Properties of Zr isotopes near the neutron drip line and beyond it

The properties of neutron-rich Zr isotopes up to the neutron drip line and beyond it have been investigated on the basis of the Hartree-Fock method with the Skyrme forces (Ska, Sly4), taking into account the deformation. By the example of chains of Zr isotopes, good agreement is shown for the two-neutron separation energies and mean-square radii with the known results of Hartree-Fock-Bogolyubov calculations with the Sly4 forces. For the extremely neutron-rich Zr isotopes, states with very large deformation parameters (β ≈ 0.4?0.45) of neutron and proton density distributions can be realized. Beyond the neutron drip line with respect to emission of two neutrons, the existence of ^150,152Zr isotopes, which are stable with respect to one-neutron emission, is predicted.     

Broad states beyond the neutron drip line

Theoretical studies of broad states in the few-body systems beyond the neutron drip line have been performed. We introduce a theoretical model which allows to incorporate the initial structure of colliding nuclei, reaction mechanism, few-body effects and final-state interactions in studies of broad unbound states. The model is directly related to the sudden-removal approximation for high-energy knock-out or break-up reactions. We apply this model to qualitative studies of some general properties of broad few-body states including correlations for emitted fragments. The theoretical ideas are illustrated mainly using the example of ⁵H. The prospect for observation of broad continuum structures corresponding to the tetraneutron ⁴n is also discussed.Received: 9 April 2003, Revised: 21 August 2003, Published online: 26 January 2004PACS: 21.45. + v Few-body systems - 21.60.Gx Cluster models - 25.10. + s Nuclear reactions involving few-nucleon systems     

Parity violation in neutron scattering on lead isotopes

On the basis of an analysis of experimental data, it is shown that a subthreshold (negative) p-wave resonance can exist in the ²⁰⁴Pb isotope at E _p =?16 eV. The presence of this resonance may explain unexpectedly large values observed experimentally for the P-odd angle of the rotation of the spin of transversely polarized neutrons about their momentum that are scattered on a natural mixture of lead isotopes. No available data are at odds with the existence of the E _p =?16 eV subthreshold resonance in ²⁰⁴Pb.     

Exploring the neutron drip line at the Ne-Mg region

The neutron drip line in the neon-magnesium region has been explored by the projectile fragmentation of a ⁴⁸Ca(59.8 A MeV) beam using the new fragment separator LISE-2000 at GANIL. New neutron-rich isotopes, ³⁴Ne and ³⁷Na, have been observed together with some evidence for the particle instability of ³³Ne and ³⁶Na.     

E2 moments andg-factors in neutron deficient lead isotopes

The half-lives of the (vi _13/2)² I ^π=12⁺ isomers in^{190, 192}pb were measured asT _1/2 =7.9(4) μs and 1.10(5) μs, respectively. The quadrupole moment and theg-factor of the corresponding state in¹⁹⁴Pb were determined toQ=49(3) efm² andg=?0.167(2) using the perturbed angular distribution method. Additionalg-factor and lifetime information was obtained for a number of isomers in^{194, 195, 196, 197, 198, 199, 200}Pb also involvingi _13/2 neutrons. The results are discussed in comparison to a two-quasiparticle Tamm-Dancoff calculation in the full shell using the surface delta interaction.     

Cross-section prediction for isotopes near neutron drip line in ~(70, 80)Zn projectile fragmentation reactions

The cross sections for ~(59,60)Ca, recently measured in the 345 A MeV ~(70)Zn+~9 Be reaction,were estimated using the FRACS parametrization and an empirical formula,which are in good agreement.The FRACS parametrization and the empirical formula are combined to predict the cross sections for extreme calcium isotopes ~(66,70)Ca in the~(70,80)Zn+~9 Be reactions at the incident energies of ~(60,80),and 345 A MeV.The dependence of emperical formula parameters on the reaction system,as well as the incident energy,are discussed.The results indicate that ~(66,70)Ca can be discovered in reactions of ~(60,80)A MeV ~(80)Zn+~9 Be.The predicted binding energy for extreme neutron-rich isotopes by the spherical relativistic continuum Hartree-Bogoliubov theory was adopted in the calculation.Hence,the planned Beijing Isotope-Separation-On Line Neutron-Rich Beam Facility(BISOL),which is a third generation radioactive ion beam facility,could provide the opportunity to discover ~(66,70) Ca and neighboring neutron-drip line nuclei.     

Neutron-deficient and neutron-rich Fe and Ni isotopes near the drip line

Positions of the proton and neutron drip lines and characteristics of the neutron-deficient and neutron-rich Fe and Ni isotopes have been studied by the Hartree-Fock method with the Skyrme forces (Ska, SkM*, Sly4) with allowance for deformation. Pairing was taken into account on the basis of the BCS approximation with the pairing constant G = (19.5/2)[1 ± 0.51(N ? Z)/A]. Manifestation of magic numbers for the nickel isotopes ⁴⁸Ni, ⁵⁶Ni, and ⁷⁸Ni and for the nucleon-stable isotope ¹¹⁰Ni which is beyond the drip line is discussed.     

Nuclear moments and change in the charge-radii of neutron deficient lead isotopes

The nuclear moments of¹⁹⁵Pb and the change in the charge radii of^{192,194,195,196}Pb have been determined from the isotope shift and hyperfine structure of the 723 nm line of neutral lead by collinear laser spectroscopy.     

Laser spectroscopy and the properties of light nuclei near the neutron drip line

The interesting nuclear structure phenomena observed in some light nuclei at the neutron drip line suggest the measurement of basic ground state properties such as spins, magnetic moments and electric quadrupole moments. Here it will be discussed what experiments are presently feasable using laser spectroscopy. For the outstanding example of a halo nucleus,¹¹Li, the development of a technique combining -asymmetry detected optical pumping in a fast beam with NMR spectroscopy has yielded pertinent results, the most recent of which is an experimental value of the nuclear quadrupole moment.     

New magic number, N = 16, near the neutron drip line

We have surveyed the neutron separation energies (S(n)) and the interaction cross sections (sigma(I)) for the neutron-rich p-sd and the sd shell region. Very recently, both measurements reached up to the neutron drip line, or close to the drip line, for nuclei of Z/=3), which shows the creation of a new magic number. A neutron-number dependence of sigma(I) shows a large increase of sigma(I) for N = 15, which supports the new magic number. The origin of the new magic number is also discussed.     

β-delayed neutron decay of drip line nuclei 11Li and 14Be

Experimental studies of β-decay measurements of the neutron drip line nuclei ¹¹Li and ¹⁴Be are presented. β- decay schemes of these nuclei are determined by measuring β-rays, delayed neutrons and γ-rays in triple coincidence. The decay schemes of both ¹¹Li and ¹⁴Be associated with single neutron emission are unambiguously determined. New levels in their daughter nuclei, ¹¹Be and ¹⁴B, are found. In addition, the deduced level scheme of 14B indicates the lowering of 2s _1/2 single neutron orbital with respect to 1p _1/2 orbital in the N=9 isotones. Such behavior is known to exist in N=7.     

New isotopes,secondary reactions and spectroscopy for medium-mass nuclei at the proton drip line

This paper reviews two experiments performed at GSI and GANIL. Using a primary beam of ⁵⁸Ni at 650 MeV/nucleon impinging on a beryllium target, production cross sections of proton-rich fragments from projectile fragmentation have been measured at the projectile-fragment separator FRS at GSI. The production rates measured demonstrate that counting rates much higher than expected can be obtained at the proton drip line. The results from spectroscopy measurements show that no Thomas-Ehrmann effect is present in our data which means that the decay energies and masses can be well predicted by using e.g. the IMME. The secondary reactions evidence a slight increase of the interaction cross sections when approaching the proton drip line. Finally, in an experiment performed at the SISSI/LISE facility at GANIL using a ⁷⁸Kr primary beam, we have observed the 5 new isotopes ⁶⁰Ga, ⁶⁴As, ^69,70Kr, and ⁷⁴Sr. However, we have not found any evidence for ⁶⁹Br which was reported to be observed with a few counts at MSU. These new findings change our understanding of the path and of the ending point of the rp process.     

Peninsulas of the neutron stability of nuclei in the vicinity of neutron magic numbers

On the basis of the Hartree-Fock method as implemented with Skyrme forces (Ska, SkM*, Sly4, and SkI2) and with allowance for an axial deformation and nucleon pairing in the Bardeen-Cooper-Schrieffer approximation, the properties of extremely neutron-rich even-even nuclei were calculated beyond the neutron drip line known earlier from theoretical calculations. It was shown that the chains of isotopes beyond the neutron drip line that contain N = 32, 58, 82, 126, and 184 neutrons form peninsulas of nuclei stable against the emission of one neutron and, in some cases, peninsulas of nuclei stable against the emission of two neutrons. The neutron- and proton-density distributions in nuclei forming stability peninsulas were found to be spherically symmetric. A mechanism via which the stability of nuclei might be restored beyond the neutron drip line was discussed. A comparison with the results of calculations by the Hartree-Fock-Bogolyubov method was performed for long chains of sulfur and gadolinium isotopes up to the neutron drip line.     

Shape transition in the neutron rich sodium isotopes

Mass spectrometer measurements of the neutron rich sodium isotopes show a sudden increase at ³¹Na in the values of the two-neutron separation energies. The spherical shell model naturally predicts a sudden decrease at ³²Na after the N = 20 shell closure. We propose that the explanation for this disagreement lies in the fact that sodium isotopes in this mass region are strongly deformed due to the filling of negative parity orbitais from the 1f_{$\text{7}\text{2}$} shell. Hartree-Fock calculations are presented in support of this conjecture.     

Extrapolation of a self-consistent energy density mass formula to the superheavy region and to the neutron drip line

The previously developed energy density mass formula is extrapolated to the superheavy region and to the neutron drip line. Fission barriers,α-decay andβ-decay energies of isotones withN=184 are computed and half-lives are estimated by using simple corrections to the half-lives obtained by Fiset and Nix. According to the assumption made to extrapolate the pairing parameters, the most stable nucleus atN=184 is found to be either₁₈₄X ₁₁₀ ²⁹⁴ with a half-life of 67 years, or_{184 108} ²⁹² with a half-life of 13 years. The position of the neutron drip line for 8≦Z≦44 is also studied and compared to the predictions of other mass formulae. Important differences are found, particularly for the influence of the shell effects in this region.     

Thermal neutron capture in isotopes of nickel

The gamma-ray spectra associated with thermal neutron capture in targets of⁵⁸Ni,⁶⁰Ni,⁶²Ni, and⁶⁴Ni have been observed with a high-sensitivity pair-spectrometer. Level schemes for the four product nuclei are discussed, correlation of widths with neutron single-particle states examined, and estimates forM1 andE2γ-ray strength functions obtained.     

Elastic neutron transfer in the scattering of Si isotopes

Angular distributions of the elastic scattering of ²⁸Si on ²⁹Si and ³⁰Si have been measured for incident beam energies at E = 65 and 70 MeV with a time-of-flight spectrometer for heavy ions. At 70 MeV the neutron transfer ³⁰Si(²⁸Si, ²⁹Si)²⁹Si was observed in addition to the elastic channel. The pronounced oscillations in the elastic scattering distributions are interpreted as being due to an elastic transfer of neutrons between the colliding nuclei during the scattering process. This assumption is in accordance with some general features of the data and allows for the extraction of spectroscopic factors of the transferred neutrons.     

Study of neutron rich neon isotopes

The half-lives andP _n -values of the neutron rich isotopes^26–29Ne have been determined. The results are compared to shell-model calculations and good agreement is found except for²⁹Ne, where the half-life exceeds the predictions by more than an order of magnitude. This unexpectedly long half-life can be explained as due to a fp intruder configuration.     

Investigation of the properties of nuclei with extreme neutron excess in the vicinity of neutron magic numbers

Properties of even-even nuclei with extreme neutron excess in the vicinity of neutron magic numbers up to and beyond the neutron drip line (NDL) are calculated by the Hartree-Fock (HF) method using Skyrme forces (Ska, SkM*, Sly4, SkI2, SkP) with allowance for axial deformation and BCS-approximation pairing. It is shown that chains of isotones with the neutron numbers N = 32, 58, 82, 126, 184, and 258 beyond the NDL form peninsulas of nuclei stable with respect to emission of one neutron, and occasionally peninsulas of nuclei stable with respect to the emission of two neutrons. The length of these peninsulas in (N, Z) space depends on the choice of the Skyrme forces, while their locations are at the same N = 32, 58, 82, 126, 184, and 258 and do not depend on the choice of forces. The investigated isotones restore stability beyond the NDL due to the complete filling of subshells with high angular momentum and to the intrusion of corresponding neutron levels in the region of discrete bound states. The stability of the numerical solution to the HF equations for nuclei belonging to the peninsulas of stability is analyzed.