Energy Dependence of MeV He+ Ion-Induced Re-Emission of Hydrogen Isotopes Implanted into Graphite

The energy dependence of MeV He⁺ ion-induced re-emission of hydrogen isotopes (H and D) implanted into graphite has been measured by means of the elastic recoil detection (ERD) technique in order to clarify the collision process for the ion-induced detrapping. The experimental re-emission profiles have been analyzed by solving the mass balance equations, in which the ion-induced detrapping cross section σ _d and the rate constants of the retrapping Σ ^T and local molecular recombination K between an activated hydrogen atom and a trapped one are taken into account. The values of σ _d and K/Σ _T have been determined from the best-fit analytical solution to the experimental re-emission profiles. It has been found that the average values of σ _d and K/Σ _T for H are twice as large as those for D, which is the so-called isotope effect.

It has been shown that the experimental values of σ _d and their energy dependence agree well with the theoretical ones, which are calculated using the power-law approximations for Thomas-Fermi potential, on the assumption that the ion-induced detrapping of hydrogen isotopes takes place due to elastic displacement collisions with energetic carbon recoils produced by incident MeV He⁺ ions.     

Isothermal Re-Emission of Hydrogen Implanted into Isotropic Graphite

Isothermal re-emission of hydrogen from graphite implanted with 5 keV H₂ ⁺ ion beam up to saturation (3×10¹⁸/cm²) at room temperature has been studied by means of the elastic recoil detection (ERD) technique at temperatures of 450, 500, 550 and 600°C. It is found that the concentration of retained hydrogen decreases rapidly in the beginning and then decreases very gradually as the annealing time increases.

The re-emission profiles have been analyzed taking into account local molecular recombination between activated hydrogen atoms and that between an activated hydrogen atom and a trapped one together with retrapping of the activated hydrogen atom. It is shown that the re-emission of hydrogen by isothermal annealing occurs mainly due to the former type of local molecular recombination and that the activation energy of the thermally activated detrapping rate constant is 0.50±0.04eV. Moreover, it is shown that an analytical expression for the re-emission profile reproduces reasonably well the observed thermal desorption spectra.     

Low-energy helium implantation of aluminum

A series of 20 keV He⁺ implantations was conducted on well-annealed MARZ grade aluminum at fluxes of 6 × 10¹⁴ and 8 × 10¹³He⁺/cm² sec. Three distinct, temperature dependent He release mechanisms were found by He re-emission measurements during implantation, and by subsequent SEM and TEM investigations. At 0.08 of the melting temperature (T_m) gas re-emission rose smoothly after a critical dose of 3 × 10¹⁷He⁺/cm², with extensive blistering. The intermediate temperature range (~0.3 T_m) was characterized by repeated flake exfoliation and bursts of He after a dose of 3 × 10¹⁷He⁺/cm². Rapid He evolution, with hole formation, was found above 0.7 T_m. No significant differences in either gas re-emission or surface deformation were found between the two fluxes employed.     

Helium and hydrogen re-emission during implantation of molybdenum,vanadium and stainless steel

He re-emission has been measured during 300 keV He⁺ implantation from 400°C to 1200°C in V and Mo between ?170°C and 700°C in 316 SS. H re-emission has been measured during 150 keV H⁺ implantation of Mo, V, and 316 SS between ?90°C and 115 °C. In general the He re-emission is low until a critical dose is reached, at which time the re-emission abruptly rises to higher values. Both the critical dose and re-emission are strongly temperature dependent. The H re-emission generally rises smoothly to an equilibrium value which is both temperature and material dependent. Only at the lowest temperature does the H re-emission in Mo bear a qualitative resemblance to He re-emission.     

Charge exchange of helium ions channeled in diamond

We have examined the effect of charge exchange on the planar channeling of 0.2 to 1.2 MeV He⁺ ions in diamond. In this energy range, the He⁺ → He²⁺ and He²⁺ → He⁺ cross sections in the random medium are comparable. We investigated the dependence of the ratio of the exchange cross sections on the channel wavelength using a suitable potential and of the rms sum of the charge exchange cross sections on the width of the channeling peaks in the backscattered spectrum. We find that the helium is primarily in the He⁺ state at 1 MeV. and that the average cross section for stripping from He⁺ to He²⁺ in the channel is at least a factor 1.5 larger than for a random medium.     

He+ ion energy loss parameter in thin films of UO2

The stopping cross sections of He⁺ ions in vacuum evaporated films of UO₂ have been measured at incident energies from 0.3 to 2.0 MeV. A procedure has been described for obtaining true stopping cross sections by using thin films having various thicknesses. The energy loss parameter of He⁺ ions was determined using slopes at the origin in relative energy loss vs. thickness curves.     

Charge exchange of medium energy H and He ions emerging from solid surfaces

Charge exchange of medium energy H and He ions emerging from clean solid surfaces is studied extensively using a toroidal electrostatic analyzer with an excellent energy resolution. The charge distributions of He ions scattered from sub-monolayers near a surface are non-equilibrated, resulting in a surface peak even for poly-crystal solids. By solving simultaneous rate equations numerically, we derive electron capture and loss cross sections for Ni and Au surfaces. Based on a free electron gas model, non-equilibrated He⁺ fractions dependent on emerging angle reveals uniform electronic surfaces for metals and corrugated surfaces for Si and graphite with covalent bonds. It is also found that equilibrium charge fractions of H⁺ are independent of surface materials (Z₂) and in contrast equilibrium He⁺ fractions depend pronouncedly on Z₂. The data obtained are compared with semi-empirical formulas.     

Stoichiometry evolution of polyethylene terephtalate under 3.7 MeV He irradiation

A 3.7 MeV He⁺ ion beam was simultaneously used for Polyethylene Terephtalate (PET) film degradation and characterization. To enhance the potentialities of the characterization method, a multi-detector Ion Beam Analysis (IBA) technique was used. The stoichiometry change of the PET target following the irradiation is quantified at a beam fluence varying between 7 × 10¹³ and 1.8 × 10¹⁶ He⁺ cm⁻². The damage induced in PET films by He⁺ bombarding was analyzed in-situ simultaneously through Rutherford Backscattering Spectrometry (RBS), Particle Elastic scattering Spectrometry (PES) and Hydrogen Elastic Recoil Detection (ERD).Appropriate experimental conditions for the observation of absolute changes in composition and thickness during irradiation were determined. The oxygen and carbon content evolution as a function of the ion fluence was monitored by He⁺ backscattering whereas the hydrogen content was measured by H(α, H)α collisions in which both the scattered He⁺ ions and the recoiling H could be observed. The present study reveals that, at the highest fluence 1.8 × 10¹⁶ He⁺ cm⁻², the PET films have lost approximately 15% of the carbon, more than 45% of the hydrogen and 85% of the oxygen of the amount contained in the pristine sample. The energy shift of recoiling H⁺ ions at a forward angle 45° was followed in order to study the mass loss effect.The experimental results are consistent with the bulk molecular recombination model. Based on the results, hydrogen, oxygen and carbon release cross sections are determined. For hydrogen, comparison with deuteron irradiation indicates a cross section linear dependence on the stopping power.     

Investigation of the projectile atomic number dependence of the L-subshell ionization

Relative L-subshell ionization cross sections have been measured using a series of ions at a fixed collision velocity in order to study the projectile nuclear charge dependence. Thin Sm, Er, and Au targets were bombarded by ²H⁺, ⁴He⁺, ¹²C⁺, and ¹⁴N⁺ ions $\text{MeV}\text{u}$ impact energy. The cross sections were normalized to the data obtained by deuterons. The experimental results have been compared with calculations in the perturbed stationary state approximations with Coulomb deflection, binding and relativistic corrections included (CPSSR) as well as with the recent second order calculations. Particularly large discrepancies have been found for the L₂ subshell comparing the CPSSR results with the measured values. These discrepancies have been reduced significantly by the inclusion of the second order effects.     

Angular distribution of elastic and inelastic scattering of 2.34 Mev neutrons from chromium,iron, and lead

Spectra are obtained for neutrons with initial energy E₀=2.34 Mev scattered by chromium, iron, and lead nuclei, and the corresponding differential cross sections for elastic and inelastic scattering are measured for angles from 30 ° to 135 °. The neutrons were obtained from the D (d, n) He³ reaction with an initial deuteron energy of 1 Mev and a neutron emission angle of 110 °. Nuclear photographic emulsion was used as the detectorspectrometer and as the monitor of the incident neutron flux. The scatterers were 2.8 cm diameter spheres. In calculating the cross sections for elastic and inelastic scattering, corrections for self-absorption and multiple scattering were introduced to the incident neutron flux in the scatterer. The angular distribution of inelastically scattered neutrons from chromium and iron, as well as from lead when the 0.53 Mev level was excited, was found to be isotropic within the limits of the experiment. Inelastic scattering from lead when the 0.805 and 0.890 Mev levels are excited cannot be considered isotropic.     

Single electron loss in the collisions of lowly charged ions and atomic hydrogen

In this paper, the projectile electron loss cross sections of He⁺, Li⁺, Li²⁺ and C²⁺ colliding with atomic hydrogen are studied in the frame work of the extended over-barrier model at intermediate velocities (25–600 keV/u). The electron loss cross sections are calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between the projectile electron and the target electron. Compared with the measurements, this model satisfactorily reproduces the experimentally obtained energy dependence of the single electron loss cross sections over the energy range investigated here.     

Sputtering of materials by H+, D+, T+ and He+ Ions

The sputtering yields of various materials by H⁺, D⁺, T⁺, He⁺ ions were calculated using the theoretical dependence of sputtering yields on energy and ion mass and taking into account experimental sputtering yield values, whenever available, as measured for H⁺, D⁺, T⁺, He⁺ ions.     

One-electron capture and target-ionization in He-neutral-atom collisions

One-electron capture and target-ionization cross-sections in collisions of He⁺ ions with neutral atoms: He⁺ + A → He + A⁺ and He⁺ + A → He⁺ + A⁺ + e, A = H, He(1s², 1s2s), Ne, Ar, Kr, Xe, are calculated and compared with available experimental data over the broad energy range E = 0.1 keV/u–10 MeV/u of He⁺ ions. The role of the metastable states of neutral helium atoms in such collisions, which are of importance in plasma physics applications, is briefly discussed. The recommended cross-section data for these processes are presented in a closed analytical form (nine-order polynomials) which can be used for a plasma modeling and diagnostics.     

Measurement of 4He-p recoil cross sections

The recoil cross sections of protons induced by ⁴He⁺ particles were experimentally determined in an energy range of 1.3–2.1 MeV at recoil angles of 20° and 30 ° . The angular dependence of the recoil cross section at 2.0 MeV was also measured. By using the principle of detailed balance, a calculation of ⁴He-p recoil cross sections were calculated from p-⁴He phase shift data. Good agreement between the experimental data and theoretical values was found. The results show that recoil cross sections at low energies are non-Rutherford and are larger than the Rutherford value by approximately a factor of 2 at 2.0 MeV.     

Fragmentation by Beta-Decay in Tritium-Labelled Compounds, (III)

The potential energy curves of LiHe⁺, BeHHe⁺ and FHe⁺ in the ground state are calculated with a scale factor optimized STO-6G basis set, and these potential curves are compared with those of HHe⁺, CH₃He⁺, NH₂He⁺ and OHHe⁺ already reported. In the T→He⁺ β-decay, ground state daughter ions HHe⁺, LiHe⁺ and BeHHe⁺ are found to be bound, whereas CH₃He⁺, NH₂He⁺, OHHe⁺ and FHe⁺ ions dissociate into a He atom and residual fragments.     

Grazing-incidence electron microscopy of surface blisters in single- and polycrystalline tungsten formed by H, D and He irradiation

Blisters on single- and polycrystalline tungsten surfaces formed by hydrogen and helium ion irradiation were investigated by grazing-incidence electron microscopy (GIEM) with an ultra-high-voltage transmission electron microscope. It was found that the blister skin thickness formed by D⁺ irradiation of polycrystalline tungsten (PCW) was considerably larger than the calculated ion range of the implants; however, this skin thickness (or blister depth) is not related to the pre-existing grain boundaries in the PCW. Blister formation was also observed with GIEM for single crystal tungsten (SCW) irradiated with H⁺, D⁺, and He⁺. The critical ion fluence for blister formation in SCW is estimated to be ∼10²³ H⁺(D⁺)/m² for H(D) and ∼10²¹ He⁺/m² for He. The size of the blisters and their skin structure depends on the irradiating conditions. Typical skin thickness was about 50-150 nm. Based on the assumption that gas particles (H₂, D₂, and He) accumulate within the blisters during H⁺, D⁺, and He⁺ irradiation, the GIEM measurements provide a means to derive an estimate of the amount of gas so accumulated, by reproducing the observed blister shapes with finite element method (FEM) calculations. From the GIEM images and FEM calculations we have estimated the number of implanted ions being retained in the blisters, and compared these amounts with published retention measurements. A mechanism for the blister formation is proposed based on the present results.     

An investigation of the differential cross section for elastic scattering of 19.4-Mev protons by nuclei of T,He3, He4, N14, and O16

The differential cross section has been measured for elastic scattering of 19.4-Mev protons by nuclei of T, He³, He⁴, N¹⁴, and O¹⁶. The proton source was a linear accelerator. The protons were scattered on A gas target at pressures from 520 to 620 mm Hg. The scattered protons were recorded by two CsI(T1) crystal scintillation counters. The differential cross sections for T, He³, and He⁴ have about the same variation, but the minimum for He⁴ is somewhat higher and is displaced in the direction of small angles. At small angles the cross section for T is considerably higher than for He³ and He⁴.The differential cross sections for N¹⁴ and O¹⁶ are also of similar structure, but the depth of the minima and height of the maxima are greater for O¹⁶ than for N¹⁴. Comparison of our data with that of other authors shows that as the atomic number increases for a given proton energy, the positions of the minima for all elements are displaced towards smaller angles. As the energy of the scattered protons increases, the minima in the differential cross section for T and He³ is displaced towards larger angles, whereas for all heavier nuclei this displacement is towards smaller angles.In conculsion the authors express their gratitude to L. V. Kurchatov, K. D. Sincinikov, E. K. Zavoisky, and A. P. Klyucharev for valuable advice and constant attention to the work, to P. V. Chikin who carried out the work on the clectronic circult, and to the maintenance personnel of the accelerator under the direction of A. M. Smirnov, and to A. L. Bazi for discussion of the results.     

Measurement of Effective Capture Cross Section of Americium-243 for Thermal Neutrons

The effective capture cross section of ²⁴³Am for thermal neutrons was measured with an activation method. A sample of ²⁴³Am was irradiated for 10 hrs at Kyoto University Reactor, KUR. After the irradiation, the sample was cooled for one month. In the cooling time, ^244mAm and ^244gAm produced by the irradiation decayed out to ²⁴⁴Cm. The α rays emitted from ²⁴³Am and ²⁴⁴Cm were measured with a silicon surface barrier detector. The neutron flux at the irradiation position was monitored using Au/Al and Co/Al wires. The effective capture cross section was deduced as 174.5±5.3b from the α-ray counts and the neutron flux. The quantity r√T/T₀ in Westcott's convention was 0.037±0.004. The present result was compared with the effective capture cross sections from the JENDL-3.3 and the Mughabghab evaluations.     

Fragmentation by β-Decay in Tritium-Labelled Compounds, (II)

The behaviors of CH₃He⁺ and C₂H₅He⁺ formed by the decay of CH₃T and C₂H₅T were studied theoretically using the STO-3G molecular orbital method and was compared with that of HHe⁺ in the decay of HT. It was clearly shown that the ground state daughter ions CH₃He⁺ and C₂H₅He⁺ dissociate instantly to give CH₃ ⁺ and C₂H₅ ⁺ because their potential energy curves are repulsive, whereas the daughter ion HHe⁺ in the ground state does not dissociate. The transition probability to the ground state ions of CH₃He⁺ and of C₂H₅He⁺ are computed to be 66.5 and 64.8%, respectively. These values are in fairly good accordance with those obtained experimentally.     

Influence of screening length modification on the scattering cross section in LEIS

Scattering cross sections for He⁺ ions in the energy range of 100 eV to 100 keV and for Al, Cu and Au target atoms were calculated. Employing the Thomas-Fermi-Molière model the potential strength was tuned by variation of the screening length. The resulting change in scattering cross section was analyzed and the absolute value is compared to cross sections obtained from potentials commonly employed in the medium-energy ion scattering (MEIS) regime. A large influence on the scattering cross section is observed for targets with large atomic number in the very low energy range. For instance, the scattering cross section for 100 eV He⁺-ions scattered from Au by 129° changes by a factor of 2.5 between different potential strengths claimed in the literature to be suitable for low-energy ion scattering (LEIS) energies. An experiment to determine electronic energy loss of very slow ions in metals is presented. It shows how uncertainties in the scattering potential strength can lead to systematically wrong results, although perfect agreement between experimental data and simulations is found. The impact of these results on quantitative surface structure and composition analysis is discussed.