Inelastic scattering on 12Be and disappearance of the N = 8 magic number

Experimental studies on in-beam γ-ray spectroscopy using a ¹²Be radioactive beam are presented. Inelastic scattering of the neutron-rich ¹²Be nucleus on ²⁰⁸Pb, ¹²C, and (CH₂)_n targets has been studied by measuring de-excitation γ-rays in coincidence with scattered particles. The level schemes and transition probabilities are determined for low-lying excited states in ¹²Be. The present paper presents a brief review of the spectroscopic results, which may be associated with the N = 8 shell quenching near the drip line. Received: 1 May 2001 / Accepted: 4 December 2001     

Direct evidence for the breakdown of the N = 8 shell closure in 12Be

We derive the formalism to obtain spatial distributions of collisional correlation times for macroscopic particles undergoing granular flow from pulsed gradient spin echo nuclear magnetic resonance diffusion data. This is demonstrated with an example of axial motion in the shear flow regime of a 3D granular flow in a horizontal rotating cylinder at one rotation rate.     

Mechanism of the 12C(11B,15N)8Be reaction and 8Be + 15N optical-model potential

Angular distributions of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction were measured at the energy E_lab(¹¹B) = 49 MeV for the transitions to the ground and 2.94 MeV (2⁺) excited state of ⁸Be and to the ground and 5.270 MeV (5/2⁺) + 5.299 MeV (1/2⁺), 6.324 MeV (3/2^-), 7.155 MeV (5/2⁺) + 7.301 MeV (3/2⁺), 7.567 MeV (7/2⁺) excited states of ¹⁵N. The data were analyzed by the coupled-reaction-channel method. The elastic, inelastic scattering and one- and two-step transfers were included in the coupling scheme. The data of the ¹²C( ¹¹B, ⁸Be) ¹⁵N reaction at E_cm = 9.4-17.8 MeV known from the literature, were also included in the analysis. The mechanism of the ¹²C( ¹¹B, ¹⁵N) ⁸Be reaction and the optical-model potential parameters for the ¹⁵N + ⁸Be channel were deduced. The energy dependence of the optical-model parameters for the ¹⁵N + ⁸Be channel was obtained.     

Cancellations beyond finiteness in N=8 supergravity at three loops

We construct the three-loop four-point amplitude of N=8 supergravity using the unitarity method. The amplitude is ultraviolet finite in four dimensions. Novel cancellations, not predicted by traditional superspace power-counting arguments, render its degree of divergence in D dimensions no worse than that of N=4 super-Yang-Mills theory--a finite theory in four dimensions. Similar cancellations can be identified at all loop orders in certain unitarity cuts, suggesting that N=8 supergravity may be a perturbatively finite theory of quantum gravity.     

Shape coexistence and intruder states: theZ=82 region

The phenomena associated with shape coexistence and intruder states are illustrated with examples from the Pt, Au, Hg, Tλ, Pb and Bi isotopes which lie far from beta stability midway between closed neutron shells 82 and 126. The type of spectroscopic measurements which have already been developed to investigate these structures as well as those types of measurements needed to extend and quantify the analysis, such as on-line nuclear orientation, are discussed.     

Structure studies of 11Be and 12Be: Observation of molecular rotational bands

Excited states of ¹¹Be have been studied with several transfer reactions. Nine states between 3.96 and 25.0 MeV excitation energy show the characteristic energy dependence of a rotational band. The deduced large moment of inertia is consistent with a 2α structure with large deformation. In ¹²Be four high-lying states observed at 7.3, 10.7, 14.6, and 21.7 MeV in the reaction ⁹Be(¹⁵N,¹²N)¹²Be, also form a rotational band with almost the same moment of inertia; tentative spin assignments of 2⁺–8⁺ are used.     

Gauged N = 8 supergravity in five dimensions

We construct gauged N = 8 supergravity theories in five dimensions. Instead of the twenty-seven vector fields of the ungauged theory, the gauged theories contain fifteen vector fields and twelve second-rank antisymmetric tensor fields satisfying self-dual field equations. The fifteen vector fields can be used to gauge any of the fifteen-dimensional semisimple subgroups of SL(6,R), specially SO(p, 6?p) for p = 0, 1, 2, 3. The gauged theories also have a physical global SU(1,1) symmetry which survives from the E₆₍₆₎ symmetry of the ungauged theory. This SU(1,1) for the SO(6) gauging is presumably related to that of the chiral N = 2 theory in ten dimensions. In our formalism we maintain a composite local USp(8) symmetry analogous to SU(8) in four dimensions.     

Resonant effects in the12C+12C→8Be gs +16O gs reaction aroundE cm=32.5 MeV

The¹²C+¹²C→⁸Be _gs +1¹⁶O _gs reaction has been studied at c.m. energies of 27.9, 32.5 and 35.0 MeV. The energy dependence of the cross section is consistent with a resonant behaviour like the one observed for two¹²C nuclei excited in a very deformed state in the final channel. The angular distributions for the two reactions show also a similar oscillatory behaviour. The analysis suggests that the reaction proceeds through the formation of an intermediate state with a complex nature, involving largely deformed configurations as well as almost spherical ones.     

The N = Z nucleus 76 Sr: Gamow-Teller strength and nuclear deformation

Challenged by recent theoretical work, a study of the -EC decay of the N = Z even-even nucleus ⁷⁶ Sr has been undertaken in order to assign the parent ground-state deformation. In this study 13 levels with have been located in ⁷⁶ Rb. The beta-delayed proton emission has been observed for the first time in an N = Z even-even nucleus. The p branching ratio has been determined to be . The ⁷⁶ Sr half-life has been remeasured and a more precise value of s has been obtained. On the basis of the comparison of measured Gamow-Teller strength with calculated strength distributions for both oblate and prolate deformation, it is concluded that the ⁷⁶ Sr has strong prolate deformation in the ground state.Received: 26 June 2003, Revised: 5 November 2003, Published online: 18 June 2004PACS: 21.10.Hw Spin, parity, and isobaric spin - 23.20.Lv transitions and level energies - 23.40.Hc Relation with nuclear matrix elements and nuclear structure - 27.50. + e J. Giovinazzo: Present address: Centre d Etudes Nucléaires de Bordeaux-Gradignan, Le Haut Vigneau, F-33175 Gradignan Cedex, France.M. Ramdhane: Present address: University Mentouri, 25000 Constantine, Algeria.     

Radiative width of the first T = 2 state in 8Be

The lowest T = 2 state (J^π = 0⁺) in ⁸Be was observed as a resonance at E_d = 6962.8 ± 3.0 keV (E_X = 27495.8 ± 2.4 keV, λ_c.m. = 5.5 ± 2.0 keV) in the ⁶Li(d, γ)⁸Be reaction. The resonant capture yield to the 17.6 MeV J^π = 1⁺, T = 1 state gives λ_γ = 23 ± 4 eV (1.14 ± 0.20 W.u.), in good agreement with the intermediate coupling model.     

Structure of neutron-rich Ar isotopes beyond N = 28

Results from the gamma-ray spectroscopy of {47,48}Ar exemplifying new limits of sensitivity for characterizing neutron-rich nuclei at energies around the Coulomb barrier are presented. The present results, along with interacting shell model calculations, highlight the role of cross-shell excitations and indicate the presence of a nonaxial deformation in 48Ar.     

The reaction 14N(n,t)12C at En = 14.4 MeV

Triton spectra from the ¹⁴N(n, t)¹²C reaction at E_n = 14.4 MeV have been measured by the counter telescope technique. The angular distributions of tritons for the transitions to the ground and first excited state of ¹²C have been compared with the theories of Butler and Newns. The analysis indicates that L = 2 angular momentum transfer is favoured over L = 0 transfer, which is also allowed. The integrated cross section for the transition to the first excited state is larger than that to the ground state by a factor of 3.6. The plane wave analysis indicates that the process involves a direct transfer of a n-p pair.     

Properties of8Be and12C deduced from the folding-potential model

Theα-α differential cross sections are analyzed in the optical model using a double-folded potential. With the knowledge of this potential bound and resonance-state properties ofα-cluster states in⁸Be and¹²C as well as astrophysical S-factors of⁴He(α,γ)⁸Be and⁸Be(α,γ)¹²C are calculated. Γ_γ-widths and B(E2)-values are deduced.