Showing papers by "Chong Qi published in 2018"

Recent developments in radioactive charged-particle emissions and related phenomena

Abstract: The advent and intensive use of new detector technologies as well as radioactive ion beam facilities have opened up possibilities to investigate alpha, proton and cluster decays of highly unstable nuclei. This article provides a review of the current status of our understanding of clustering and the corresponding radioactive particle decay process in atomic nuclei. We put alpha decay in the context of charged-particle emissions which also include one- and two-proton emissions as well as heavy cluster decay. The experimental as well as the theoretical advances achieved recently in these fields are presented. Emphasis is given to the recent discoveries of charged-particle decays from proton-rich nuclei around the proton drip line. Those decay measurements have shown to provide an important probe for studying the structure of the nuclei involved. Developments on the theoretical side in nuclear many-body theories and supercomputing facilities have also made substantial progress, enabling one to study the nuclear clusterization and decays within a microscopic and consistent framework. We report on properties induced by the nuclear interaction acting in the nuclear medium, like the pairing interaction, which have been uncovered by studying the microscopic structure of clusters. The competition between cluster formations as compared to the corresponding alpha-particle formation are included. In the review we also describe the search for super-heavy nuclei connected by chains of alpha and other radioactive particle decays.

Lifetime Measurements of Excited States in ^{172}Pt and the Variation of Quadrupole Transition Strength with Angular Momentum.

Abstract: Lifetimes of the first excited 2(+) and 4(+) states in the extremely neutron -deficient nuclide Pt-172 have been measured for the first time using the recoil-distance Doppler shift and recoil-decay tagging techniques. An unusually low value of the ratio B(E2: 4(1)(+) -> 2(1)(+)/B(E2: 2(1)(+) -> 0(gs)(+)) = 0.55(19) was found, similar to a handful of other such anomalous cases observed in the entire Segre chart. The observation adds to a cluster of a few extremely neutron -deficient nuclides of the heavy transition metals with neutron numbers N approximate to 90-94 featuring the effect. No theoretical model calculations reported to date have been able to explain the anomalously low B(E2: 4(1)(+) -> 2(1)(+)/B(E2: 2(1)(+) -> 0(gs)(+)) ratios observed in these cases. Such low values cannot, e.g., be explained within the framework of the geometrical collective model or by algebraic approaches within the interacting boson model framework. It is proposed that the group of B(E2: 4(1)(+) -> 2(1)(+)/B(E2: 2(1)(+) -> 0(gs)(+)) ratios in the extremely neutron-deficient even-even W, Os, and Pt nuclei around neutron numbers N approximate to 90-94 reveal a quantum phase transition from a seniority-conserving structure to a collective regime as a function of neutron number. Although a system governed by seniority symmetry is the only theoretical framework for which such an effect may naturally occur, the phenomenon is highly unexpected for these nuclei that are not situated near closed shells.

Partial seniority conservation and solvability of single-j systems

Abstract: The seniority symmetry is known to be partially conserved in two special cases in the ${(9/2)}^{4}$ system, which can lead to striking features in the corresponding structure and electromagnetic transition properties. However, it is still quite difficult to derive those kind of solvable states, in general, especially towards higher-$j$ orbits. We have developed a novel and effective way to confront this challenge by starting from the $m$ scheme and making use of the angular momentum projection method. It also allows us to explore another special family of seniority conserving states in the midshell besides the specific case of the ${(9/2)}^{4}$ configuration. Moreover, we have studied systemically all states in single-$j$ systems up to $j=15/2$ and derived the analytic expressions for the eigenvalues of all solvable states with a focus on those in $j=9/2$ and $j=11/2$. Such studies can also be useful for the experimental search of relevant states and for the understanding of their electromagnetic transition properties.

M1 and E2 transition rates from core-excited states in semi-magic 94Ru

Abstract: Lifetimes of high-spin states have been measured in the semi-magic $(N=50)$ nucleus 94Ru Excited states in 94Ru were populated in the 58Ni(40Ca, 4p)94Ru* fusion-evaporation reaction at the Grand Accelerateur National d’Ions Lourds (GANIL) accelerator complex DSAM lifetime analysis was performed on the Doppler broadened line shapes in energy spectra obtained from $\gamma$ -rays emitted while the residual nuclei were slowing down in a thick 6mg/cm^2 metallic 58Ni target In total eight excited-state lifetimes in the angular momentum range $I = (13-20)\hbar$ have been measured, five of which were determined for the first time The corresponding B(M1) and B(E2) reduced transition strengths are discussed within the framework of large-scale shell model calculations to study the contribution of different particle-hole configurations, in particular for analyzing contributions from core-excited configurations

Investigation of high spin states in 133 Cs

Abstract: High spin states in 133Cs nucleus have been studied with the reaction 130Te (7Li, 4n) at a beam energy of 38 MeV. The level scheme has been expanded with spin up to 31/2 $\hbar$ . Compared with a recent paper, ground state band and other two collective band structures at lower spin states have been confirmed. Another collective band structure at higher spin states as well as some levels and transitions are updated. Compared with the experimental data, large-scale shell model and tilted axis cranking model calculations have been carried out. The results show that the band-head configuration of yrast band based on $7/2^{+}$ ground state and the side band built on the $5/2^{+}$ state are a pair of pseudospin partner states with $\pi \tilde{f}_{7/2,5/2}$ . The negative parity band based on 1071.5 keV level originates from $\pi h_{11/2}$ orbital. Another band built on 2642.9 keV level at high spin states has been proposed with oblate deformation. Other characteristics for these bands were also discussed.

Shell model description of heavy nuclei and abnormal collective motions

Abstract: In this contribution I present systematic calculations on the spectroscopy and electromagnetic transition properties of intermediate-mass and heavy nuclei around 100 Sn and 208 Pb. We employed the large-scale configuration interaction shell model approach with realistic interactions. Those nuclei are the longest isotopic chains that can be studied by the nuclear shell model. I will show that the yrast spectra of Te isotopes show a vibrational-like equally spaced pattern but the few known E2 transitions show rotational-like behaviour. These kinds of abnormal collective behaviors cannot be reproduced by standard collective models and provide excellent background to study the competition of single-particle and various collective degrees of freedom. Moreover, the calculated B(E2) values for neutron-deficient and heavier Te isotopes show contrasting different behaviours along the yrast line, which may be related to the enhanced neutron-proton correlation when approaching N=50. The deviations between theory and experiment concerning the energies and E2 transition properties of low-lying 0+ and 2+ excited states and isomeric states in those nuclei may provide a constraint on our understanding of nuclear deformation and intruder configuration in that region.

Reinvestigation of the excited states in the proton emitter Lu-151: Particle-hole excitations across the N = Z = 64 subshell (vol 96, 064307, 2017)

Abstract: The excited states of the proton emitter Lu151 were reinvestigated in a recoil-decay tagging experiment at the Accelerator Laboratory of the University of Jyvaskyla (JYFL). The level scheme built on the ground state of Lu151 was updated with five new γ-ray transitions. Large-scale shell model calculations were carried out in the model space consisting of the neutron and proton orbitals 0g7/2, 1d5/2, 1d3/2, 2s1/2, and 0h11/2 with the optimized monopole interaction in order to interpret the experimental level scheme of Lu151. It is found that the excitation energies of states above the 27/2- and 23/2+ isomeric levels in Lu151 can be sensitive to excitations from g7/2 and d5/2 to single-particle orbitals above N=Z=64.