MeV 4He Ions Research Articles

Stopping powers of GaAs for 0.3–2.5 MeV 1H and 4He ions

Ion backscattering and foil transmission methods have been used to determine the energy loss of 0.3–2.5 MeV 1H and 4He ions in crystalline bulk GaAs and thin foil GaAs grown by molecular beam epitaxy (MBE). The self-supporting GaAs sample foil was produced by floating the MBE-grown GaAs film from an AlAs GaAs backing by a lift-off process. The stopping powers, corresponding to energy loss of the ions in a nonchanneling direction in the crystal were extracted from the measurements. Ion backscattering and channeling were employed to study the effect of the crystal structure of the thin foil sample on the stopping powers deduced. Deviations from semi-empirical SRIM calculations (version 96.04) were observed in the case of 4He ions for which the stopping powers fall clearly below the calculated values. An average deviation of about 5% is found for the energies studied, from below the stopping power maximum at 0.8 to 2.2 MeV. The results obtained by the two independent experimental methods have been compared and discussed.

Ion beam analysis of MgAl 2O 4 spinel irradiated with fast neutrons to 50–250 dpa

Non-destructive ion beam analysis techniques have been employed to examine the radiation damage in MgAl 20 4 spinel single crystals irradiated with fast neutrons at 400 and 750°C to high fluences (≥ 5 × 10 22 n/cm 2, E n > 0.1 MeV). Rutherford backscattering and ion channeling measurements using 1–4 MeV He-ion beams revealed that the radiation damage saturated after irradiation at 400°C to 50 displacements per atom. The energy dependence of dechanneling indicated the dominant extended defects present in the highly irradiated spinel are in the form of dislocations. Channeling angular scans of particle induced X-ray emission further suggested that neutron irradiation tends to randomize cation distribution for Mg 2+ and Al 3+ cations on the lattice sites. These results are compared to the microstructure observations of Kinoshita et al. and the neutron scattering results of Sickafus et al.

Water diffusion at short times in hydrophilic polymer studied by an ion beam analysis technique

Gravimetric measurements establishing D 2O as a good model for H 2O diffusion in a hydrophilic polymer are reported. An MeV 3He ion microbeam technique is described which enables the measurement of the diffusion coefficient for initial Fickian diffusion of deuterated water into hydrophilic polymer. A raster scanned microbeam is used to bombard a sample cooled by liquid nitrogen. Spectra from 3He interactions with both deuterium and carbon in the polymer matrix are collected. The latter is used to normalise water concentrations resulting from exposure of the polymer to heavy water for different times. A diffusion coefficient is deduced for initial water diffusion into tetrahydrofurfylmethacrylate/Poly(ethylmethacrylate) (THFM/PEM) at 37°C.

Structural transformations in leached uranium dioxide

The effects of water interaction with UO 2 single crystals (leaching) were studied by means of 7.53 MeV 4He-ion channeling. Upon leaching at temperatures above 150°C transformation of the surface layer into a higher oxide, namely β-U 3O 7, was observed. The lattice mismatch between the bulk crystal and the transformed layer produced important changes in the 〈110〉 axial channeling direction. Based on these data and Monte Carlo channeling simulations a model structure of the transformed layer was developed.

Characterization of SiO xN yH z and SiN yH z using RBS, ERDA and NRA In memoriam Priv.-Doz. Dr. Rainer P.H. Garten

Rutherford backscattering (RBS) measurements, elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA) were applied to determine the composition, the thickness and the density of plasma-deposited SiO x N y H z and SiN y H z layers on silicon substrate. RBS-measurements were carried out with 2.3 MeV 4He-ions in random and channeling geometry to determine the atomic ratios of N Si in the surface layers, and the layer thicknesses. Depth profiling for hydrogen in the samples was performed by means of α-ERDA using 2.3 or 2.9 MeV 4He-ions. In order to determine the atomic ratios of H N , heavy ion elastic recoil detection (HIERDA) using 32 MeV 28Si 7+-ions was applied. The oxygen contents were measured using the nuclear reaction 16O(d,p) 17O. Depth profiling for nitrogen was also performed by means of NRA using the reaction 14N(d,α) 12C. In addition, ellipsometric measurements of the layer thicknesses were utilized to determine the densities in absolute units (atoms/m 3 or kg/m 3).

High depth resolution Rutherford scattering using forward angles

Rutherford scattering of MeV 4He ions at forward angles has been used to determine the thickness and composition of single Si 1 − x Ge x layers in Si. With scattering angles of about 20° the obtained depth resolution is up to 25% better than with standard RBS at glancing backward angles. The large scattering cross section at forward angles allows for the use of a small solid angle while maintaining good count rates and short acquisition times. Geometrical broadening of the energy spectra, due to the finite acceptance angle of the detector, is thus negligible. The kinematic factor is close to 1 for almost all elements in this geometry. Layers with different composition are therefore only distinguishable by the differences in spectrum height. The best accuracy for the stoichiometry is obtained by combining the measured energy loss in a layer with the areal density of the heavier element in that layer, which can be determined by standard RBS. The results are compared to RBS and X-ray diffraction measurements. The influence of multiple scattering is illustrated and a comparison to depth resolution calculations with the DEPTH code is made.

Carbon analysis in MOCVD grown HgCdMnTe by charged particle activation

Charged Particle Activation Analysis (CPAA) has been used for the determination of the concentration of carbon in HgCdMnTe grown by Metal Organic Chemical Vapour Deposition (MOCVD). The samples were irradiated with a beam of 3.0 MeV 3He ions. 11C is produced by the reaction 12C( 3He, α) 11C and is a positron (β +) emitting radionuclide with a half-life of 20.38 min. At the same time the reaction 16O( 3He, p) 18F produces 18F which is also a β + emitter and has a half-life of 109.72 min. A post-irradiation etching technique has been developed to enable removal of surface contaminants. The radioactivity is determined by a β + spectrometer consisting of two NaI γ-ray detectors (3 × 3 in.) oriented at 180°. The two coincident 511 keV γ-rays emitted at 180° during the positron annihilation are detected. The initial 11C and 18F activities, and hence the oxygen and carbon contributions, can be separated by analysis of the count rate versus time. Analysis shows significant carbon levels in the HgCdMnTe samples.

Exfoliation of GaAs caused by MeV 1H and 4He ion implantation at 〈100〉, 〈110〉 axial and random orientations

The exfoliation procedure of the ion range determination of gaseous implants in single crystal GaAs is investigated. The correlation of the observed crater depth with the ion range is studied for random, 〈100〉 and 〈110〉 axial orientation high dose implantations of 1.5–2.5 MeV 1H and 4He ions. Depending on the experimental conditions, the crater depths corresponded to range values between the modal range and the range maximum. The observed crater depths could be related to the actual He concentration depth distributions by determining the profiles of the 4He implants by 2.7 MeV proton backscattering. The implantation parameters affecting the exfoliation process, and especially the increase rate of the sample temperature, are investigated. The range distribution parameters for the 1.5 MeV 4He implants are presented.

Cross sections for 165° backscattering of 2.0–9.0 MeV 4He ions from nitrogen

The cross sections for 165° backscattering of 4He ions from N in ZrN films with the energy ranging from 2.0 MeV to 9.0 MeV were measured, and the excitation curves were plotted. The results were used to analyse the N implanted in the steel, and a computer simulation was performed.

Ranges of 1.0–2.7 MeV 1H and 4He ions in GaAs

Ranges of 1H and 4He ions in GaAs single crystals are reported in the high energy implantation range of 1.0–2.7 MeV. Experiments were performed at random and for 1H ions also at 〈100〉 axial channeling orientation. Ion beam irradiation of the samples resulted in exfoliation of the surface layer and craters were produced. The crater depths corresponding to the projected range were measured by a profilometer. At 〈100〉 channeling orientation the 1H ions penetrated slightly deeper than in the random case. The random ranges are compared to transport equation and Monte Carlo calculations (PRAL and TRIM codes). In the 1H ion case, the experimental data were well reproduced by the calculations, while the data for 4He ions clearly exceeded the calculations.

Azimuthally averaged backscattering yield near the 〈100〉 axis in Si

An azimuthally averaged backscattering yield near the 〈100〉 axis in Si is studied experimentally and by Monte Carlo simulation, for 1.5 MeV 4He ions and tilt angles θ up to 16°. A complete compensation of the 〈100〉 dip is obtained within 0° ⩽ θ⩽ 6°, in agreement with the rule of angular compensation. For larger tilts a fine structure in the yield, with a number of distinct secondary dips, is observed. The results of two simulation programs (CXX and FLUX) are found to be fully consistent with each other, and in excellent agreement with the experimental data. The extension of the simulation methods to include trajectories that substantially deviate from the string direction is presented.

Coulomb excitation of states in 238U

Twenty-two states in 238U have been observed with 18 MeV 4He ions on a thick target. Eight 2 + states between 966 and 1782 keV and three 3 − states are populated by direct E2 and E3, respectively. The remaining states are either weakly excited by multiple Coulomb excitation and /or populated by the γ-ray decay of the directly excited states. Spin assignments are based on γ-ray angular distributions. Reduced transition probabilities have been deduced from the γ-ray yields. The B(E2) values for excitation of the 2 + states range from 0.10 to 3.0 W.u. (281 W.u. for the first 2 + state). For the 3 states, the B(E3, 0 → 3 −) values are 7.1, 7.8, and 24.2 W.u. Several of the 2 + states have decay branches to the one-phonon states with B(E2) values between 27 and 56 W.u. which are an order of magnitude larger than the B(E2) values between the one- and zero-phonon states. This disagrees with our present understanding of collectivity in nuclei if these 2 + states are considered to be collective two-phonon excitations. However, the excitation energies of these 2 + states with respect to the one-phonon states are only 1.3 to 1.6. The B(E1) values for 17 transitions between the positive- and negative-parity states range between 10 −3 and 10 −7 W.u. The B(E1) branching ratios for many of these transitions have large deviations from the Alaga-rule predictions. These deviations can be understood by the strong Coriolis coupling between the states of the one-phonon octupole quadruplet in deformed nuclei. The general features of the experimental results for the B(E3) values are reproduced by the microscopic calculations of Neergård and Vogel when the Coriolis coupling between the states of the octupole quadruplet is included.

Close encounter processes in Monte Carlo simulations of ion channeling

The backscattering yield is analyzed in terms of the depth-dependent distribution of ion trajectories, characterized by instantaneous values of such parameters as the ion energy, the scattering angle, the angle between the ion direction and the surface normal, and the impact parameter with respect to the equilibrium site of the nearest lattice atom. The normalized nuclear encounter probability (NEP) is reformulated as the expected value of the normalized NEP for a single trajectory, and the definition is extended for directions tilted by a large angle with respect to a major axis. It is shown that the normalized NEP cannot be defined to correspond to a physically measurable quantity if the energy and angular spreads of the beam in the crystal are taken into account. The scattering yield measured in the energy spectrum — to be used in comparisons of simulations with experiment — is also expressed as the expected value of the respective yield associated with a single trajectory. Examples of the distribution of trajectories are given for 1.5 MeV 4He ions traversing a Si(100) single crystal.

Damage Production and Annealing During MeV He Ion RBS/Channeling Analysis of GaP

The creation of defects in GaP bombarded with MeV 4He ions has been studied under ultra-high-vacuum condition by Rutherford backscattering/channeling (RBS/C) method. Our experimental results suggest that the defect generation rate is strongly related to the surface condition. When the GaP is covered with a thin amorphous layer, a significant ion beam annealing effect will be observed. The present results show that for GaP the damage is produced mainly by nuclear collision.

Improved hydrogen and deuterium depth profiling in polymers using low energy forward recoil spectrometry

We demonstrate that the hydrogen and deuterium depth resolution of forward recoil spectrometry (FRES) is greatly improved by utilizing 1.3 MeV 4He ions and sample tilting. The near-surface depth resolution is measured on samples of alternating deuterated and protonated polystyrene layers, each ca 250 Å thick. The depth resolution at the surface is dramatically improved (from 800 Å to 250 Å) as the incident beam energy decreases from 3.0 to 1.3 MeV. We call this technique low-energy FRES (LE-FRES) to differentiate it from standard FRES, which uses 3 MeV 4He ions incident at θ = 75°. Furthermore, by using a glancing incident or exit geometry, the LE-FRES depth resolution is improved to ca 125 Å, which is comparable to the resolution of secondary ion mass spectrometry and nuclear reaction analysis. Measurements of the energy and glancing angle dependence of the depth resolution are in quantitative agreement with theoretical predictions. The probing depth of LE-FRES is found to have a maximum at θ ≈ 71° and decreases at glancing exit and incident angles. A contour plot illustrates the relationship between depth resolution and probing depth and provides guidance for future users of LE-FRES.

Radiation-induced damage in GaP by MeV Helium ions

The creation of defects in GaP bombarded with MeV 4He ions has been studied under ultra-high-vacuum condition by Rutherford backscattering/channeling (RBS/C) method, and the absolute damage cross section σd for GaP has been determined under bombardment of 1.0 MeV 4He ions, with the measured value 8.0×10-18 cm2. Our experimental results suggest that the defect generation rate is strongly related to the surface condition. When the GaP is covered with a thin amorphous layer, a significant ion beam annealing effect will be observed. The present results show that for GaP the damage is produced mainly by nuclear collision.

A combined technique of nuclear reaction analysis and plasma-driven permeation for a quantitative study on deuterium trapping

A new method of the in-situ depth-profiling technique was developed, in which the deuterium depth profile by nuclear reaction analysis (NRA) and the deuterium permeation flux of the plasma-driven permeation (PDP) were observed at the same time for a sample membrane, one side of which was continuously exposed to a deuterium plasma. The dissolved deuterium concentration in the sample was known from the permeation flux of the PDP and the trapped deuterium concentration was obtained from NRA. The trapping energy for deuterium in nickel, which was prebombarded with 0.8 MeV 4He ions, was determined from the observations at the steady state at sample temperatures of 398 to 685 K. It was 0.24 eV, which suggested that the traps were associated with vacancies produced by the 4He bombardment. This was supported by the fact that the depth profile of the trapped deuterium agreed well with the estimated distribution of the displaced host atoms.

Tumorigenic Potential of High-Z, High-LET Charged-Particle Radiations

The potential for radiogenic neoplasia from charged-particle irradiation has been estimated using the Harderian gland of the mouse as a test system. Particles ranging in Z from Z = 1 (proton) to Z = 41 (niobium), in energy from 228 to 670A MeV, and in LET from 0.4 to 464 keV/microns were produced at the Lawrence Berkeley Laboratory BEVALAC. Expression of the tumorigenic potential of the initiated cells was enhanced by hormones from isogeneic grafts of pituitaries. The goal of the studies was to estimate the initial slope of the relationship between increased tumor prevalence at 16 months after irradiation and the dose received. Initial slopes were measured with good precision for 60Co gamma rays and the Bragg plateau beams of 228A MeV 4He ions, 600A MeV 56Fe ions, and 350A MeV 56Fe ions. The ratio of the initial slope for these ions to that of 60Co gamma rays give an estimate of the maximum RBE for radiogenic neoplasia. These values were 2.3 for the 4He ions, 40 for 600A MeV 56Fe, and 20 for 350A MeV 56Fe. In the studies reported here the prevalence of tumors as the result of pituitary isografts was not enhanced after irradiation with 56Fe ions. It remains to be seen how effective pituitary isografts are for enhancement of radiogenic neoplasia from other ions at different LET values. A risk analysis was undertaken using particle fluence rather than dose as the independent variable. This analysis provides a value for a "cross section" expressed in microns 2. This parameter expresses as the increase in proportion of mice with one or more Harderian gland tumors per unit increase in particle fluence. The plot of the cross section (risk coefficient) as a function of LET is monotonic, with no clear evidence of a maximum value of the risk coefficient for even the highest LET particle used.

Absolute cross section measurements for H and D elastic recoil using 1 to 2.5 MeV 4He ions, and for the 12C(d,p) 13C and 16O(d,p 1) 17O nuclear reactions

Absolute differential cross sections dσ H/dΩ and dσ D/dΩ for elastic recoil of H and D using 1 to 2.5 MeV 4He ions have been measured at 10°, 20° and 30° laboratory scattering angles, with accuracies of ± 4 to 7% for dσ D/dΩ, and ± 6 to 9.5% for dσ H/dΩ, depending on the geometry; their ratios (dσ H/dΩ/(dσ D/dΩ) have been measured to ± 2% in the same energy range and geometries. Cross sections for several nuclear reactions of interest in materials science have also been measured at scattering angle θ lab = 150°. That of 12C(d,p) 13C at 968 keV is dσ c/dΩ = 29.5 ± 1.2 mb/sr ; that of D(d,p)T in the same conditions is 8.6 ± 0.5 mb/sr; that of 16O(d,p 1) 17O at 857 keV is dσ o/dΩ = 5.3 ± 0.4 mb/sr . The latter value was deduced from a measurement of the ratio R c o = (dσ C/dΩ) 968 keV/(dσ O/dΩ) 857 keV = 5.56 ± 5% .

Coulomb excitation of states in 232Th

Twenty-five states in 232Th have been observed with 18 MeV 4He ions on a thick target. Eleven 2 + states between 774 and 1554 keV and three 3 − states are populated by direct E2 and E3, respectively. The remaining states are either weakly excited by multiple Coulomb excitation and/or populated by the decay of the directly excited states. Spin assignments are based on γ-ray angular distributions. Reduced transition probabilities have been deduced from the γ-ray yields. The B(E2) values for excitation of the 2 + states range from 0.024 to 3.5 W.u. (222 W.u. for the first 2 + state). For the 3 − states, the B(E3,0 → 3 −) values are 1.7, 11, and 24 W.u. A possible two-phonon state at 1554 keV, which is nearly harmonic, decays to four members of the one-phonon states, to the ground-state band, and to the K = 0 − octupole band. The B(E2) value for excitation of this state is 0.66 ± 0.05 W.u. and the B(E1) values for decay of this state are (2 and 6)×10 −4 W.u. The B(E2) values between two- and one-phonon vibrational states range between 16 and 53 W.u. which are an order of magnitude larger than the B(E2) values between the one- and zero-phonon states. This disagrees with our present understanding of collectivity in nuclei if this 2 + state is considered to be a collective two-phonon excitation. The 2 + states at 1477 and 1387 keV, which are also nearly harmonic, are possible candidates with two-phonon structure. The agreement between the experimental results and the microscopic calculations by Neergård and Vogel of the B(E3,0 → 3) for the 3 − members of the one-phonon octupole quadruplet is satisfactory when the Coriolis coupling between the states with K and K ± 1 is included. The B(E1) branching ratios for transitions from the 3 − and 1 − states to the ground-state band have large deviations from the Alaga-rule predictions. These deviations can be understood by the strong Coriolis coupling between the states of the octupole quadruplet in deformed nuclei.