1.7-MeV Anomaly in theC12(p, p)C12Reaction

Abstract

The ${\mathrm{C}}^{12}(p, p){\mathrm{C}}^{12}$ reaction cross section has been measured for laboratory proton energies from 1.48 to 2.02 MeV at center-of-mass scattering angles of 25°, 35°, 45°, 54.7°, 70°, 80°, 90°, and 125.2°. Thin self-supporting carbon films were used as targets and the scattered protons were analyzed with a double-focusing magnetic spectrometer. The data were fitted with a theoretical one-level formula. Resonances included in the theoretical expression were the ${\mathrm{J}}^{\pi }={\frac{1}{2}}^{+}$, ${\mathrm{E}}_{\mathrm{p}}$=0.461-MeV resonance (unobserved in this experiment) and the ${\mathrm{J}}^{\pi }={\frac{3}{2}}^{-} \mathrm{and} {\frac{5}{2}}^{+}$ resonances associated with the 1.7-MeV anomaly. Parameters for the ${\mathrm{J}}^{\pi }={\frac{1}{2}}^{+}$ resonance were taken from previous experiments, and parameters for the ${\mathrm{J}}^{\pi }={\frac{3}{2}}^{-} \mathrm{and} {\frac{5}{2}}^{+}$ resonances were varied to obtain a best fit to the data. The final parameter values are: ${\mathrm{E}}_{\mathrm{p}}$=1686±6 keV, ${\gamma }^2=(0.515\mathrm{}\pm 0.02)\times {10}^{-13\mathrm{}}$ MeV cm for the ${\mathrm{J}}^{\pi }={\frac{3}{2}}^{-}$ level, and ${\mathrm{E}}_{\mathrm{p}}$=1734±6 keV, ${\gamma }^2=(3.55\mathrm{}\pm 0.18)\times {10}^{-13\mathrm{}}$ MeV cm for the ${\mathrm{J}}^{\pi }={\frac{5}{2}}^{+}$ level. These reduced-width values are approximately 20% larger than those previously measured. Although the error in the absolute energy measurements is 6 keV, the errors in energy differences are smaller and the measured difference ${\mathrm{E}}_{\mathrm{p}}$ values for the ${\frac{3}{2}}^{-}$ and ${\frac{5}{2}}^{+}$ levels is 48±2 keV.