Engineering topics
Xu, F. R.
Publications and source records attributed to Xu, F. R..
First observation of isomeric states in 111 Zr, 113 Nb, and 115 Mo
Isomeric states in the neutron-rich nuclei 111 Zr [T 1/2 =0.10 (7) μs] , 113 Nb [T 1/2 =0.7 (4) μs], 115 Mo [T 1/2 =46 (3) μs] were first identified at the Radioactive Ion Beam Factory (RIBF) of RIKEN by using in-flight fission and fragmentation of a 238 U beam at an energy of 345 MeV/u. Here is a brief report of the γ transitions de-exciting from isomeric states and half-lives measurements, which provides the first spectroscopy in the nuclear region of prolate-to-oblate shape-phase transition around mass A ≈110.
Identification of excited states in Bi 188 and Po 188
The neutron-deficient 188 Bi and 188 Po isotopes have been studied by γ-ray spectroscopy using the recoil-decay tagging technique with the Argonne Gas-Filled Analyzer. A new 0.25(5)-μs isomeric state and a prompt cascade formed by 319-, 366-, and 462-keV γ rays have been established on top of the (10¯) α -decaying isomer in 188 Bi. The first excited (2 + ) state in 188 Po was identified, its excitation energy of 242(2) keV continues the nearly constant trend for the first 2 + states in 190, 192,194 Po. Here, the state is most likely a member of a prolate rotational band built on the ground state, albeit mixing with other coexisting configurations cannot be excluded. The new results obtained in the present work provide new information to shape coexistence in bismuth and polonium isotopes near the neutron midshell at N = 104 . In this mass region, a reduction in the prompt γ-ray yield obtained with recoil decay tagging was observed for a few nuclides, and the possible reasons are presented.
First observation of the decay of the 13/2 + isomer in 183 Hg and B(M 2) systematics of neutron transitions across the nuclear chart
The decay of the 13/2 + isomeric state in 183 Hg was observed for the first time following the a decay of the 13/2 + isomer in 187 Pb produced in the 142 Nd( 50 Cr, 2p3n) reaction. Using $\alpha - \gamma$ delayed coincidence measurements, the half-life of this isomer was measured to be 290(30) μs. This isomer is proposed to deexcite by an unobserved low-energy M2 transition to the known 9/2 - member of a strongly prolate-deformed 7/2 - [514] band, followed by a 105-keV M1 transition to the bandhead. A lower limit of B(M2) >= 0.018 W.u. was deduced for the unobserved transition. The presumed retardation is proposed to be due to the notable shape change between the initial, nearly spherical, and the final, strongly deformed, states. A similar scenario is also considered for the 13/2 + isomer in 181 Hg, suggesting both are cases of shape isomers. The B(M2) systematics of neutron transitions across the nuclear chart is discussed.
First observation of a shape isomer and a low-lying strongly-coupled prolate band in neutron-deficient semi-magic 187 Pb
Prompt and delayed gamma-ray spectroscopy of the neutron-deficient, semi-magic isotope 187Pb has been performed using the recoil-decay and isomer-decay tagging techniques at the Argonne Gas-Filled Analyzer. A new 5.15(15)-mu s isomeric state at only 308 keV above the spherical 3/2 - ground state is identified and classified as a shape isomer. A strongly-coupled band is observed on top of the isomer, which is nearly identical to the one built on the prolate 7/2 - [514] Nilsson state in the isotone 185 Hg. Based on this similarity and on the result of the potential-energy surface calculations, the new isomer in 187 Pb is proposed to originate from the same configuration. The retarded character of the 308-keV (7/2( - )→ 3/2$_{gs}^{-}$ transition with a deduced B(E2) = 5.6(2) x 10 -4 W.u. can be well explained by the significant difference between the prolate parent and spherical daughter configurations, leading to the shape isomerism. The excitation energy of the isomer is surprisingly low, being roughly half of the excitation energies of the known 0 + intruder bandheads in the neighboring 186,188 Pb isotopes. The combined results of the present work and the previous alpha-decay and laser spectroscopy studies present evidence for triple shape coexistence at low energy in the negative-parity configurations of 187 Pb, which is well reproduced by the potential-energy surface calculations.
The roles of three-nucleon force and continuum coupling in mirror symmetry breaking of oxygen mass region
With both three-nucleon force and continuum coupling included, we have developed a self-consistent ab initio Gamow shell model within the Gamow Hartree-Fock (GHF) basis obtained by the realistic interaction itself. With the chiral two-nucleon N 3 LO and three-nucleon N 2 LO interactions, the Gamow shell model has been applied to the mirror systems of Z = 8 neutron-rich isotopes and N = 8 proton-rich isotones, giving good agreements with data in binding energies, dripline positions and excitation spectra. The GHF calculated that the 0 ds/2 , 1 S1/2 and 1 p3/2 orbitals are resonances. The resonance states and their interplay with nonresonant continua play a crucial role in the descriptions of nuclei around driplines. Excitation spectra and Thomas-Ehrman shifts observed can be better described when both three-nucleon force and continuum coupling are considered in calculations. The three-nucleon force and continuum coupling produce a combined effect on the Thomas-Ehrman shift, e.g., for the 1/2 + resonance level of 19Na. The calculations help the understandings of related nuclear astrophysical processes.
Resonances of A = 4T = 1 isospin triplet states within the ab initio no-core Gamow shell model
Here, the A = 4 nuclei, i.e., 4 H, 4 He, and 4 Li , establish an interesting isospin T = 1 isobaric system. 4 H and 4 Li are unbound broad resonances, whereas 4 He is deeply bound in its ground state but unbound in all its excited states. The present situation is that experiments so far have not given consistent data on the resonances. Few-body calculations have well studied the scatterings of the 4N systems. In the present work, we provide many-body calculations of the broad resonance structures, in an ab initio framework with modern realistic interactions. It occurs that, indeed, 4 H, 4 Li, and excited 4 He are broad resonances, which is in accordance with experimental observations. The calculations also show that the first 1 - excited state almost degenerates with the 2 - ground state in the pair of mirror isobars of 4 H and 4 Li, which may suggest that the experimental data on energy and width are the mixture of the ground state and the first excited state. The T = 1 isospin triplet formed with an excited state of 4 He and ground states of 4 H and 4 Li is studied, focusing on the effect of isospin symmetry breaking.
Tensor force role in β decays analyzed within the Gogny-interaction shell model
The half-life of the famous C 14 β decay is anomalously long, with different mechanisms: the tensor force, cross-shell mixing, and three-body forces, proposed to explain the cancellations that lead to a small transition matrix element. In this study, we revisit and analyze the role of the tensor force for the β decay of C 14 as well as of neighboring isotopes. We add a tensor force to the Gogny interaction, and derive an effective Hamiltonian for shell-model calculations. The calculations were carried out in a p – s d model space to investigate cross-shell effects. Furthermore, we decompose the wave functions according to the total orbital angular momentum L in order to analyze the effects of the tensor force and cross-shell mixing. The inclusion of the tensor force significantly improves the shell-model calculations of the β -decay properties of carbon isotopes. In particular, the anomalously slow β decay of C 14 can be explained by the isospin T = 0 part of the tensor force, which changes the components of N 14 with the orbital angular momentum L = 0 , 1 , and results in a dramatic suppression of the Gamow-Teller transition strength. At the same time, the description of other nearby β decays are improved. Decomposition of wave function into L components illuminates how the tensor force modifies nuclear wave functions, in particular suppression of β -decay matrix elements. Cross-shell mixing also has a visible impact on the β -decay strength. Inclusion of the tensor force does not seem to significantly change, however, binding energies of the nuclei within the phenomenological interaction.
Reexamining the variational two-particle reduced density matrix for nuclear systems
We report that in most nuclear many-body methods, observables are calculated using many-body wave functions explicitly. The variational two-particle reduced density matrix method is one of the few exceptions to the rule. Ground-state energies of both closed-shell and open-shell nuclear systems can indeed be evaluated by minimizing a constrained linear functional of the two-particle reduced density matrix. However, it has virtually never been used in nuclear theory, because nuclear ground states were found to be well overbound, contrary to those of atoms and molecules. Consequently, we introduced new constraints in the nuclear variational two-particle reduced density matrix method, developed recently for atomic and molecular systems. Our calculations then show that this approach can provide a proper description of nuclear systems where only valence neutrons are included. For the nuclear systems where both neutrons and protons are active, however, the energies obtained with the variational two-particle reduced density matrix method are still overbound. The possible reasons for the noticed discrepancies and solutions to this problem will be discussed.
First observation of collective rotational bands in neutron-rich 142 La and the study of octupole/triaxial deformations in 142,143 La
Collective rotational bands were observed for the first time in neutron-rich odd–odd 142 La by means of γ-γ-γ and γ-γ-γ-γ coincidence measurements of prompt fission γ rays from 252 Cf using multi-detector array Gammasphere. Similarity between the yrast band of 142 La and those of neighboring 144 La, 144 Ce was found and interpreted as quasiparticle excitations. Here, PES calculations suggested triaxial deformation for the ground state of 142La. Considerable triaxial and near zero octupole deformations were deduced for the yrast band of 142 La by the TRS model calculations. The band-crossing observed in the yrast band of 142 La was interpreted to be caused by alignment of the (h 11/2 ) 2 proton pair, and the crossing frequency was best reproduced by the TRS calculations taking into account triaxial deformations. In contrast to the interpretations for the band crossing of 142 La, alignments of the (i 13/2 ) 2 neutron pair were found to be responsible for the band-crossing of the yrast band in the neighboring even-N 143 La, and triaxial degree of freedom was found to play a more significant role than octupole deformations in the nucleus, although its β 3 values deduced are large.
Proton decays in 16 Ne and 18 Mg and isospin-symmetry breaking in carbon isotopes and isotones
We report that proton-rich nuclei possess unique properties in the nuclear chart. Due to the presence of both continuum coupling and Coulomb interaction, phenomena such as halos, Thomas-Ehrman shift, and proton emissions can occur. Relevant experimental data are difficult to obtain, so that theoretical calculations are needed to understand nuclei at drip lines and to guide experimentalists. In particular, the 16 Ne and 18 Mg isotopes are supposed to be one-proton and/or two-proton emitting nuclei, but associated experimental data are either incomplete or even unavailable. Consequently, we performed Gamow shell model calculations of carbon isotones bearing A = 15 - 18 . Isospin-symmetry breaking occurring in carbon isotones and isotopes is also discussed. It is hereby shown that the mixed effects of continuum coupling and Coulomb interaction at drip lines generate complex patterns in isospin multiplets. Added to that, it is possible to determine the one-proton and two-proton widths of 16 Ne and 18 Mg . Obtained decay patterns are in agreement with those obtained in previous experimental and theoretical works. Moreover, to our knowledge, this is the first theoretical calculation of binding energy and partial decay widths of 18 Mg in a configuration interaction picture.
Observation of the near-threshold intruder 0 – resonance in 12 Be
A resonant state at $3.21^{+0.12}_{–0.04}$ MeV, located just above the one-neutron separation threshold, was observed for the first time in 12 Be from the 11 Be(d,p) 12 Be one-neutron transfer reaction in inverse kinematics. This state is assigned a spin-parity of 0 – according to the systematics of the level scheme of the N=8 isotones and decay-width analysis. Gamow coupled-channel and Gamow shell-model calculations show the importance of the continuum coupling, which dramatically influences the excitation energy and ordering of low-lying states. Various exotic structures associated with cross-shell intruding configurations in 12 Be and in its isotonic nucleus 11 Li are comparably discussed.
Unbound spectra of neutron-rich oxygen isotopes predicted by the Gamow shell model
The Gamow shell model has shown to efficiently describe weakly bound and unbound nuclear systems, as internucleon correlations and continuum coupling are both taken into account in this model. In the present work, we study neutron-dripline oxygen isotopes. It is hereby demonstrated that the presence of continuum coupling is important for the description of oxygen isotopes at dripline, and especially to assess the eventual bound or unbound character of 28 O. Our results suggest that the ground state of 28 O is weakly unbound and is similar to the narrow resonant 26 O ground state. Predictions of weakly bound and resonance excited states in 24–26 O are also provided. The asymptotes of the studied many-body states are analyzed via one-body densities, whereby the different radial properties of well bound, loosely bound, resonance states are clearly depicted.
Direct observation of the exotic β – γ – α decay mode in the T z = – 1 nucleus Na 20
The exotic β – γ – α decay mode of 20 Na has been directly observed for the first time in the Day-one experiment at the Beijing Radioactive Ion-beam Facility. The 20 Na source was produced by using a 100-MeV proton beam bombarding a stack of microporous MgO thick target and delivered as an intense mass separated beam after online ionization. A high-efficiency simultaneous measurement of β, γ, and α transitions enables the β-delayed γ – γ and α – γ coincidence spectroscopy. Here three β – γ – α exotic decay sequences in 20 Na are discovered, which expands the rare decay modes observed in β decay. Moreover, a β – α –decay sequence to the 6130-keV 3 – state of 16 O is observed, which is likely through the 12367-keV 1 + state in 20 Ne. The experimentally deduced B(F) and B(GT) are compared to the shell-model calculation, the allowed β transition strengths can be well accounted for by using sd shell-model space interactions.
Shell-model study of calcium isotopes toward their drip line
Here, we report in this paper a study in terms of the nuclear shell model about the location of the calcium isotopes drip line. The starting point is considering the realistic two-body potential derived by Entem and Machleidt within chiral perturbation theory at next-to-next-to-next-to-leading order (N 3 LO), as well as a chiral three-body force at next-to-next-to-leading order (N 2 LO) whose structure and low-energy constants are consistent with the two-body potential. Then we construct the effective single-particle energies and residual interaction needed to diagonalize the shell-model Hamiltonian. The calculated two-neutron separation energies agree nicely with experiment until 56 Ca, which is the heaviest isotope whose mass has been measured, and do not show any sign of two-neutron emission until 70 Ca . We discuss the role of the choice of the model space in determining the neutron drip line, and also the dependence of the results on the parameters of the shell-model Hamiltonian.
Properties of 187 Ta Revealed through Isomeric Decay
Mass-separated 187 Ta114 in a high-spin isomeric state has been produced for the first time by multi-nucleon transfer reactions, employing an argon gas stopping cell and laser ionisation. Here, internal $γ$ rays revealed a $T$ 1/2 = 7.3±0.9 s isomer at 1778±1 keV, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer decay reduced $E$2 hindrance factor, $f$ $ν$ = 27±1, supports the interpretation that axial symmetry is approximately conserved.