Neutron Drip Line Nuclei Research Articles

Discovery of ^{39}Na.

The new isotope ^{39}Na, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ^{48}Ca beam at 345 MeV/nucleon on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ^{39}Na events have been unambiguously observed in this work and clearly establish the particle stability of ^{39}Na. Furthermore, the lack of observation of ^{35,36}Ne isotopes in this experiment significantly improves the overall confidence that ^{34}Ne is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ^{39}Na has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ^{39}Na at the neutron number N=28, which is normally a magic number.

Hyperspherical three-body model calculation for the bound and resonant states of the neutron dripline nuclei 42,44Mg using isospectral potential

In this paper, an effective theoretical scheme is presented to investigate ground and resonant levels of weakly bound nuclei near the neutron-dripline. The hyperspherical harmonics expansion method is employed for the bound state while supersymmetric quantum mechanics (SSQM) is used for the resonant state. The three-body Schrödinger equation is solved for the lowest bound state with chosen set of two-body potentials to obtain the energy and normalized wavefunction. The three-body effective potential has a very shallow well following a low-wide barrier giving a broad resonance due to far extended wavefunction in the asymptotic region. Numerical computation of such broad resonance is very challenging due to large computational errors. Hence, a one-parameter family of isospectral potential is constructed with the ground state energy and wavefunction, by the use of the algebra of SSQM. This enhanced potential facilitates a more accurate computation of the resonance energy and width. Effectiveness of the scheme has been checked for the J=π0+ bound and resonant states of neutron-rich nuclei 42,44Mg in the coreNN three-body cluster model where core=40,42Mg. In the present calculation, two-neutron separation energies obtained with very large number of partial waves for the J=π0+ bound states of neutron-rich nuclei 42,44Mg are found to be 0.41433 MeV and 0.44975 MeV respectively. Results of the calculation have been compared with those found in the literature.

Exploring the halo phenomena of medium-mass nuclei having approximately Z= 40 with point-coupled parameters in complex momentum representations * *Supported by the National Natural Science Foundation of China (11875070) and the Natural Science Foundation of Anhui Province (1908085MA16)

To explore the properties of neutron-rich nuclei with approximately 40 protons, the density-dependent point coupling (DD-PC1) effective interaction parameter is adopted in the relativistic mean-field theory with the complex momentum representation (RMF-CMR). The calculated two-neutron separation energy (S 2n ) and root-mean-square (rms) radii support the halo structure that appear in Mo and Ru isotopic chains. Besides, the neutron skin structures appear in Kr and Sr isotopes. The conclusions drawn are also supported by the single-particle energy levels and their occupancy probability and density distribution. Particularly, the energy levels, which reduce to bound states or are approximately 0 MeV with a small orbital angular momentum, are suggested to provide the primary contribution to increasing the neutron radius. Moreover, the single-particle energy levels significantly reflect the shell structure. In addition, the neutron drip line nuclei for Kr, Sr, Mo, and Ru elements are proposed via the changes in S 2n .

A study of nuclear radii and neutron skin thickness of neutron-rich nuclei near the neutron drip line

We studied the charge radius ([Formula: see text]), neutron radius ([Formula: see text]), and neutron skin-thickness ([Formula: see text]) over a chain of isotopes from C to Zr with the stable region to the neutron drip line. Theoretical calculations are done with axially deformed self-consistent relativistic mean-field theory (RMF) and effective nonlinear NL3 and NL3* interactions. The theoretically estimated values are compared with available experimental data and a reasonable agreement is noted. We additionally assessed the two-neutron separation energy ([Formula: see text]) to mark the drip line nuclei of the considered isotopic series. In the reference of [Formula: see text], neutron magicity is also discussed. The calculated neutron radii are compared with empirical estimation made by [Formula: see text] to examine the abnormal trend of the radius for neutron drip line nuclei. In view to guide the long tails, the density distribution for some skin candidates is analyzed. Finally, neutron skin thickness is observed for the whole considered isotopic series.

Cross-section prediction for isotopes near neutron drip line in 70, 80Zn projectile fragmentation reactions * * Supported by the National Natural Science Foundation of China (U1732135) and Natural Science Foundation of Henan Province (162300410179)

The cross sections for 59, 60Ca, recently measured in the 345 A MeV 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 in the 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 can be discovered in reactions of 60, 80 A MeV . 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 and neighboring neutron-drip line nuclei.

Study of spin-isospin responses of radioactive nuclei with the background-reduced neutron spectrometer, PANDORA

The status of a project to measure spin-isospin responses of neutron drip-line nuclei using a new low-energy neutron detector, PANDORA (Particle Analyzer Neutron Detector Of Real-time Acquisition), is reported. The performance of PANDORA was characterized by the 6He(p,n)6Li reaction in inverse kinematics at the HIMAC facility in Chiba. Observation of the strong transition to the ground state in 6Li is discussed. Preliminary results of 11Li(p,n)11Be and 14Be(p,n)14B experiments in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center are also presented including the exotic decay channel of 11Be→9Li + d. Details of the experimental setup based on PANDORA and the SAMURAI large-acceptance magnetic spectrometer, as well as the combined data-acquisition system are described. The neutron-gamma discrimination capability of PANDORA was evaluated, Figure-of-Merit (FoM) values higher than those found in the literature for similar materials were derived from experimental data.

Rotational excitations in near neutron-drip line nuclei: The birth and death of particle-bound rotational bands and the extension of nuclear landscape beyond spin zero neutron drip line

Two new mechanisms active in rotating nuclei located in the vicinity of neutron drip line have been discovered. Strong Coriolis interaction acting on high-j orbitals transforms particle-unbound (resonance) nucleonic configurations into particle-bound ones with increasing angular momentum. The point of the transition manifests the birth of particle-bound rotational bands. Alternative possibility of the transition from particle-bound to resonance rotational band (the death of particle-bound rotational bands) with increasing spin also exists but it is less frequent in the calculations. The birth of particle-bound rotational bands provides a mechanism for the extension of nuclear landscape to neutron numbers which are larger than those of the neutron drip line in non-rotating nuclei.

Shape and shell structure of lighter ( N≲90 ) neutron-rich nuclei based on a phenomenological Woods-Saxon potential

By using a phenomenologically successful Woods-Saxon potential, I study the shape and shell structure of (A) neutron drip line nuclei with 10 \leq N \leq 60, (B) neutron-rich nuclei related to the r- process with 40 \leq N \leq 90, and (C) one-particle spectra in the potential provided by the nucleus 70Fe as a representative of so-called N=40 "island of inversion (IoI)" nuclei. First, the shell structure that is unique in very weakly-bound neutrons is systematically studied, and the approximate neutron number of odd-N nuclei at which spherical (or deformed) halos can be found is pinned down. Second, the difference of the shell structure in r-process nuclei from that in stable nuclei is examined. Third, the similarity and the difference between the shell structure of N=20 IoI nuclei and that of N=40 IoI nuclei are analyzed. As a result of it, it is concluded that in Fe and Cr isotopes the deformation called "N=40 IoI" continuing up to N=50 is unlikely.

Odd–even staggering in neutron drip line nuclei

We have done systematic Hartree–Fock–Bogoliubov calculations in coordinate space on the one-quasi-particle energies and binding energy odd–even staggering (OES) in semi-magic nuclei with the zero-range volume, mixed and surface pairing forces in order to explore the influence of their density dependence. The odd-N isotopes are calculated within the blocking scheme. The strengths for the pairing forces are determined in two schemes by fitting locally to reproduce pairing gap in 120Sn and globally to all available data on the OES of semi-magic nuclei with Z≥8. In the former calculations, there is a noticeable difference between the neutron mean gaps in neutron-rich O, Ca, Ni and Sn isotopes calculated with the surface pairing and those with the mixed and volume pairing. The difference gets much smaller if the globally optimized pairing strengths are employed. The heavier Pb isotopes show the opposite trend. Moreover, large differences between the mean gap and the OES may be expected in both calculations when one goes towards the neutron drip line.

Nuclear rotation in the continuum

${\textbf{Background:}}$ Atomic nuclei often exhibit collective rotational-like behavior in highly excited states, well above the particle emission threshold. What determines the existence of collective motion in the continuum region, is not fully understood. ${\textbf{Purpose:}}$ In this work, by studying the collective rotation of the positive-parity deformed configurations of the one-neutron halo nucleus $^{11}$Be, we assess different mechanisms that stabilize collective behavior beyond the limits of particle stability. ${\textbf{Method:}}$ To solve a particle-plus-core problem, we employ a non-adiabatic coupled-channel formalism and the Berggren single-particle ensemble, which explicitly contains bound states, narrow resonances, and the scattering continuum. We study the valence-neutron density in the intrinsic rotor frame to assess the validity of the adiabatic approach as the excitation energy increases. ${\textbf{Results:}}$ We demonstrate that collective rotation of the ground band of $^{11}$Be is stabilized by (i) the fact that the $\ell=0$ one-neutron decay channel is closed, and (ii) the angular momentum alignment, which increases the parentage of high-$\ell$ components at high spins; both effects act in concert to decrease decay widths of ground-state band members. This is not the case for higher-lying states of $^{11}$Be, where the $\ell=0$ neutron-decay channel is open and often dominates. ${\textbf{Conclusion:}}$ We demonstrate that long-lived collective states can exist at high excitation energy in weakly bound neutron drip-line nuclei such as $^{11}$Be.

Effects of pairing correlation on the low-lying quasiparticle resonance in neutron drip-line nuclei

Effects of pairing correlation on the low-lying quasiparticle resonance in neutron drip-line nuclei

Neutron occupancy of 0d5/2orbital in24O

The partial one-neutron removal cross section of 24 O leading to the first ex- cited state in 23 O has been measured to be 65.7 5.3 mb with a proton target in inverse kinematics at the beam energy of 62 MeV/nucleon. The decay energy spectrum of the neutron unbound state of 23 O was reconstructed from the measured four momenta of 22 O and the emitted neutron. A sharp peak was observed at Edecay = 48 3 keV, confirming the previous measurements. The measured partial longitudinal momentum distribution of 23 O showed the d-wave knockout character, providing a support for the J assignment of 5/2 + for this state. The spectroscopic factor for the neutrons in 0d5=2 orbital was deduced, for the first time, to be C 2 S (0d5=2) = 3.7 0.3 (preliminary) by comparing with the result of Glauber model. The relatively large spectroscopic factor supports the N = 16 shell closure in the neutron drip-line nucleus 24 O.

Extending the nuclear chart by continuum: From oxygen to titanium

Nuclear masses ranging from O to Ti isotopes are systematically investigated with relativistic continuum Hartree-Bogoliubov (RCHB) theory, which can provide a proper treatment of pairing correlations in the presence of the continuum. From O to Ti isotopes, there are 402 nuclei predicted to be bound by the density functional PC-PK1. For the 234 nuclei with mass measured, the root mean square (rms) deviation is 2.23 MeV. It is found that the proton drip-lines predicted with various mass models are roughly the same and basically agree with the observation. The neutron drip-lines predicted, however, are quite different. Due to the continuum couplings, the neutron drip-line nuclei predicted are extended further neutron-rich than other mass models. By comparison with finite-range droplet model (FRDM), the neutron drip-line nucleus predicted by RCHB theory has respectively 2(O), 10(Ne), 10(Na), 6(Mg), 8(Al), 6(Si), 8(P), 6(S), 14(K), 10(Ca), 10(Sc), and 12(Ti) more neutrons.

Breakup reactions of neutron drip-line nuclei near N = 20, 28

The cross sections of one-neutron removal via Coulomb and nuclear breakup from the very neutron-rich nuclei 29,31Ne, 33,35,37Mg and 39,41Si and the momentum distributions of the fragments have been measured at around 240 MeV/nucleon. These reactions are combined to identify halo nuclei and obtain the spectroscopic factors, separation energies and spin-parities of ground states. The Coulomb breakup cross sections showed significant enhancement for 31Ne and 37Mg, which suggests halo formation. Further analysis is in progress.

Expansion of the 16O-core in neutron drip-line nuclei: 23O and 24O

Abstract We study an effect of the expansion of the core nucleus to the radius of 23 O and 24 O. In order to clarify the importance of the expansion of the core, we perform two calculations; an m -scheme approach of the cluster-orbital shell model for O 16 + X n systems and a simplified model approach for core + n and core + 2 n systems. We calculate the energy and r.m.s. radius of oxygen isotopes by using these two approaches and discuss the effect of the expansion of the 16 O-core and the relation to the presence of valence neutrons. Furthermore, we propose a new approach of a coupled-channel model including a particle–hole excitation in the 16 O-core and the motion of the valence neutron.

Neutron Halo Structure Probed by Breakup Reactions

The neutron drip line represents the boundary of nuclei in the neutron-rich side of the nuclear chart. In the vicinity of the neutron drip line, we often observe 'neutron halo' structure. We discuss how the neutron halo nuclei are studied by the breakup reactions at relativistic energies. Coulomb breakup is the dominant process in the breakup with a heavy target, such as Pb. In the Coulomb breakup of halo nuclei, enhancement of the electric dipole strength at low excitation energies (soft E1 excitation) is observed as a unique property for halo nuclei. The mechanism of the soft E1 excitation and its spectroscopic significance is shown as well as the applications of the Coulomb breakup to the very neutron rich 22C and 31Ne, which was measured at 230-240 MeV/nucleon at the new-generation RI beam facility, RIBF(RI Beam Factory), at RIKEN. Evidence of halo structures for these nuclei is provided as enhancement of the inclusive Coulomb breakup, which is a useful tool for the low-intense secondary beam. We also show that the breakup with a light target (C target), where nuclear breakup is a dominant process, can be used to extract the spectroscopic information of the removed neutron. The combinatorial analysis was found very useful to extract more-detailed information such as the spectroscopic factor and the separation energy. Prospects of the breakup reactions on neutron-drip line nuclei at RIBF at RIKEN are also briefly presented.

Vorticity of threshold strength in neutron drip-line nuclei

By adopting a finite square-well potential, the analytic expressions of vorticity strengths, which are related to transverse currents, are derived for some selected E1 and E2 transitions. From these analytic expressions it can be seen clearly that, like the threshold strengths, the vorticity strengths are also sharply distributed just above the threshold in nuclei near the drip-line and mainly come from the orbits with small binding energy.

Breakup Reactions of Neutron Drip Line Nuclei Near N=20

Coulomb breakup at intermediate energies is a useful experimental tool for investigating the microscopic structure of neutron drip-line nuclei. Here, results from the inclusive Coulomb breakup experiment of 31Ne on a lead target at RIBF(RI Beam Factory) at RIKEN are presented. The experiment was performed as one of day-one campaign experiments at RIBF, using a 48Ca primary beam at 345 MeV/nucleon. A unique feature of a halo nucleus is the enhanced electric dipole strength of the order of 1 W.u.(Weisskopf unit) at very low excitation energies around 1 MeV (soft E1 excitation). Owing to high sensitivity of the Coulomb breakup to the soft E1 excitation, a measurement of inclusive Coulomb breakup cross section can be used to identify the halo structure of a certain drip-line nucleus. We have indeed observed a strong enhancement of the Coulomb breakup cross section of 540(70) mb for 31Ne on Pb at 230 MeV/nucleon, nearly as high as that for the known halo nucleus 19C, thereby giving evidence of the halo structure in 31Ne. The finding of a new halo structure for such a heavy system, compared to the known halo nuclei, is the first step for the understanding of halo phenomena along the neutron drip line towards heavier nuclei. We discuss also the change of shell structure in 31Ne, as a nucleus in the island of inversion.

Properties of Zr hypernuclei in deformed Skyrme Hartree–Fock approach

Properties of the even-N Zr isotopes and their corresponding single-Λ and double-Λ hypernuclei are studied in the deformed Skyrme–Hartree–Fock approach. Binding energy, the two-neutron separation energies, radii are studied from beta-stable zone to the neutron drip line in this paper. The neutron drip line nuclei predicted with SLy4 and SkI4 interactions are 122Zr and 138Zr, respectively. The neutron drip line of single-Λ and double-Λ hypernuclei are 139ΛZr and 1422ΛZr with SkI4 interaction, respectively. The predicted hyperon drip line hypernuclei with 80Zr and 138Zr cores are 10020ΛZr and 18850ΛZr, respectively.

One-particle properties of deformedN≈28odd-Nnuclei with weakly bound or resonant neutrons

Possible deformation of odd-N nuclei with N approximate to 28 toward the neutron drip line is investigated using the Nilsson diagram based on deformed Woods-Saxon potentials. Both weakly-bound and resonant one-particle levels are properly obtained by directly solving the Schrodinger equation in mesh of space coordinate with the correct boundary condition. If we use the same diffuseness of the potential as that of beta-stable nuclei, the energy difference between the neutron 2p(3/2) and 1f(7/2) levels becomes very small or the N = 28 energy gap almost disappears, as the binding energies of those levels approach zero. This suggests that the ground states of those neutron drip line nuclei are likely to be deformed. In particular, the spin-parity and the magnetic moment of the ground state of odd-N nuclei, S-43(16)27 and S-45(16)29, are examined. Moreover, it is suggested that in Mg-39(12)27 lying outside the drip line the lowest resonant state may have 5/2(-), if the N = 28 energy gap almost vanishes. (Less)