Beta Decay ofBe11

Abstract

${\mathrm{Be}}^{11}$ has been made in the reaction ${\mathrm{B}}^{11}(n, p){\mathrm{Be}}^{11}$ and its radiations have been studied with plastic and NaI(Tl) scintillation spectrometers both singly and in coincidence. The half-life as determined from measurements on both beta rays and gamma rays is 13.57±0.15 sec. Decay takes place by beta-ray emission to the ground state of ${\mathrm{B}}^{11}$ (61%, $logft=6.77\mathrm{}$, ${\beta }_{max}=11.48\mathrm{}$ Mev), to the 2.14-Mev first excited state (29%, $logft=6.63\mathrm{}$), to the 6.76-6.81-Mev doublet (6.5%, $logft=5.93\mathrm{}$) and to the 7.99-Mev level (4.1%, $logft=5.53\mathrm{}$). Limits are set on the decay to several other levels, particularly those at 4.46 and 5.03 Mev (≤0.2%). Gamma rays of 2.121, 4.64, 5.86, 6.76, and 7.97 Mev are observed both singly and in coincidence with beta-ray branches. An energy separation ${\mathrm{Be}}^{11}$-${\mathrm{B}}^{11}$=11.48±0.15 Mev is derived from the various beta and gamma-ray energy measurements. It is concluded that the assignment $J=\frac{1}{2}-$ for ${\mathrm{Be}}^{11}$, as expected from the shell model, is possible but cannot be established firmly on the basis of the present evidence.