Search

We report on the progress in conceptual and mathematical solutions of the theory of nuclear geometrical symmetries -- more precisely: nuclear point-group symmetries -- obtained within the TetraNuc Collaboration in recent years. We shortly summarise the basic concepts, the strategic lines of the solutions as well as the link between theoretical and experimental considerations. [ABSTRACT FROM AUTHOR]

Odd-parity core excited states have been identified in two close neighbors of 100Sn: 96Pd and 97Ag. This was done in an fusion-evaporation experiment, using a 58Ni beam on a 45Sc target. Even-parity core excited states in these nuclei are very well reproduced in large scale (LSSM) calculations in which particle-hole excitations are allowed with up to five g9/2 protons and neutrons across the N = Z = 50 gap, to the g9/2, d5/2, d3/2, and s½ orbitals. The odd-parity states can only be qualitatively interpreted though, employing calculations in the full fpg shell model space, but with just one particle-hole core excitation allowed. A more complete model including odd-parity orbitals is need for the description of core excited states in the region of 100Sn. [ABSTRACT FROM AUTHOR]

A great deal of research work has been undertaken in the α-clustering study since the pioneering discovery, half a century ago, of 12C+12C molecular resonances. Our knowledge in the physics of nuclear molecules has increased considerably and nuclear clustering remains one of the most fruitful domains of nuclear physics, facing some of the greatest challenges and opportunities in the years ahead. In this work, the occurrence of "exotic" shapes in light N = Z α-like nuclei is investigated. Various approaches of superdeformed and hyperdeformed bands associated with quasimolecular resonant structures are presented. Results on clustering aspects are also discussed for light neutron-rich oxygen isotopes. [ABSTRACT FROM AUTHOR]

Heavy elements above Fm (Z = 100) are nuclei with vanishing liquiddrop fission barriers and are therefore entirely stabilized by quantum shell effects. Due to the large density of single-particle levels and strong polarized Coulomb fields, theoretical predictions of magic numbers are extremely model dependent. Furthermore, shell closures for one nucleon species depend strongly on the number of the other species. Reliable experimental data is needed in order to test and constrain theory. As there is a lack of such data in the region, new data is needed along with the confirmation of previous data. A detection system dedicated to the spectroscopy of transfermium nuclei was constructed in 2004 and installed at the focal plane of the VASSILISSA separator at the FLNR, Dubna, by a Franco-Russian collaboration. The results from the 2009 campaign will be presented. [ABSTRACT FROM AUTHOR]

Following a series of experiments launched by the TetraNuc collaboration to possibly demonstrate the existence of high-rank symmetries in subatomic physics, the first experimental results on the Rare Earth region appear in publications. Meanwhile an important progress has been made on the theory side strongly suggesting that the original criterion of the static tetrahedral symmetry in the form of vanishing quadrupole moments may need to be revised to include explicitly the vibrational motion. The Actinide region seems of particular interest because of the extra stability provided by the octahedral symmetry. In this article a summary of the current experimental efforts on both the Rare-Earth and Actinide regions is given. Finally the ELMA project addressing the experimental search for the symmetries in the Actinides is briefly discussed. [ABSTRACT FROM AUTHOR]

The nuclear mean-field theory and the group representation theory can be used to optimise the search for strong nuclear shell effects. The two theories allow to correlate the symmetry aspects with the presence of large gaps in the single-particle spectra, facilitate in this way the conditions of search for strong nucleonic- and nuclear-binding and thus for an increased nuclear stability. In this article we give a short overview of the related on-going research, focussing on the results of the TetraNuc Collaboration. [ABSTRACT FROM AUTHOR]

Theoretical predictions suggest the presence of tetrahedral symmetry as an explanation for the vanishing intra-band E2 transitions at the bottom of the odd-spin negative-parity band in 156Gd. The present study reports on experiment performed to address this phenomenon. It allowed to remove certain ambiguouities related to the intra-band E2 transitions in the negative-parity bands, to determine the new inter-band transitions and reduced probability ratios B(E2)/B(E1) and, for the first time, to determine the experimental uncertainties related to the latter observable. [ABSTRACT FROM AUTHOR]

We present what we call a new theory of nuclear stability enabled by the combination of the realistic nuclear mean-field and the group theory approaches. It allows us to simplify searches for the strong quantum shell effects at nuclear shapes that result from spectral properties deduced from group theory and geometrical symmetries rather than through 'brute force numerical search'. Illustrations are presented and discussed. [ABSTRACT FROM AUTHOR]

Large scale calculations based on the microscopic-macroscopic method with Woods-Saxon single particle potential guided by the use of the discrete point group symmetries allow us to find the new islands of nuclear stability. These new stability regions are the consequence of particularly strong shell effects which are obtained in the calculations when the nuclear mean field is allowed to deform by respecting some special symmetries related to the so called high-rank point groups. The underlying mechanism is illustrated together with the full chain of the symmetry-associated magic numbers. [ABSTRACT FROM AUTHOR]

The γ-decay from the warm rotation in the transition region between order and chaos is studied in the superdeformed (SD) nuclei 151Tb and 196Pb, using the EUROBALL IV array. A number of observables, testing the decay dynamics in the SD well, are compared with predictions from a Monte Carlo simulation of the γ-decay based on microscopic calculations of discrete levels and decay probabilities. Agreement with the data is found only assuming an enhancement of the B(E1) strength around 1 MeV by a factor of 10-100, which is consistent with the evidence for octupole vibrations in both mass regions. The work shows the relevance of γ-spectroscopy in the order-to-chaos regime to highlight specific nuclear structure effects. [ABSTRACT FROM AUTHOR]

The 81, 82, 83, 85Se nuclei have been produced as fission fragments in the fusion reaction 18O + 208Pb at 85MeV bombarding energy and studied with the Euroball IV array. Their high-spin level schemes have been built from the triple $ \gamma$ -ray coincidence data and $ \gamma$ - $ \gamma$ angular correlations have been analyzed in order to assign spin and parity values to many observed states. The lowest-spin states of the two-neutron and three-neutron configurations are strongly mixed with two-proton excitations among the fp orbits. On the other hand, the highest-spin states of these neutron configurations are found to remain almost pure. Neutron excitation across the N = 50 gap is observed both in 83Se49 and in 85Se51 . [ABSTRACT FROM AUTHOR]

High-spin states in the 142Ce and 140Ba nuclei have been populated in the 12C + 238U and 18O + 208Pb fusion-fission reactions at 90MeV and 85MeV bombarding energy, respectively. The emitted γ -radiation was detected using the Euroball III and IV arrays. The high-spin yrast and near-to-yrast structures of 142Ce have been considerably extended. The level scheme of 140Ba has been extended by six new levels. The newly observed structures in these N = 84 isotones are discussed by analogy with the neighbouring nuclei. [ABSTRACT FROM AUTHOR]

Independent fission fragment yields have been obtained for the six fragment pairs ( Z L/ Z H = 44/46 , 42/48, 40/50, 38/52, 36/54 and 34/56) in 208Pb(18O, f ) at the beam energy E = 85 MeV. γ -ray spectroscopy technique was applied to analyze the experimental data. With use of these data the mass distribution of fission fragments was restored. Data on average fragment spin distribution and total neutron multiplicity are presented. The agreement of the obtained results with other experimental data proves the validity of the applied method to solve this kind of problems. [ABSTRACT FROM AUTHOR]

Exotic-deformation effects in 46Ti nucleus were investigated by analysing the high-energy γ-ray and the α-particle energy spectra. One of the experiments was performed using the charged-particle multi-detector array ICARE together with a large volume (4 inx4 in) BGO detector. The study focused on simultaneous measurement of light charged particles and γ-rays in coincidence with the evaporation residues. The experimental data show a signature of very large deformations of the compound nucleus in the Jacobi transition region at the highest spins. These results are compared to data from previous experiments performed with the HECTOR array coupled to the EUROBALL array, where it was found that the GDR strength function is highly fragmented, strongly indicating a presence of nuclei with very large deformation. [ABSTRACT FROM AUTHOR]

Lifetimes of exited states in 134Pr were measured be means of the recoil distance Doppler-shift and Doppler-shift attenuation techniques. The experiments were performed at IReS, Strasbourg using the EUROBALL IV spectrometer, in conjunction with the inner BGO ball and the Cologne coincidence plunger apparatus. Exited states in 134Pr were populated in the fusion-evaporation reaction 119Sn(19F, 4n)134Pr. Reduced transition probabilities in 134Pr are compared to the predictions of the two quasiparticle+triaxial rotor and interacting boson fermion-fermion models. The experimental results do not support the presence of static chirality in 134Pr underlying the importance of shape fluctuations. Only within a dynamical context the presence of intrinsic chirality in 134Pr can be supported. [ABSTRACT FROM AUTHOR]

The 87Kr nucleus has been produced as fission fragment in the fusion reaction 18O + 208Pb at 85MeV bombarding energy and studied with the Euroball IV array. High-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 6.3MeV excitation energy and spin I ∼ 23/2ℏ. Its structure is interpreted by analogy with those of the heavier isotones. The proposed configurations involve both proton and neutron excitations from several sub-shells located close to the Fermi levels, particularly ν d 5/2, π p 3/2 f 5/2 and π g 9/2. Moreover, a revised spin value of 5/2- for the 87Br ground state is proposed. [ABSTRACT FROM AUTHOR]

The neutron-deficient nucleus68As was populated at high spin in two experiments using the reaction40Ca(32S, 3pn) at beam energies of 105 and 95 MeV. A self-supporting and a gold-backed, highly enriched40Ca target were used. Gamma rays were detected with the EUROBALL array, combined with the charged-particle detector array EUCLIDES and the Neutron Wall. The68As level scheme was considerably extended, especially at negative parity and many previous spin-parity assignments were confirmed or rejected. The total-Routhian-surface (TRS) calculations find shape coexistence and ? softness for the negative- and positive-parity states, respectively. [ABSTRACT FROM AUTHOR]

The84Se nucleus has been produced as fission fragment in the fusion reaction18O +208Pb at 85MeV bombarding energy and studied with the Euroball IV array. Medium-spin states of this neutron-rich isotope have been identified for the first time. Its level scheme has been obtained up to 4.9MeV excitation energy and spinI~ 7?. Its structure is interpreted by analogy with those of the stable heavier isotones. The evolution of the energy of theN= 50 neutron-core excitation is discussed as a function of the proton number. [ABSTRACT FROM AUTHOR]

An experiment has been performed to investigate the structure at high spin of the neutron-deficient polonium isotopes using the Recoil Filter Detector coupled to the EUROBALL IV gamma array. High spin states of [sup 200,201]Po have been populated by the [sup 30]Si+[sup 176]Yb reaction at a beam energy of 160 MeV. We report here the performances of the RFD + EUROBALL IV coupling. [ABSTRACT FROM AUTHOR]

The first experimental information about excited states in the N = Z + 1 nucleus 93Pd is presented. The experiment was performed using a 205 MeV 58Ni beam from the Vivitron accelerator at IReS, Strasbourg, impinging on a bismuth-backed 40Ca target. Gamma-rays, neutrons and charged particles emitted in the reactions were detected using the Ge detector array Euroball, the Neutron Wall liquid-scintillator array and the Euclides Si charged-particle detector system. The experimental level scheme is compared with the results of new shell model calculations which predict a coupling scheme with aligned neutron-proton pairs to greatly influence the level structure of N ≈ Z nuclei at low excitation energies. [ABSTRACT FROM AUTHOR]

Intruder rotational bands in 45Sc and 45Ti have been investigated up to the maximum aligned angular momentum by means of EUROBALL IV and the Recoil Filter Detector (RED). The use of the RED allowed for a reduction of the γ-line Doppler broadening and, moreover, for a determination of very short level lifetimes. In the studied nuclei, the estimated deformation shows a gradual disappearance of the collectivity at the highest available spins. [ABSTRACT FROM AUTHOR]

The γ-rays following the reaction 105 McV 18O | 28Si have been measured using the EUROBALL IV, HECTOR and EUCLIDES arrays in order to investigate the predicted Jacobi shape transition. The high-energy &gama;-ray spectrum from the GDR decay indicates the presence of large deformations in the hot 46Ti nucleus, in agreement with new theoretical calculations based on the rotating liquid-drop model. [ABSTRACT FROM AUTHOR]

Abstract: The level structures of the 83As, 82Ge, and 81Ga isotones have been investigated by means of multi-nucleon transfer reactions. A first experiment was performed with the CLARA–PRISMA setup to identify these nuclei. A second experiment was carried out with the GASP array in order to deduce the γ-ray coincidence information. The results obtained on the high-spin states of such nuclei are used to test the stability of the shell closure in the region of 78Ni (). The comparison of the experimental level schemes with the shell-model calculations yields an energy gap value of 4.7(3) MeV at . This value, in a good agreement with the prediction of the finite-range liquid-drop model as well as with the recent large-scale shell model calculations, does not support a weakening of the shell gap down to . [Copyright &y& Elsevier]

Abstract: High-spin states in 105Rh were populated by the 96Zr(13C, p3n) reaction at beam energies of 51 and 58 MeV, and studied using the EUROBALL IV γ-ray spectrometer and the DIAMANT charged particle array. A pair of nearly degenerate three-quasiparticle bands with the same spins and parity have been observed. Comparison of the experimental results with tilted axis cranking calculations confirms the chiral character of the two bands, while arguments based on the excitation of particles within the configuration of the yrast band and comparison with the previously observed γ band exclude the other possible interpretations. This is the first experimental evidence for three-quasiparticle chiral structure in the A ∼ 100 region, and the first simultaneous observation of a γ band and chiral partner bands in one nucleus. [Copyright &y& Elsevier]

The γ-decay properties of 24Mg excited states are investigated in the inverse reaction 24Mg+12C at Elab(24Mg) = 130 MeV. At this energy the direct inelastic scattering populates a 24Mg* energy region where 12C+12C breakup resonances can occur. Very exclusive data were collected with the Binary Reaction Spectrometer (BRS) in coincidence with EUROBALL installed at the VIVITRON Tandem facility of the IReS at Strasbourg. The experimental detection system is described and preliminary results of binary reaction coincidence data are presented. [Copyright &y& Elsevier]

The γ-ras from the decay of the GDR in the compound nucleus reaction 18O+28Si at bombarding enery of 105 MeV have been measured in an experiment using a setup consisting of the combined EUROBALL IV, HECTOR and EUCLIDES arrays. The shape of the rotating compound nucleus, 46Ti, is expected to undergo the Jacobi transition around spin 28 h̵. A comparison of the GDR lineshape data with the predictions of the thermal shape fluctuation model, based on the most recent rotating liquid drop LSD calculations, shows evidence for such Jacobi shape transition. In addition to the previously found broad structure in the GDR lineshape region at 18–25 MeV caused by large deformations, the presence of a low energy component (around 10 MeV), due to the Coriolis splitting in prolate, well deformed shape has been identified for the first time. [Copyright &y& Elsevier]