arXiv subjects

A. D. Ayangeakaa

Publications and source records attributed to A. D. Ayangeakaa.

At least 19 recordsLinked to original sources

arXiv2026

Independent Validation of Octupole Collectivity in radium-224 through lifetime measurements of low-lying negative-parity states

The nucleus $^{224}$Ra is a key benchmark for octupole deformation and for theoretical descriptions of enhanced Schiff moments in reflection-asymmetric nuclei. While Coulomb-excitation measurements have established strong octupole collectivity in $^{224}$Ra, theoretical models predict that its intrinsic electric-dipole moment should be strongly quenched by a cancellation between macroscopic and microscopic contributions. Direct fast-timing measurements of the low-lying $J^π= 1^-_1$ and $3^-_1$ states populated following the $β$-decay of $^{224}$Fr at TRIUMF-ISAC were performed. Using the LaBr$_3$(Ce) detectors of the GRIFFIN array, mean lifetimes of $τ(1^-_1) = 444(6)$~ps and $τ(3^-_1) = 460(18)$~ps were obtained. The corresponding reduced transition probabilities agree with values inferred from Coulomb excitation, but are determined with substantially improved precision. These results provide an independent validation of the electromagnetic matrix elements associated with octupole collectivity in $^{224}$Ra and confirm a strongly-quenched intrinsic dipole moment of $D_0 \simeq 0.032~e\mathrm{fm}$. The present measurements therefore provide a stringent experimental benchmark for nuclear-structure models used in the interpretation of Schiff moments and future searches for non-zero electric dipole moments.

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arXiv2026

Core Breaking at Low Spin in $^{68}$Zn from Nuclear Resonance Fluorescence

Low-spin excited states in $^{68}$Zn have been studied at the High Intensity Gamma-Ray Source (HI$γ$S) from the ground state up to the particle emission threshold using the nuclear resonance fluorescence technique (NRF) and the newly developed Clover Array. Low-spin levels were excited by linearly-polarized, $2.90 - 9.79$ MeV photon beams. Spin-parity quantum numbers as well as associated $M1$ and $E1$ decay strengths were determined for a large fraction of the 158 states observed. In addition, long-duration coincidence measurements at 9.46 and 9.79 MeV enabled the investigation of the level scheme near the ground state. The results have been interpreted with shell-model calculations using two different model spaces and several effective interactions often used to describe nuclei in this mass region. While the structure near the ground state can be understood in terms of excitations involving solely valence nucleons, core breaking is required to account for the evolution of the total $M1$ strength at excitation energies above $\sim5$ MeV.

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arXiv2026

Evolution of chirality from transverse wobbling in $^{135}$Pr

Chirality is a distinct signature that characterizes triaxial shapes in nuclei. We report the first observation of chirality in the nucleus $^{135}$Pr using a high-statistics Gammasphere experiment with the $^{123}$Sb($^{16}$O,4n)$^{135}$Pr reaction. Two chiral-partner bands with the configuration $π(1h_{11/2})^1\otimesν(1h_{11/2})^{-2}$ have been identified in this nucleus. Angular distribution analyses of the $ΔI = 1$ transitions connecting the two bands reveal a dominant dipole character, and quasiparticle triaxial rotor model calculations show good agreement with the data. Since the simultaneous observation of chirality and transverse wobbling in $^{135}$Pr relies critically on these angular distribution results, we also address and refute the experimental and theoretical criticisms raised in a recent work by Lv et al., presenting additional evidence that further strengthens our interpretation. This marks the first observation of both hallmarks of triaxiality-chirality and wobbling-in the same nucleus.

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arXiv2025

Beta-decay Half Lives beyond $^{54}$Ca: A Systematic Survey of Decay Properties approaching the Neutron Dripline

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator (FDSi), 15 new half lives of isotopes near $^{54}$Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown $β$-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N =34. Beyond N =34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results for Z$>$19 nuclei, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed QRPA model with potential for making global predictions were also tested against the experimental results and good agreement was found.

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arXiv2025

Triaxiality and shape dynamics in $^{70}$Ge

The electromagnetic properties of low-lying states in $^{70}$Ge were investigated via multi-step Coulomb excitation of a $^{70}$Ge beam impinging on a $^{208}$Pb target at the ATLAS facility of the Argonne National Laboratory. A total of 27 transitional elements and six diagonal matrix elements coupling 11 low-lying states, were extracted from the measured cross sections. These were used to calculate reduced transition probabilities, spectroscopic quadrupole moments, and rotational invariant shape parameters, providing enhanced precision and expanding on previous studies. The experimental data were compared within several theoretical frameworks, including the generalized triaxial rotor model, configuration interaction shell-model calculations, and computations within the combined frameworks of relativistic density functional theory and the five-dimensional collective Hamiltonian. The results demonstrate a good agreement with the experimental data and, in conjunction with calculations using a two-state mixing model, support significant triaxiality and strong mixing between the $0^+_1$ and $0^+_2$ states. This results in the magnitudes of their respective quadrupole deformations $[β_\text{rms}(0^+_1) = 0.228\,(3),\,β_\text{rms}(0^+_2) = 0.273\,(1)]$ being more similar than previously observed. The implications of these results for understanding the complex shape coexistence phenomena, the role of triaxiality, and shape evolution along the Ge isotopic chain are discussed.

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arXiv2025

Universal Effective Charges in the $sd$ and $fp$ Shells

The 247-keV state in $^{54}$Sc, populated in the $β$ decay of $^{54}$Ca, is reported here as a nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the $1^+$ member of the $πf_{7/2}\otimesνf_{5/2}$ spin-coupled multiplet, which decays to the $3^+,πf_{7/2} \otimes νp_{1/2}$ ground state. The new half-life corresponds to a pure $E2$ transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous $B(E2)$ value in the neutron-rich $fp$ region to date for a nucleus with valence protons above $Z=20$. Notably, it is roughly four times larger than the $B(E2; 1/2^{-} \rightarrow 5/2^{-})$ value in $^{55}$Ca. The results, as compared to semi-empirical and ab initio shell-model calculations, indicate (1) a weak $N=34$ sub-shell gap relative to $N = 32$, (2) a large $E2$ enhancement in Sc as compared to Ca due to $1p-1h$ proton excitations across $Z=28$, and (3) empirical effective proton and neutron charges, $e_π$ = 1.30(8)$e$ and $e_ν$ = 0.452(7)$e$, respectively, that are in contrast to reports of $e_π\approx 1.1-1.15e$ and $e_ν\approx 0.6-0.8e$ for $fp$-shell nuclei near $N = Z$. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the $sd$ and $fp$ shells.

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arXiv2025

Gamma decay of the $^{154}$Sm Isovector Giant Dipole Resonance: Smekal-Raman Scattering as a Novel Probe of Nuclear Ground-State Deformation

Gamma decays of the isovector giant dipole resonance (GDR) of the deformed nucleus $^{154}$Sm from $2^+_1$-Smekal-Raman and elastic scattering were measured using linearly polarized, quasimonochromatic photon beams. The two scattering processes were disentangled through their distinct angular distributions. Their branching ratio and cross sections were determined at six excitation energies covering the $^{154}$Sm GDR. Both agree with the predictions of the geometrical model for the GDR and establish $γ$ decay as an observable sensitive to the structure of the resonance. Consequently, the data place strong constraints on the nuclear shape, including the degree of triaxiality. The derived $^{154}$Sm shape parameters $β=0.2926(26)$ and $γ=5.0(14)$ agree well with other measurements and recent Monte Carlo Shell-Model calculations.

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arXiv2024

Determination of the spins and parities for the 0$_{4}^{+}$ and 0$_{5}^{+}$ states in $^{100}$Zr

Two 0$^{+}$ states at 1294.5 and 1774.0 keV, together with three 2$^{+}$ and one 4$^{+}$ levels, were identified or unambiguously spin-parity assigned for the first time in $^{100}$Zr utilizing $γ$-ray spectroscopy and $γ$-$γ$ angular correlation techniques with the Gammasphere spectrometer, following the $β^{-}$ decay of neutron-rich, mass separated $^{100,100m}$Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around $\approx$1500 keV. According to the measured relative B(E2)$_{relative}$ transition probabilities, the 0$_{5}^{+}$ state exhibits decay properties which more closely align with those predicted for a spherical shape, while the 0$_{4}^{+}$ level is suggested to be associated with a weakly-deformed shape similar to one related to the 0$_{2}^{+}$ state.

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arXiv2022

E2 Rotational Invariants of $0^+_1$ and $2^+_1$ states for $^{106}$Cd: the Emergence of Collective Rotation

The collective structure of $^{106}$Cd is elucidated by multi-step Coulomb excitation of a 3.849 MeV/$A$ beam of $^{106}$Cd on a 1.1 mg/cm$^2$ $^{208}$Pb target using GRETINA-CHICO2 at ATLAS. Fourteen $E2$ matrix elements were obtained. The nucleus $^{106}$Cd is a prime example of emergent collectivity that possesses a simple structure: it is free of complexity caused by shape coexistence and has a small, but collectively active number of valence nucleons. This work follows in a long and currently active quest to answer the fundamental question of the origin of nuclear collectivity and deformation, notably in the cadmium isotopes. The results are discussed in terms of phenomenological models, the shell model, and Kumar-Cline sums of $E2$ matrix elements. The ${\langle 0_2^+ ||E2||2_1^+ \rangle}$ matrix element is determined for the first time, providing a total, converged measure of the electric quadrupole strength, $\langle Q^2 \rangle$, of the first-excited $2_1^+$ level relative to the $0_1^+$ ground state, which does not show an increase as expected of harmonic and anharmonic vibrations. Strong evidence for triaxial shapes in weakly collective nuclei is indicated; collective vibrations are excluded. This is contrary to the only other cadmium result of this kind in $^{114}$Cd by C. Fahlander et al., Nucl. Phys. A485, 327 (1988), which is complicated by low-lying shape coexistence near midshell.

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arXiv2021

Ground-state and decay properties of neutron-rich 106Nb

The ground-state properties of neutron-rich 106Nb and its beta decay into 106Mo have been studied using the CARIBU radioactive-ion-beam facility at Argonne National Laboratory. Niobium-106 ions were extracted from a 252Cf fission source and mass separated before being delivered as low-energy beams to the Canadian Penning Trap, as well as the X-Array and SATURN beta-decay-spectroscopy station. The measured 106Nb ground-state mass excess of -66202.0(13) keV is consistent with a recent measurement but has three times better precision; this work also rules out the existence of a second long-lived, beta-decaying state in 106Nb above 5 keV in excitation energy. The decay half-life of 106Nb was measured to be 1.097(21) s, which is 8% longer than the adopted value. The level scheme of the decay progeny, 106Mo, has been expanded up to approximately 4 MeV. The distribution of decay strength and considerable population of excited states in 106Mo of J >= 3 emphasises the need to revise the adopted Jpi = 1- ground-state spin-parity assignment of 106Nb; it is more likely to be J => 3.

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arXiv2020

Search for the $1/2^+$ intruder state in $^{35}$P

The excitation energy of deformed intruder states (specifically the 2p2h bandhead) as a function of proton number $Z$ along $N=20$ is of interest both in terms of better understanding the evolution of nuclear structure between spherical $^{40}$Ca and the Island of Inversion nuclei, and for benchmarking theoretical descriptions in this region. At the center of the $N=20$ Island of Inversion, the npnh (where n=2,4,6) neutron excitations across a diminished $N=20$ gap result in deformed and collective ground states, as observed in $^{32}$Mg. In heavier isotones, npnh excitations do not dominate in the ground states, but are present in the relatively low-lying level schemes. With the aim of identifying the expected 2p2h$\otimes\mathrm{s}_{1/2^+}$ state in $^{35}$P, the only $N=20$ isotone for which the neutron 2p2h excitation bandhead has not yet been identified, the $^{36}$S(d,$^3$He)$^{35}$P reaction has been revisited in inverse kinematics with the HELical Orbit Spectrometer (HELIOS) at the Argonne Tandem Linac Accelerator System (ATLAS). While a candidate state has not been located, an upper limit for the transfer reaction cross-section to populate such a configuration within a 2.5 to 3.6\,MeV energy range, provides a stringent constraint on the wavefunction compositions in both $^{36}$S and $^{35}$P.

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arXiv2020

Shape Coexistence at Zero Spin in 64Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (3463 keV), with a collective 2+ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

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arXiv2020

Longitudinal Wobbling Motion in $^{187}$Au

The rare phenomenon of nuclear wobbling motion has been investigated for the nucleus $^{187}$Au. A longitudinal wobbling-bands pair has been identified and clearly distinguished from the associated signature-partner band on the basis of angular distribution measurements. Theoretical calculations in the framework of the Particle Rotor Model (PRM) are found to agree well with the experimental observations. This is the first experimental evidence for longitudinal wobbling bands where the expected signature partner band has also been identified, and establishes this exotic collective mode as a general phenomenon over the nuclear chart.

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arXiv2019

Probing the role of proton cross-shell excitations in 70Ni using nucleon knockout reactions

The neutron-rich Ni isotopes have attracted attention in recent years due to the occurrence of shape or configuration coexistence. We report on the difference in population of excited final states in 70Ni following gamma-ray tagged one-proton, one-neutron, and two-proton knockout from 71Cu, 71Ni, and 72Zn rare-isotope beams, respectively. Using variations observed in the relative transition intensities, signaling the changed population of specific final states in the different reactions, the role of neutron and proton configurations in excited states of 70Ni is probed schematically, with the goal of identifying those that carry, as leading configuration, proton excitations across the Z = 28 shell closure. Such states are suggested in the literature to form a collective structure associated with prolate deformation. Adding to the body of knowledge for 70Ni, 29 new transitions are reported, of which 15 are placed in its level scheme.

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arXiv2019

Evidence for rigid triaxial deformation in $^{76}$Ge from a model-independent analysis

An extensive, model-independent analysis of the nature of triaxial deformation in $^{76}$Ge, a candidate for neutrinoless double-beta ($0νββ$) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and $γ$ bands. Both sequences were determined to be characterized by the same $β$ and $γ$ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in $^{76}$Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured $E2$ matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to $0νββ$.

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arXiv2019

New $γ$-ray Transitions Observed in $^{19}$Ne with Implications for the $^{15}$O($α$,$γ$)$^{19}$Ne Reaction Rate

The $^{15}$O($α$,$γ$)$^{19}$Ne reaction is responsible for breakout from the hot CNO cycle in Type I x-ray bursts. Understanding the properties of resonances between $E_x = 4$ and 5 MeV in $^{19}$Ne is crucial in the calculation of this reaction rate. The spins and parities of these states are well known, with the exception of the 4.14- and 4.20-MeV states, which have adopted spin-parities of 9/2$^-$ and 7/2$^-$, respectively. Gamma-ray transitions from these states were studied using triton-$γ$-$γ$ coincidences from the $^{19}$F($^{3}$He,$tγ$)$^{19}$Ne reaction measured with GODDESS (Gammasphere ORRUBA Dual Detectors for Experimental Structure Studies) at Argonne National Laboratory. The observed transitions from the 4.14- and 4.20-MeV states provide strong evidence that the $J^π$ values are actually 7/2$^-$ and 9/2$^-$, respectively. These assignments are consistent with the values in the $^{19}$F mirror nucleus and in contrast to previously accepted assignments.

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arXiv2019

Key $^{19}$Ne states identified affecting $γ$-ray emission from $^{18}$F in novae

Detection of nuclear-decay $γ$ rays provides a sensitive thermometer of nova nucleosynthesis. The most intense $γ$-ray flux is thought to be annihilation radiation from the $β^+$ decay of $^{18}$F, which is destroyed prior to decay by the $^{18}$F($p$,$α$)$^{15}$O reaction. Estimates of $^{18}$F production had been uncertain, however, because key near-threshold levels in the compound nucleus, $^{19}$Ne, had yet to be identified. This Letter reports the first measurement of the $^{19}$F($^{3}$He,$tγ$)$^{19}$Ne reaction, in which the placement of two long-sought 3/2$^+$ levels is suggested via triton-$γ$-$γ$ coincidences. The precise determination of their resonance energies reduces the upper limit of the rate by a factor of $1.5-17$ at nova temperatures and reduces the average uncertainty on the nova detection probability by a factor of 2.1.

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arXiv2018

Understanding the Low-Energy Enhancement of the $γ$-ray Strength Function of $^{56}$Fe

A model-independent technique was used to determine the $γ$-ray Strength Function ($γ$SF) of $^{56}$Fe down to $γ$-ray energies less than 1 MeV for the first time with GRETINA using the $(p,p')$ reaction at 16 MeV. No difference was observed in the energy dependence of the $γ$SF built on $2^{+}$ and $4^{+}$ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

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