Structure Of Neutron-rich Nuclei Research Articles

Structure of neutron-rich nuclei near N=40

Recent experimental results obtained for neutron-rich nuclei of mass A~60-70 at LNL are presented together with the shell model interpretation. The development of a new region of deformation approaching the neutron number N = 40 in Cr, Fe and Co isotopes is discussed. A new effective interaction (LNPS) that considers the full fp shell model space for protons and the p3/2, p1/2, f5/2, g9/2, d5/2 orbitals for the neutrons allows to reproduce, for the first time, the different structure properties of these nuclei. The configurations of the calculated states suggest the development of a new island of inversion at N ~ 40.

Proton resonance elastic scattering in inverse kinematics on the medium heavy nucleus 68Zn

An isobaric analog resonance of 69Zn was studied by the resonance elastic scattering of p ( 68Zn , p with a 5.5MeV/nucleon 68Zn beam and a thick polyethylene target. The excitation function of the differential cross-section of proton elastic scattering was measured around 0 degrees in the laboratory frame by the thick target inverse kinematics method. The angular momentum, and proton and total widths of the resonance assigned using an R -matrix calculation are in good agreement with earlier measurements performed using normal kinematics, demonstrating that the thick target inverse kinematics method is a useful tool for studying the single-particle structures of neutron-rich nuclei.

THE HELIOS SPECTROMETER AND THE RADIOACTIVE BEAM PROGRAM AT ARGONNE

The near-term radioactive beam capabilities of ATLAS include radioactive beams produced in flight in a gas cell, or starting in the fall of 2009, re-accelerated beams of 252 Cf fission fragments provided by the new CARIBU injector. The availability of such exotic beams will allow for detailed studies of the single-particle aspects of nuclear structure in neutron-rich nuclei reaching out to the astrophysical r-process path by employing light-ion reactions in inverse kinematics. The HELIOS spectrometer is based on a new concept that is especially well suited for such studies. This concept was recently demonstrated using the reactions D (28 Si , p )29 Si with a (stable) 168 MeV 28 Si beam. Since then D (12 B , p )13 B , D (17 O , p )18 O , and D (15 C , p )16 C have been studied successfully. The combination of neutron-rich beams from CARIBU and the HELIOS spectrometer opens a fertile research area of precision studies of the single particle strengths and collective excitations in exotic nuclei, and is likely to have applications in other reactions as well.

New subshell closure atN=58emerging in neutron-rich nuclei beyondNi78

The structure of neutron-rich nuclei beyond $^{78}\mathrm{Ni}$ was studied using postaccelerated radioactive beams of $^{83,84,85}\mathrm{Ga}$ utilizing $\ensuremath{\beta} \ensuremath{\gamma}$ and $\ensuremath{\beta}\ensuremath{-}n \ensuremath{\gamma}$ spectroscopy. Our data, when combined with energy level systematics, suggests a possible new spherical subshell closure at $N=58$ is created by the nearly degenerated $\ensuremath{\nu}3{s}_{1/2}$ and $\ensuremath{\nu}2{d}_{5/2}$ orbitals being well separated from other orbitals above $N=50$. The near degeneracy of these states could be evidenced by isomerism in this region. The energies of the ${2}_{1}^{+}$ and proposed ${4}_{1}^{+}$ states observed in $N=52$ $^{84}\mathrm{Ge}$ are interpreted as an indication of a possible weakening of the doubly magic $^{78}\mathrm{Ni}$ core for nuclei beyond $N=50$. The experimental evidence is supported by spherical HFB calculations using the ${\mathrm{SkO}}_{T}$ functional involving the tensor term in the nucleon-nucleon interaction.

Structure of neutron-rich nuclei around the N = 126 closed shell; the yrast structure of 205Au126 up to spin-parity $I^{\pi}$ = (19/2+)

Heavy neutron-rich nuclei have been populated through the relativistic fragmentation of a $\ensuremath ^{208}_{\ 82}{\rm Pb}$ beam at $\ensuremath E/A = 1$ GeV on a $\ensuremath 2.5 {\rm g/cm^2}$ thick Be target. The synthesised nuclei were selected and identified in-flight using the fragment separator at GSI. Approximately 300 ns after production, the selected nuclei were implanted in an $\ensuremath \sim 8$ mm thick perspex stopper, positioned at the centre of the RISING $\ensuremath \gamma$ -ray detector spectrometer array. A previously unreported isomer with a half-life $\ensuremath T_{1/2} = 163(5)$ ns has been observed in the N = 126 closed-shell nucleus $\ensuremath ^{205}_{\ 79}{\rm Au}$ . Through $ \gamma$ -ray singles and $ \gamma$ - $ \gamma$ coincidence analysis a level scheme was established. The comparison with a shell model calculation tentatively identifies the spin-parity of the excited states, including the isomer itself, which is found to be $\ensuremath I^{\pi} = (19/2^+)$ .

(h11/2,g7/2)9−neutron excitation inSr92,94,96

Medium-spin structure of neutron rich nuclei $^{92}\mathrm{Sr}$, $^{94}\mathrm{Sr}$, and $^{96}\mathrm{Sr}$ have been studied in spontaneous fission of $^{248}\mathrm{Cm}$. New spin and parity assignments done in this work differ significantly from those reported in other works. The ${9}^{\ensuremath{-}}$ excitations, involving the $\ensuremath{\nu}({g}_{7/2}{h}_{11/2}){}_{{9}^{\ensuremath{-}}}$ maximum aligned configuration, were proposed in the three nuclei at 4930.2 keV, 4858.6 keV, and 3523.9 keV, respectively. Shell-model calculations, with the $^{78}\mathrm{Ni}$ core, support the proposed nature of the ${9}^{\ensuremath{-}}$ excitations. The new data allow testing the single-particle energies in the region of $^{78}\mathrm{Ni}$, and in particular, the position of the $\ensuremath{\nu}{h}_{11/2}$ orbital, which was not uniquely determined to date.

Disturbance of the shell structure of neutron-rich nuclei in the oxygen-magnesium region

The effects related to nuclear deformation and their influence on the shell structure and nuclear properties have been investigated. Calculations were performed within the self-consistent theory of finite Fermi systems. The possibility of existence of strongly deformed nuclei at the neutron drip line and behind it, such as 28O, 39Na, and 41–43Na, is discussed.

Gamma spectroscopy of neutron-rich nuclei with the CLARA-PRISMA setup

The CLARA-PRISMA experimental set-up combines the large acceptance magnetic spectrometer for heavy ions PRISMA with the highly efficient CLARA array of 25 Clover detectors. Selected results on the structure of neutron-rich nuclei, populated in multinucleon transfer and deep-inelastic reactions, are presented here. Some technical developments of the setup are also discussed.

New results from MINIBALL

Preliminary results from the study of collectivity and single-particle structure in neutron-rich nuclei are presented from selected experiments. The experiments discussed here employed the MINIBALL Germanium detector array for gamma-ray detection. Coulomb-excitation experiments at REX-ISOLDE focused on the collectivity of Cd and Xe isotopes near 132Sn as well as the shell structure near N = 20 , N = 40 , and N = 50 . Another experiment was performed to probe the single-particle structure of 47Ca, 55Ti, and 73Cu at the GSI Fragmentseparator, using single-nucleon knock-out reactions at relativistic energies.

Structure of unstable nuclei from nuclear moments and β decays

The structure of neutron-rich nuclei around the N = 20 magic number is studied through the measurements of static electromagnetic moments. Recent data on the magnetic dipole moments and electric quadrupole moments for neutron-rich Al isotopes are discussed to clarify whether the emergence of a strong deformation reported for Z = 10−12 nuclei in this region might happen also in these Z = 13 nuclei. Possibility to study the nuclear structure via the β transition matrix elements is also discussed.

First results of the CLARA-PRISMA setup installed at LNL

Spring 2004 has seen the first experiments performed with the CLARA-PRISMA setup installed at LNL (Legnaro). The setup consists of CLARA, an array of 25 Clover (EUROBALL type) Ge detectors, placed at the target position of the large acceptance PRISMA magnetic spectrometer. The setup is an excellent tool to investigate the structure of neutron-rich nuclei, populated in multinucleon transfer reactions and deep inelastic collisions with stable beams. PRISMA allows the identification of the reaction products opening the possibility to go further away from stability in comparison with previous experimental activities using the aforementioned reactions. The setup has been commissioned in the first three months of the year and since March is fully operational. Five experiments had been performed, with beams delivered by the LNL tandem and the ALPI linac. In this contribution the main features of the setup as well as the preliminary outcome of the first experiments will be described.

Electric dipole transitions in neutron-rich nuclei

The structure of neutron-rich nuclei in several isotopes is investigated by shell model calculations. We study the electric dipole (E1) transitions in C isotopes focusing on the interplay between the low-energy Pigmy strength and the giant dipole resonance (GDR). Reasonable agreement is obtained with available experimental data for the photoreaction cross sections in 12C, 13C, and 14C with the inclusion of the quenching effects. A low-energy peak in the dipole strength in 15C is associated with a single-particle motion of the 1s1/2 valence neutron relative to the 14C core. The calculated transition strength below the GDR in C isotopes heavier than 15C is found to exhaust about 50–80% of the cluster sum rule value and 12–16% of the classical Thomas-Reiche-Kuhn sum rule value. Next, we point out that the quadrupole and magnetic moments in the odd C isotopes strongly depend on configuration, which will be useful to determine the spin parities and the deformations of the ground states of these nuclei. The electric quadrupole (E2) transitions in even C isotopes are also studied. The isotopic dependence of the E2 transition strength is found to be reasonably well explained, although the calculated strength largely overestimates the unexpectedly small strength observed in 16C. The E1 strength in 18N and 19N as well as in Ne isotopes is also investigated.

A New Possible Probe for Investigating the Exotic Structure ofNeutron-Rich Nuclei by Using the Hanbury–Brown–Twiss Method

Hanbury–Brown–Twiss (HBT) results of the nucleon–nucleon correlation functionhave been presented for the nuclear reactions with neutron-richprojectiles using an event-generator, i.e. the isospin-dependentquantum molecular dynamics model. A good agreement of ourcalculation for the two-halo-neutron correlation function of 11Liwith the experimental data has been reached. In addition, we explorethe relationship between the single-neutron separation energy and thestrength of the HBT results. It is feasible to investigate the exoticstructure of neutron-rich nuclei with the HBT method.

Probing shell structure in neutron-rich nuclei with in-beam γ-spectroscopy

The structure of neutron-rich light nuclei around N = 20 and 28 has been investigated at GANIL by means of in-beam gamma-spectroscopy using fragmentation reactions of 36S and 48Ca beams on a Be target. Gamma-decay of relatively high-lying excited states have been measured for the first time in nuclei around 32Mg and 44S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28.

Structure of neutron-rich nuclei around N=20, 28 and 40 from in-beam γ-spectroscopy

Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for 23N, 23O, 23F, 23Ne, 23Na, 23Mg, 23Al, and 23Si. This evaluation for A=23 supersedes the earlier one by R. B. Firestone (2007Fi02).Highlights of this evaluation are the following:The recommended absolute γ-ray emission probability (%Pγ) in the 23Ne β-decay was reported with 3.0% systematic uncertainty in 1986BrZQ. In 2007Fi02, the Pγ for the most intense γ ray was recommended with 4% uncertainty. These uncertainties are comparable with the total excited state β feeding uncertainty of 33 l, resulting from 100–67 l (g.s. β feeding) in 23Na (1963Ca06). In this evaluation, notes are added for relative and absolute γ-ray emission probabilities.From new measurements 2020Kw01 constrain the spin of the 7788 keV level, the dominant resonance state for the 22Na(p,γ)22Mg reaction rate at stellar temperature, in 23Mg to (3/2+,5/2+). Earlier assignment was 7/2+.The 23Al ε decay scheme appears to be incomplete. Some of the latest reported data on the 23Al ε decay study in the literature need verification.

Core excitation in Coulomb breakup reactions

Within the pure Coulomb breakup mechanism, we investigate the one-neutron removal reaction of the type A(a,b$\gamma$)X with $^{11}$Be and $^{19}$C projectiles on a heavy target nucleus $^{208}$Pb at the beam energy of 60 MeV/nucleon. Our intention is to examine the prospective of using these reactions to study the structure of neutron rich nuclei. Integrated partial cross sections and momentum distributions for the ground as well as excited bound states of core nuclei are calculated within the finite range distorted wave Born approximation as well as within the adiabatic model of the Coulomb breakup. Our results are compared with those obtained in the studies of the reactions on a light target where the breakup proceeds via the pure nuclear mechanism. We find that the transitions to excited states of the core are quite weak in the Coulomb dominated process as compared to the pure nuclear breakup.

In-beam γ-spectroscopy using projectile fragmentation: Structure of neutron-rich nuclei around N=20

In-beam γ-spectroscopy using projectile fragmentation: Structure of neutron-rich nuclei around N=20

Varying shell gap and deformation inN∼20unstable nuclei studied by the Monte Carlo shell model

The structure of neutron-rich nuclei in the $N\ensuremath{\sim}20$ region is studied by the Monte Carlo shell model based on the quantum Monte Carlo diagonalization method. We present a comprehensive description of even-even isotopes of O, Ne, Mg, and Si. It is demonstrated that, for different nuclei, various particle-hole excitations from the $\mathrm{sd}$ to $\mathrm{pf}$ shell are mixed in different ways, producing distinct effects sometimes, for instance, in ${}^{28}\mathrm{Ne}.$ The monopole interaction is examined and modified, resulting in the shell gap changing from nucleus to nucleus. The drip line of O isotopes is then reproduced. The interplay between the $T=0$ and $T=1$ monopole interactions is discussed from the viewpoint of the potential energy surface and the effective single-particle energy. The extension of the neutron drip line of Ne isotopes is explained, and the boundary of the ``island of inversion'' is shown to be rather indistinct.

Structure of neutron-rich nuclei around A ⋍ 100

Neutron-rich isotopes of Mo (Z=42) around A ⋍ 100 have been investigated within the formalisms of cranked Nilsson Strutinsky and CHFB, to study several interesting features of nuclear structure in this mass region. The total energy/routhian surfaces have been generated for the isotopes of Mo ranging from A ⋍ 96 − 112, as a function of deformation (β 2 and γ) for ground state and higher angular momentum states. Results of calculations using two different formalisms have been compared and combined to have a better understanding of the underlying mechanism of shape evolution.

Reply to “Comment on ‘Structure of neutron-rich nuclei around132Sn’ ”

We reply to the preceding Comment. Received 22 September 1998DOI:https://doi.org/10.1103/PhysRevC.59.548©1999 American Physical Society