Neutron-rich Nuclei Research Articles

Systematic researches on the ground state properties of the medium-mass neutron-rich nuclei

Abstract The recently developed relativistic-mean-field in complex momentum representation with the functional NL3* was used to explore the exotic properties of neutron-rich Pd, Cd, Te, and Xe isotopes. The results were compared with those obtained using the relativistic Hartree-Bogoliubov (RHB) calculations, and available experimental data. The single-particle levels were obtained for the bound and resonant states. The two neutron separation energies S2n and root mean square (rms) radii, agree with the experimental data. It is shown that a halo structure in the extremely neutron-rich 164-180Te and 164-182Xe, as well as a thick neutron skin in the extremely neutron-rich Pd and Cd isotopes. From the numbers of neutrons (Nλ) and (N0 ) occupying the levels above the Fermi surface and the zero-potential energy level, it was found that pairing correlations play an important role in the formation of halo phenomena. These findings are further supported by investigating S2n, rms radii, occupation probabilities, contributions of single-particle levels to the neutron rms radii, and density distributions. The neutron rms radii increased sharply, obviously deviating from the traditional rule r ∝A1/3, and the density distributions were very diffuse. Finally, the contributions of different single-particle levels to the total neutron density and wavefunction were discussed. It was found that the sudden increase in the neutron rms radii and diffuse density distributions mainly come from the resonant levels with a lower orbital angular momentum near the continuum threshold.

Production of neutron-rich nuclei in the vicinity of 78Ni: Fragmentation reactions of unstable 81Ga and 82Ge beams

The fragmentation reactions of neutron-rich unstable nuclei 81Ga and 82Ge at 250 MeV/nucleon have been studied in order to search the optimal mean for the production of very neutron-rich nuclei in the vicinity of 78Ni. The newly measured cross sections were compared with various calculations, showing a good agreement with EPAX3.01. The present work provides experimental insights into the two-step scheme—fragmentation following fission—as a method to produce very neutron-rich nuclei through a combination of ISOL and fragmentation of post-accelerated beams of unstable nuclei. Our results enable an evaluation of the potential of the two-step scheme in the production of very neutron-rich nuclei for the 78Ni region. The two-step scheme using fragmentation of a post-accelerated 81Ga beam could be an option to produce neutron-rich nuclei around 78Ni when the 81Ga beam intensity reaches 1.0×108 pps, compared to the one-step scheme with fission of 238U.

Relativistic mean field analysis of triaxial deformation for nuclei near the neutron drip line

The present study focuses on the deformation of neutron-rich nuclei near the neutron drip line. The nuclei of interest include 28O, 42Si, 58Ca, 80Ni, 100Kr, 122Ru, 152Ba, 166Sm, and 176Er. The relativistic Hartree - Bogoliubov (RHB) approach with effective density-dependent point coupling is utilized to investigate the triaxial deformation, and Skyrme - Hartree - Fock + Bardeen - Cooper - Schrieffer is used to analyze the axial deformation. The study aimed to understand the interplay between nuclear forces, particle interactions, and shell structure to gain insights into the unique behavior of neutron-rich nuclei. Despite these nuclei containing magic numbers, their shapes are still affected by the nucleons' collective behavior and energy levels. As the number of neutrons increases, the shape smoothly transitions from spherical to triaxial and then to prolate. The axial deformation analysis confirmed the results of the triaxial deformation analysis using the RHB method. An imbalance in the number of protons and neutrons can affect pairing energy, where extra neutrons can reduce overall pairing energy, and protons can disrupt the nucleon pairing due to stronger Coulomb repulsion between them.

A coherent microscopic picture for the exotic structure of 11Be

The neutron-rich nucleus 11Be is one of the most enigmatic exotic nuclei in the periodic table. It exhibits a multitude of unique and intriguing phenomena within a single nucleus, including the halo phenomenon, clustering structure, and parity inversion. A unified description of these exotic properties represents a significant challenge for the nuclear physics field. By employing the axially deformed relativistic Hartree-Fock-Bogoliubov (D-RHFB) model, we have successfully reproduced the parity inversion and the one-neutron halo in 11Be, and revealed the underlying cluster structure. It is found that the intrusion of the sd-shell, which results in the parity inversion, is coherently connected with the halo and cluster structures. In particular, the s-wave is responsible for the halo formation, and the mixing of 2s1/2- and 1d5/2-waves enhances the cluster structure. This work elucidates the coherence between the parity inversion and the halo/cluster structures of 11Be, and addresses the long-standing challenge of providing a unified explanation for the multiple exotic phenomena observed in 11Be.

Understanding the relationship between electric dipole polarizability and photonuclear (-2) moment in odd-A actinide nuclei

The nuclear electric dipole (E1) polarizability (α E1) is mainly dominated by the dynamics of the giant dipole resonance (GDR). α E1 is proportional to the (-2) moment of the total photo nuclear cross-section (σ −2). This research investigates the relationship between α E1 and σ −2, along with the effects of the Pygmy Dipole Resonance (PDR) and GDR in odd-mass actinide nuclei. For the first time, α E1 and σ −2 values have been calculated using the Translational and Galilean Invariant Quasiparticle Phonon Nuclear Model (TGI-QPNM) approach for odd-A actinide nuclei. According to TGI-QPNM results, E1 dipole transitions in the GDR region significantly contribute to σ −2 due to the energy weighting factor. Below the neutron separation threshold, the PDR in neutron-rich nuclei shows a contribution of about 5% to σ −2 values. In this region, E1 polarizability can reach values of 20%–25%. The α E1 values indicate the presence of PDR in these nuclei. Additionally, the Adaptive Neuro-Fuzzy Inference System (ANFIS), a new machine learning method, has been performed to analyze the relationship between α E1 and σ −2. The ANFIS results have been compared with those from the TGI-QPNM and experimental data. The TGI-QPNM model achieves an R2 of 0.85–0.95, while the ANFIS model achieves an R2 of 0.99. Moreover, the study suggests that the ANFIS model, consistent with TGI-QPNM results, could be an effective tool for estimating σ −2 in odd-A actinide nuclei.

How do mirror charge radii constrain density dependence of the symmetry energy?

It has recently been suggested that differences in the charge radii of mirror nuclei (ΔRchmirr) are strongly correlated with the neutron-skin thickness (Rskin) of neutron-rich nuclei and with the slope of the symmetry energy (L). To test this assumption, we present ab initio calculations of Rskin in 48Ca and 208Pb, ΔRchmirr in 36Ca−36S, 38Ca−38Ar, 41Sc−41Ca, 48Ni−48Ca, 52Ni−52Cr, and 54Ni−54Fe mirror pairs, and L. Employing the recently developed 34 chiral interaction samples, identified by the history matching approach, we conduct rigorous statistical analysis of correlations among ΔRchmirr, Rskin and L, accounting for quantified uncertainties from low-energy constants of chiral interaction, chiral effective field theory truncation and many-body method approximation. The ab initio results reveal an appreciable ΔRchmirr−L correlation in fp-shell mirror pairs. However, contrary to previous studies, the present calculation finds that the studied sd-shell mirror pairs do not exhibit any ΔRchmirr−L correlation.

First Exploration of Monopole-Driven Shell Evolution above the N=126 Shell Closure: New Millisecond Isomers in ^{213}Tl and ^{215}Tl.

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19)keV in ^{213}Tl and 874(5)keV in ^{215}Tl. The measured half-lives of 1.34(5)ms in ^{213}Tl and 3.0(3)ms in ^{215}Tl suggest spins and parities 11/2^{-} with the single proton-hole configuration πh_{11/2} as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2^{+}, interpreted as arising from a single πs_{1/2} proton hole coupled to the 8^{+} seniority isomer in the ^{A+1}Pb cores. The lowering of the 11/2^{-} states is ascribed to an increase of the πh_{11/2} proton effective single-particle energy as the second νg_{9/2} orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the πh_{11/2} and νg_{9/2} orbitals. The new ms isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly magic ^{208}Pb.

Ion-optical simulations for the NEXT solenoid separator

In this paper, we present ion-optical simulation for the design studies of a new solenoid separator as part of the NEXT experiment. Our aim is to separate Neutron-rich, EXotic, heavy nuclei produced in multinucleon Transfer reactions (NEXT) from the primary beam and un-wanted by-products within the magnetic field. The goal of the simulation is to find the optimum arrangement of the components within the NEXT solenoid separator to achieve the highest transmission efficiencies of transfer products and a good suppression of background. In our simulations, we focused on two complementary reactions to produce transfermium isotopes and to produce neutron-rich nuclei around the neutron number N = 126. Transmission yields above 50% for the transfermium isotopes of interest were reached in the simulations. The simulated yields for N = 126 lie between 5% and 20%.

Capture rates of highly degenerate neutrons

At the low temperature and high density conditions of a neutron star crust neutrons are degenerate. In this work, we study the effect of this degeneracy on the capture rates of neutrons on neutron rich nuclei in accreted crusts. We use a statistical Hauser–Feshbach model to calculate neutron capture rates and find that neutron degeneracy can increase rates significantly. Changes increase from a factor of a few to many orders of magnitude near the neutron drip line. We also quantify uncertainties due to model inputs for masses, γ-strength functions, and level densities. We find that uncertainties increase dramatically away from stability and that degeneracy tends to increase these uncertainties further, except for cases near the neutron drip line where degeneracy leads to more robustness. As in the case of capture of classically distributed neutrons, variations in the mass model have the strongest impact. Corresponding variations in the reaction rates can be as high as 3–4 orders of magnitude, and be more than 5 times larger than under classical conditions. To ease the incorporation of neutron degeneracy in nucleosynthesis networks, we provide tabulated results of capture rates as well as analytical expressions as function of temperature and neutron chemical potential, for proton numbers between 3 ≤ Z ≤ 85, derived from fits to our numerical results. Fits are based on a new parametrization that complements previously employed power law approximations with additional Lorentzian terms that account for low energy resonances, significantly improving accuracy.

Inner fission barriers of uranium isotopes in the deformed relativistic Hartree-Bogoliubov theory in continuum

Abstract The inner fission barriers of the even-even uranium isotopes from the proton to the neutron drip line are studied with the deformed relativistic Hartree-Bogoliubov theory in continuum. A periodic evolution for the ground state shapes is shown with the neutron number, i.e., spherical shapes at shell closures N=126, 184, 258, and prolate dominated shapes between them. In analogy to the shape evolution, the inner fission barriers also exhibit a periodic behavior: peaks at the shell closures and valleys in the mid-shells. The triaxial effect to the inner fission barrier is evaluated using the triaxial relativistic mean field calculations plus a simple BCS method for pairing. With the triaxial correction included, good consistency in the inner barrier heights is found with the available empirical data.
Besides, the evolution from the proton to the neutron drip line is in accord with the results by the multi-dimensionally constrained relativistic mean field theory. A flat valley in the fission barrier height is predicted around the neutron-rich nucleus 318U which may play a role of fission recycling in the astrophysical r-process nucleosynthesis.

Opportunities for production and property research of neutron-rich nuclei around N = 126 at HIAF

Opportunities for production and property research of neutron-rich nuclei around N = 126 at HIAF

Fast rotation of nuclei with extreme isospin in the vicinity of neutron and proton drip lines

The analysis of the present understanding of collective rotation in very neutron-rich nuclei is presented. It is shown that collective rotation can lead to the increase of stability of rotational states with increasing spin. The detailed investigation of rotational excitations in very proton-rich nuclei confirms this conclusion and indicates that experimental studies of such features are more feasible in the nuclei near proton drip line. They also show that rotational bands which are proton quasi-bound at zero or low spins can be transformed into proton bound ones at high spin by collective rotation of nuclear systems. This is due to strong Coriolis interaction which acts on high-j or strongly mixed M orbitals and drives the highest in energy occupied single-particle states into negative energy domain. These physical mechanisms lead to a substantial extension of the nuclear landscape beyond the spin zero proton drip line. In addition, a new phenomenon of the formation of giant proton halos in rotating nuclei emerges: it is triggered by the occupation of strongly mixed M intruder orbitals.

Collective Neutrino Oscillations and Heavy-element Nucleosynthesis in Supernovae: Exploring Potential Effects of Many-body Neutrino Correlations

In high-energy astrophysical processes involving compact objects, such as core-collapse supernovae or binary neutron star mergers, neutrinos play an important role in the synthesis of nuclides. Neutrinos in these environments can experience collective flavor oscillations driven by neutrino–neutrino interactions, including coherent forward scattering and incoherent (collisional) effects. Recently, there has been interest in exploring potential novel behaviors in collective oscillations of neutrinos by going beyond the one-particle effective or “mean-field” treatments. Here, we seek to explore implications of collective neutrino oscillations, in the mean-field treatment and beyond, for the nucleosynthesis yields in supernova environments with different astrophysical conditions and neutrino inputs. We find that collective oscillations can impact the operation of the ν p-process and r-process nucleosynthesis in supernovae. The potential impact is particularly strong in high-entropy, proton-rich conditions, where we find that neutrino interactions can nudge an initial ν p-process neutron-rich, resulting in a unique combination of proton-rich low-mass nuclei as well as neutron-rich high-mass nuclei. We describe this neutrino-induced neutron-capture process as the “ν i-process.” In addition, nontrivial quantum correlations among neutrinos, if present significantly, could lead to different nuclide yields compared to the corresponding mean-field oscillation treatments, by virtue of modifying the evolution of the relevant one-body neutrino observables.

Nuclear structure properties and weak interaction rates of even–even Fe isotopes

Nuclear structure properties and weak interaction rates of neutron-rich even–even iron (Fe) isotopes (A = 50 – 70) are investigated using the Interacting Boson Model-1 (IBM-1) and the proton–neutron Quasiparticle Random Phase Approximation (pn-QRPA) model. The IBM-1 is used for the calculation of energy levels and the B(E2) values of neutron-rich Fe isotopes. Later their geometry was predicted within the potential energy formalism of the IBM-1 model. Weak interaction rates on neutron-rich nuclei are needed for the modeling and simulation of presupernova evolution of massive stars. In the current study, we investigate the possible effect of nuclear deformation on stellar rates of even–even Fe isotopes. The pn-QRPA model is applied to calculate the weak interaction rates of selected Fe isotopes using three different values of deformation parameter. It is noted that, in general, bigger deformation values led to smaller total strength and larger centroid values of the resulting Gamow–Teller distributions. This later translated to smaller computed weak interaction rates. The current finding warrants further investigation before it may be generalized. The reported stellar rates are up to 4 orders of magnitude smaller than previous calculations and may bear astrophysical significance.

Production of p Nuclei from r -Process Seeds: The νr Process

We present a nucleosynthesis process that may take place on neutron-rich ejecta experiencing an intensive neutrino flux. The nucleosynthesis proceeds similarly to the standard r process, a sequence of neutron captures and beta decays with, however, charged-current neutrino absorption reactions on nuclei operating much faster than beta decays. Once neutron-capture reactions freeze out the produced r process, neutron-rich nuclei undergo a fast conversion of neutrons into protons and are pushed even beyond the β stability line, producing the neutron-deficient p nuclei. This scenario, which we denote as the νr process, provides an alternative channel for the production of p nuclei and the short-lived nucleus Nb92. We discuss the necessary conditions posed on the astrophysical site for the νr process to be realized in nature. While these conditions are not fulfilled by current neutrino-hydrodynamic models of r-process sites, future models, including more complex physics and a larger variety of outflow conditions, may achieve the necessary conditions in some regions of the ejecta. Published by the American Physical Society 2024

COeCO: A new [formula omitted]-delayed conversion-electron spectroscopy setup for low-energy ISOL beams at the ALTO facility in Orsay

A new β-decay station, COnversion electrons Chasing at Orsay (COeCO), has been developed at ALTO to perform conversion electron spectroscopy studies of neutron-rich nuclei produced by photo-fission of a uranium carbide target. It is based on the collection of a low-energy ISOL beam on a mylar tape, and the transportation of the electrons emitted by the produced radioactive source through a magnetic field induced by two copper coils, towards a cooled Si(Li) detector. In this article, a detailed description of the new decay station and its components is given. The magnetic field induced by the coils was measured and compared to simulations performed with the COMSOL® software. The efficiency of the detection setup was estimated using a 207Bi and a 152Eu source as an off-line commissioning. Finally, the results obtained with a 96Rb radioactive beam for the on-line commissioning are presented.

Proton Shell Gaps in N=28 Nuclei from the First Complete Spectroscopy Study with FRIB Decay Station Initiator.

The first complete measurement of the β-decay strength distribution of _{17}^{45}Cl_{28} was performed at the Facility for Rare Isotope Beams (FRIB) with the FRIB Decay Station Initiator during the second FRIB experiment. The measurement involved the detection of neutrons and γ rays in two focal planes of the FRIB Decay Station Initiator in a single experiment for the first time. This enabled an analytical consistency in extracting the β-decay strength distribution over the large range of excitation energies, including neutron unbound states. We observe a rapid increase in the β-decay strength distribution above the neutron separation energy in _{18}^{45}Ar_{27}. This was interpreted to be caused by the transitioning of neutrons into protons excited across the Z=20 shell gap. The SDPF-MU interaction with reduced shell gap best reproduced the data. The measurement demonstrates a new approach that is sensitive to the proton shell gap in neutron rich nuclei according to SDPF-MU calculations.

A new approach for deducing rms proton radii from charge-changing reactions of neutron-rich nuclei and the reaction-target dependence

We report the charge-changing cross sections (σcc) of 24 p-shell nuclides on both hydrogen and carbon at about 900A MeV, of which 8,9Li, 10–12Be, 10,14,15B, 14,15,17–22N and 16O on hydrogen and 8,9Li on carbon are for the first time. Benefiting from the data set, we found a new and robust relationship between the scaling factor of the Glauber model calculations and the separation energies of the nuclei of interest on both targets. This allows us to deduce proton radii (Rp) for the first time from the cross sections on hydrogen. Nearly identical Rp values are deduced from both target data for the neutron-rich carbon isotopes; however, the Rp from the hydrogen target is systematically smaller in the neutron-rich nitrogen isotopes. This calls for further experimental and theoretical investigations.

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.

Isoscaling properties for neutron-rich fragments in highly asymmetric heavy ion collision systems* *Supported by the National Natural Science Foundation of China (12375123, 11975091) and the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (21IRTSTHN011), China

Traditionally, isoscaling has been interpreted and applied within the framework of the grand canonical ensemble, based on the assumption that fragment production occurs following the attainment of a statistical equilibrium state. However, the influence of the symmetry energy can lead to differences in the neutron and density distribution in neutron-rich nuclei. This in turn may impact the isoscaling parameters (usually denoted by α and β). We examine the isoscaling properties for neutron-rich fragments produced in highly asymmetric systems on inverse kinematics, namely Ca and Ni + Be at 140 MeV per nucleon. We evaluate α and β values and sort them as a function of the neutron excess . The significant differences in α extracted from fragments within different ranges of I emphasize the importance of understanding the dependence of isoscaling parameters on fragments generated in various collision regions. Furthermore, the value for a specific fragment in small size and highly isospin asymmetry systems can serve as a probe to detect the variations in neutron density and proton density in different regions of the nucleus and indicate the limitations of theoretical models in investigating these issues.