The region of N Z 40 has long been associated with strongly deformed nuclear configurations. The presence of this strong deformation was recently confirmed through lifetime measurements in N Z Sr and Zr nuclei. Theoretically, however, these nuclei present a challenge due to the vast valence space required to incorporate all necessary orbitals. Recent state-of-the-art predictions indicate a near axial prolate deformation for N = Z and N = Z + 2 nuclei between N = Z = 36 and N = Z = 40. In this work we investigate the shores of this island of deformation through a sub-barrier Coulomb excitation study of the N = Z +4 nucleus, 80Sr. Extracting a spectroscopic quadrupole moment of Qs(2+1 ) = 0.5+0.8-0.9 eb, we find that 80Sr is inconsistent with significant axial prolate deformation with a significance of 1.5 sigma. This result, albeit with a large uncertainty, indicates that the predicted region of strong prolate deformation around N = Z = 40 is tightly constrained to the quartet of nuclei: 76,78Sr and 78,80Zr.

High-precision lifetime measurements in 28Mg were performed to study neutron shell evolution in Mg isotopes and the onset of the =20 island of inversion. Using both the recoil distance and Doppler shift attenuation methods, five lifetimes were measured in addition to six upper limits. The observation of two long-lived, negative-parity states demonstrate the importance of studying Mg isotopes for the contribution of intruder configurations to -shell nuclei. Lifetimes of the 2+1 and 4+1 states of 1.81(5) ps and 172(+11-10)stat.(4)stop.(8)feed.(4)targ.fs, respectively, demonstrate a loss of collectivity with increasing spin in the yrast band, permitting for distinguishing between current theoretical models. These measurements also highlight the progression of yrast structure across the Mg isotopic chain from rotational at =12 to large shape mixing at =16 and back to collective behavior at =20 but with dominating intruder configurations.

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 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.

Motivated primarily by the large uncertainties in the thermonuclear rate of the 30P(p,γ)31S reaction that limit our understanding of classical novae, we carried out lifetime measurements of 31S excited states using the Doppler Shift Lifetimes (DSL2) facility at the TRIUMF Isotope Separator and Accelerator (ISAC-II) facility. The 31S excited states were populated by the 3He(32S,α)31S reaction. The deexcitation γ rays were detected by a clover-type high-purity germanium detector in coincidence with the α particles detected by a silicon detector telescope. We have applied modern Markov chain Monte Carlo-based Bayesian statistical techniques to perform lineshape analyses of Doppler-shift attenuation method γ-ray data for the first time. We have determined the lifetimes of the two lowest-lying 31S excited states. First experimental upper limits on the lifetimes of four higher-lying states have been obtained. The experimental results were compared to shell-model calculations using five universal sd-shell Hamiltonians. Evidence for γ rays originating from the astrophysically important Jπ=3/2+, 260-keV 30P(p,γ)31S resonance with an excitation energy of Ex=6390.2(7) keV in 31S has also been observed, although strong constraints on the lifetime will require better statistics.

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 $^{160}\mathrm{Eu}$.

The properties of a nanosecond isomer in $^{32}$Si, disputed in previous studies, depend on the evolution of proton and neutron shell gaps near the `island of inversion'. We have placed the isomer at 5505.2(2) keV with $J^{\pi} = 5^-$, decaying primarily via an $E3$ transition to the $2^+_1$ state. The $E3$ strength of 0.0841(10) W.u. is unusually small and suggests that this isomer is dominated by the $(\nu d_{3/2})^{-1} \otimes (\nu f_{7/2})^{1}$ configuration, which is sensitive to the $N=20$ shell gap. A newly observed $4^+_1$ state is placed at 5881.4(13) keV; its energy is enhanced by the $Z=14$ subshell closure. This indicates that the isomer is located in a `yrast trap', a feature rarely seen at low mass numbers.

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.

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.

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.

To analyze the cause of the destruction of thin, carbon-backed lithium fluoride targets during a measurement of the fusion of 7Li and 17O, we estimate theoretically the lifetimes of carbon and LiF films due to sputtering, thermal evaporation, and lattice damage and compare them with the lifetime observed in the experiment. Sputtering yields and thermal evaporation rates in carbon and LiF films are too low to play significant roles in the destruction of the targets. We estimate the lifetime of the target due to lattice damage of the carbon backing and the LiF film using a previously reported model. In the experiment, elastically scattered target and beam ions were detected by surface silicon barrier (SSB) detectors so that the product of the beam flux and the target density could be monitored during the experiment. The areas of the targets exposed to different beam intensities and fluences were degraded and then perforated, forming holes with a diameter around the beam spot size. Overall, the target thickness tends to decrease linearly as a function of the beam fluence. However, the thickness also exhibits an increasing interval after SSB counts per beam ion decreases linearly, extending the target lifetime. The lifetime of thin LiF film as determined by lattice damage is calculated for the first time using a lattice damage model, and the calculated lifetime agrees well with the observed target lifetime during the experiment. In experiments using a thin LiF target to induce nuclear reactions, this study suggests methods to predict the lifetime of the LiF film and arrange the experimental plan for maximum efficiency.

Background: Electric-quadrupole (E2) strengths relate to the underlying quadrupole deformation of a nucleus and present a challenge for many nuclear theories. Mirror nuclei in the vicinity of the line of N = Z represent a convenient laboratory for testing deficiencies in such models, making use of the isospin symmetry of the systems. Purpose: Uncertainties associated with literature E2 strengths in Mg-23 are some of the largest in T-z = vertical bar 1/2 vertical bar nuclei in the sd shell. The purpose of the present paper is to improve the precision with which these values are known, to enable better comparison with theoretical models. Methods: Coulomb-excitation measurements of Mg-23 and Na-23 were performed at the TRIUMF-ISAC facility using the TIGRESS spectrometer. They were used to determine the E2 matrix elements of mixed E2/M1 transitions. Results: Reduced E2 transition strengths, B(E2), were extracted for Mg-23 and Na-23. Their precision was improved by factors of approximately 6 for both isotopes, while agreeing within uncertainties with previous measurements. Conclusions: A comparison was made with both shell-model and ab initio valence-space in-medium similarity renormalization group calculations. Valence-space in-medium similarity renormalization group calculations were found to underpredict the absolute E2 strength, in agreement with previous studies.