States Of 8Be Research Articles

Probing Configuration Mixing inBe12with Gamow-Teller Transition Strengths

We present a novel technique for studying the quenching of shell gaps in exotic isotopes. The method is based on extracting Gamow-Teller (ΔL=0, ΔS=1) transition strengths [B(GT)] to low-lying states from charge-exchange reactions at intermediate beam energies. These Gamow-Teller strengths are very sensitive to configuration mixing between cross-shell orbitals, and this technique thus provides an important complement to other tools currently used to study cross-shell mixing. This work focuses on the N=8 shell gap. We populated the ground and 2.24 MeV 0+ states in 12Be using the 12B(1+) (7Li, 7Be) reaction at 80 MeV/u in inverse kinematics. Using the ground-state B(GT) value from β-decay measurements (0.184±0.007) as a calibration, the B(GT) for the transition to the second 0+ state was determined to be 0.214±0.051. Comparing the extracted Gamow-Teller strengths with shell-model calculations, it was determined that the wave functions of the first and second 0+ states in 12Be are composed of 25±5% and 60±5% (0s)4(0p)8 configurations, respectively.

Signatures of α-particle condensation in N = Z nuclei

At the thresholds for multi-α particle decay a phase transition of second order appears in N = Z nuclei, because the chemical potentials for nucleons in nuclear matter and in α-particles become equal. The Bose gas created by the α's strongly interacting via 0+ states in 8Be and 12C* has coherent properties because of the fact that the de-Broglie wavelength is larger than the nuclear radius. Experimental observabels are discussed, which show these coherent properties, like large radial extensions and the coherent emission of several α's.

Resonance parameters of the first1/2+state inBe9and astrophysical implications

Spectra of the 9Be(e,e') reaction have been measured at the S-DALINAC at an electron energy E_0 = 73 MeV and scattering angles of 93{\deg} and 141{\deg} with high energy resolution up to excitation energies E_x = 8 MeV. The astrophysically relevant resonance parameters of the first excited 1/2+ state of 9Be have been extracted in a one-level approximation of R-matrix theory resulting in a resonance energy E_R = 1.748(6) MeV and width Gamma_R = 274(8) keV in good agreement with the latest 9Be(gamma,n) experiment but with considerably improved uncertainties. However, the reduced B(E1) transition strength deduced from an extrapolation of the (e,e') data to the photon point is a factor of two smaller. Implications of the new results for a possible production of 12C in neutron-rich astrophysical scenarios are discussed.

Above threshold s-wave resonances illustrated by the [formula omitted] states in 9Be and 9B

We solve the persistent problem of the structure of the lowest 1/2+ resonance in 9Be which is important to bridge the A=8 gap in nucleosynthesis in stars. We show that the state is a genuine three-body resonance even though it decays entirely into neutron–8Be relative s-waves. The necessary barrier is created by “dynamical” evolution of the wave function as the short-distance α-5He structure is changed into the large-distance n-8Be structure. This decay mechanism leads to a width about two times smaller than table values. The previous interpretations as a virtual state or a two-body resonance are incorrect. The isobaric analog 1/2+ state in 9B is found to have energy and width in the vicinity of 2.0 MeV and 1.5 MeV, respectively. We also predict another 1/2+ resonance in 9B with similar energy and width.

Concepts of nuclearα-particle condensation

Certain aspects of the recently proposed antisymmetrised alpha particle product state wave function, or THSR alpha cluster wave function, for the description of the ground state in 8Be, the Hoyle state in 12C, and analogous states in heavier nuclei, are elaborated in detail. For instance, the influence of antisymmetrisation in the Hoyle state on the bosonic character of the alpha particles is studied carefully. It is shown to be weak, so that bosonic aspects are predominant. The de Broglie wave length of alpha particles in the Hoyle state is shown to be much larger than the inter-alpha distance. It is pointed out that the bosonic features of low density alpha gas states have measurable consequences, one of which, that is enhanced multi-alpha decay properties, likely already have been detected. Consistent with experiment, the width of the proposed analogue to the Hoyle state in 16O at the excitation energy of E_x=15.1 MeV is estimated to be very small (34 keV), lending credit to the existence of heavier Hoyle-like states. The intrinsic single boson density matrix of a self-bound Bose system can, under physically desirable boundary conditions, be defined unambiguously. One eigenvalue then separates out, being close to the number of alpha's in the system. Differences between Brink and THSR alpha cluster wave functions are worked out. No cluster model of the Brink type can describe the Hoyle state with a single configuration. On the contrary, many superpositions of the Brink type are necessary, implying delocalisation towards an alpha product state. It is shown that single alpha particle orbits in condensates of different nuclei are almost the same. It is thus argued that alpha particle antisymmetrised product states of the THSR type are a very promising novel and useful concept in nuclear physics.

Cluster models of light nuclei and the method of hyperspherical harmonics: Successes and challenges

Hyperspherical-harmonics method to investigate the lightest nuclei having three-cluster structure is discussed together with recent experiments. Properties of bound states and methods to explore three-body continuum are presented. The challenges created by large neutron excess and halo phenomena are highlighted. Astrophysical aspects of the 7Li + n → 8Li + γ reaction and the solar-boron-neutrinos problem are analyzed. Three-cluster structure of highly excited states in 8Be is shown to be responsible for extreme isospin mixing. Progress in studies of 6He- and 11Li-induced inclusive and exclusive nuclear reactions is demonstrated, providing information on the nature of continuum structures of Borromean nuclei.

Reply to ‘Comment on ‘Low-lying 0+, T = 2 states of 12Be, 12C and 12O’’

We reply to the preceding paper.

Comment on ‘Low-lying 0+, T = 2 states of 12Be, 12C and 12O’

A description of the low-lying A = 12, 0+, T = 2 states from 30 years ago has recently been criticized and an alternative description proposed that is claimed to agree better with experimental data. We here discuss this claim and suggest a different interpretation of some of the data.

Multiple particle break-up study of low excited states in 9Be: The ghost peak in the 8Be excitation energy spectrum visited

The aim of this work is to find out the origin of the anomalous resonance in Be-8 seen in the reactions through excited states in Be-9. We have populated the Be-9 excited states by beta-decay of Li-9. Energy and direction of the two alpha particles has been detected and the neutron spectra reconstructed. In our work we identified the "anomalous resonance" in Be-8 observed in several reaction studies as coming from the decay of the 2.43 and 2.78 MeV states in Be-9. This anomalous resonance appears when the two detected alpha particles are assumed to form a resonance in Be-8. We argued that the main decaying channels for these two levels in Be-9 do not involve Be-8.

Investigation of the reaction mechanism for the four-particle photodisintegration of a carbon nucleus

The four-particle photodisintegration of a carbon nucleus in the reactions 12C(γ, p)3H2α and 12C(γ, n)3H2α is investigated by a method that employs a diffusion chamber in a magnetic field. It is shown that these reactions proceed according a sequential-type scheme: excited states of 11B and 11C nuclei decay to weakly excited states of 8Be, 7Li, and 7Be nuclei. It is concluded that nucleons are knocked out from the s shell. In the excitation curve for the 2α system in the reaction 12C(γ, p)3H2α, a resonance is found between the maxima corresponding to the ground and the first excited state of the 8Be nucleus, and this resonance is identified as a ghost anomaly. The branching fractions of the decay modes are determined. The angular distributions of nucleons in the reaction c.m. frame are measured. The energy dependence of the asymmetry coefficient for the angular distributions is obtained. A fast increase in this coefficient is observed in the energy range 38–40 MeV. It is concluded that the asymmetry coefficient depends on the excitation energy of the final nucleus in the region of intermediate photon energies.

In-beam spectroscopy of exotic nuclei probed by α-induced direct reactions

The α inelastic scatterings and the (α,t) reaction on exotic nuclei at 50–60 A MeV are discussed. A new multipole decomposition analysis taking into account the angular correlation of decaying process originated from the alignments of the residual nucleus is applied for a study of cluster states in 12Be. The proton transfer reaction on the 12Be nucleus populates a possible proton single-particle state bound by a deformed core in the 13B nucleus of which ground state is spherical.

The triple-α process and the origin of the elements

The triple-α process is the nuclear fusion of helium to carbon. Two 4He nuclei collide and form an unstable state of 8Be. Before this can decay back a third α-particle collides with it to form a 12C nucleus. The reaction rate is probably the most temperature sensitive known to physics because there is a resonant state of the 12C nucleus at just the energy of the 8Be* and the 4He that have the greatest chance of penetrating the Coulomb barrier. The reaction takes place in stars that have exhausted their central hydrogen fuel. Most interesting are the asymptotic giants that can dredge the carbon produced to the surface and return it, perhaps first processing it to nitrogen, to the interstellar medium. The fortuitous energy of the resonant state of 12C and its consequences are crucial to the way stars evolve and the abundance of carbon on the Earth.

Alpha-particle states in 16O and 20Ne

Studies of the 12C(12C,8Be+8Be)8Be and 12C(12C,8Be+12C(0+2))α reactions, using a large array of strip detectors, have provided evidence for states in 16O and 20Ne which decay to α-cluster states in 8Be and 12C. The spins of the resonances have been determined which allows the energy-spin systematics to be characterized.

The reaction of triple radiative capture αα(n, γ) 9Be studied in a β decay of 9Li

A new way of study the reaction of triple radiative capture αα(n,γ)9Be is demonstrated. At the GSI-ISOL facility, we have measured the γ decays of excited 1/2+, 5/2− and 1/2− states in 9Be, which are strongly populated in the β decay of 9Li. The respective partial γ-widths are of astrophysical interest in order to reliably estimate the αα(n,γ)9Be rates in SN type-II as a possible site of the synthesis of heavy elements in the Universe. The 9Li nuclei were implanted into a foil viewed by high-granularity arrays of β and γ detectors. The γ-ray spectra were measured in coincidence with the β-particles by selecting large β-γ angles in order to suppress the bremsstrahlung background. For the first time, the γ-rays from the 1/2+ and 5/2− states in 9Be were observed in 9Li decay, with the 1/2+-state energy and width measured to be 1689(10) and 224(7) keV, respectively, while the latter transition provided a reference point. The upper limit of the 1/2− partial γ-width is measured to be 7 ⋅ 10 6 MeV . An improved sensitivity experiment is feasible at facilities with the larger yield of 9Li.

Low-lying resonance states in the 9Be continuum

Excited states in 9Be from 2 to 9 MeV are studied via beta delayed particle emission from 9Li. The broad overlapping particle unbound states are investigated using an extension of an experimental method developed for dealing with three-body decays of broad isolated levels. The results confirm the existence of a broad state at 5 MeV, with a width of 2 MeV. Angular correlations are used for firm spin determinations for this and other levels.

The β-decay of 9Li to the high lying states in 9Be

Mirror β -transitions should have similar strengths as long as binding energy, Coulomb energy and other explicitly breaking effects are small. However, large β -decay asymmetries have been observed in the decay of 9 C [Phys. Rev. C 37 (1988) 766] and the decay of its mirror nucleus 9 Li [Nucl. Phys. A 510 (1990) 189]. These asymmetries could be the indication of structural differences between mirror states. We have focussed on the beta-decays of 9 C and 9 Li to the high lying region (between 11-12 MeV) in the daughter nuclei 9 B and 9 Be. The difficulty in the study of these decays is that they are accompanied by the emission of three particles and one needs to determine the breakup mechanism to extract information about the position, width and feeding of the states. The β -decays of 9 C [Nucl. Phys. A 692 (2001) 427] and 9 Li [Phys. Lett. B. 576 (2003) 22] to the high lying region of the daughter nuclei have been remeasured with setups that allow the study of the complete kinematics of the processes and, hence, the breakup mechanism of the states in the daughter nuclei can be determined. The most recent experiments on 9 C [Nucl. Phys. A 692 (2001) 427, Phys. Rev. C 61 (2000) 064310] gave B GT values of 1.58(16) and 1.20 (15) for the transition 9 C → 9 B (12.2 MeV) whereas the only early determination for 9 Li→ 9 Be (11.81 MeV) [Nucl. Phys. A 510 (1990) 189] gave 5.6 (1.2). The experiments on 9 C employed coincidence detection of charged particles thereby allowing a detailed analysis, whereas the older 9 Li experiment only included singles spectra. The only previous 9 Li experiment where α -particle coincidences were measured [Nucl. Phys. A 366 (1981) 449] did not have sufficient angular resolution and no detailed analysis of the breakup mechanism was performed.

SU(3) model description of Be isotopes

Experimental data on light nuclei close to dripline suggests that as the nucleon number asymmetry increases, the shell structure from stability line is not preserved. In contrast with spherical shell model, Elliott's SU(3) model, uses a deformed multi-nucleon basis to describe nuclear states. The SU(3) symmetry is a strong feature of 1p shell nuclei, where symmetry breaking spin-orbit force is rather weak. We have calculated the binding energies and low-lying energy spectra of Be isotopes (A=6 to A=14), within the framework of Elliott's SU(3) model, with special emphasis on effects due to the presence of intruder orbit 1S1/2 in the region. The model space includes SU(3) basis states that maximize the quadrupole-quadrupole interaction (-chiQ.Q). An extended model space includes, in addition, a set of 2p-2h excitations, with excited nucleons occupying lowest energy SU(3) states in harmonic oscillator shell N=2. Group theoretical methods have been used to classify the states and calculate the nuclear interaction matrix elements. Good angular momentum states are projected out from intrinsic deformed SU(3) states by using standard angular momentum projection techniques. The interaction used contains the monopole-monopole, quadrupole-quadrupole and isospin dependent terms. Interaction parameters are fixed so as to reproduce the binding of 4 nucleons in N=1 orbit for the N=Z isotope-8Be in ground state and first excited 2+ state, along with the known systematics of single neutron separation energies for Be isotopes. The calculated energy espectra are compared with available experimental data. The calculated excitation energies of intruder states in 8Be and 10Be, are discussed in the light of available experimental evidence and theoretical results from shell model as well as deformed oscillator model calculations of Fayache et al..

Resonance structure of 9Be and 10Be in a microscopic cluster model

Structure of the excited states in 9Be and 10Be have been theoretically explored by means of the α+α+n and α+α+n+n microscopic cluster model respectively. Resonance excited states in 9Be and 10Be are localized by solving the two‐body scattering problems of {8Be(0+, 2+, 4+)+n, 5He(3/2−, 1/2−)+α} and {9Be(3/2−, 1/2+, 5/2−)+n, 6He(0+, 2+)+α} respectively. Our models have reproduced satisfactorily the experimental low‐energy spectrum in 9Be and given various resonance excited states in 10Be. Our model shows that the two and three competing configurations are quite essential to reproduce the anomalous 1/2+ state in 9Be and the second 0+ state in 10Be respectively. One of these configurations has a strong core(8Be) distortion which is induced by the valence neutron and is responsible for lowering the s1/2 orbit in these states. Another configuration produces a spatially extended neutron distribution outside the core like the neutron halo.

Large asymmetry in the strongest β-transition for A=9

A new measurement of the β-decay of 9Li has clarified the feeding to the highest accessible states in 9Be. It is found that the β-decay mainly populates the 11.8 MeV state, whose spin is determined as 5/2−. The extracted BGT value of 5.3(0.9) is a factor 4.4(1.0) larger than that of the mirror transition from 9C. A theoretical explanation of such a pronounced β-decay asymmetry is presently lacking.

4Hedecay of excited states in14C

A study of the 7Li(9Be,4He 10Be)2H reaction at E{beam}=70 MeV has been performed using resonant particle spectroscopy techniques and provides the first measurements of alpha-decaying states in 14C. Excited states are observed at 14.7, 15.5, 16.4, 18.5, 19.8, 20.6, 21.4, 22.4 and 24.0 MeV. The experimental technique was able to resolve decays to the various particle bound states in 10Be, and provides evidence for the preferential decay of the high energy excited states into states in 10Be at ~6 MeV. The decay processes are used to indicate the possible cluster structure of the 14C excited states.