Parity ofBe11

Abstract

Two measurements have been made which are relevant to the parity of ${\mathrm{Be}}^{11}$. In one experiment the gamma-ray spectrum from a mixed source of 13.6-sec ${\mathrm{Be}}^{11}$ and 7.4-sec ${\mathrm{N}}^{16}$ activities was examined with a 3-mm-thick lithium-drifted germanium detector having an experimental linewidth of 13 keV for 6-MeV gamma rays. Based on a calibration from lines due to the 6.132- and 7.116-MeV gamma rays in the decay of ${\mathrm{N}}^{16}$, the 6.8-MeV gamma ray in the decay of ${\mathrm{Be}}^{11}$ has an energy of 6.792\ifmmode\pm\else\textpm\fi{}0.006 MeV. This is in agreement with the excitation energy of the ${\mathrm{\textonehalf{}}}^{+}$ or ${\frac{3}{2}}^{+}$ upper member of the 6.752-6.804-MeV doublet in ${\mathrm{B}}^{11}$. In the other experiment a magnetic pair spectrometer measurement was made on the ground-state and first-excited-state transitions from the ${\mathrm{B}}^{11}$ 7.99-MeV level as excited in the ${\mathrm{Be}}^{9}({\mathrm{He}}^{3}, p){\mathrm{B}}^{11}$ reaction. It was shown that these transitions are $E1$, thereby requiring even parity for the 7.99-MeV state, and that the spin of the state is $\frac{3}{2}$. Since the beta-ray transitions of ${\mathrm{Be}}^{11}$ to the 6.804- and 7.99-MeV levels of ${\mathrm{B}}^{11}$ are both known to have allowed $log\mathrm{ft}$ values, the parity of ${\mathrm{Be}}^{11}$ is even.