Decay ofBe11

The $\beta$ decay of ${\mathrm{Be}}^{11}$, formed in the ${\mathrm{Be}}^9\mathrm{}(t,p)\mathrm{}{\mathrm{Be}}^{11}$ reaction, has been investigated using Ge(Li) and NaI(Tl) $\gamma$-ray detectors and Si $\alpha$-particle detectors. Eight $\gamma$-ray transitions were measured including three weak $\gamma$ rays with energies of 1772, 2893, and 5019 keV not observed previously in ${\mathrm{Be}}^{11}$ decay. The relative $\gamma$ intensities show that the 5020-keV state of ${\mathrm{B}}^{11}$ is populated by a ${\mathrm{Be}}^{11}$ first-forbidden $\beta$-ray branch of 0.28% ($logft=7.94\mathrm{}\pm 0.14$), as well as by a cascade $\gamma$-ray branch of 4.1% from the ${\mathrm{B}}^{11}$ 6793-keV state. Other previously observed $\beta$-ray branches have been confirmed. Accurate energies have been obtained for the ${\mathrm{B}}^{11}$ states, including a value of 7978.1±1.9 keV for the seventh excitated state. This is 18 keV lower than in previous reports. Delayed $\alpha$ particles are also found in ${\mathrm{Be}}^{11}$ decay; the experimentally observed $\alpha$-particle spectrum is a continuum extending from an instrumental lower limit of 0.3 MeV up to about 1.0 MeV, and with a total $\alpha$-particle intensity of 3.0% per decay. These $\alpha$ particles are not in coincidence with 478-keV $\gamma$ rays from ${\mathrm{Li}}^7$. It is suggested that this $\beta$-ray branch of ${\mathrm{Be}}^{11}$ proceeds to the 9870-keV $J^{\pi }={\frac{3}{2}}^{+}$ state of ${\mathrm{B}}^{11}$ ($logft=4.03\mathrm{}\pm 0.15$) followed by $\alpha$-particle emission to the ground state of ${\mathrm{Li}}^7$, although an $\alpha$-decay branch to the 478-keV state of ${\mathrm{Li}}^7$ cannot be excluded. The new features of ${\mathrm{Be}}^{11}$ decay are discussed. It appears that the analog in ${\mathrm{B}}^{11}$ of the ground state of ${\mathrm{Be}}^{11}$ has no antianalog.