Background: The nuclear structure of low-lying excited states in I-139, a neutron-rich nucleus with Z = 53 and N = 86 above Sn-132 and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for I-139 is obtained after the first beta-decay measurement of Te-139. Transitions in I-138 are detected after the beta-delayed neutron emission of Te-139. The Pn ratio is investigated based on the gamma-ray emissions. Methods: beta-delayed. -ray spectroscopy is employed to study excited states in I-139, populated in the decay of a mass-separated beam of Te-139, produced in the in-flight fission of U-235 on a Be-9 target. Results: The new level scheme of I-139 with 26 new transitions, established for the first time in beta decay, is reported. Two new transitions are observed also in I-138. The beta-delayed neutron emission probability P-n of Te-139 is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter I-138 nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

Background: The nuclear structure of low-lying excited states in 139 I, a neutron-rich nucleus with Z = 53 and N = 86 above 132 Sn and in the proximity of A similar to 140, is investigated. Purpose: The decay scheme for 139I is obtained after the first /3-decay measurement of 139 Te. Transitions in 138I are detected after the /3-delayed neutron emission of 139 Te. The Pn ratio is investigated based on the gamma-ray emissions. Methods: /3-delayed gamma-ray spectroscopy is employed to study excited states in 139 I, populated in the decay of a mass-separated beam of 139 Te, produced in the in-flight fission of 235U on a 9 Be target. Results: The new level scheme of 139I with 26 new transitions, established for the first time in /3 decay, is reported. Two new transitions are observed also in 138 I. The /3-delayed neutron emission probability Pn of 139 Te is deduced to be 17.6(48)%. New and/or more restrictive spin/parity assignments of states in the daughter 138I nucleus are reported as well. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell-model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

Nuclei in the vicinity of 78Ni are important benchmarks for nuclear structure, which can reveal changes in the shell structure far from stability. Spectroscopy of the odd-odd isotope 78Cu was performed for the first time in an experiment with the EURICA setup at the Radioactive Isotope Beam Factory at RIKEN Nishina Center. Excited states in the neutron-rich isotope were populated following the beta decay of 78Ni produced by in-flight fission and

Isomeric states were observed in nuclei produced in an experiment at the RIKEN Nishina Center Radioactive Isotope Beam Factory following the in-flight fission of a 345 MeV/nucleon ^238 U beam. Isomers reported in nuclei spanning a predicted prolate-oblate shape change boundary, ^111 Zr ( E=283.1 keV; τ =0.326(63) s), ^112 Nb ( E=44.2 keV; τ =0.094(26) s), ^113 Nb ( E=135.4 keV; τ =0.846(80) s), and ^115 Mo ( E=198.6 keV; τ =63(4) s), are compared to potential-energy surface calculations which gave a selection of low-lying configurations for each nucleus. Tentative assignments of ground and excited states were made based on energy similarities to the calculations, reduced transition probabilities of the decays, and constraints of transition multipolarities from γ -ray coincidence measurements. These assignments are suggestive of significant deformation being persistent for N>70 in this region. In addition, isomers in ^108 Nb, ^109 Nb, ^113 Tc, ^117 Ru, ^119 Ru, ^120 Rh, and ^122 Rh, not spanning the prolate-oblate transition discussed, are presented.

Excited states in Cu-78 were observed for the first time following the ss decay of Ni-78 created by in-flight fission of U-238. Based on the coincidence relationships between the observed gamma-ray transitions, it was possible to construct a level scheme comprising eight excited states with tentative spin assignments for 5 of them. In addition to the gamma-decaying states, an isomeric state with a lifetime of 3.8(4) ms was found to decay by internal conversion.

The $\ensuremath{\beta}$ decay of $^{142}\mathrm{Te}_{90}$ to $^{142}\mathrm{I}_{89}$ was investigated for the first time. The parent nucleus was produced by the in-flight fission of a $^{238}\mathrm{U}$ beam with an energy of 345 MeV per nucleon, impinging on a $^{9}\mathrm{Be}$ target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in $^{142}\mathrm{I}$ were established by $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$-ray spectroscopy. The observed $({1}^{+})$ states in $^{142}\mathrm{I}$ could be interpreted to be predominantly the $\ensuremath{\nu}0{h}_{9/2}\ensuremath{\bigotimes}\ensuremath{\pi}0{h}_{11/2}$ configuration formed by a Gamow-Teller transition between a neutron in the $0{h}_{9/2}$ orbital and a proton in the $0{h}_{11/2}$ orbital. Additional features of the $({1}^{+})$ states are discussed by comparing with neighboring heavier isotones, such as $^{144}\mathrm{Cs}$ and $^{146}\mathrm{La}$. In the context of deformed shell-model calculations, the $({1}_{1}^{+})$ state is closely related to the $\ensuremath{\nu}[5,3,2]3/2\ensuremath{\bigotimes}\ensuremath{\pi}[5,5,0]1/2$ configuration, which may be related to the weak Gamow-Teller transition strength.

The beta decay of 142Te90 to 142I89 was investigated for the first time. The parent nucleus was produced by the in-flight fission of a 238U beam with an energy of 345 MeV per nucleon, impinging on a 9Be target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in 142I were established by beta-delayed gamma-ray spectroscopy. The observed (1+) states in 142I could be interpreted to be predominantly the nu 0h9/2 (R) pi 0h11/2 configuration formed by a Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Additional features of the (1+) states are discussed by comparing with neighboring heavier isotones, such as 144Cs and 146La. In the context of deformed shell-model calculations, the (1+1 ) state is closely related to the nu[5, 3, 2]3/2 (R) pi[5, 5, 0]1/2 configuration, which may be related to the weak Gamow-Teller transition strength.

The beta-decay scheme of Te-138 and the level structure of I-138 is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN Cluster Array campaigns. Secondary radioactive ions, including Te-138 and Sb-138, were produced by the in-flight fission of a U-238 beam with the energy of 345 MeV per nucleon. From the beta decay of Te-138, the level scheme of I-138 was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1(+) state. This 1(+) state can be interpreted as being associated with the pi 0h(11/2) circle times nu 0h(9/2) partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h(9/2) orbital and a proton in the 0h(11/2) orbital. Details of the structure of I-138 are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.

Structure of the neutron-rich $N=86$ isotope ${}^{140}\mathrm{Xe}$, located northeast of a doubly-magic nucleus ${}^{132}Sn$, is investigated by $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy. Two $\ensuremath{\beta}$-decay isomers in $^{140}\mathrm{I}$ are newly found in the study of two different $\ensuremath{\beta}$ decays of $^{140}\mathrm{I}$ which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) $\ensuremath{\beta}$ decay of $^{140}\mathrm{Te}$ at an active stopper. Half-lives of the $\ensuremath{\beta}$ decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the $\ensuremath{\beta}$ decay of the high-spin isomer and of the mixed $\ensuremath{\beta}$ decays of the ground state and the low-spin isomer in $^{140}\mathrm{I}$ to $^{140}\mathrm{Xe}$ are constructed using the information on $\ensuremath{\gamma}$-ray coincidence relation and $\ensuremath{\gamma}$-ray intensity. Nuclear structures of the low-lying states in $^{140}\mathrm{Xe}$ and $^{140}\mathrm{I}$ are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)$\ensuremath{\gamma}$-band members of ${2}^{+}$ and ${4}^{+}$ states and the octupole collective ${1}^{\ensuremath{-}}$ state are proposed in $^{140}\mathrm{Xe}$. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.

Structure of the neutron-rich N = 86 isotope Xe-140, located northeast of a doubly-magic nucleus Sn-132, is investigated by beta-gamma spectroscopy. Two beta-decay isomers in I-14(0) are newly found in the study of two different beta decays of I-140 which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) beta decay of Te-140 at an active stopper. Half-lives of the beta decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the beta decay of the high-spin isomer and of the mixed beta decays of the ground state and the low-spin isomer in I-140 to Xe-140 are constructed using the information on gamma-ray coincidence relation and gamma-ray intensity. Nuclear structures of the low-lying states in Xe-140 and I-140 are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)gamma-band members of 2(+) and 4(+) states and the octupole collective 1(-) state are proposed in Xe-140. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.

The $\ensuremath{\beta}$-decay scheme of $^{138}\mathrm{Te}$ and the level structure of $^{138}\mathrm{I}$ is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN Cluster Array campaigns. Secondary radioactive ions, including $^{138}\mathrm{Te}$ and $^{138}\mathrm{Sb}$, were produced by the in-flight fission of a $^{238}\mathrm{U}$ beam with the energy of 345 MeV per nucleon. From the $\ensuremath{\beta}$ decay of $^{138}\mathrm{Te}$, the level scheme of $^{138}\mathrm{I}$ was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity ${1}^{+}$ state. This ${1}^{+}$ state can be interpreted as being associated with the $\ensuremath{\pi}0{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}0{h}_{9/2}$ partner orbital configuration populated by the Gamow-Teller transition between a neutron in the $0{h}_{9/2}$ orbital and a proton in the $0{h}_{11/2}$ orbital. Details of the structure of $^{138}\mathrm{I}$ are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.

We studied the proton-rich T_z=-1 nucleus ^70Kr through inelastic scattering at intermediate energies in order to extract the reduced transition probability, B(E2; 0^+ → 2^+). Comparison with the other members of the A=70 isospin triplet, ^70Br and ^70Se, studied in the same experiment, shows a 3σ deviation from the expected linearity of the electromagnetic matrix elements as a function of T_z. At present, no established nuclear structure theory can describe this observed deviation quantitatively. This is the first violation of isospin symmetry at this level observed in the transition matrix elements. A heuristic approach may explain the anomaly by a shape change between the mirror nuclei ^70Kr and ^70Se contrary to the model predictions.

Newly observed decay schemes of the nuclei Sb-137 and Sb-138 are reported. The neutron-rich Sb isotopes were produced by the in-flight fragmentation of a U-238 primary beam with an energy of 345 MeV/nucleon. Several new excited states of Te-137 with tentatively assigned spin-parities of (5/2(-)), (9/2(-)), and (7/2) have been established which play an important role in the evolution of neutron levels beyond N = 82. The study of the beta decay of Sb-138 led to a considerable extension of the level scheme of Te-138 including the identification of several nonyrast states. The structure of Te-137 and Te-138 is discussed on the basis of large-scale shell-model calculations performed using two different effective interactions.

Received 23 November 2017DOI:https://doi.org/10.1103/PhysRevC.97.019901©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsIsomer decaysLifetimes & widthsNuclear structure & decaysProperties59 ≤ A ≤ 8990 ≤ A ≤ 149TechniquesShell modelNuclear Physics

Neutron-rich Pm ( Z = 61) isotopes were studied by delayed γ -ray spectroscopy at RIBF, RIKEN Nishina Center using the in-flight fission of a 345 MeV / nucleon 238 U beam. A cluster-type Ge detector array, EURICA, was used to measure the delayed γ rays from stopped ions. Isomeric γ decays were observed in 159 Pm and 161 Pm with half-lives of 4.97(12) μ s and 0.79(4) μ s, respectively. Level schemes for 159 Pm and 161 Pm were constructed in this study. The isomeric states of 159 Pm and 161 Pm could be interpreted as two quasiparticle excitations of neutrons with the configurations of ν (7 / 2[633] ⊗ 5 / 2[523]) and ν (7 / 2[633] ⊗ 1 / 2[521]), respectively. They are analogous to the isomers that have been observed systematically in other even-mass N = 98 and N = 100 isotones in this region. A projected shell model calculation was performed and it reproduced the order of three-quasiparticle states only if new Nilsson parameters with an N -dependent spin-orbit interaction were used.

Using a fusion-evaporation reaction and a gas-filled recoil separator, an isomeric state [T-1/2 = 83(8) mu s] with a most likely spin and parity of 13(+)/2 has been identified in Th-211. The isomeric state is mainly depopulated via a hindered internal M2 transition [B(M2) = 0.0025(5) W.u.], but also a weak alpha-decay branch of 4(3)% was observed. The present observations fit well to the systematic pattern set by the previously identified states of the same spin and parity in this region of the nuclear chart.