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.     

Pyrolytic Graphite Irradiated with 4 keV Hydrogen Ions Using Reflection High-Energy Electron Diffraction

Separation of barium isotopes with a selective two-step photoionization process was accomplished using a continuous wave dye laser and ultra-high pressure mercury lamp. Narrow line-width laser light was tuned to the 6s^{2 1}S₀--6s6p ¹P₁ resonance line (553.6 nm), and only a single isotopic component in an atomic beam was excited through the isotope shift. The excited atoms were successively ionized by uv radiation and deflected by a static electric field.

The spectrum of the ion current separated isotopically agreed well with spectroscopic data within the system resolution of 65 MHz. The isotopic enrichment of ¹³⁸Ba was 97%, which corresponded to the selectivity of 1.36. The ionization rate defined as what portion of the incident atoms was ionized was approximately 4x10^?5%. The photoionization cross section was estimated from the experimental results by using the least squares method. The resultant value was (4±1)x10^?23m².     

C 1s Binding-Energy Shift of Pyrolytic Graphite Bombarded by keV-Deuterium Ions

As a basic study of plasma-wall interactions, chemical state of graphite basal face after deuterium bombardment are studied by means of X-ray photoelectron spectroscopy. As the fluence of deuterium ions with energy of 1 or 4keV is increased from 10¹⁴ to 10¹⁸ ions/cm², the C Is line is observed to shift towards the lower binding energy at first, down to the minimum by about 0.2eV and then towards the higher energy up to an asymptotic value of about 0.2eV higher than the initial position. The first negative shift is due to lattice displacement at the target surface, and the subsequent positive shift is due to the deuterium trapping at the graphite surface. The fluence dependence of the observed C 1s shift is explained by a simple saturation model.