Shell Gap Research Articles

Nuclear shape evolution in even-even Pd isotopic chain

The covariant density functional theory is utilized to examine the evolution of shape in even–even 96−130 Pd isotopes by using the density-dependent meson-exchange DD-ME2 and the density-dependent point-coupling DD-PC1. This research is carried out by considering the evolution of the ground-state shapes derived from calculations of the axial and triaxial potential energy surfaces. The shape transition in the palladium isotopic chain is very clearly manifested. In addition, various ground-state properties, such as binding energy, charge radius, two-neutron separation energy, and two-neutron shell gap have been calculated and have been observed to adequately match the existing experimental data. Moreover, a robust shell closure is distinctly observed at the magic neutron number N = 82.

Weakening of N = 28 shell gap and the nature of 02+ states

The work reports a novel application of the interacting boson model in light-mass region around 48Ca, that takes into account intruder states and configuration mixing. The model is shown to provide a reasonable description of the observed low-lying yrast and yrare states of the N = 28 even–even isotones from Si to Fe, and even–even Ca isotopes. The nature of 02+ states is addressed in terms of the competition between spherical and deformed intruder configurations, and the rigidity of the N = 28 shell gap is tested, particularly for 44S. The 02+ isomer in 44S is shown to arise from a weak mixing between the two configurations and called as shape isomer. The unresolved low-lying spectra in 46Ar is also approached using the core-excitations across N = 28 shell. The SU(3) structure in 42Si is supported by the present calculation. The nearly spherical nature of 50Ti, 52Cr, 54Fe and 42,44,46Ca isotopes is found, while the core excitations are found to be essential for the description of yrare states. Shell model calculation is also performed for comparison.

Exploring asymmetric fission in 180Hg⁎ compound nucleus using dynamical cluster-decay model

The observed asymmetric fission of the 180Hg⁎ compound nucleus challenges conventional expectations of symmetric fission, which are attributed to the presence of shell closures at Z=40 (semi-magic) and N=50 (magic). To comprehend this novel phenomenon, the dynamical cluster-decay model has been employed. For the first time, this model incorporates the bulk and neutron-proton asymmetry coefficients of the nuclear shape-dependent mass excess formula which are tuned recently to the ground state mass excess data of AME2020 and/or FRDM(2012) along with the temperature dependence for the nuclear shape and the surface energy coefficient of the nuclear proximity potential. The calculations have considered nuclear shapes as both spherical and deformed (quadrupole), with and without temperature dependence for the quadrupole deformation. A new minimum appears in the symmetric mass region of the fragmentation potential for masses (80, 100), when the fragments are deformed and optimally oriented, at an energy lower than that obtained for masses (90, 90) where the fragments are assumed to be spherical or nearly spherical at higher temperatures. This new minimum seems equivalent to the appearance of a new shell gap with deformation and is responsible for the asymmetric fission of 180Hg⁎. The most probable fission channel and the transition from asymmetric to symmetric mass distribution at higher excitation energies are consistent with experiments for the current temperature dependence assigned to the quadrupole deformation parameter.

Prediction of two-neutron halos in the N = 28 isotones 40Mg and 39Na

The ground states of the nuclei Mg40 and Na39 are investigated using the hyperspherical formalism. Since they are located at the edge of the “big island of inversion”, we concentrate on whether we are likely to find two-neutron Borromean halos in these nuclei. A three-body model with effective n-n and Mg38+n interactions is built for Mg40 based on the available data. We also give predictions for the low-lying spectrum of Na38=37Na+n and two-neutron separation energy of the Na39 nucleus. Depending on parameter choice, we report an increase in the matter radii in the range 0.1-0.5 fm relative to those of the core nuclei. The results suggest a two-neutron halo structure in Mg40 for a subset of parameters, reinforcing the prediction of a Borromean halo nucleus. The calculations indicate that a two-neutron halo is even more likely for Na39. As expected, the halo is linked to the disappearance of the shell gap in these nuclei due to the inversion of the 2p3/2 and 1f7/2 orbitals. We study the total cross section for scattering of these nuclei from a carbon target using a Glauber model and show that these provide a clear signal to assess the halo structure.

A systematic study of the ground state properties of gold nuclei near the neutron drip line using HFB formalism

In this theoretical work, the ground state properties like binding energy per nucleon, two-neutron separation energy, two-neutron shell gap, neutron-pairing gap, neutron rms radii, proton rms radii, charge radii, neutron skin thickness and nucleon density distributions of odd-even and odd-odd gold isotopes (A165−265u) were systematically studied. Computations were performed for a wide mass range of gold nuclei, spanned from the proton-rich side to the neutron-rich side, following the Hartree-Fock-Bogoliubov theory. Based on this approach, the nuclear structure of gold isotopes lying up to the exotic neutron rich region where the experimental data are not available, was also investigated. Calculations, taking into account the UNEDF0 Skyrme effective interaction, reproduce the available experimental data and results of other nuclear model based estimations, such as the Relativistic-Continuum-Hartree-Bogoliubov theory and the Finite Range Droplet Models, reasonably well.

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.

Shape evolution and shape coexistence in even-even Mo isotopic chain

The phenomena of shape evolution and shape coexistence within the Molybdenum isotopic chain are investigated using the covariant density functional theory with the parameterizations DD-ME2 and DD-PC1. Furthermore, various ground state properties of this chain are investigated, including binding energy, two-neutron separation energy, charge radii, and two-neutron shell gap. A robust agreement is observed when comparing with the available experimental data. The ground state deformation of Mo isotopes evolves smoothly and correlates with the continuous and gradual changes observed in the physical properties. A noticeable phenomenon of shape coexistence can be observed in certain isotopes, marked by the simultaneous existence of both a triaxial and an oblate axial minimum. A pronounced and well-defined shell closure is prominently evident at the neutron magic number N=82.

Shell-model study for 204–210Tl isotopes and core excitations across the Z = 82 and N = 126 shell gaps

In the present work, the 204–210Tl isotopes have been investigated by performing large-scale shell-model calculations, including configurations allowing both neutron and proton core excitations across the Z=82 and N=126 shell gaps. Inspired by the recent high-spin experimental data, the structure of Tl isotopes has been studied for a considerably large model space. The KHHE interaction has been used for 204–206Tl isotopes, KHH7B interaction for 204–210Tl isotopes, and additionally KHM3Y interaction has been used for 208Tl isotope. The core excitation has been performed using the KHH7B and KHM3Y interactions. The level spectra of 204–210Tl isotopes are comprehensively described and explained by multi-nucleon couplings of single-particle-hole orbitals within the valence space and by core excitations across 208Pb core. The well-known isomeric states are analyzed in terms of the shell model configurations.

The 46Ar(3He, d)47K direct reaction as a probe of the 46Ar proton wavefunction

The discrepancy between shell-model calculations and intermediate-energy Coulomb excitation measurements in 46Ar still stands as an unsolved puzzle in understanding the N = 28 shell evolution. This phenomenon has significant relevance considering the remarkable achievements of the shell model and the SDPF-U interaction in the region which is able to predict the fading of the N = 28 shell gap in neutron-rich 44S. Recent measurements narrowed down this discrepancy to an overestimation of the proton amplitude to the quadrupole transition matrix element. The current work aims to propose a different perspective on the puzzle, by studying a direct proton-transfer reaction on 46Ar as a means to directly probe the proton wavefunction of the ground state this isotope. By measuring the amount of l = 0 transfer to the ground state (1/2+) of 47K with respect to the l = 2 to the first excited state (3/2+), we aim to gain insight into the ground state proton wavefunction of 46Ar. We will present a brief description of the experiment performed at the SPIRAL1 facility in GANIL (France). The experimental apparatus allowed a full reconstruction of the two-body reaction thanks to the combination of AGATA, VAMOS, MUGAST, CATS2, and HECTOR.

N=16 Magicity Revealed at the Proton Drip Line through the Study of ^{35}Ca.

The last proton bound calcium isotope ^{35}Ca has been studied for the first time, using the ^{37}Ca(p,t)^{35}Ca two neutron transfer reaction. The radioactive ^{37}Ca nuclei, produced by the LISE spectrometer at GANIL, interacted with the protons of the liquid hydrogen target CRYPTA, to produce tritons t that were detected in the MUST2 detector array, in coincidence with the heavy residues Ca or Ar. The atomic mass of ^{35}Ca and the energy of its first 3/2^{+} state are reported. A large N=16 gap of 4.61(11)MeV is deduced from the mass measurement, which together with other measured properties, makes ^{36}Ca a doubly magic nucleus. The N=16 shell gaps in ^{36}Ca and ^{24}O are of similar amplitude, at both edges of the valley of stability. This feature is discussed in terms of nuclear forces involved, within state-of-the-art shell model calculations. Even though the global agreement with data is quite convincing, the calculations underestimate the size of the N=16 gap in ^{36}Ca by 840keV.

Cross-shell states in 15C: A test for p-sd interactions

The low-lying structure of 15C has been investigated via the neutron-removal 16C(d,t) reaction. Along with the known bound neutron sd-shell states, unbound p-shell hole states have been observed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the [(p)−1(sd)2] neutron configurations in 15C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for 15C. However, this same interaction predicted energies that were too low for the corresponding hole states in the N=9 isotone 17O and adjustment of the p-sd and sd-sd monopole terms was required to match the 17O energies. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of 15C with the ab initio self-consistent Green's function method employing the NNLOsat interaction. The results show the sensitivity to the size of the N=8 shell gap and highlight the need to go beyond the current truncation scheme.

Deformed shell stabilities influencing the fragment formation in sub-lead nuclei

The fragment mass distributions from fission in the sub-lead nuclei gave traces of the co-existence of symmetric and asymmetric fission modes. To investigate the major drivers behind the observed asymmetry, we have accumulated the fission data of 19 neutron-deficient lead island nuclei from the last few years and compared them with the corresponding predictions made by the GEF (General description of fission observables) model. Consequently, both experimental and theoretical analysis performed to extract asymmetric fission properties led to similar results, showing the deformed proton shell gaps, at Z = 34 for light fragments and Z = 42, 44, and 46 for heavier fragments, associated with large quadrupole deformations as the possible origin for the asymmetric split for all isotopes ranging from 178−198Hg, 176−186Pt, and 179−191Au. These observations contradicted the earlier theoretical report where neutron numbers were declared to play a leading role in deciding asymmetric fission. This disagreement motivated us to get better insight by extending the examination over a large domain around Pb covering Hg, Pt, Au, Pb, and Po nuclides. Although protons in the light fragments are still spotted to be constant and influenced by the deformed shell gaps at Z = 34, protons in the heavier fragments are found increasing with rising ZCN and gave evidence of negligible influence from Z = 42, 44, and 46 shell gaps. Interestingly, the excellent compatibility of GEF results with the experimental outcomes proved it an efficient model for extracting fragment properties. However, it could only reproduce the experimental symmetric fission fraction (SFF) within the 30–52 MeV energy window. The GEF's inadequate predictability of the multichance fission contributions might be considered as one of the possible reasons behind the observed disagreements between the GEF and experimental SFF above 52 MeV. Moreover, the other reason might be the inadequate modeling of the evolution of the fission fragment mass/charge distribution with excitation energy.

Intruder configurations in 29Ne at the transition into the island of inversion: Detailed structure study of 28Ne

Detailed γ-ray spectroscopy of the exotic neon isotope 28Ne has been performed for the first time using the one-neutron removal reaction from 29Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.

Dispersive optical model analysis of nucleon scattering on Zr90

$^{90}\mathrm{Zr}$ is a double-magic spherical nucleus. Nucleon scattering on such a nucleus can be well described by spherical optical model potentials. Nucleon scattering data including neutron total cross section, elastic scattering angular distributions and analyzing powers, and low-energy scattering observables are used to derive a new Lane-consistent spherical optical model potential that obeys dispersive constraints, features nonlocality of the real part, and considers the shell gap in the potential. It is shown that the derived dispersive optical potential gives a good description of the energies of the bound single-particle states of the target nucleus $^{90}\mathrm{Zr}$.

A Study of the Weakly Bound Structure of Nuclei Around the Magic Number N=50

An investigation of the quadrupole deformation of Kr, Sr, Zr, and Mo isotopes has been conducted using the HFB method and SLy4 Skyrme parameterization. The primary role of occupancy of single particle state 2d5/2 in the existence of the weakly bound structure around N=50 is probed. Shell gaps are performed using a few other calculations for the doubly magic number 100Sn using different Skyrme parameterizations. We explore the interplays among neutron pairing strength and neutron density profile in two dimensions, along with the deformations of 100Sn.

Merging of the island of inversion at N = 40 and N = 50

We performed a quantitative study of the nuclei with proton numbers from Z=20 (Ca) to Z=28 (Ni) and neutron numbers ranging from N=38 to 52 using the realistic shell model, which merged the descriptions of nuclei around N=40 and N=50 within an extended model space. The 21+ excitation energy (E(21+)) is calculated for neutron-rich Cr, Fe, and Ni isotopes. The results suggest that the islands of inversion at N=40 and N=50 merge around the Cr chain, and the N=50island of inversion is located below 78Ni. To demonstrate the prediction of the N=50island of inversion, we calculated and compared the E(21+), B(E2;01+→21+), effective single-particle energies (ESPEs), the probability of particle-hole excitation, and the average occupations in the N=40 and N=50 isotones. Similar parabolic trends of E(21+) and B(E2;01+→21+) in the N=40 and 50 isotones are obtained. The calculated ESPEs give that the N=40 and N=50 shell gaps both increase with the variation of proton number from Z=20 (Ca) to Z=28 (Ni). However, our calculations indicate that different particle-hole excitations dominate in the ground states of the N=40 and N=50islands of inversion, with 2p2h and 4p4h configurations being prominent in N=40 isotones, whereas 2p2h excitations are most important in the N=50 isotones. Furthermore, large occupations of the intruder quadrupole partner orbitals are also obtained in N=40 and N=50 isotones in our realistic shell model calculations.

Discovery of New Isotope ^{241}U and Systematic High-Precision Atomic Mass Measurements of Neutron-Rich Pa-Pu Nuclei Produced via Multinucleon Transfer Reactions.

The new isotope ^{241}U was synthesized and systematic atomic mass measurements of nineteen neutron-rich Pa-Pu isotopes were performed in the multinucleon transfer reactions of the ^{238}U+^{198}Pt system at the KISS facility. The present experimental results demonstrate the crucial role of the multinucleon transfer reactions for accessing unexplored neutron-rich actinide isotopes toward the N=152 shell gap in this region of nuclides.

Validation of HIV-1 MA Shell Structural Arrangements and Env Protein Interactions Predict a Role of the MA Shell in Viral Maturation.

The molecular structure of the type 1 human immunodeficiency virus (HIV-1) is tightly linked to the mechanism of viral entry. The spike envelope (Env) glycoproteins and their interaction with the underlying matrix (MA) shell have emerged as key components of the entry mechanism. Microscopy evidence suggests that the MA shell does not span the entire inner lipid surface of the virus, producing a region of the virus that completely lacks an MA shell. Interestingly, evidence also suggests that Env proteins cluster during viral maturation and, thus, it is likely that this event takes place in the region of the virus that lacks an MA shell. We have previously called this part of the virus a fusion hub to highlight its importance during viral entry. While the structure of the MA shell is in contention due to the unaddressed inconsistencies between its reported hexagonal arrangement and the physical plausibility of such a structure, it is possible that a limited number of MA hexagons could form. In this study, we measured the size of the fusion hub by analysing the cryo-EM maps of eight HIV-1 particles and measured the size of the MA shell gap to be 66.3 nm ± 15.0 nm. We also validated the feasibility of the hexagonal MA shell arrangement in six reported structures and determined the plausible components of these structures that do not violate geometrical limitations. We also examined the cytosolic domain of Env proteins and discovered a possible interaction between adjacent Env proteins that could explain the stability of cluster formation. We present an updated HIV-1 model and postulate novel roles of the MA shell and Env structure.

Quasi-SU(3) Coupling Induced Oblate-Prolate Shape Phase Transition in the Casten Triangle

Shapes and shape evolution in the mass-130 region, including the Te, Xe, and Ba isotopes, have long been a focus of discussion in nuclear physics. This mass region consists of complex many-body systems that can behave in astonishingly simple and regular ways, as classified in the Casten symmetry triangle. By applying the shell model Hamiltonian proposed recently, we carry out calculations using the Hartree-Fock-Bogolyubov plus generator coordinate method, in the large model space containing the (1g_{9/2},1g_{7/2},2d_{5/2},2d_{3/2},3s_{1/2},1h_{11/2},2f_{7/2}) orbits. Based on good reproduction of the experimentally known energy levels, spectroscopic quadrupole moments, and E2 transition probabilities, we identify the quasi-SU(3) couplings across the N=50 and 82 shell gaps, which play a role in driving shape evolution and phase transition discussed in the extended Casten triangle. Specifically, we demonstrate that the quasi-SU(3) coupling mechanism in the proton partner orbits (1g_{9/2}, 2d_{5/2}) tends to drive the system to be more γ soft, and that in the neutron partner orbits (1h_{11/2}, 2f_{7/2}) are responsible for the oblate-to-prolate shape phase transition. With an emphasis on discussing spectroscopic quadrupole moments, our Letter uncovers hidden symmetries from the vast shell-model configurations and adds microscopical insights into the empirical symmetry triangle.

Study of the N=32 and N=34 Shell Gap for Ti and V by the First High-Precision Multireflection Time-of-Flight Mass Measurements at BigRIPS-SLOWRI.

The atomic masses of ^{55}Sc, ^{56,58}Ti, and ^{56-59}V have been determined using the high-precision multireflection time-of-flight technique. The radioisotopes have been produced at RIKEN's Radioactive Isotope Beam Factory (RIBF) and delivered to the novel designed gas cell and multireflection system, which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For ^{56,58}Ti and ^{56-59}V, the mass uncertainties have been reduced down to the order of 10keV, shedding new light on the N=34 shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species ^{58}Ti and ^{59}V. With the new precision achieved, we reveal the nonexistence of the N=34 empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied νp_{3/2} orbit is identified as a feature unique to Ca. We perform new MonteCarlo shell model calculations including the νd_{5/2} and νg_{9/2} orbits and compare the results with conventional shell model calculations, which exclude the νg_{9/2} and the νd_{5/2} orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at N=34.