From a coincidence study of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$(${}_{}{}^3{}_{}{}^{}\mathrm{He}$,$\mathrm{np}$)${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reaction we obtain branching ratios of (9.6±1.4)% and (16.4±3.6)% respectively for the isospin forbidden proton decays of the lowest $T=\frac{3}{2}$ level of ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$ to negative parity levels of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ at 9.64 and 10.84 MeV. The reduced widths for these $s$- or $d$-wave decays are much larger than those for the $p$-wave decays to the lower lying positive parity levels of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. This establishes the importance of ${\left(2s1d\right)}^{2}1p^7$ configurations in the isospin impurity in ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$($T=\frac{3}{2}$). We argue that isospin impurities containing pairs of particles excited into higher shells are a widespread phenomenon and present a qualitative explanation of this effect which has been ignored in essentially all isospin mixing calculations.

NUCLEAR REACTIONS ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$(${}_{}{}^3{}_{}{}^{}\mathrm{He}$,$\mathrm{np}$)${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$; $E_{{}_{}{}^3{}_{}{}^{}\mathrm{He}}=7.0\mathrm{}$ MeV; ${}_{}{}^{13}{}_{}{}^{}\mathrm{N}$($T=\frac{3}{2}$) proton decays; isospin mixing

• Received 11 September 1978

DOI:https://doi.org/10.1103/PhysRevC.19.1