Molecular-orbital structure in neutron-rich Be and C isotopes

The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In ¹⁰Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0⁺ state of ¹⁰Be has a large α-α cluster structure, and this is characterized by a (1/2⁺ _σ)² configuration. An enlargement of the α-α distance due to two-valence neutrons along the α-α axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the S _z = 0 and the S _z = 1 configurations, is important. In the ground state of ¹²Be, the calculated energy exhibits similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2^- _π)²(1/2⁺ _σ)² configuration properly stronger in comparison with the closed p-shell (3/2^- _π)²(1/2^- _π)² configuration. This is related to the breaking of the N = 8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic α+α+α+n+n+ ^{. . .} model. The combination of the valence neutrons in the π- and the σ-orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of ¹⁶C are the most promising candidates for such structure. Received: 1 May 2001 / Accepted: 4 December 2001     

Intruder states in even-even neutron-rich Pd isotopes

The decays of ¹¹²Rh ^g and ¹¹²Rh ^m obtained as on-line mass separated fission products have been investigated by standard β and γ spectroscopic techniques and γ-γ angular correlations. Low-lying 0⁺ and 2⁺ states are identified in the daughter nucleus ¹¹²Pd. Systematics suggests the existence of an intruder band which is the lowest in ¹¹⁰Pd, i.e. two neutrons before the midshell. Received:13 March 1998     

Production of neutron-rich Ca isotopes in transfer-type reactions

The production cross sections of neutron-rich isotopes ^{52, 54, 56, 58, 60}Ca in the diffusive nucleon transfer reactions ⁴⁸Ca + ¹⁹⁷Au and ⁴⁸Ca + ²³⁸U at incident energies close to the Coulomb barrier are predicted. The global trend of production cross-section with respect to the charge (mass) number of target in reactions with ⁴⁸Ca beam is analysed for the future experiments.     

The strength of octupole correlations in neutron-rich Xe isotopes

Excited levels in ¹⁴⁰Xe and ¹⁴²Xe nuclei, populated in the spontaneous fission of ²⁴⁸Cm, were studied by means of prompt γ-ray spectroscopy, using EUROGAM2 array. We report the first observation of an octupole band in ¹⁴²Xe and extend the octupole band in ¹⁴⁰Xe. Level schemes of ¹⁴⁰Xe and ¹⁴²Xe obtained in this work show patterns characteristic of ocupole-vibrational bands. Properties of octupole bands in Xe isotopes indicate that octupole correlations in these nuclei are lower than in the corresponding Ba nuclei. The electric dipole moment of ¹⁴²Xe was found to be larger than in other Xe isotopes, contrary to theoretical predictions. This may be due to the special role of the N = 88 neutron number. Received: 18 November 2002 / Accepted: 15 December 2002 / Published online: 25 February 2003 RID="a" ID="a"e-mail: urban@fuw.edu.pl Communicated by D. Schwalm     

Haloes and clustering in light, neutron-rich nuclei

Clustering is a relatively widespread phenomenom which takes on many guises across the nuclear landscape. Selected topics concerning the study of halo systems and clustering in light, neutron-rich nuclei are discussed here through illustrative examples taken from the Be isotopic chain. The production and detection of multineutron clusters is also briefly presented. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: orr@caelav.in2p3.fr     

High-spin states in neutron-rich Dy isotopes populated in deep-inelastic reactions

High-spin states in neutron-rich Dy isotopes, populated in deep-inelastic processes produced by the interaction of 234 MeV ³⁷Cl ions with a ¹⁶⁰Gd target, have been studied using the highly sensitive EUROBALL IV gamma-ray detector array. The previously known level schemes for ^{159,160,161,162}Dy have been extended to significantly higher spin ( ? 30?) and the i _13/2 band crossing in ¹⁵⁹Dy has been observed for the first time. The experimental results are discussed within the framework of cranked shell model and projected shell model calculations with particular reference to the observed delayed band crossing in ¹⁶²Dy. Received: 20 October 2000 / Accepted: 16 January 2001     

Effects of phonon-phonon coupling on low-lying states in neutron-rich Sn isotopes

Starting from an effective Skyrme interaction we present a method to take into account the coupling between one- and two-phonon terms in the wave functions of excited states. The approach is a development of a finite rank separable approximation for the quasiparticle RPA calculations proposed in our previous work. The influence of the phonon-phonon coupling on energies and transition probabilities for the low-lying quadrupole and octupole states in the neutron-rich Sn isotopes is studied.     

Direct evidence for the onset of intruder configurations in neutron-rich Ne isotopes

We report on direct experimental evidence of the population of the 3/²⁻$3/2^{-}$ intruder state in ²⁷Ne in the knockout of a single neutron from the ground state of ²⁸Ne. This low-lying negative parity state is consistent with a narrower shell gap for exotic nuclei with Z?N$Z?N$ and N≈20$N\approx 20$. Monte Carlo shell-model calculations with the modern SDPF-M interaction successfully describe neutron-rich nuclei in the vicinity of N=20$N=20$ where normal and intruder configurations coexist at low excitation energy. This observation demonstrates the importance of direct reactions for the study of exotic nuclei and the predictive power of these large-scale shell-model calculations.     

Two-quasiparticle K-isomers and pairing strengths in the neutron-rich isotopes Er and Er

Isomeric two-quasiparticle states have been identified in the neutron-rich isotopes ¹⁷²Er and ¹⁷⁴Er using multi-nucleon transfer reactions with ¹³⁶Xe beams incident on various targets, and γ  -ray spectroscopy with Gammasphere. A candidate for the Kπ=⁶⁺$K^{\pi }=6^{+}$ two-quasineutron state in ¹⁷²Er is found at 1500 keV. In ¹⁷⁴Er, a nuclide whose level scheme was previously unknown, a long-lived isomer is identified at 1112 keV decaying via an inhibited E1 transition and revealing the yrast sequence of ¹⁷⁴Er. This isomer is proposed to be a Kπ=⁸⁻$K^{\pi }=8^{-}$ two-quasineutron state, defining a sequence in the N=106$N=106$ isotones extending from the well-deformed neutron-rich isotope ¹⁷⁴Er to the neutron-deficient isotope ¹⁸⁸Pb, where the presence of the isomer signifies a prolate minimum in an otherwise spherical well. Configuration-constrained potential-energy surface calculations are used to predict the excitation energies of the 6⁺ and 8⁻ intrinsic states and as a basis for extracting the pairing force strength, Gn$G_n$, in the N=104$N=104$ and N=106$N=106$ isotones.     

Finite data-size scaling of clustering in earthquake networks

An earthquake network is known to be of the small-world type. The values of the network’s characteristics, however, depend not only on the cell size (i.e., the scale of coarse graining needed for constructing the network) but also on the size of a seismic data set. Here, discovery of a scaling law for the clustering coefficient in terms of the data size, which is referred to here as finite data-size scaling, is reported. Its universality is shown to be supported by the detailed analysis of the data taken from California, Japan and Iran. Effects of setting a threshold of magnitude are also discussed.     

How does Europe Make Its Mind Up? Connections,cliques, and compatibility between countries in the Eurovision Song Contest

We investigate the complex relationships between countries in the Eurovision Song Contest, by recasting past voting data from 1992–2003 in terms of a dynamical network. Our analysis shows that the UK is remarkably compatible, or ‘in tune’, with other European countries during the period of study. Equally surprising is our finding that some other core countries, most notably France, are significantly ‘out of tune’ with the rest of Europe during the same period. In addition, our analysis enables us to confirm a widely-held belief that there are unofficial cliques of countries; however, these cliques are not always the expected ones, nor can their existence be explained solely on the grounds of geographical proximity. The complexity in this system emerges via the group ‘self-assessment’ process, and in the absence of any central controller. One might therefore speculate that such complexity is representative of many real-world situations in which groups of ‘agents’ establish their own inter-relationships and hence ultimately decide their own fate. Possible examples include groups of individuals, societies, political groups or even governments.     

Dense mode clustering in brain maps

A mode-based clustering method is developed for identifying spatially dense clusters in brain maps. This type of clustering focuses on identifying clusters in brain maps independent of their shape or overall variance. This can be useful for both localization in terms of interpretation and for subsequent graphical analysis that might require more coherent or dense regions of interest as starting points. The method automatically does signal/noise sharpening through density mode seeking. We also discuss the problem of parameter selection with this method and propose a new method involving 2-parameter control surface, in which we show that the same cluster solution results from tradeoff of these 2 parameters (the local density k and the radius r of the spherical kernel). We benchmark the new dense mode clustering by using several artificially created data sets and brain imaging data sets from an event perception task by perturbing the data set with noise and measuring three kinds of deviation from the original cluster solution. We present benchmark results that demonstrate that the mode clustering method consistently outperforms the commonly used single-linkage clustering, k means method (centroid method) and Ward's method (variance method).     

Production of neutron-rich isotopes in fission of uranium induced by neutrons of 20 MeV average energy

In the context of a parameter study conducted by several laboratories for future European radioactive beam facilities based on fast-neutron induced fission, in particular for the SPIRAL-II project at GANIL, we have measured the yields of neutron-rich isotopes in the mass range of 88 to 144. These nuclei were obtained as fission products of natural uranium bombarded by neutrons of 20 MeV average energy emitted by a thick carbon target irradiated by 50 MeV deuterons. Yields have been measured using on-line mass separation with the ion-guide method. Compared with proton-induced fission at 25 MeV the magnitude of cross-sections, except for the symmetric region, is similar. Z-distributions of isobars have the same width, 0.7 charge units, but their maxima are shifted by about 0.8 charge units, favouring production of the neutron-richer isobars. Our data allow calculations of absolute cross-sections for fission of natural uranium induced by neutrons of about 20 MeV. Received: 10 July 2000 / Accepted: 27 October 2000     

Yields of neutron-rich isotopes around Z = 28 produced in 30 MeV proton-induced fission of 238U

Heavy ^65-70Co, ^68-74Ni, ^70-76Cu and ^74-81Ga isotopes were produced at the LISOL facility by means of 30 MeV proton-induced fission of ²³⁸U. Production rates were deduced and compared to two types of cross-section calculations: the empirical model (V. Rubchenya, private communication) and the PROFI code. Comparison with experimental data favors the latter model. Yields using different beam-target combinations and different energies are calculated and discussed. Received: 11 February 2002 / Accepted: 26 April 2002     

New neutron-rich isotopes in the scandium-to-nickel region,produced by fragmentation of a 500 MeV/u86Kr beam

We have measured production cross-sections of the new neutron-rich isotopes⁵⁸Ti,⁶¹V,⁶³Cr,⁶⁶Mn,⁶⁹Fe,⁷¹Co and neighbouring isotopes that have been identified as projectile fragments from reactions between a 500 MeV/u⁸⁶Kr beam and a beryllium target. The isotope identification was performed with the zero-degree magnetic spectrometer FRS at GSI, using in addition time-of-flight and energy-loss measurements. The experimental production cross-sections for the new nuclides and neighbouring isotopes are compared with an empirical parametrization. The resulting prospects for reaching even more neutron-rich isotopes, such as the doubly-magic nuclide⁷⁸Ni, are discussed.This work is part of the Ph.D. Thesis of M. Weber     

Molecular dynamics simulation of the pressure-induced phase transition in BaFCl

The structural phase transition in BaFCl under high-pressure up to 30 v GPa has been studied using molecular dynamics (MD) method. It was found that BaFCl transforms from a tetragonal structure to a monoclinic structure in the upstroke process and then the tetragonal structure is recovered upon releasing the pressure. The atomistic mechanisms of the transformation have been examined using the pair-correlation functions and the coordination numbers for the lattices with or without vacancies in the MD cell. It was also demonstrated that the Cl atoms between the adjacent weakly bonded Cl layers shift in the compressed direction and move to positions with a 2-fold coordination number at a nearest-neighbour site after the transition.     

Consequences of an attractive force on collective modes and dust structures in a strongly coupled dusty plasma

We present investigations of the combined effects of Debye–Hückel repulsive and overlapping Debye spheres attractive interaction potentials around charged dust particles on collective modes, phase separation and ordered structures in a strongly coupled dusty plasma. We obtain static and dynamical information via Molecular Dynamics simulations in the liquid and crystallized phases and identify the onset of an instability in the transverse mode, by using lattice summation method. The results are useful for understanding the origin of coagulation/agglomeration of charged dust particles and the formation of ordered dust structures in low-temperature laboratory and space plasmas.     

Determination of microquantities of Be in samples using α-sensitive track etch detectors

A new technique of determining Be content in samples using 4.66 MeV γ-rays is proposed. This energy range is sufficient to knock out the neutron from the ⁹Be nucleaus and to excite the first 2.9 MeV level of the ⁸Be nuclei. 1.5 MeV alpha particles are registered by means of CR-39 or cellulose nitrate detectors. Alpha particles produced in the reaction of γ-rays with Li nuclei which are contained in samples may cause the background. The latter is excluded by heating of samples before irradiation for Li molecules to be evaporated. ¹¹B nuclei do not produce alpha particles at the energy range of γ-rays presented above.The results obtained in the experiments of determining Be content in natural beryl crystal samples are in good agreement with their known chemical composition. Microparticles containing Beryllium were found in soil samples from the Ust-Kamenogorsk are deposit region, which contains 10⁻³ − 10⁻⁴ g/g of Be. The sensitivity of the method is 10⁻⁶ g/g of Be in solids.     

A molecular dynamics study of ion migration in a doped electroactive polymer

Simulating the \hbox{BF}_{4}^{-} -doped poly(3-octylthiophene) lattice by molecular dynamics results in a structure in which dopant ions intercalate as a sandwich between thiophene rings on adjacent polymer chains. The ions occupy sites in channels parallel to the polymer main chain, which retains a high degree of planarity in contrast to the pristine (undoped) polymer. Even when lattice imperfections are created by expanding the cell, Coulomb forces ensure that the intercalation features containing the dopant channels are largely retained. On applying electric fields in the principal directions of the 'perfect lattice' it is found that the ions migrate most readily along the ion-channel directions (the lattice c axis), leaving the lattice undisturbed. Although higher electric fields cause dopant migration to occur perpendicular to the channel directions they destroy the intercalated lattice. In the reduced-order lattice regions substantial motion of the ions are predicted at a critical value of the lattice parameter.