Decay modes of ^9Li and states of ^9Be

Abstract

The β-decay of the ^9Li nucleus and the subsequent particle decays of the unbound states of the ^9Be nucleus have been investigated with two complementary techniques, a recoil-particle method and a beta-neutron coincidence method. ^9Li nuclei were produced by the ^(18)O(^7Li, ^9Li) ^(16)O reaction at a bombarding energy of 20 MeV and by the ^3H(^7Li, ^9Li)p reaction with a 14 MeV bombarding energy. The energy spectrum of the delayed alphas and the time-of-flight spectrum of the delayed neutrons were obtained. The half-life was measured to be 177±3 ms. Besides the known decay branches to the ground state and to the 2.43 MeV state in ^9Be, a new decay branch to an unbound state in 9Be at an excitation energy of 2.78±0.12 MeV was observed. The branching ratios and the log ft values were found to be (65.0_(−2.4)^(+2.7)) %, (32.0_(−3.7)^(+2.7))%, (3.0_(−0.3)^(+2.7))% and 5.12_(−0.02)^(+0.01), 5.00_(−0.05)^(+0.04), 5.97_(−0.28)^(+0.05) to the ground state, the 2.43 MeV state and the 2.78 MeV state, respectively. The observed properties of the 2.78 MeV state favor an assignment of 1/2. This state decays mainly by neutron emission to the ground state of ^8Be and has a c.m. width of 1.10±0.12 MeV. The fraction of the decays of the 2.43 MeV state in ^9Be involving emission of a neutron to the ground state of ^8Be was found to be (6.4±1.2)%.

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