Hydrogen depth profiling using the 6.385 MeV resonance in the ¹H(¹⁵N, αγ)¹² carbon nuclear reaction.

Abstract

The use of ¹⁵N analysis ion beams as a probing beam can provide a powerful and non-destructive technique for detection of hydrogen in the near surface region of materials. The strong, isolated resonance at 6.385 MeV in the ¹H(¹⁵N,αγ) ¹²C nuclear reaction induces a γ-ray yield which is proportional to the hydrogen content of the material at the depth where the nuclear reaction occurs. The ¹H depth profiling using this nuclear reaction resonance is done by increasing the energy of the ¹⁵N analysis ion beam from 6.385 MeV in steps of several KeV. The measured induced 4.43 MeV γ-rays from this nuclear reaction are representative of the hydrogen presence in the material at the depth where the ion beam has the resonance energy. Hydrogen depth profiling using this reaction is done in the energy range of 6 to 10 MeV. The measured γ-ray yield of a standard sample, with a well known amount of hydrogen, and the measured γ-ray yield of the "unknown sample" and its non-hydrogen stoichiometry are used to determine the H atomic fraction of the "unknown sample". The non-hydrogen stoichiometry of the "unknown sample" is frequently obtained by Rutherford backscattering spectrometry (RBS). A Penning ion source assembly was installed in the 5.5 MeV Van de Graaff (VDG) accelerator to provide ¹⁵N⁺⁺ analysis beams for hydrogen depth profiling. The Penning ion source assembly was tested with 60 Hz and 400 Hz power supplies in order to determine the best voltage settings of the power supplies for obtaining the optimum beam currents for ¹⁵N⁺⁺, ⁴He⁺ and ⁴He⁺⁺ analysis beams. The testing was accomplished before the Penning ion source assembly was installed in the VDG accelerator. Investigation for suitability as a standard was carried out on several hydrogenated samples. Several thin film samples, such as Si:H, WO₃ and HiT(c) were also hydrogen depth profiled in order to determine the hydrogen atomic fraction present in the films, uniformity of the hydrogen distribution throughout the films and the stability of hydrogen in the films under ¹⁵N ion beam bombardment. A multilayer film of hydrogenated Si and SiO₂ was also hydrogen depth profiled. The results show that the hydrogen incorporated in either Si or SiO₂ layers is extremely stable. The hydrogen depth profiling of this multilayer also shows that hydrogen depth profiling with the 6.385 MeV resonance in the ¹H(¹⁵N,αγ) ¹²C nuclear reaction can have a good depth resolution of 700 Å at depth of 1.7 μm.