Use of the ¹⁶O(³He, α)¹⁵O reaction for studying oxygen-containing thin films

Abstract

The quantitative analysis of ¹⁶O in near-surface regions of samples is usually performed with the ¹⁶(d, p)¹⁷O^∗ reaction, at rather low energies (⩽ 900 keV). In many IBA facilities this technique cannot be used because of the absence of adequate biological shielding against the neutrons produced by the deuteron beam. On the other hand ¹⁶O depth profiling is often carried out with the resonance near 3.05 MeV of ¹⁶C(α,α)¹⁶O. With this technique the overall oxygen content is difficult to measure with precision on high-Z targets like high-T_c superconductors, due to the strong RBS background under the ¹⁶O peak. Moreover, α-energies above 3 MeV are not available in many facilities. In this article a method is presented using the ¹⁶O(³He, α)¹⁵O reaction for the background-free determination of oxygen. The cross section of this reaction, which was remeasured at θ_lab = 90° using thin SiO₂/Si targets, presents at this angle a peak near 2.4 MeV and a drastic reduction below 2 MeV. The overlapping peaks from ¹⁶O(³He, p)¹⁸F, which present no cutoff at lower energies, were eliminated from the spectra by using a detector with a very thin depletion zone. The advantages of this reaction are: very low neutron yields; background-free ¹⁶O detection, hence high precision, even on oxygenated substrates, as counts from deeper regions are drastically reduced by the cross-section cutoff; possibility to carry out simultaneous³He RBS measurements, an asset for in situ analysis; reduced beam-target interaction effects as compared to hydrogen beams which might induce reduction phenomena at high temperatures.