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.

The known I^{π}=8_{1}^{+}, E_{x}=2129-keV isomer in the semimagic nucleus ^{130}Cd_{82} was populated in the projectile fission of a ^{238}U beam at the Radioactive Isotope Beam Factory at RIKEN. The high counting statistics of the accumulated data allowed us to determine the excitation energy, E_{x}=2001.2(7)  keV, and half-life, T_{1/2}=57(3)  ns, of the I^{π}=6_{1}^{+} state based on γγ coincidence information. Furthermore, the half-life of the 8_{1}^{+} state, T_{1/2}=224(4)  ns, was remeasured with high precision. The new experimental information, combined with available data for ^{134}Sn and large-scale shell model calculations, allowed us to extract proton and neutron effective charges for ^{132}Sn, a doubly magic nucleus far-off stability. A comparison to analogous information for ^{100}Sn provides first reliable information regarding the isospin dependence of the isoscalar and isovector effective charges in heavy nuclei.

The magnetic dipole moment and the electric quadrupole moment are the nuclear moments that provide us with key information about the proton and neutron configurations in a nucleus and the shape of a nucleus, respectively. In the study of nuclear structure through the measurement of the nuclear moments, a technique to produce spin orientation of rare-isotope beams has played important roles. Recently, a scheme of the two-step projectile fragmentation was developed to produce high spin alignment in RI beams and was applied to the frontier of the study for nuclear structure of neutron-rich nuclei, such as ^75 Cu and ^99 Zr. The recent activities of the nuclear-moment measurements using highly spin-aligned beams at RIKEN RIBF are reported.

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.

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 ground-state magnetic dipole moment of the neutron-rich 21O isotope has been measured via beta-ray-detected nuclear magnetic resonance (beta-NMR) spectroscopy by using a spin-polarized secondary beam of 21O produced from the 22Ne primary beam. From the present measurement, the g factor |gexp(21Og.s.)| = 0.6036(14) has been determined. Based on the comparison of this value with Schmidt values, we unambiguously confirm the nu d5/2 configuration with spin and parity assignments I pi = 5/2+ for the 21O ground state, suggested by previously reported studies. Consequently, the magnetic moment has been determined as mu exp(21Og.s.) = (-)1.5090(35)mu N. The obtained experimental magnetic moment is in good agreement with the predictions of the shell-model calculations using the USD, YSOX, and SDPF-M interactions as well as random phase approximation (RPA) calculations. This observation indicates that the 21O nucleus in its ground state does not manifest any anomalous structure and is not influenced by the proximity of the drip line.

Background: Nuclear spectroscopy of neutron-rich isotopes provides important information on their nuclear structure and has a valuable impact on the modeling of the r-process path. Particularly interesting are nuclei close to doubly-magic species, e.g., Sn-132, with only several valence particles. Such is the barely explored I-137 nucleus, investigated here in detail. Purpose: To establish excited states in I-137, ss decay of the Te-137 ground state is studied. In addition, the unknown ss-delayed neutron-emission channel of Te-137 to I-136 is inspected. Search for levels and for candidates for Gamow-Teller and first-forbidden transitions between the mother nucleus and excited states in the daughter nucleus is conducted within the experimental observations. Methods: ss-delayed gamma-ray spectroscopy is employed to study excited states in I-137. The nucleus is populated in the decay of a mass-separated beam of Te-137, produced in neutron-induced fission of U-235. Results: The new level scheme of I-137 populated in ss decay is established. The half-life T-1/2 of Te-137 is determined to be 2.46(5) s. The ss-delayed neutron-emission probability P-n value of Te-137 is deduced as a lower limit to be 2.63(85)%. 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.

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.

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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.

The gyromagnetic factor of the low-lying ${E}_{x}=684.10(19)\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ isomeric state of the nucleus $^{99}\mathrm{Mo}$ was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of ${J}^{\ensuremath{\pi}}=11/{2}^{\ensuremath{-}}$, with a half-life of ${T}_{1/2}=742(13)\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$. The state of interest was populated and spin-aligned via a single-neutron transfer on a highly enriched $^{98}\mathrm{Mo}$ target. A magnetic moment ${\ensuremath{\mu}}_{\mathrm{expt}.}=\ensuremath{-}0.627(20){\ensuremath{\mu}}_{\mathrm{N}}$ was obtained. This result is far from the Schmidt value expected for a pure single-particle $\ensuremath{\nu}{h}_{11/2}$ state. A comparison of experimental spectroscopic properties of this nucleus is made with results of multishell Interacting boson-fermion Model (IBFM-1) calculations. In this approach, the ${J}^{\ensuremath{\pi}}=11/{2}^{\ensuremath{-}}$ isomeric state in $^{99}\mathrm{Mo}$ has a pure $\ensuremath{\nu}{h}_{11/2}$ configuration. Its magnetic moment, as well as that of other two excited states could be reasonably well reproduced by reducing the free neutron spin $g$ factor with a quenching factor of 0.45. This low value is not appropriate only for this case, similar values for the quenching factor being also required in order to describe magnetic moments in other nuclei from the same mass region.

The gyromagnetic factor of the low-lying E-x = 684.10(19) keV isomeric state of the nucleus Mo-99 was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of J(pi) = 11/2(-), with a half-life of T-1/2 = 742(13) ns. The state of interest was populated and spin-aligned via a single-neutron transfer on a highly enriched Mo-98 target. A magnetic moment mu(expt.) = -0.627(20)mu(N) was obtained. This result is far from the Schmidt value expected for a pure single-particle nu h(11/2) state. A comparison of experimental spectroscopic properties of this nucleus is made with results of multishell Interacting boson-fermion Model (IBFM-1) calculations. In this approach, the J(pi) = 11/2(-) isomeric state in Mo-99 has a pure nu h(11/2) configuration. Its magnetic moment, as well as that of other two excited states could be reasonably well reproduced by reducing the free neutron spin g factor with a quenching factor of 0.45. This low value is not appropriate only for this case, similar values for the quenching factor being also required in order to describe magnetic moments in other nuclei from the same mass region.

The $\beta$-decay half-lives of 55 neutron-rich nuclei $^{134-139}$Sn, $^{134-142}$Sb, $^{137-144}$Te, $^{140-146}$I, $^{142-148}$Xe, $^{145-151}$Cs, $^{148-153}$Ba, $^{151-155}$La were measured at the Radioactive Isotope Beam Factory (RIBF) employing the projectile fission fragments of $^{238}$U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly-measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.