Parity of the 2.13-MeV First Excited State ofB11

The positron-electron pair-line spectrum of the 2.13-MeV transition in ${\mathrm{B}}^{11}$ has been measured by means of an intermediate-image pair spectrometer using the reaction ${\mathrm{B}}^{11}(p,p^{\prime }){\mathrm{B}}^{11*}$ at the 2.66-MeV proton resonance. Comparison measurements were made on the pair line of the 3.09-MeV $E1\mathrm{}$ transition in the ${\mathrm{C}}^{12}\mathrm{}(d,p)\mathrm{}{\mathrm{C}}^{13}$ reaction at $E_d=1.7\mathrm{}$ MeV. Gamma-ray yields from the two reactions were monitored by means of a 5-in.×5-in. NaI crystal spectrometer. The ratio $R\equiv \frac{{\left(\frac{N_{\mathrm{pair}\mathrm{line}}}{N_{\gamma }}\right)}_{3.09}}{{\left(\frac{N_{\mathrm{pair}\mathrm{line}}}{N_{\gamma }}\right)}_{2.13}}$ was found to be 3.3±0.7. Calculations of the spectrometer pairline efficiency versus transition energy were made for $E0\mathrm{}$, $E1\mathrm{}$ to $E4\mathrm{}$, and $M1\mathrm{}$ to $M4\mathrm{}$ multipoles based on angular correlation formulas of Oppenheimer and of Rose. The values of $R$ derived from the efficiency curves are 1.81, 3.51, 2.78, and 5.42 corresponding to $E1\mathrm{}$, $M1\mathrm{}$, $E2\mathrm{}$, and $M2\mathrm{}$ multipolarities, respectively, for the 2.13-MeV ${\mathrm{B}}^{11}$ transition. The experimental result establishes that there is no parity change between the ground and first excited states and therefore that the latter is of odd parity since the ground state of ${\mathrm{B}}^{11}$ is ${3/2}^{-}$.