Spectroscopy ofB9via high-resolution ejectile-tagged recoil break-up

Abstract

An experiment has been carried out using the 9Be(3He, r)9B* reaction at a beam energy of 33 MeV. A large acceptance silicon-strip array was used to detect the 9B* break-up in coincidence with the triton ejectiles in the high-resolution Munich-Q3D spectrograph. The excitation energy regime <3 MeV has been explored and the spectrum resulting from proton decaying states, isolated and characterized. Additional resonance strength is observed at 1.86 MeV ±70 keV(stat) ±35 keV(syst), in agreement with two other recent measurements at higher energies and different angles. The consequences for the "missing" >/2+ first excited state are discussed. Additionally, the branching ratios for the 2.36 MeV 5/i~ state have been measured as r»o/r = 0.98 ± 0.12 and i> /r = 0.016 ± 0.008, in close agreement with earlier work.

Evidence for a 3.8 MeV state inBe9

“…These orbitals are analogous to electron orbits in atomic molecules [1]. In agreement with this picture, antisymmetrised molecular dynamics (AMD) and no-core ab initio calculations have explicitly illustrated the emergence of α clustering in 9 Be without assuming this structure a priori [2,3].…”

“…Mean-field-type calculations have been shown to satisfactorily reproduce the natural parity experimental spectrum of 9 Be [6], and calculations which utilise an extended Nilsson model space obtain a good fit to the ground state charge form factor for electron scattering * Email: r.smith.3@pgr.bham.ac.uk [7]. However, it should be noted that in order to correctly describe the properties of the excited 5/2 − 1 level at 2.43 MeV (interpreted as a rotational excitation of the ground state in the molecular picture), higher order deformation terms of the Nilsson potential (beyond r 2 Y 20 prolate shapes) were required.…”

“…From a theoretical perspective the α:n:α molecular structure of 9 Be has been successfully described in terms of an exchange neutron being in either σ-or π-type orbitals about two α particle cores. These orbitals are analogous to electron orbits in atomic molecules [1].…”

“…Despite these exciting advances in nuclear theory 9 Be remains an exceptionally difficult nucleus to study experimentally. Measured states have been tentatively classified into rotational bands with reasonable success [4,5].…”

“…However, since the excited states exist as shortlived resonances above the particle decay threshold, evidence of enhanced γ transition rates between band members -a key indicator of collective rotation -is scarce. Therefore, ambiguity remains as to whether the low-lying states in 9 Be are α-clustered or shell-model-like.…”