Hydrogen Depth-Profiling with 19F, 15N Induced γ-Ray Resonances

Abstract

The l6.44MeV ^1H(^19F,αγ)^l6O and 6.4MeV ^1H(^15N,αγ)^12C resonance reactions were investigated for the purposes of hydrogen depth-profiling. The problems of calibrating the γ-ray yield from the reaction in terms of hydrogen concentration and of deconvoluting the yield curves to obtain the distribution of hydrogen with depth in the target were solved. The calibration involved using an ammonium chloride target as a hydrogen reference standard. With this standard the sensitivity limits of the fluorine resonance for hydrogen determinations were estimated to be 750, 740 and 520 atomic ppm in iron, titanium and silicon respectively with an uncertainty of ~±25%. The target also formed the basis of a new energy calibration procedure for heavy ion beams. With this procedure a magnetic calibration constant for the magnetic analysing system of the EN Tandem was determined to be Km = (199.6 ± 0.3)xl0^-4 MeV/(MHz)^2 over the range 0.5 to 0.8 Tesla. A considerably more accurate value Km = (199.4 ± 0.1)x10^-4 MeV/(MHz)^2 is obtainable for the higher magnetic field region of 0.7 to 0.8 Tesla. Large beam energy spreads were also measured and attributed to non-uniformities in the sripper foil. Yield curve deconvolution was achieved with two iterative schemes: the Van Cittert and least-squares methods. The least-squares method incorporated smoothing factors to enable the efficient handling of noise variations in the raw data. Experimental depth-profiling studies were restricted to the use of the fluorine resonance because of poor accelerator transmission at low energies. Gas stripping is recommended as one way to improve the low energy transmission. The fluorine resonance was used to profile hydrogen in titanium, in stainless steel and in obsidian based materials. Reproducible profiles were obtained for titanium hydride but no hydrogen could be detected in hydrogen charged (cathodic and implantation) stainless steel samples. The hydrogen profile in the obsidian samples was unstable under the probing beam. A surface layer of hydrogen existed on all samples investigated and the dynamic nature of this layer under the probing beam was studied. The presence of a surface layer makes hydrogen determinations in the near surface region of a target very inaccurate. Finally several exploratory investigations using ion beam analysis were undertaken in addition to the hydrogen profiling studies that were the main subject of the research. These investigations addressed problems of interest to researchers in the Wellington area and included a search for a resonance at ~340keV for the ^12C(d,p)^13C reaction, deuterium depth-profiling with the D(d,p)T reaction, nitrogen depth-profiling with the ^14N(d,α v1)^l2C reaction and the production of ion beams (44MeV ^79Br^9+ and 35MeV ^36•Cl^6+) for elastic recoil detection analysis.