphitic materials having different densities suggest that the stopping cross section of carbon is sensitive not to the target density but to the chemical bond.

Energy loss of 4He ions in H-implanted materials

Rutherford backscattering spectrometry (RBS) combined with the elastic recoil detection (ERD) is applied to the characterization of hydrogen implanted into Be, Si, Al-3.5 at.% Li and He-implanted Si. By monitoring the hydrogen concentration profile in the target with ERD, the backscattering measurements of 1.5 MeV 4He ions provide information on the energy loss in the implanted hydrogen atoms. The results indicate that the energy loss for hydrogen varies with each target materials implanted; the measured stopping cross sections for the implanted hydrogen in Be and Si are considerably smaller than those in Al-3.5 at.% Li and the He-implanted Si. SEM observations reveal that the physical state of the implanted hydrogen leads to the difference in the stopping cross sections determined from the RBS spectra.

Stopping power measurements of 20–180 keV 1H and 4He in indium phosphide using thick target backscattering

The stopping power of InP for 20–180 keV 1H and 4He was determined by measuring the absolute backscattering yield of thick InP targets. Several nonideal aspects of surface barrier detectors were observed which should be accounted for when making low to medium energy backscattering yield measurements. These are: non-Gaussian detector resolution, nonlinear detector energy calibration, and ion reflection in the detector Au window. The present 1H in InP stopping cross section measurements are in good agreement with those recently obtained by Khodyrev et al. using a relative, bulk yield, single spectrum approach. The 4He in InP stopping cross section measurements were found to be velocity proportional below 30 keV/amu. The estimated standard error of the measurements is ±7% at 20 keV and ± 4% at 180 keV.

How to measure absolute stopping cross sections by backscattering and by transmission methods: Part I. backscattering

The stopping cross section of solids for swift ions may be deduced either from the energy loss in a transmission experiment or from the width of a Rutherford backscattering (RBS) spectrum. The two parts of this article discuss the advantages and the limitations of these complementary measuring methods. This first part deals with RBS measurements using various target elements and protons or deuterons in the energy range around the stopping cross section maximum (30 keV ≦ E ≦ 600 keV). The peculiarities of measurements using light elements (like carbon) and heavy elements (like gold) are discussed. Realistic estimates are given for the accuracy, which may be achieved in this kind of stopping cross section measurements (2–3% for a series of measurements using one well characterized target).

The influence of different experimental methods on the measured energy dependence of stopping powers

The stopping cross section data measured by foil transmission often show an energy dependence different from that of RBS measurements. This cannot be due to problems connected with foil thickness-determination. Therefore, we investigate in this contribution, how the energy loss data in these types of measurements are influenced by the different projectile paths due to impact-parameter selection, plural and multiple scattering, by target properties such as bulk and surface impurities, texture and desity differences and by the use of different energy analyzers. Experiments have been performed which prove the reliability of our target preparation, stopping cross section measurements and data evaluation with respect to these influences.

Stopping cross-sections of liquid water and water vapour for alpha particles within the energy region 0.3 to 5.5 MeV

Phase effects in the stopping power of heavy charged particles are important in dosimetry and, in particular, in cavity chamber theory and in the design and operation of homogeneous gas-filled ionisation chambers for neutron dosimetry. A single apparatus has been designed to allow measurement of stopping cross-sections of materials in liquid and vapour phase for alpha particles. Experimental stopping powers of water vapour are presented for energies from 0.3 to about 5.5 MeV and of liquid-phase water from 1.75 to 4.75 MeV. The molecular stopping cross-sections of the vapour tend to be greater than those for the liquid. The differences between the stopping powers for the two states of H2O indicate phase effects of no more than about 4% in the energy region of comparison (down to 1.75 MeV), with an increasing trend towards lower energies. Major fluctuations in stopping power ratio are not observed in this energy region.