Structure Of Neutron-rich Nuclei Research Articles

Evolution of nuclear shell structure in neutron-rich nuclei N=32 and N=34

This paper presents an overview of the experimental setup to study the structure of unstable nuclear isotopes in the region from 47Cl to 63V of the SEASTAR 3 project and the evolution of shell structure in neutron-rich nuclei N=32 and N=34. The first excited energies of Cl, Ar, and K isotopes around N=32, N=34 obtained from the SEASTAR 3 project were determined. The evolution of shell structure in neutron-rich nuclei N=32 và N=34 is explained by the contribution of tensor forces in addition to central and spin-orbit forces. The appearance of new magic neutron N=34 in 54Ca is explained due to proton-neutron tensor force that pulls neutron 0f5/2 lower than 1p1/2 level, leading to not only causes the spin inversion of neutrons at 1p1/2 and 0f5/2 levels and enlarges an energy gap between these levels. The change of proton and neutron energy levels in Cl, Ar, and K isotopes around N=32, N=34 driven by tensor force was also discussed in this article.

Spectroscopy of neutron-rich nuclei produced by multinucleon transfer reactions at KISS

Properties such as lifetimes and masses of neutron-rich nuclei are key parameters for elucidating the astrophysical rapid neutron capture process (r-process). Due to the lack of experimental nuclear data in the relevant extremely neutron-rich region, especially near and above N = 126, the predictions of theoretical nuclear models are crucial for simulating r-process nucleosynthesis. Experimental studies of the properties and structures of neutron-rich nuclei from near the β stability line up to the r-process path provide important inputs to these theoretical models, improving the accuracy of predictions for the nuclear properties involved in the rprocess. We are developing the KEK Isotope Separation System (KISS) at the RIKEN RIBF facility to produce and separate these nuclei to perform spectroscopy experiments. The nuclei of interest are produced by multinucleon transfer reactions, which have been recently attracting renewed interest as they provide a way to access neutron-rich nuclei that are difficult to produce by other methods such as complete fusion or fragmentation. We have performed nuclear and laser spectroscopy, lifetime and mass measurements of neutron-rich nuclei of refractory elements near the N = 126 region using 198,natPt, natTa, natW, and natIr targets irradiated with 136Xe beams. We also extend our spectroscopic studies into the neutron-rich actinide region using 238U beams. This paper introduces the KISS facility and provides an overview of the nuclear spectroscopy experiments carried out there. We also introduce the KISS-1.5 project, which was started in FY2024 to further extend the research into the neutron-rich region.

Ab initio calculations for well deformed nuclei: 40Mg and 42Si

We performed ab initio valence-space in-medium similarity renormalization group calculations based on chiral nucleon-nucleon and three-nucleon interactions to study the neutron-rich nuclei around N=28, where the experimental and theoretical evidence point to a rapid transition from spherical to rotational regimes. First, the rotational spectra of 40Mg, 42Si, and 44S are established theoretically, anchored by the calculated E2 transition strength. The results suggest that 40Mg and 42Si exhibit large deformations. After that, the erosion of the N=28 shell closure, especially the evolutions of shape deformation and the emergence of shape coexistence, are discussed in N=28 even-even isotones ranging from 40Mg to 48Ca. Finally, the shell evolution of N=28 in Mg and Si chains is discussed based on the experimental and predicted excitation energies of low-lying states. The results from our ab initio calculations align well with available experimental data, furnishing essential insights into the structure of neutron-rich nuclei, particularly deformations in the vicinity of 40Mg and 42Si.

Role of triaxiality in deformed halo nuclei

It is known that nuclear deformation plays an important role in inducing the halo structure in neutron-rich nuclei by mixing several angular momentum components. While previous theoretical studies on this problem in the literature assume axially symmetric deformation, we here consider non-axially symmetric deformations. With triaxial deformation, the $\Omega$ quantum number is admixed in a single-particle wave function, where $\Omega$ is the projection of the single-particle angular momentum on the symmetric axis, and the halo structure may arise even when it is absent with the axially symmetric deformation. In this way, the area of halo nuclei may be extended when triaxial deformation is considered. We demonstrate this idea using a deformed Woods-Saxon potential for nuclei with neutron number N=13 and 43.

Structure of Nd155 and Gd163 from Cf252 spontaneous fission

Background: A puzzle has arisen recently caused by the apparent shift in maximum deformation from the expected $^{66}\mathrm{Dy}$ isotopic chain to the ${}_{60}\mathrm{Nd}$ isotopic chain in the $82lNl126$ and $50lZl82$ midshell region.Purpose: This work provides data for two specific nuclei with odd neutron numbers, $^{155}\mathrm{Nd}$ and $^{163}\mathrm{Gd}$, useful for constraining parameters in models that seek to answer the six proton shift in maximum deformation.Method: Data from the spontaneous fission of $^{252}\mathrm{Cf}$ were taken by the Gammasphere detector array at Lawrence Berkeley National Laboratory to observe the excited states of $^{155}\mathrm{Nd}$ and $^{163}\mathrm{Gd}$.Results: The structure of $^{163}\mathrm{Gd}$ has been expanded with the addition of two new levels and three new $\ensuremath{\gamma}$ rays, which are found to be consistent with previously published calculations and the structure of $^{165}\mathrm{Dy}$. In $^{155}\mathrm{Nd}$, nine new levels and 12 new $\ensuremath{\gamma}$ rays are observed. The spins and parities of the previously known levels in $^{155}\mathrm{Nd}$ have been reassigned from a $\ensuremath{\nu}3/{2}^{\ensuremath{-}}[521]$ ground state configuration to a $\ensuremath{\nu}5/{2}^{+}[642]$ isomeric configuration by comparison of these newly observed levels with levels in $^{153}\mathrm{Nd}$ and $^{155}\mathrm{Sm}$.Conclusion: Further experimentation is required to determine the energy of the newly reassigned $\ensuremath{\nu}5/{2}^{+}[642]$ level in $^{155}\mathrm{Nd}$ with respect to the suspected $\ensuremath{\nu}3/{2}^{\ensuremath{-}}[521]$ ground state. Additionally, more experiments should be conducted to further determine the structure of neutron rich nuclei, rarely produced in the spontaneous fission of $^{252}\mathrm{Cf}$, such as $^{163}\mathrm{Gd}$.

New magicity N = 32 and 34 due to strong couplings between Dirac inversion partners

The novel nuclear structure in neutron-rich nuclei — the new magicity N=32 and 34 has been confirmed by the intensive experiment measurements (F. Wienholtz et al. (2013) [17]; D. Steppenbeck et al. (2013) [22]; S. Michimasa et al. (2018) [28]; etc.). However, the underlying mechanism of the new magicity is still under discussion. In this letter, we present a new mechanism that the strong couplings between the s1/2 (including both neutron and proton ones with different principle numbers) and neutron (ν) ν2p1/2 orbits, referred as “Dirac inversion partners” (DIPs) which are of the same total angular momentums but opposite parity, play a key role in opening both subshells at N=32 and 34. Such strong couplings originate from the inversion similarity between the DIPs, that the upper component of the Dirac spinor of one partner shares the same angular momentum as the lower component of the other, and vice versa. Following the revealed mechanism, it is predicted that on the proton deficient side (Z⩽20) the magicity N=32 is reserved from 52Ca until 48S, but vanishes in 46Si.

Suddenly shortened half-lives beyond Ni78:N=50 magic number and high-energy nonunique first-forbidden transitions

$\beta$-decay rates play a decisive role in understanding the nucleosynthesis of heavy elements and are governed by microscopic nuclear-structure information. A sudden shortening of the half-lives of Ni isotopes beyond $N=50$ was observed at the RIKEN-RIBF. This is considered due to the persistence of the neutron magic number $N=50$ in the very neutron-rich Ni isotopes. By systematically studying the $\beta$-decay rates and strength distributions in the neutron-rich Ni isotopes around $N=50$, I try to understand the microscopic mechanism for the observed sudden shortening of the half-lives. The $\beta$-strength distributions in the neutron-rich nuclei are described in the framework of nuclear density-functional theory. I employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the proton-neutron Quasiparticle Random-Phase Approximation (pnQRPA) for the transitions. Not only the allowed but the first-forbidden (FF) transitions are considered. The experimentally observed sudden shortening of the half-lives beyond $N=50$ is reproduced well by the calculations employing the Skyrme SkM* and SLy4 functionals. The sudden shortening of the half-lives is due to the shell gap at $N=50$ and cooperatively with the high-energy transitions to the low-lying $0^-$ and $1^-$ states in the daughter nuclei. The onset of FF transitions pointed out around $N=82$ and 126 is preserved in the lower-mass nuclei around $N=50$. This study suggests that needed is a microscopic calculation where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are selfconsistenly taken into account for predicting $\beta$-decay rates of exotic nuclei in unknown region.

Rotational bands in neutron-rich Mo, Ru, and Pd nuclei * * Supported by the National Key R&D Program of China (2018YFA0404401), the National Natural Science Foundation of China (11835001, 11575007, 11847203) and the China Postdoctoral Science Foundation (2018M630018)

The collective rotations of the configuration in neutron-rich Mo, Ru and Pd isotopes were systematically investigated by the configuration-constrained cranking shell model based on the Skyrme Hartree-Fock method with pairing treated by shell-model diagonalization. The calculations efficiently reproduce the experimental moments of inertia of both the ground-state and side bands. Rotational bands built on two-particle configurations have been the subject of intense study. Possible configurations were assigned to the observed bands in 102-106Mo, 108-112Ru and 112-114Pd. We predict the existence of the bands in 108,110Mo. These results provide deep insights into the structure of neutron-rich nuclei, and provide useful information for future experiments.

Structural changes at large angular momentum in neutron-richCd121,123

Prompt γ rays of isotopically identified neutron-rich isotopes of Cd, produced in transfer- and fusion-fission induced by the 238U beam at 6.2 MeV/u on a 9Be target, were measured using the EXOGAM γ -ray detector array and the magnetic spectrometer VAMOS++. New results for the level scheme of 121,123Cd, extending to relatively large angular momentum are reported. The energy levels above 2-MeV excitation energy, are found to differ from those observed in lighter isotopes of Cd indicating a change in structure in these more neutron-rich nuclei. These states are not explained by large-scale shell model calculations, that explain well the structure of the underlying Sn isotopes and the neighboring even-A Cd isotopes. The present data, especially for the odd-A nuclei, point to a deficiency in the matrix elements related to the p-n residual interaction and provide a new domain for testing widely used shell model interactions employed for understanding the evolution of structure in neutron-rich nuclei.

Development of a Gas Filled Magnet spectrometer within the FIPPS project

The Fission Product Prompt γ-ray Spectrometer, FIPPS, is under development to enable prompt γ-ray spectroscopy correlated with fission fragment identification. This will open new possibilities in the study of fission and of nuclear structure of neutron rich nuclei. FIPPS will consist of an array of γ and neutron detectors coupled with a fission fragment filter. The chosen solution for the filter is a Gas Filled Magnet (GFM). Both experimental and modeling work was performed in order to extract the key parameters of such a device and design the future GFM of the FIPPS project. Experiments performed with a GFM behind the LOHENGRIN spectrometer demonstrated the capability of additional beam purification.

Response of the TETRA 4π detector to neutrons

The MCNP model of the 4π neutron array TETRA is discussed. TETRA is coupled to BEDO decay station to study both the β-decay properties and the nuclear structure of neutron rich nuclei produced at ALTO ISOL-facility. β-, γ- and neutron radioactivity from β-decay of nuclei accumulated at the center of TETRA can be recorded simultaneously. The MCNP model was validated on measurements of spontaneous fission neutrons of 252Cf source. The model is employed to optimize performance of TETRA coupled to BEDO decay system.

Response of the TETRA 4π detector to neutrons

The MCNP model of the 4π neutron array TETRA is discussed. TETRA is coupled to BEDO decay station to study both the β-decay properties and the nuclear structure of neutron rich nuclei produced at ALTO ISOL-facility. β-, γ- and neutron radioactivity from β-decay of nuclei accumulated at the center of TETRA can be recorded simultaneously. The MCNP model was validated on measurements of spontaneous fission neutrons of 252Cf source. The model is employed to optimize performance of TETRA coupled to BEDO decay system.

A new topological “twist” to BR scaling

When vector mesons are considered on the same footing as pions as suggested by hidden local symmetry, the property of the nuclear tensor forces is strongly controlled by the behavior of the vector mesons in dense medium. This led to BR scaling in 1991. When baryons as skyrmions are put on crystal, there can be a phase transition from skyrmions to half-skyrmions at a density above that of normal nuclear matter. This topology change can induce fundamental changes to the parameters in hidden local symmetric Lagrangian, hence BR scaling, and brings a drastic modification to the structure of nuclear forces, in particular, the tensor forces. This can have far-reaching consequences on the EoS of compact-star matter and the structure of neutron-rich nuclei.

Structure of Neutron-rich Nuclei Beyond $N=50$

The measurement of the $\beta$-decay scheme of $^{85}$Ga triggered questions on the properties of the low-lying states in $^{85}$Ge. In order to inspect the sensitivity of the results to the neutron $d_{5/2}$ and $s_{1/2}$ single-particle states, we performed an analysis of the level structure in the $N=51$ $^{83}$Ge and $N=53$ $^{85}$Ge isotopes.

Structure analysis for hole-nuclei close to132Sn by a large-scale shell-model calculation

The structure of neutron-rich nuclei with a few holes in respect of the doubly magic nucleus ${}^{132}$Sn is investigated by means of large-scale shell-model calculations. For a considerably large model space, including orbitals allowing both neutron and proton core excitations, an effective interaction for the extended pairing-plus-quadrupole model with monopole corrections is tested through detailed comparison between the calculation and experimental data. By using the experimental energy of the core-excited $21/{2}^{+}$ level in ${}^{131}$In as a benchmark, monopole corrections are determined that describe the size of the neutron $N=82$ shell gap. The level spectra, up to 5 MeV of excitation in ${}^{131}$In, ${}^{131}$Sn, ${}^{130}$In, ${}^{130}$Cd, and ${}^{130}$Sn, are well described and clearly explained by couplings of single-hole orbitals and by core excitations.

Tracking changes in shell structure in neutron-rich nuclei as a function of spin

Taking advantage of the resolving power of modern gamma-ray spectrometers in combination with different types of nuclear reactions, it has been possible to investigate to fairly high-spin neutron-rich nuclei in a number of regions of the nuclear chart. The primary motivations for such studies are to characterize changes in shell structure as a function of the neutron-to-proton ratio and to document the impact of a large neutron excess on global properties, such as the nuclear shape. Recent data on nuclei close to the ‘island of inversion’ near 32Mg are discussed first. They highlight challenges in describing the observations with modern effective interactions. Subsequently, the nature of excitations in neutron-rich fpg-shell nuclei between Ca and Ni is reviewed. A neutron sub-shell closure at N = 32 has been attributed to the monopole tensor force. Furthermore, the presence of collective excitations at moderate spin in neutron-rich Cr and Fe isotopes illustrates the role of the g9/2 orbital in driving the nuclear shape. The observations suggest that mixing between ‘deformed’ and ‘shell-model’ states needs to be considered. Finally, recent results in the direct vicinity of 68Ni indicate that the impact of the N = 40 neutron shell closure is rather modest.

Transfer Reactions and the Structure of Neutron-rich Nuclei

Author(s): Kay, BP; Alcorta, M; Back, BB; Bedoor, S; Bertone, PF; Baker, SI; Clark, JA; Deibel, CM; Di Giovine, BJ; Freeman, SJ; Hoffman, CR; Lee, HY; Lighthall, JC; MacChiavelli, A; Marley, ST; Muller, P; Pardo, R; Rehm, KE; Rojas, A; Rogers, AM; Rohrer, J; Santiago-Gonzalez, D; Schiffer, JP; Sharp, DK; Shetty, DV; Thomas, JS; Wiedenhover, I; Wuosmaa, AH | Abstract: The study of transfer reactions in inverse kinematics is a major focus of existing and future radioactive-ion-beam facilities. One of the obstacles in such measurements is poor Q-value resolution, often several hundred keV, which can prevent the extraction of useful information. At Argonne National Laboratory, it has recently been demonstrated that good Q-value resolution can be achieved by transporting the outgoing ions through a high-field solenoid, measuring their position as a function of energy. This provides several advantages over conventional Si arrays, such as large acceptance, good particle identification, and most importantly a Q-value resolution of better than 100 keV in most cases, including reactions with moderately heavy beams. In this paper, the concept of the solenoidal spectrometer, called HELIOS, will be discussed along with highlights of recent results.

Studies on the exotic structure of neutron-rich nuclei via direct reactions

Direct reaction is particularly useful in studying the nuclear structure of neutron-rich nuclei. By the use of the recoiled proton tagging technique, knockout reaction was developed into a clean spectroscopic probe and was applied to the study of the exotic structures of 6He and 8He. The most accurate parameters and related spectroscopic factor was obtained for the ground state of 7He. The Multi-neutron detection technique was also implemented, with which the resonant states of 8He and the corresponding neutron-neutron correlation function were measured. The cluster structure of 12Be was studied via inelastic excitation followed by coincidently recording the decay products, and molecular-like resonant states were observed for α -8He and 6He-6He decaying channels. These states are part of the molecular rotational bands and therefore clearly support the highly clustering structure of 12Be.

Antisymmetrized molecular dynamics and its applications to cluster phenomena

Structure and reaction studies using the antisymmetrized molecular dynamics (AMD) method are reviewed. Applications of time-independent and time-dependent versions of AMD are described. In applications of time-independent AMD to nuclear structure studies, the structures of neutron-rich nuclei such as Be, C, Ne, and Mg isotopes are described, focusing on cluster aspects. The important roles of valence neutrons are discussed. The results suggest that a variety of cluster structures appear in unstable nuclei as well as in stable nuclei. Deformation and cluster phenomena in Z ∼ N nuclei in p- and sd-shell regions are also discussed. The applications of time-dependent AMD include various topics such as fragmentation in heavy-ion collisions as well as nuclear responses. The AMD calculations successfully describe multifragmentation, which is one of the remarkable phenomena observed in heavy-ion collisions. The approach is able to link reactions and nuclear matter properties. The studies suggest the important balance between single-particle motions and correlations to form clusters and fragments.

Large-scale shell-model calculation with core excitations for neutron-rich nuclei beyond132Sn

The structure of neutron-rich nuclei with a few nucleons beyond ${}^{132}$Sn is investigated by means of large-scale shell-model calculations. For a considerably large model space, including neutron core excitations, a new effective interaction is determined by employing the extended pairing-plus-quadrupole model with monopole corrections. The model provides a systematical description for energy levels of $A=133$--135 nuclei up to high spins and reproduces available data of electromagnetic transitions. The structure of these nuclei is analyzed in detail, with emphasis of effects associated with core excitations. The results show evidence of hexadecupole correlation in addition to octupole correlation in this mass region. The suggested feature of magnetic rotation in ${}^{135}$Te occurs in the present shell-model calculation.