A very narrow resonance in 18O(p, α) 15N near 150 keV: Application to isotopic tracing : I. Resonance width measurement

Abstract

The narrow resonance in the 18O(p, α) 15N nuclear reaction reported by the Münster group at 152 keV looks very promising for high resolution depth profiling of 18O, being isolated and corresponding to large dE dx values. Its width was investigated usin the 2.5 MV Van de Graaff accelerator of GPS that has a proton beam energy spread of ≈ 80 eV FWHM at this energy. The α counting rate from this rather weak resonance was enhanced by operating an annular surface barrier detector subtending a large solid angle. The measurements were carried out on Ta 2O 5 targets prepared by anodizing polished high purity tantalum foils in 2% KCN aqueous solutions enriched to 98% 18O, using a vacuum line dedicated to the handling of H 2 18O chemically purified with proper distillation techniques. The thicknesses of the various targets were controlled by the voltage applied to the special anodizing cell. Assuming a Breit-Wigner shape for this resonance, its width was deduced from a careful measurement of the practically background free low energy tails of thick target excitation curves. The use of the stochastic theory of energy loss for interpreting the results and the fit of the very short low energy tail found suggests that the most probable value of the width is Г = 50 eV . This unexpectedly narrow resonance appears to be an exceptionally efficient tool for very high depth resolution isotopic tracing of 18O, as will be shown in the second part of this paper.