Helium clustering in neutron-rich Be isotopes

Measurements of the helium-cluster breakup and neutron removal cross-sections for neutron-rich Be isotopes ^10–12,14Be are presented. These have been studied in the 30 to 42 MeV/nucleon energy range where reaction measurements are proposed to be sensitive to the cluster content of the ground-state wave-function. These measurements provide a comprehensive survey of the decay processes of the Be isotopes by which the valence neutrons are removed revealing the underlying – core-cluster structure. The measurements indicate that clustering in the Be isotopes remains important up to the drip-line nucleus ¹⁴Be and that the dominant helium-cluster structure in the neutron-rich Be isotopes corresponds to –Xn–.     

Mass measurement of neutron-rich isotopes from51Ca to72Ni

The ground state masses of thirty-nine neutronrich nuclei from⁵¹Ca to⁷²Ni have been measured using the Time-of-Flight Isochronous (TOFI) spectrometer. Eight of these masses have been measured for the first time and thirty-one are remeasurements of neutron-rich nuclei previously reported. Good agreement between these results and a previous TOFI experiment was observed except for the most neutron-rich isotopes of vanadium through iron with the present results being more bound and in better agreement with theory. The low binding energy of⁶⁸Ni, as indicated by an unreasonably low two-neutron separation energy, suggests the presence of a high-lying, long-lived isomeric state in this nucleus.     

Fission decay properties of ultra neutron-rich uranium isotopes

The fission decay of highly neutron-rich uranium isotopes is investigated which shows interesting new features in the barrier properties and neutron emission characteristics in the fission process. ²³³U and ²³⁵U are the nuclei in the actinide region in the beta stability valley which are thermally fissile and have been mainly used in reactors for power generation. The possibility of occurrence of thermally fissile members in the chain of neutron-rich uranium isotopes is examined here. The neutron number N = 162 or 164 has been predicted to be magic in numerous theoretical studies carried out over the years. The series of uranium isotopes around it with N = 154–172 are identified to be thermally fissile on the basis of the fission barrier and neutron separation energy systematics; a manifestation of the close shell nature of N = 162 (or 164). We consider here the thermal neutron fission of a typical representative ²⁴⁹U nucleus in the highly neutron-rich region. Semiempirical study of fission barrier height and width shows that ²⁵⁰U nucleus is stable against spontaneous fission due to increase in barrier width arising out of excess neutrons. On the basis of the calculation of the probability of fragment mass yields and the microscopic study in relativistic mean field theory, this nucleus is shown to undergo exotic decay mode of thermal neutron fission (multi-fragmentation fission) whereby a number of prompt scission neutrons are expected to be simultaneously released along with the two heavy fission fragments. Such properties will have important implications in stellar evolution involving r-process nucleosynthesis.      

Study of neutron-rich Mo isotopes by the projected shell model approach

The projected shell model (PSM) calculations have been performed for the neutron-rich even–even ^102?110Mo and odd—even ^103?109Mo isotopes. The present calculation reproduces the available experimental data on the yrast bands. In case of even–even nuclei, the structure of yrast bands is analysed and electromagnetic quantities are compared with the available experimental data. The g-factors have been predicted for high spin states. For the odd-neutron nuclei, the structures of yrast positive- and negative-parity bands are analysed and found to be in reasonable agreement with the experiments for ^103?107Mo. The disagreement of the calculated and observed plots for energy staggering quantity clearly establishes the occurrence of sizable triaxiality in ^103,105Mo and also predicts a decrease in the quantum of triaxiality with increasing neutron number and angular momentum for odd mass neutron-rich Mo isotopes.     

Some regularities in the production of isotopes with 4 ≦ Z ≦ 9 in the interaction of 22Ne with 232Th

In the bombardment of ²³²Th with ²²Ne ions with an energy of 172 MeV the energy spectra and production cross sectionslfor isotopes of elements ranging ifrom Be to F have been measured at an emission angle of 12°. It is shown that all of the isotopes detected have been produced by deep inelastic collisions of the initial nuclei, i.e. the kinetic energies of the reaction products are close to the exit Coulomb barriers. It is found that the energy spectra widths (FWHM), relative yields at 12° and 40° and the Q_gg dependences of isotopic production cross sections differ considerably for stable, neutron-deficient and neutron-rich isotopes. This difference can be interpreted as being due to a contribution from secondary processes such as α-particle and nucléon evaporation from the excited ²²Ne and light transfer reaction products. The data obtained can be employed to choose optimal conditions for the detection of the extremely neutron-rich isotopes of light elements produced in multinucleon transfer reactions.     

Decay of neutron-rich Ga isotopes near N = 50 at PARRNe

The PARRNe facility has been used to produce neutron-rich isotopes ^83,84Gaby the ISOL method. Their decay has been studied, and β-γ coincidence and γ-γ coincidence data were collected as a function of time. The first two excited levels in ⁸³Ge and the first excited level in ⁸⁴Ge have been measured for the first time.     

Impact of Tensor Interaction on Beta-Delayed Neutron Emission from Neutron-Rich Nickel Isotopes

The Skyrme energy density functional including tensor interaction is used to describe microscopically multineutron delayed-neutron emission accompanying beta decay of even–even neutron-rich nickel isotopes of mass number in the range of A = 74?80. The respective calculations are performed in the quasiparticle random-phase approximation with allowance for the two-phonon components of the wave function for states of the daughter nucleus. The properties of the lowest quadrupole excitation of ^74,76,78,80Ni are also studied. It is shown that a decrease in the strength of neutron–proton tensor interaction leads to a substantial hindrance of beta decay and to an increase in the probability for delayedneutron emission.     

The beta-decay scheme of <Superscript>232</Superscript>Fr and the K = 0 ground-state band in <Superscript>232</Superscript>Ra

The beta-decay of ²³²Fr to excited states in ²³²Ra has been studied using gamma-gamma coincidence detection combined with the isotope separator on-line technique at the ISOLDE PSB facility at CERN. Earlier findings are confirmed and three new gamma lines are reported. In addition to the beta-decay characteristics of ²³²Fr, the K = 0 ground-state band in ²³²Ra is identified. A yield survey of neutron-rich Fr isotopes, important also for the EURISOL project, is incorporated.Received: 20 April 2004, Revised: 12 May 2004, Published online: 13 July 2004PACS: 21.10.-k Properties of nuclei; nuclear energy levels - 23.20.-g Electromagnetic transitions - 28.60. + s Isotope separation and enrichment - 29.25.Rm Sources of radioactive nuclei     

Experimental indication of the onset of nuclear deformation in neutron-rich Sr isotopes at mass 98

A partial decay scheme for 0.1 sec ⁹⁸Rb has been deduced from γ-singles, γ-multispectra ana γγ coincidence measurements taken at the OSTIS mass-separator system at the Institut Laue-Langevin. The low-lying levels of ⁹⁸Sr follow the energy level systematics of the even-AN = 60 isotones. The onset of nuclear deformation in even-A neutron-rich Sr isotopes appears to occur at mass 98, as the first 2⁺ level energy drops from the nearly constant value of about 800 keV for masses 90–96 to 144 keV at mass 98. Energy level systematics indicate that a transition in the nuclear structures of the more neutron-rich nuclei near mass 100 occurs rather sharply at neutron number N = 60.     

Structure analysis of 159Sm and properties of odd-mass neutron-rich nuclei in mass-160 region

Recent fission experiment data provide interesting structure information for neutron-rich nuclei in the mass A ∼ 160 region. We apply the projected shell model to study the strongly-deformed, neutron-rich Sm isotopes. We perform calculations for rotational bands up to spin I = 20 (29/2) for even-even (odd-neutron) Sm isotopes, and analyze the band structure of low-lying states with quasiparticle excitations. Emphasis is given to rotational bands based on one-quasiparticle (1-qp) configurations in the odd-mass ¹⁵⁹Sm. The ¹⁵⁹Sm result is discussed together with those of the even-even isotopes ^158,160Sm. New bands in ¹⁵⁹Sm based on neutron 1-qp 1/2⁻ and 5/2⁺ configurations are predicted. Electromagnetic transition probabilities are discussed.

     

Calculation of beta-decay properties of neutron-rich nickel isotopes

Gamow-Tellerβ-decay properties of neutron-rich Ni isotopes have been calculated in the RPA with Nilsson model wave functions. On the basis of the results for the known isotopes^67–69Ni, half-lives and neutron-emission probabilities for^70–82Ni are predicted and compared to earlier model estimates. Possible implications onr-process nucleosynthesis are discussed.     

Two-electron self-energy contribution to the ground-state energy of helium-like ions

The two-electron self-energy contribution to the ground-state energy of helium-like ions is calculated both for a point nucleus and an extended nucleus in a wide interval of Z. All the two-electron contributions are compiled to obtain most accurate values for the two-electron part of the ground-state energy of helium-like ions in the range Z = 20–100. The theoretical value of the ground-state energy of ²³⁸U⁹⁰⁺, based on currently available theory, is evaluated to be −261382.9(8) eV, without higher-order one-electron QED corrections.     

Properties of lead isotopes in the vicinity of the neutron drip line

On the basis of the Hartree-Fock method with Skyrme forces of the Ska, SkM*, and Sly4 type, the position of the neutron drip line and the properties of neutron-rich lead isotopes are studied with allowance for deformations. It is shown that, in extremely neutron-rich nuclei, the neutron and proton density distributions are characterized by an anomalously large deformation parameter of β ～ 0.6. Also, nuclei of superdeformed lead isotopes have anomalously large root-mean-square radii. The existence of the isotopes ^266–288Pb, which are stable to the emission of one neutron, is predicted.     

Mass splittings of nuclear isotopes in chiral solition approach

The differences in the masses of isotopes with atomic numbers between ～10 and ～30 can be described within the chiral soliton model in satisfactory agreement with data. The rescaling of the model is necessary for this purpose—a decrease in the Skyrme constant by ～30%, providing the “nuclear variant” of the model. The asymmetric term in the Weizsäcker-Bethe-Bacher mass formula for nuclei can be obtained as the isospin-dependent quantum correction to the nucleus energy. Some predictions of the binding energies of neutron-rich isotopes are made in this way from, e.g., ¹⁶Be, ¹⁹B to ³¹Ne or ³²Na. The neutron-rich nuclides with high values of isospin are unstable relative to decay owing to strong interactions. The SK4 (Skyrme) variant of the model, as well as the SK6 variant (sixth-order term in the derivatives of the chiral field in the Lagrangian as soliton stabilizer), is considered; the rational-map approximation is used to describe multi-Skyrmions.     

Laser spectroscopy measurements of neutron-rich tellurium isotopes by COMPLIS

Laser spectroscopy based on resonant ionization of laser-desorbed atoms has been used to study the neutron-rich tellurium isotopes with the COMPLIS facility at ISOLDE-CERN. Isotope shifts and hyperfine structures of several neutron-rich Te isotopes: ^120?136Te and ^{123 m?133 m}Te have been measured. From the hyperfine structure we have extracted magnetic and quadrupole moments. Changes in the mean square charge radii have been deduced and their comparison with the known data for the other elements near Z?=?50 is presented. The experimental δ?<?r ²> values are compared with those obtained from relativistic mean field calculations.     

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     

High-spin states following multi-nucleon transfer

High-spin states in neutron-rich nuclei, populated following deep-inelastic multi-nucleon transfer, have been studied using the GAMMASPHERE array at the LBNL, USA. A ⁶⁴Ni beam at an energy ∼ 15% above the Coulomb barrier was incident upon a thick ²⁰⁸Pb target, leading to the population of more than 130 different nuclei. The strongest channels correspond to nuclei close to the projectile and target, although transfer of up to 50 nucleons has been observed. New high-spin states in neutron-rich ^60,62Fe and ^68,70,72Zn nuclei have been observed. Some limitations of this method of high-spin spectroscopy are discussed, including the apparent difficulty of populating odd-odd and odd-even isotopes via this type of reaction. The data have been searched for superdeformed (SD) states in the A = 190-200 region, but no evidence for their presence has been found. Received: 14 April 2000 / Accepted: 12 September 2000     

Ground-state properties of closed-shell nucleus 56Ni with realistic nucleon–nucleon interactions

We have calculated the ground-state energy of the doubly magic nucleus ⁵⁶Ni within the framework of the Green’s function using the CD-Bonn and N³LO nucleon–nucleon potentials. For the sake of comparison, the same calculations are performed using the Brueckner–Hartree–Fock approximation. Both the continuous and conventional choices of single particle energies are used. Additional binding energy is obtained from the inclusion of the hole–hole scattering term within the framework of the Green function approach. In this study, comparison of the calculated ground-state energies, obtained by using the Brueckner–Hartree–Fock approach using continuous choice and different nucleon–nucleon potentials, with the experimental value is accomplished. The results show good agreement between the calculated values and the experimental one for the ⁵⁶Ni nucleus. The sensitivity of our results to the choice of the model space is examined.     

Search for superheavy hydrogen isotopes in pion absorption reactions

Review of the experimental results on the search for and the spectroscopy of the superheavy hydrogen isotopes ^4–7H obtained in stopped π^?-meson absorption by ⁹Be and ¹¹Be nuclei is presented. Study of light neutron-rich nuclei is a principal line in developing modern views on the properties of nuclear forces and determination of nuclear properties near the drip line. The present method of investigation relies on precision measurements of energy of charged particles emitted after pion absorption by nuclei. Important advantages of this method are the practically accurate initial state energy and momentum, as well as the possibility to study a wide range of excitation energy. In the frames of this method new results were obtained on level structures of the isotopes ^4–6H and indications on ⁷H production. Comparison with other experimental and theoretical results is performed.     

On the Possibility of Delayed Fission of Nuclei in the Region of Superheavy Transuranium Elements

Delayed fission of atomic nuclei was discovered in 1966. It is observed primarily in odd–odd nuclei for which the energy released in beta decay (K capture) is commensurate with the fission barrier in the nucleus formed after this process. Delayed fission was found in four nuclide regions: neutrondeficient isotopes in the Pb region, neutron-deficient isotopes in the Ac and Pa regions, and neutrondeficient and neutron-rich isotopes of transuranium elements. In the wake of investigations into the properties of isotopes of superheavy transuranium elements, numerous calculations were performed in order to determine the masses of new nuclei and to predict their decay properties. Explored and predicted properties of superheavy-element nuclides, where, for some odd–odd nuclei of transuranium elements, the K-capture energy is commensurate with the fission barriers in the corresponding daughter nuclei formed after K capture, are analyzed. Estimates of the delayed-fission probability are presented for some isotopes of elements whose charge number Z ranges from 103 to 107.