INTERNATIONAL SYMPOSIUM ON EXOTIC NUCLEI
912(2007); http://dx.doi.org/10.1063/1.2746575View Description Hide Description
Investigation of unbound nuclear systems 9He, 7H and 4n was performed at GANIL‐SPIRAL using the 8He beam at 15.3 A MeV and a CD2 target. The missing mass spectra were deduced from kinetic energies and emission angles of light ejectiles detected by the Silicon array MUST. In addition to previously known low‐lying narrow resonant states in 9He, the d(8He,p) reaction displays a structure just above neutron emission threshold, identified with the “true” ground state of 9He. The analysis of angular distributions shows that the inversion of s1/2 and p1/2 neutron shells previously observed in 11Be and 10Li also exists in the lightest N=7 isotone 9He. The d(8He,3He) and d(8He,6Li) reactions were used to search for resonant states in the t+4n (7H) and 4n systems, respectively. The missing mass spectrum of the 4n system does not give evidence for the existence of a bound “tetraneutron”. However the comparison with the results of 5‐body phase‐space calculations emphasizes the existence of correlations in the 4n system. The broad structure observed at ≃2 MeV above the t+4n emission threshold is proposed to be the ground state of 7H.
912(2007); http://dx.doi.org/10.1063/1.2746576View Description Hide Description
Recent experimental and theoretical advances have been achieved in the field of multinucleon transfer reactions among heavy‐ions. Detailed A,Z and Q‐value distributions allow to identify the relevant degrees of freedom acting in the transfer process. The advent of the large solid angle magnetic spectrometer PRISMA coupled to the large γ‐array CLARA was a major experimental breakthrough, as reaction mechanism and nuclear structure studies of neutron‐rich nuclei can be performed via γ‐particle coincidences. Examples of recent studies, mainly focused on the reaction mechanism, are presented.
912(2007); http://dx.doi.org/10.1063/1.2746577View Description Hide Description
The 7H resonance was produced via one‐proton transfer reaction between a 8He beam at 15.4A MeV and a 12C gas target. The experimental setup was based on the active‐target MAYA which allowed a complete reconstruction of the reaction kinematics. The characterization of the identified 7H events resulted in a resonance energy of 600 keV above the 3H+4n threshold and a resonance width of 100 keV. This study represents the first unambiguous proof of the existence of the 7H state.
912(2007); http://dx.doi.org/10.1063/1.2746578View Description Hide Description
Secondary 8He beam delivered by the fragment separator ACCULINNA was used in an experiment designed for the simultaneous study of the 7H and 9He exotic nuclei. Attempt to observe 7H produced in the reaction 2H(8He,3He)7H resulted only in setting a cross section limit, dσ/dΩ < 20 μb/sr, for the reaction channel which could populate a resonance lying between 0 and 3 MeV above the 7H decay threshold. The spectrum of 9He was studied by means of the 2H(8He,p)9He reaction. Energy and angular correlations were obtained for the 9He decay products by complete kinematical reconstruction. The lowest resonant state of 9He is found at about 2 MeV with a width of ∼2 MeV and is identified as 1/2−. The observed angular correlation pattern is uniquely explained by the interference of the 1/2− resonance with a virtual state 1/2+ (a limit of a0 > −20 fm on the scattering length is obtained), and with a 5/2+ resonance located at ∼4.2 MeV.
912(2007); http://dx.doi.org/10.1063/1.2746579View Description Hide Description
The 4He(6He,2α)2n reaction was studied in experiments designed for the observation of a quasi‐free scattering (QFS) occurring between the α core of the Borromean 6He nuclei and the 4He target nuclei. The helium gas target cooled down to 14 K was bombarded by a 25A MeV secondary beam of 6He nuclei produced at the fragment separator ACCULINNA (Dubna). The data were analyzed with the use of the Plane Wave Impulse Approximation (PWIA) and the three‐body wave function of 6He and taking into account the n − n final state interaction. Correlation obtained in the reaction exit channel between the momentum values characteristic to the α‐2n and n‐n subsystems were found to be consistent with the PWIA predictions. This suggests that QFS could be suitable for the study of other Borromean nuclei.
912(2007); http://dx.doi.org/10.1063/1.2746580View Description Hide Description
Excitation functions have been measured for evaporation residues from compound nuclei formed in the interaction of 6He with 197 Au and 206 Pb. The transfer reactions leading to 194 Au, 196 Au and 198 Au were also studied for the case of 6He + 197 Au. The experiments were performed at the JINR accelerator complex for radioactive beams DRIBs, capable of providing 6He beams of about 10 AMeV maximum energy and an intensity of up to 2×107 pps. The stacked foil activation technique was used directly in the beam extracted from the cyclotron or in the focal plane of the magnetic spectrometer MSP‐144. The identification of the reaction products was done by their radioactive γ‐ or α‐decay. The fusion reactions with the evaporation of two neutrons were characterized by an increase in the cross section compared to statistical model calculations. The analysis of the data in the framework of the statistical model for the decay of excited nuclei when the sequential fusion of 6He was taken into account has shown good agreement between the experimental and the calculated values of the cross sections for the case of sub‐Coulomb‐barrier fusion in the 206Pb+6He reaction. At the Coulomb barrier unusually high cross section was measured for the production of 198 Au, whereas only an upper limit was determined for the formation of 199 Au. Possible mechanisms of formation and decay of transfer reaction products are discussed.
912(2007); http://dx.doi.org/10.1063/1.2746581View Description Hide Description
Near‐barrier fusion of neutron reach nuclei was studied within the semi‐empirical channel coupling model of intermediate neutron rearrangement and within the time dependent three‐body Schrödinger equation. A possibility of neutron transfer with positive Q‐values considerably increases the barrier penetrability. A huge enhancement of deep sub‐barrier fusion probability was found for light neutron reach weakly bound nuclei (such as 6He) that may be extremely important for astrophysical primordial and supernova nucleosynthesis.
912(2007); http://dx.doi.org/10.1063/1.2746582View Description Hide Description
Recent studies of clustering in light nuclei with an initial energy above 1 A GeV in nuclear treack emulsion are overviewed. The results of investigations of the relativistic 9Be nuclei fragmentation in emulsion, which entails the production of He fragments, are presented. It is shown that most precise angular measurements provided by this technique play a crucial role in the restoration of the excitation spectrum of the α particle sysytem. In peripheral interactions 9Be nuclei are dissociated practically totally through the 0+ and 2+ states of the 8Be nucleus.
The results of investigations of the dissociation of a 14N nucleus of momentum 2.86 A GeV/c in emulsion are presented as example of more complicated system. The momentum and correlation characteristics of α particles for the 14N→3α + X channel in the laboratory system and the rest systems of 3α particles were considered in detail.
Topology of charged fragments produced in peripheral relativistic dissociation of radioactive 8B, 7Be nuclei in emulsion is studied.
912(2007); http://dx.doi.org/10.1063/1.2746583View Description Hide Description
We have extracted the ground state to ground state neutron spectroscopic factors for 80 nuclei ranging in Z from 3 to 24 by analyzing the past measurements of the angular distributions from (d,p) and (p,d) reactions. We demonstrate an approach that provides systematic and consistent values with a minimum of assumptions. A three‐body model with global optical potentials and standard geometry of n‐potential is applied. For the 60 nuclei where modern shell model calculations are available, such analysis reproduces, to within 20%, the experimental spectroscopic factors for most nuclei. If we constraint the nucleon‐target optical potential and the geometries of the bound neutron‐wave function with the modern Hartree‐Fock calculations, our deduced neutron spectroscopic factors are reduced by 30% on average.
912(2007); http://dx.doi.org/10.1063/1.2746584View Description Hide Description
Masses of nuclides covering a large area of the chart of nuclides can be measured in storage rings where many ions circulate at the same time. In this paper the recent progress in the analysis of Schottky mass spectrometry data is presented as well as the technical improvements leading to higher accuracy for isochronous mass measurements with a time‐of‐flight detector. The high sensitivity of the Schottky method down to single ions allows to measure lifetimes of nuclides by observing mother and daughter nucleus simultaneously. In this way we investigated the decay of bare and H‐like 140Pr. As we could show the lifetime can be even shortened compared to those of atomic nuclei despite of a lower number of electrons available for internal conversion or electron capture.
All these techniques will be implemented with further improvements at the storage rings of the new FAIR facility at GSI in the future.
912(2007); http://dx.doi.org/10.1063/1.2746585View Description Hide Description
The electric quadrupole moment Q for the ground state of neutron‐rich 31,32Al has been measured using β‐NMR method applied to spin‐polarized projectile fragments. The obtained quadrupole moments are smaller than those of 27,28Al, which suggests spherical shapes of 31,32Al.
912(2007); http://dx.doi.org/10.1063/1.2746586View Description Hide Description
Nuclei with equal number of protons and neutrons have many interesting properties connected with high probability of alpha‐particle correlations. However, no such nuclei heavier than 100Sn were identified until now. Meanwhile, they can demonstrate some new unusual features, e.g. the existing of alpha‐particle skin, the appearance of a new domain of cluster radioactivity and the most aspiring one, alpha‐particle Bose condensate. Current situation in the field and possible experimental approaches are discussed.
912(2007); http://dx.doi.org/10.1063/1.2746587View Description Hide Description
Detailed spectroscopic information of excited nuclear states in deformed transfermium nuclei is scarce. Most of the information available today has been obtained from investigations of fine structure α decay. Although α decay gives access to hindrance factors and lifetimes which are strongly correlated to shell/subshell closures and the presence of isomers, only the combined use of γ and conversion electron spectroscopy allows the precise determination of excitation energy, spin and parity of nuclear levels. In particular, the study of odd isotopes gives crucial information on the single particle states available at and around the Fermi surface. In the following, the preliminary results of decay studies using α ‐ γ and α ‐ beta coincidences at the focal plane of the VASSILISSA recoil separator are presented.
912(2007); http://dx.doi.org/10.1063/1.2746588View Description Hide Description
The recent developments of radioactive nuclear beams has opened up the possibility of exploring a wide variety of nuclei far from the valley of beta‐stability. Of late several theoretical studies have come up in the past for describing nuclei in these exotic region. Extension of traditional shell model techniques to explain the exotic feature in the structure of these nuclei such as, formation of neutron halo large r.m.s radii, soft dipole resonances etc have confirmed the necessity to modify the single particle shell model potential when dealing with nuclei far from stability. The ground state nuclear structure properties of 4Be11 have been calculated in the framework of shell model using analytically soluble mean field potential given by Ginocchio. The potential is highly versatile in nature and depends on four parameters, which define its depth, range and shape. Potential parameters, which generate highly diffilse shape, account for the small binding and halo structure of the valence neutron in 4Be11. The problem of ground state parity inversion is also addressed.
Rotation of Nuclei as Observed in Ternary Fission of the Reaction 235U(nth,f) Induced by Polarized Neutron912(2007); http://dx.doi.org/10.1063/1.2746589View Description Hide Description
Ternary fission of the standard reaction 235U(nth,f) induced by cold polarized neutrons has been investigated. Fission fragments and light charged particles were recorded in coincidence. Following cold neutron capture the compound nucleus 236U* has spin 3− or 4−. At the saddle point of the fissioning 236U* nucleus these states are collective. They are expected to retain a sizable collectivity down to the scission point. In fact, a collective rotation has been sensed by the shift in the angular distribution of the light charged particles which depends on the orientation of neutron polarization. Direct observation of the rotation of 236U* excited in a cold neutron reaction is reported here for the first time. It is proposed to call the new phenomenon the “ROT‐effect”.
912(2007); http://dx.doi.org/10.1063/1.2746590View Description Hide Description
In the present paper we describe an experiment performed on the reaction 238U + 4He (40 MeV) using a two‐arm TOF‐E (time‐of‐flight vs. energy) spectrometer with micro‐ channel plate detectors and mosaics of PIN diodes. This experiment has aimed at searching for fine structures in the total kinetic energy vs. fragment mass (TKE ‐ M) distributions of binary events, and disclosing rare ternary decay modes others than already known from ternary fission. Details of the procedures of data handling are briefly described as well.
912(2007); http://dx.doi.org/10.1063/1.2746591View Description Hide Description
Systematic calculations of the beta‐ and neutrino‐induced rates are performed for the nuclei at Z=92–96 approaching the possible r‐process paths in vicinity of the spherical neutron shell at N=184. The nuclear ground states are treated self‐consistently in the framework of the local energy‐density functional (DF) theory. The beta‐strength‐functions of the Gamow‐Teller and first‐forbidden decays are calculated within the continuum QRPA approach of the finite Fermi system theory. The beta‐decay half‐lives and beta‐delayed neutron emission probabilities are analyzed simultaneously. Within the same approach the weak interaction rates are calculated for the fission products around Z≈50, N=82 forming the A=130 peak in the r‐process isotopic abundance. A shell‐model study of selected Pd isotopes is performed. An analysis of available experimental data and theoretical predictions from the FRDM is also presented.
912(2007); http://dx.doi.org/10.1063/1.2746592View Description Hide Description
Nuclei properties for neutron rich exotic nuclei near the neutron number N=20, 28 (the chains of Oxygen and Magnesium isotopes), and the border of stability have been investigated in the frame of mean‐field (relativistic mean‐field and deformed Hartree‐Fock with Skyrme forces) approach. It is shown, the stability border may have a more complicated irregular structure as it is used lo think. The neutron rich systems are predicted to be very soft (isomeric stales may exist), The deformation effects are very important near the drip‐line, where the neutron‐matter distribution becomes very diffuse and of large size giving rise to “neutron skin”.
Microscopic model analysis of the 6He, 6Li+28Si total reaction cross sections at the energy range 5–50 A MeV912(2007); http://dx.doi.org/10.1063/1.2746593View Description Hide Description
The existing and some preliminary experimental data on the total cross sections of the 4,6He, 6,7Li+28Si reactions at energies E=5–50 A MeV are demonstrated. The data on 6Li,6He+28Si are analyzed in the framework of the microscopic optical potential with real and imaginary parts obtained with a help of the double‐folding procedure and by using the current models of densities of the projectile nuclei. Besides, the microscopic double‐folding Coulomb potential is calculated and its effect on cross sections is compared with that when one applies the traditional Coulomb potential of the uniform charge distribution. The semi‐microscopic potentials are constructed from both the renormalized microscopic potentials and their derivatives to take into account collective motion effect and to improve an agreement with experimental data.