Materials analysis using the (³He,p) and (α,p) nuclear reactions

Abstract

Rutherford backscattering spectrometry (RBS) is a very versatile and popular technique in materials characterization. ⁴He⁺ ion beams in the keV or MeV range have been widely used to obtain quantitative information regarding the composition and depth distribution of elemental constituents and impurities in thin films. However, in many cases, RBS is ineffective for light element analysis due to overlapping signals caused by heavy elements in the film or backing material. This project proposes using the (³He,p) and (α,p) reactions to develop nuclear reaction techniques for light element analysis in cases where regular RBS cannot accurately determine elemental content. The (³He,p) nuclear reaction for boron, nitrogen, carbon and oxygen in thin films was investigated using incident beams between 2 and 4 MeV. Absolute cross sections were measured at reaction angles of 90° and 135°. These reactions were observed to have regions of constant cross section suitable for elemental content determination. The B(³He,p)C and ¹⁴N(³He,p)¹⁶O reactions were applied to thin films containing boron and nitrogen, and were proven to be an accurate means of determining elemental areal density in thin films in cases where regular RBS was ineffective due to signal interference from heavier elements in the film or backing substrate. Advantages and limitations of the application of the (³He,p) reaction to B, N, C and O will be discussed for each of these elements. The ¹⁹F(α,p)²²Ne nuclear reaction was investigated over the energy range 2200-2500 keV. Cross sections for the ¹⁹F(α,p₀) reaction were measured at a reaction angle of 135°. A strong, isolated resonance near 2315 keV was observed which is suitable for fluorine depth profiling. A computer program was also used to generate simulated yield curves. Resonance parameters were empirically fit to the yield curve obtained using a target with known areal density (atoms/cm²). The program, with these parameters, was applied to accurately simulate yield curves obtained from other targets. The advantages, limitations and applications of this reaction will be discussed.