Beta Decay ofLi9

Abstract

A beryllium "rabbit" irradiated with neutrons from the $t+d$ reaction was transferred repeatedly to remote scintillation detectors by means of a timed pneumatic system. The spectrum of beta rays emitted by the Be sample and detected by means of a Pilot-B scintillator displays a strong component with a half-life of ∼1 sec and ${\beta }_{max}\sim 3.5$ MeV and a weaker component with a half-life of 0.19±0.03 sec and ${\beta }_{max}=13.5\mathrm{}\pm 0.3$ MeV. These activities are identified as ${\mathrm{He}}^6$ from the ${\mathrm{Be}}^9(n, \alpha ){\mathrm{He}}^6$ reaction and as ${\mathrm{Li}}^9$ from the ${\mathrm{Be}}^9(n, p){\mathrm{Li}}^9$ reaction, respectively. A much weaker component is assigned to ${\mathrm{N}}^{16}$ resulting from oxygen in the sample. Beta rays in coincidence with neutrons detected in a second Pilot-B crystal have an end-point energy of 11.0±0.4 MeV. The coincidence spectrum from the neutron-detecting crystal displays a principal component corresponding to a neutron energy of 0.7±0.2 MeV and gives some evidence for neutrons having an energy of 3-4.5 MeV. From these data, together with a shape analysis of the beta-ray singles spectrum, it is deduced that ${\mathrm{Li}}^9$ decays with a 25% branch to the ground state of ${\mathrm{Be}}^9(logft=5.5\mathrm{}\pm 0.2)$ and with a 75% branch mostly to the known 2.430-MeV level ($logft=4.7\mathrm{}\pm 0.2$). Both $log\mathrm{ft}$ values require allowed transitions and are compatible with a probable shell-model spin-parity assignment of $\frac{3}{2}-$ to ${\mathrm{Li}}^9$ and with the tentative assignment of $\frac{5}{2}-$ given previously to the 2.430-MeV level. The cross section for forming ${\mathrm{Li}}^9$ with neutrons of about 15.5 MeV is ∼0.7 mb.