A VERY NARROW RESONANCE IN O-18(P, ALPHA)N-15 NEAR 150 KEV - APPLICATION TO ISOTOPIC TRACING .1. RESONANCE WIDTH MEASUREMENT

The narrow resonance in the O-18(p, alpha)N-15 nuclear reaction reported by the Munster group at 152 keV looks very promising for high resolution depth profiling of O-18, being isolated and corresponding to large dE/dx values. Its width was investigated using the 2.5 MV Van de Graaff accelerator of GPS that has a proton beam energy spread of almost-equal-to 80 eV FWHM at this energy. The alpha 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 Ta2O5 targets prepared by anodizing polished high purity tantalum foils in 2% KCN aqueous solutions enriched to 98% O-18, using a vacuum line dedicated to the handling of (H2O)-O-18 chemically purified with proper distillation techniques. The thickness 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 GAMMA = 50 eV. This unexpectedly narrow resonance appears to be an exceptionally efficient tool for very high depth resolution isotopic tracing of O-18 as will be shown in the second part of this paper.