In the selenium isotopes various shape phenomena are present, in particular, the emergence of a dominant oblate deformation in the most neutron-deficient isotopes has been observed. The scenario of shape coexisting oblate and prolate bands has been proposed across the isotopic chain, with the crossing point of such bands being located near Se70, where no coexistence has yet been identified. To determine the presence or absence of any low-lying 0+ state in Se70, confirm the level structure, and interpret the nuclear deformation with theoretical models. A combined internal-conversion-electron and γ-ray spectroscopy study was undertaken with the SPICE and TIGRESS spectrometers at the TRIUMF-ISAC-II facility. Nuclear models were provided by the generalized triaxial rotor model (GTRM) and the collective generalised Bohr Hamiltonian (GBH). Despite a comprehensive search, no evidence was found for the existence of a 0+ state below 2 MeV in Se70. Significant discrepancies to the previously established positive-parity-level scheme were found. GBH calculations using UNEDF1 mass parameters were found to reproduce the revised low-lying level structure well. Se70 does not have a well-defined axial shape. The 22+ state at 1601 keV resembles a quasi-γ excitation rather than a member of a shape coexisting band; the presence of such a band is all but ruled out. Published by the American Physical Society 2024

The excited states of 209Rn (Z = 86, N = 123) have been populated by the heavy -ion induced fusion evaporation reaction 198Pt (16O, 5n) 209Rn at a beam energy of 102 MeV. The de -excited gamma rays were detected with the Compton suppressed clover HPGe detectors of the Indian National Gamma Array (INGA) set-up. The high spin spectroscopic study of 209Rn has been carried out up to an excitation energy of 7.9 MeV and spin (55/2) h over bar . Spin -parity assignments of the excited levels have been determined and are confirmed on the basis of the ratio of directional correlation and polarization asymmetry measurement. The possible presence of new isomeric states has been observed and the half-lives have been estimated. A negative parity sequence of M1 transitions has been observed which exhibits the property of magnetic rotation and is interpreted in the framework of semiclassical model calculation. The large basis shell -model calculation has been performed for all the nuclear levels and is found to be in well agreement with the experimental results.

The cross-shell excited states of 34Si have been investigated via beta decays of the 4- ground state and the 1+ isomeric state of 34Al. Since the valence protons and valence neutrons occupy different major shells in the ground state as well as the intruder 1+ isomeric state of 34Al, intruder levels of 34Si are populated via allowed beta decays. Spin assignments to such intruder levels of 34Si were established through gamma -gamma angular correlation analysis for the negative-parity states with dominant configurations (nu d3/2)-1 (R) (nu f7/2)1 as well as the positive-parity states with dominant configurations (nu sd )-2 (R) (nu f7/2p3/2)2. The configurations of such intruder states play crucial roles in our understanding of the N = 20 shell gap evolution. A configuration interaction model derived from the FSU Hamiltonian was utilized in order to interpret the intruder states in 34Si. Shell model interaction derived from a more fundamental theory with the valence space in medium similarity renormalization group method was also employed to interpret the structure of 34Si.

We have measured the cross section of the $^{83}$Rb(p,$\gamma)^{84}$Sr radiative capture reaction in inverse kinematics using a radioactive beam of $^{83}$Rb at incident energies of 2.4 and $2.7 A$ MeV. Prior to the radioactive beam measurement, the $^{84}$Kr(p,$\gamma)^{85}$Rb radiative capture reaction was measured in inverse kinematics using a stable beam of $^{84}$Kr at an incident energy of $2.7 A$ MeV. The effective relative kinetic energies of these measurements lie within the relevant energy window for the $\gamma$ process in supernovae. The central values of the measured partial cross sections of both reactions were found to be $0.17-0.42$ times the predictions of statistical model calculations. Assuming the predicted cross section at other energies is reduced by the same factor leads to a slightly higher calculated abundance of the $p$ nucleus $^{84}$Sr, caused by the reduced rate of the $^{84}$Sr($\gamma$,p)$^{83}$Rb reaction derived from the present measurement.

The excited states of N=44 ^{74}Zn were investigated via γ-ray spectroscopy following ^{74}Cu β decay. By exploiting γ-γ angular correlation analysis, the 2_{2}^{+}, 3_{1}^{+}, 0_{2}^{+}, and 2_{3}^{+} states in ^{74}Zn were firmly established. The γ-ray branching and E2/M1 mixing ratios for transitions deexciting the 2_{2}^{+}, 3_{1}^{+}, and 2_{3}^{+} states were measured, allowing for the extraction of relative B(E2) values. In particular, the 2_{3}^{+}→0_{2}^{+} and 2_{3}^{+}→4_{1}^{+} transitions were observed for the first time. The results show excellent agreement with new microscopic large-scale shell-model calculations, and are discussed in terms of underlying shapes, as well as the role of neutron excitations across the N=40 gap. Enhanced axial shape asymmetry (triaxiality) is suggested to characterize ^{74}Zn in its ground state. Furthermore, an excited K=0 band with a significantly larger softness in its shape is identified. A shore of the N=40 "island of inversion" appears to manifest above Z=26, previously thought as its northern limit in the chart of the nuclides.

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the r-process path and following freeze-out region impact the resulting r-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei. Purpose: We aim to improve the understanding of the nuclear structure of 160Gd, specifically the K & pi; = 4+ bands, as well as study the & beta; decay of 160Eu into 160Gd. Methods: High-statistics decay spectroscopy of 160Gd resulting from the & beta;-decay of 160Eu was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility. Results: Two new excited states and ten new transitions were observed in 160Gd. The & beta;-decaying half-lives of the low-and high-spin isomers in 160Eu were determined, and the low-spin state's half-life was measured to be t1/2 = 26.0(8) s, & AP;16% shorter than previous measurements. Lifetimes of the two K & pi; = 4+ bandheads in 160Gd were measured for the first time, as well as & gamma; -& gamma; angular correlations and mixing ratios of intense transitions out of those bandheads. Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the K & pi; = 4+ bandheads in 160Gd is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in 160Gd heavily populated in & beta; decay is shown to have positive parity, which raises questions regarding the structure of the high-spin & beta;-decaying state in 160Eu.

The r-process has been shown to be robust in reproducing the abundance distributions of heavy elements, such as europium, seen in ultra-metal poor stars. In contrast, observations of elements 26 < Z < 47 display overabundances relative to r-process model predictions. A proposed additional source of early nucleosynthesis is the weak r-process in neutrino-driven winds of core-collapse supernovae. It has been shown that in this site (α,n) reactions are both crucial to nucleosynthesis and the main source of uncertainty in model-based abundance predictions. Aiming to improve the certainty of nucleosynthesis predictions, the cross section of the important reaction 86Kr(α,n)89Sr has been measured at an energy relevant to the weak r-process. This experiment was conducted in inverse kinematics at TRIUMF with the EMMA recoil mass spectrometer and the TIGRESS gamma-ray spectrometer. A novel type of solid helium target was used.

Structures of negative parity multi-quasiparticle states in Xe-127 have been investigated through in-beam gamma-ray spectroscopy. Excited states were populated via the Sn-122(Be-9, 4n gamma)Xe-127 fusion-evaporation reaction at E-beam = 48 MeV. Two new negative parity bands have been identified and their structures have been discussed using the triaxial projected shell model (TPSM) approach. One of the bands is established above the I-pi = 27/2(-) state at 3702 keV and based on the experimental inferences and TPSM results, this band is interpreted as a gamma-band built on a three-quasineutron configuration. The second sequence, found above I-pi = 31/2(-) state at 4848 keV, is proposed to have a two-proton aligned configuration (nu h(11/2) circle times pi h(11/2)(2)), considering the properties of this band with the neighbouring isotopes. The analysis of the TPSM wave function substantiates the proposed interpretation of this band.

Excited states in the 84 Sr nucleus were investigated via the 76 Ge( 12 C,4n) 84 Sr reaction at a beam energy of 58 MeV. The de-excited γ -rays were detected using the Indian National Gamma Array (INGA) spectrometer at Inter-University Accelerator Center, New Delhi. Directional Correlation from Oriented (DCO) states ratio and the polarization asymmetry (Δ) measurements were done to confirm the spin-parity of the low-lying states. Eight new γ -ray transitions were placed in the level scheme of 84 Sr. The systematic behaviour of energy staggering S ( I ) of the γ -band (Band 1 and Band 2) was compared with the γ -bands in the mass A ≈ 80 region and the nuclei of other mass regions with similar behaviour (odd-I down). The E 1 character is confirmed for strong γ -ray transitions connecting Band 3 to the Yrast band. Comparison of new results such as B ( E 1)/ B ( E 2) ratio, frequency ratio ω − / ω + and energy displacement Δ E in 84 Sr with those of 72 Se, 150 Sm, 152 Gd, 220 Ra and 224 Th nuclei suggests the presence of octupole correlations in 84 Sr.

Background: Detailed spectroscopy of neutron-rich, heavy, deformed nuclei is of broad interest for nuclear astrophysics and nuclear structure. Nuclei in the $r$-process path and following freeze-out region impact the resulting $r$-process abundance distribution, and the structure of nuclei midshell in both proton and neutron number helps to understand the evolution of subshell gaps and large deformation in these nuclei.Purpose: We aim to improve the understanding of the nuclear structure of $^{160}\mathrm{Gd}$, specifically the ${K}^{\ensuremath{\pi}}={4}^{+}$ bands, as well as study the $\ensuremath{\beta}$ decay of $^{160}\mathrm{Eu}$ into $^{160}\mathrm{Gd}$.Methods: High-statistics decay spectroscopy of $^{160}\mathrm{Gd}$ resulting from the $\ensuremath{\beta}$-decay of $^{160}\mathrm{Eu}$ was collected using the GRIFFIN spectrometer at the TRIUMF-ISAC facility.Results: Two new excited states and ten new transitions were observed in $^{160}\mathrm{Gd}$. The $\ensuremath{\beta}$-decaying half-lives of the low- and high-spin isomers in $^{160}\mathrm{Eu}$ were determined, and the low-spin state's half-life was measured to be ${t}_{1/2}=26.0(8)$ s, $\ensuremath{\approx}16%$ shorter than previous measurements. Lifetimes of the two ${K}^{\ensuremath{\pi}}={4}^{+}$ bandheads in $^{160}\mathrm{Gd}$ were measured for the first time, as well as $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlations and mixing ratios of intense transitions out of those bandheads.Conclusions: Lifetimes and mixing ratios suggest that the hexadecapole phonon model of the ${K}^{\ensuremath{\pi}}={4}^{+}$ bandheads in $^{160}\mathrm{Gd}$ is preferred over a simple two-state strong mixing scenario, although further theoretical calculations are needed to fully understand these states. Additionally, the 1999.0-keV state in $^{160}\mathrm{Gd}$ heavily populated in $\ensuremath{\beta}$ decay is shown to have positive parity, which raises questions regarding the structure of the high-spin $\ensuremath{\beta}$-decaying state in $^{160}\mathrm{Eu}$.

A beta-decay experiment aiming at investigation of the low-spin structure of 100Zr was performed using the GRIFFIN spectrometer at TRIUMF-ISAC. Based on the obtained data, a new 2+ state is postulated which is degenerate in energy with the established (5+) level at 2209 keV.

Excited states in 126,127,128Xe were populated via 122Sn(9Be, xnγ) fusion-evaporation reaction at a beam energy of 48 MeV. A positive parity rotational-like sequence of ΔI=2γ-transitions, which is found to decay into the negative parity band via E1 transitions, is observed above Iπ=19/2+ state at 2307 keV in 127Xe. Such enhanced E1 γ-transitions are also observed between the antimagnetic rotational (AMR) and longitudinal wobbling (LW) bands in this nucleus. These indicate the presence of simplex symmetry with s=±i excitation in 127Xe.

The low-lying structure of 13Be has remained an enigma for decades. Despite numerous experimental and theoretical studies, large inconsistencies remain. Being both unbound and one neutron away from 14Be, the heaviest bound beryllium nucleus, 13Be is difficult to study through simple reactions with weak radioactive-ion beams or more complex reactions with stable-ion beams. Here, we present the results of a study using the 12Be(d, p)13Be reaction in inverse kinematics using a 9.5 MeV per nucleon 12Be beam from the ISAC-II facility. The solid deuteron target of IRIS was used to achieve an increased areal thickness compared to conventional deuterated polyethylene targets. The Q-value spectrum below -4.4 MeV was analyzed using a Bayesian method with GEANT4 simulations. A three-point angular distribution with the same Q-value gate was fit with a mixture of s- and p-wave, s- and d-wave, or pure p-wave transfer. The Q-value spectrum was also compared with GEANT simulations obtained using the energies and widths of states reported in four previous works. It was found that our results are incompatible with works that revealed a wide 5/2+ resonance but shows better agreement with ones that reported a narrower width.

The low-lying structure of $^{13}\mathrm{Be}$ has remained an enigma for decades. Despite numerous experimental and theoretical studies, large inconsistencies remain. Being both unbound and one neutron away from $^{14}\mathrm{Be}$, the heaviest bound beryllium nucleus, $^{13}\mathrm{Be}$ is difficult to study through simple reactions with weak radioactive-ion beams or more complex reactions with stable-ion beams. Here, we present the results of a study using the $^{12}\mathrm{Be}(d,p)^{13}\mathrm{Be}$ reaction in inverse kinematics using a 9.5 MeV per nucleon $^{12}\mathrm{Be}$ beam from the ISAC-II facility. The solid deuteron target of IRIS was used to achieve an increased areal thickness compared to conventional deuterated polyethylene targets. The $Q$-value spectrum below $\ensuremath{-}4.4$ MeV was analyzed using a Bayesian method with geant4 simulations. A three-point angular distribution with the same $Q$-value gate was fit with a mixture of $s$- and $p$-wave, $s$- and $d$-wave, or pure $p$-wave transfer. The $Q$-value spectrum was also compared with geant simulations obtained using the energies and widths of states reported in four previous works. It was found that our results are incompatible with works that revealed a wide $5/{2}^{+}$ resonance but shows better agreement with ones that reported a narrower width.

We have measured the cross section of the( 83)Rb(p, ? ) Sr-84 radiative capture reaction in inverse kinematics using a radioactive beam of Rb-83 at incident energies of 2.4 and 2.7A MeV. Prior to the radioactive beam measurement, the Kr-84(p, ? ) Rb-85 radiative capture reaction was measured in inverse kinematics using a stable beam of Kr-84 at an incident energy of 2.7A MeV. The effective relative kinetic energies of these measurements lie within the relevant energy window for the ? process in supernovae. The central values of the measured partial cross sections of both reactions were found to be 0.17-0.42 times the predictions of statistical model calculations. Assuming the predicted cross section at other energies is reduced by the same factor leads to a slightly higher calculated abundance of the p nucleus Sr-84, caused by the reduced rate of the Sr-84(? , p) Rb-83 reaction derived from the present measurement.

High-spin states of neutron-deficient trans-lead nucleus $$^{204}\hbox {At}$$ were populated up to $$\sim 8\,\mathrm{MeV}$$ excitation through the $$^{12}\hbox {C} + ^{197}\hbox {Au}$$ fusion evaporation reaction. Decay of the associated levels through prompt and delayed $$\gamma $$ -ray emissions were studied to evaluate the underlying nuclear structure. The level scheme, which was partly known, was extended further. An isomeric $$16^+$$ level with observed mean lifetime $$\tau =52 \pm 5\, \mathrm{ns}$$ , was established from our measurements. Attempts were made to interpret the excited states based on multi quasiparticle and hole structures involving $$2f_{5/2}$$ , $$1h_{9/2}$$ , and $$1i_{13/2}$$ shell model states, along with moderate core excitation. Magnetic dipole band structure over the spin parity range:  $$16^+$$ – $$23^+$$ was confirmed and evaluated in more detail, including the missing cross-over E2 transitions. Band-crossing along the shears band was observed and compared with the evidence of similar phenomena in the neighbouring $$^{202}\hbox {Bi}$$ , $$^{205}\hbox {Rn}$$ isotones and the $$^{203}\hbox {At}$$ isotope. Based on comparison of the measured B(M1)/B(E2) values for transitions along the band with the semiclassical model based estimates, the shears band of $$^{204}\hbox {At}$$ was established along with the level scheme.

The selenium isotopes lie at the heart of a tumultuous region of the nuclear chart where shape coexistence effects grapple with neutron-proton pairing correlations, triaxiality, and the impending proton drip line. In this work, a study of Se-72 by internal conversion electron and gamma-ray spectroscopy was undertaken with the SPICE and TIGRESS arrays. New measurements of the branching ratio and lifetime of the 0(2)(+) state were performed, yielding a determination of rho(2)(E-0; 0(2)(+) -> 0(1)(+))- = 29(3) milliunits. Two-state mixing calculations were performed that highlighted the importance of interpretation of such E0 strength values in the context of shape coexistence.

The level scheme of Fr-215 is extended up to 55/2 h and 4.8 MeV excitation energy with the addition of 52 new gamma-ray transitions. Previously established isomers and their half-lives, except for the 47/2(+) state, are revisited. The discrepancy in the half-life of the 39/2(-) state is resolved, and its half-life is revised to 11.4(14) ns. An overall good agreement is observed between the experimental results and the shell-model calculations performed using the CD-Bonn NN interaction derived from the V-l(ow)-k renormalization approach. A weak coupling of the odd proton to the even-even core is observed to account for the level structure at lower energies, which strongly resembles a decoupled nonrotational band. A new positive-parity sequence is also established which is observed to originate from the coupling of the i(13/12) proton at low excitation energy.

Excited states of the Po-203 (Z = 84, N = 119) have been investigated after populating them through Pt-194(C-13, 4n) fusion-evaporation reaction at E-beam = 74 MeV and using a large array of Compton-suppressed HPGe clover detectors as the detection setup for the emitted ?? rays. Standard techniques of gamma-ray spectroscopy have been applied towards establishing the level structure of the nucleus. Twenty five new gamma-ray transitions have been identified therein, through gamma-gamma coincidence measurements, and spin-parity assignments of several states have been determined or confirmed, following the angular correlation and linear polarization measurements on the observed gamma rays. The excited states have been interpreted in the framework of large basis shell-model calculations, while comparing their calculated and experimental energies. They have been principally ascribed to proton population in the h(9/2) and i(13/2) orbitals outside the Z = 82 closure and neutron occupation of the f(5/2), p(3/2), and i(13/2) orbitals in the N = 126 shell.

The half-life of the superallowed Fermi β ^+ emitter ^14 O was determined to high precision via a direct β counting experiment performed at the Isotope Separator and Accelerator (ISAC) facility at TRIUMF. The result, T_1/2 ( ^14 O) = 70619.2(76) ms, is consistent with, but is more precise than, the world average obtained from 11 previous measurements. Combining the ^14 O half-life deduced in the present work with the previous most precise measurements of this quantity leads to a reduction in the overall uncertainty, by nearly a factor of 2. The new world average is T_1/2 ( ^14 O) = 70619.6(63) ms with a reduced χ ^2 value of 0.87 obtained from 8 degrees of freedom.