The H(t, n)³He reaction for depth profiling of hydrogen by neutron time-of-flight

The usefulness of the H(t, n) reaction for depth profiling of hydrogen in solids was investigated using the pulsed T⁺ beam from the Los Alamos vertical Van de Graaff accelerator. A 2.54 cm diameter by 2.54 cm thick stilbene neutron detector was placed at 0°, 5.35 meters from the target. Pulse shape discrimination virtually eliminated gamma-ray background. Neutron spectra from targets of Ti and TiH₂ with a nominal thickness of ten micrometer were compared with targets of SiO₂, Si, C and Au to assess the importance of neutron backgrounds from triton reactions with the constituents of those targets. When the neutron spectra were transformed into profiles, the background corrected TiH₂ spectrum showed a uniform hydrogen concentration over ten microns. A measurement of the neutron spectrum produced when bombarding an Au target on which (t, n) reactions are strongly inhibited by the Coulomb barrier gives a yield equivalent to a background concentration of 3 at.% of hydrogen in Ti. This background is due to reactions on hydrogen, hydrocarbons, or other light nuclei on or in beam line components. Similar measurements on other targets gave background values equivalent to hydrogen atomic concentrations in Ti of: SiO₂-11 at.%, Si-7 at.%, C-22 at.%. Ti-5 at.%. Development of high sensitivity for depth profiling hydrogen with this technique appears possible only for high Z host materials and will require careful attention to beam line and vacuum system design.