INSTITUTE OF PHYSICS PUBLISHING PHYSICA SCRIPTA Phys. Scr. T125 (2006) 103–107 doi:10.1088/0031-8949/2006/T125/024 Clarification of the low-lying states of 9 Be M J G Borge 1 , Y Prezado 1 , O Tengblad 1 , H O U Fynbo 2 , K Riisager 2 and B Jonson 3 1 Instituto Estructura de la Materia, CSIC, Serrano 113bis, E-28006 Madrid, Spain 2 Inst. for fysik og astronomi, Aarhus Universitet, DK-8000 Aarhus C, Denmark 3 Fundamental Fysik, Chalmers Tekniska Högskola, S-412 96 Göteborg, Sweden E-mail: borge@iem.cfmac.csic.es Received 18 July 2005 Accepted for publication 1 September 2005 Published 28 June 2006 Online at stacks.iop.org/PhysScr/T125/103 Abstract A newly developed technique for dealing with three-body decays of broad isolated levels is extended to deal with the broad, overlapping levels found at 2–9MeV excitation energy in 9 Be. The levels are populated through beta-decay of 9 Li. The method gives firm evidence for the existence of several levels. Angular correlation studies allow spin values to be assigned. PACS numbers: 23.40.Hc, 27.20.+n, 24.80.+y (Some figures in this article are in colour only in the electronic version.) 1. Introduction The interest in very light nuclei has renewed recently due to the progress in ab initio calculations applied to the structure of light nuclei, now reaching the lowest energy states for several spin values in isobars with mass 9 and 10 [1, 2]. This development presents a challenge for experimentalists to complete the knowledge on the excited states. This task becomes increasingly hard when one moves to the unbound part of the spectrum. Even for a well-studied stable nucleus such as 9 Be there are large uncertainties at about 5MeV excitation energy. Ab initio calculations have made an effort [1] to reach this region. They predict more 3/2 − , 5/2 − and 7/2 − levels in the Q β -window than are seen experimentally. Between 5–8MeV excitation energy, there is an experimental indication of a tentative 3/2 − level at 5.6 MeV observed in a single experiment (p, p ′ ) at 180 MeV in which spin assignment stems from theory [3]. The first 7/2 − state at 6.4MeV has been firmly established [3] and a state at 7.94MeV observed in beta decay has a controverted spin assignment. The difficulty of the region is that all levels are broad (width about 1 MeV) and disintegrate into 3-particles. The partial decay channels of these broad levels are of interest. The properties of low-lying unbound states in 9 Be are relevant in the calculation of the 4 He(αn,γ ) 9 Be reaction rate in the stellar scenario. This reaction is one of the key reactions which could bridge the mass gap at A = 8 to produce intermediate-to-heavy mass elements in alpha- and neutron- rich environments. A ternary process hardly plays a role in the formation of 9 Be. Instead the reaction reflects the nuclear structure of 9 Be with large neutron widths. The reaction is described by α + α ⇋ 8 Be(n,γ) 9 Be which takes place during the lifetime of 8 Be (10 −16 s) in such a suitable astrophysical scenario as the neutrino driven wind [4]. So resonance states in 9 Be near threshold play an important role. Besides it has recently been pointed out [5] that the reaction α +n ⇋ 5 He(α, γ ) 9 Be with the lifetime of 5 He (10 −21 s) can have an additional contribution to the formation of 9 Be. A microscopic three-cluster study of the photoneutron cross-section for 9 Be shows that the 5 He + α channel is of growing importance at temperatures above 10 9 K(≈ 4 MeV). Lack of experimental information has prevented the incorporation of this channel in photodisintegration calculations. Therefore a good knowledge of the 9 Be system is very important with emphasis on the 5 He intermediate decay channel. Due to its selection rules, β -decay provides a clean way to feed unbound states making beta delayed particle emission an excellent source of nuclear structure information. This approach has given a lot of information. If one knows the final state from the particle spectra one can derive the beta feeding to the different states. In a two-body break-up as in the beta-decay of 8 Li by measuring the single alpha- spectrum the process is fully determined. In a seminal paper, Barker [6] presented an R-matrix analysis of the α spectra following the decay of 8 Li and 8 B, which is applicable to spectra of single-particle emission to bound states. Later the formulation was expanded for cases where the final state is also a resonance and a second particle is emitted, 0031-8949/06/125103+05$30.00 © 2006 The Royal Swedish Academy of Sciences Printed in the UK 103