A Study ofT=32States inN13

Abstract

A study of $T=\frac{3}{2}$ states in ${\mathrm{N}}^{13}$ was carried out by observing these states as resonances in the compound system ${\mathrm{C}}^{12}$+$p$ The lowest $T=\frac{3}{2}$ state was observed as a resonance in the proton elastic scattering at $E_p=14.231\mathrm{}\pm 0.004$ MeV. A search for additional $T=\frac{3}{2}$ states was carried out by measuring excitation functions for elastic scattering and for several proton-induced reactions over the proton energy range 17.0-21.5 MeV; two anomalies were found at $E_p=17.80\mathrm{} \mathrm{and} 18.459$ MeV, whose widths and locations suggest that they might correspond to $T=\frac{3}{2}$ states. In order to obtain the spins, parities, and elastic partial widths for the observed resonances, a program of phase-shift analysis was carried out using optical-model phases plus a single-level dispersion term. Such an analysis of the 14.231-MeV resonance yielded the assignment of $J^{\pi }={\frac{3}{2}}^{-}$. From a similar analysis, one obtains $J^{\pi }={\frac{3}{2}}^{+}$ for the 17.80-MeV resonance, while the 18.459-MeV resonance was limited to spin and parity assignments of ${\frac{3}{2}}^{-}$ or ${\frac{7}{2}}^{+}$. These assignments for the 17.80- and 18.459-MeV resonances, and the observed intensities for their proton decay to the 15.11-MeV state in ${\mathrm{C}}^{12}$ are consistent with their identification as $T=\frac{3}{2}$ states. Five other resonances, thought to correspond to $T=\frac{1}{2}$ states, were observed at $E_p=13.035, 15.220, 17.67, 19.40, \mathrm{and} 19.46$ MeV and were also analyzed in a similar fashion to yield spin and parity assignments of ${\frac{3}{2}}^{+}$, ${\frac{7}{2}}^{+}$, ${\mathrm{½}}^{-}$, ${\frac{5}{2}}^{-}$, and ${\frac{3}{2}}^{+}$, respectively.