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K. Starosta

Publications and source records attributed to K. Starosta.

At least 19 recordsLinked to original sources
arXiv2026

Negative-parity high-spin structure of 105Pd

Negative-parity medium- and high-spin structure of the nucleus 105Pd was studied through the 96Zr(13C,4n)105Pd reaction at incident energies of 51 and 58 MeV, using the EUROBALL IV gamma-ray spectrometer in conjunction with the DIAMANT charged particle array. New bands have been observed and the previously reported bands have been extended to higher energies and spins. Altogether six decoupled bands with E2 transitions and one strongly coupled band with M1 + E2 transitions have been observed. The observed energy spectra and B(M1)/B(E2) ratios are compared with results of quantum particle rotor model calculations. Based on these comparisons, quasiparticle configurations can be assigned to two newly observed decoupled bands as well as to the strongly coupled band. The previously emerged possible interpretation for the third decoupled band as a two-phonon wobbling excitation lacks support. The observations indicate possible gamma-band nature for this band. The strongly coupled band, consistently with the absence of another observed strongly coupled band in this experiment, does not exhibit chirality.

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arXiv2025

Intruder structures in $^{32}$Si and $^{29}$Al

We have studied $^{32}$Si and $^{29}$Al using $^{12}$C($^{22}$Ne,2p) and $^{12}$C($^{22}$Ne,$\alpha$p) fusion-evaporation reactions. In both cases, we observed significant population of high-spin structures distinct from the ground-state yrast bands. In $^{32}$Si, most of the high-energy states feed into a $J^{\pi} = 5^-$ nanosecond isomer. In $^{29}$Al, we identified a rotor-like negative-parity band with a $J^{\pi} = 7/2^-$ band-head. Doppler shift lifetime measurements were performed for all observed states. These results were compared to shell model calculations and interpreted in terms of proton and neutron cross-shell excitation.

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arXiv2023

Identifying the spin trapped character of the $^{32}$Si isomeric state

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 $J^π = 5^-$, 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 $(νd_{3/2})^{-1} \otimes (νf_{7/2})^{1}$ 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.

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arXiv2022

Neutron Emission Spectrometer To Measure Ion Temperature On The Fusion Demonstration Plant

General Fusion is building the Fusion Demonstration Plant to demonstrate a magnetized target fusion scheme in which a deuterium plasma is heated from 200 eV to 10 keV by piston-driven compression of a liquid-lithium liner. The multilayer coaxial time-of-flight (MCTOF) neutron emission spectrometer is designed to measure the ion temperature near peak compression at which time the neutron yield will approach $10^{18}$ neutrons/s. The neutron energy distribution is expected to be Gaussian since the machine uses no neutral beam or radiofrequency heating. In this case, analysis shows that as few as 500 coincidence events should be sufficient to accurately measure the ion temperature. This enables a fast time resolution of 10 $μs$, which is required to track the rapid change in temperature approaching peak compression. We overcome the challenges of neutron pileup and event ambiguity with a compact design having two layers of segmented scintillators. The error in the ion temperature measurement is computed as a function of the neutron spectrometer's geometric parameters and used to optimize the design for the case of reaching 10 keV at peak compression.

physics.plasm-ph↗
arXiv2022

Improved measurement of the $0_2^+\rightarrow0_1^+$ E0 transition strength for $^{72}$Se using the SPICE spectrometer

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 dripline. In this work a study of $^{72}$Se by internal conversion electron and $γ$-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 $ρ^2(E0;0_2^+{\rightarrow}0_1^+)=29(3)$ milliunits. two state mixing calculations were performed that highlighted the importance of interpretation of such $E0$ strength values in the context of shape-coexistence.

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arXiv2019

Shape coexistence in the neutron-deficient lead region: A systematic study of lifetimes in the even-even $^{188-200}$Hg with GRIFFIN

Lifetimes of $2^+_1$ and $4^+_1$ states, as well as some negative-parity and non-yrast states, in $^{188-200}$Hg were measured using $γ-γ$ electronic fast timing techniques with the LaBr$_3$(Ce) detector array of the GRIFFIN spectrometer. The excited states were populated in the $ε/β^+$-decay of $J^π=7^+/2^-$ $^{188-200}$Tl produced at the TRIUMF-ISAC facility. The deduced B(E2) values are compared to different interacting boson model predictions. The precision achieved in this work over previous ones allows for a meaningful comparison with the different theoretical models of these transitional Hg isotopes, which confirms the onset of state mixing in $^{190}$Hg.

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arXiv2019

Isospin symmetry in $B(E2)$ values: Coulomb excitation study of ${}^{21}$Mg

The $T_z$~=~$-\frac{3}{2}$ nucleus ${}^{21}$Mg has been studied by Coulomb excitation on ${}^{196}$Pt and ${}^{110}$Pd targets. A 205.6(1)-keV $γ$-ray transition resulting from the Coulomb excitation of the $\frac{5}{2}^+$ ground state to the first excited $\frac{1}{2}^+$ state in ${}^{21}$Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $\frac{1}{2}^+$ state yield an adopted value of $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$~=~13.3(4)~W.u. A new excited state at 1672(1)~keV with tentative $\frac{9}{2}^+$ assignment was also identified in ${}^{21}$Mg. This work demonstrates large difference of the $B(E2;\frac{5}{2}^+\rightarrow\frac{1}{2}^+)$ values between $T$~=~$\frac{3}{2}$, $A$~=~21 mirror nuclei. The difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern \textsl{ab initio} nuclear structure calculations, which have recently become applicable in the $sd$ shell.

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arXiv2019

Shell evolution approaching the N=20 island of inversion: Structure of 29Mg

The "Island of Inversion" for neutron-rich nuclei in the vicinity of N=20 has become the testing ground par excellence for our understanding and modelling of shell evolution with isospin. In this context, the structure of the transitional nucleus 29Mg is critical. The first quantitative measurements of the single particle structure of 29Mg are reported, using data from the d(28Mg,p gamma)29Mg reaction. Two key states carrying significant ell=3 (f-wave) strength were identified at 2.40 +/- 0.10 (Jpi = 5/2-) and 4.28 +/- 0.04 MeV (7/2-). New state-of-the-art shell model calculations have been performed and the predictions are compared in detail with the experimental results. Whilst the two lowest 7/2- levels are well described, the sharing of single-particle strength disagrees with experiment for both the 3/2- and 5/2- levels and there appear to be general problems with configurations involving the p3/2 neutron orbital and core-excited components. These conclusions are supported by an analysis of the neutron occupancies in the shell model calculations.

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arXiv2019

Experimental Evidence for Transverse Wobbling in $^{105}$Pd

New rotational bands built on the $ν$$(h_{11/2})$ configuration have been identified in $^{105}$Pd. Two bands built on this configuration show the characteristics of transverse wobbling: the $Δ$$I$=1 transitions between them have a predominant E2 component and the wobbling energy decreases with increasing spin. The properties of the observed wobbling bands are in good agreement with theoretical results obtained using constrained triaxial covariant density functional theory and quantum particle rotor model calculations. This provides the first experimental evidence for transverse wobbling bands based on a one-neutron configuration, and also represents the first observation of wobbling motion in the $A$$\sim$100 mass region.

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arXiv2018

The GRIFFIN Facility for Decay-Spectroscopy Studies at TRIUMF-ISAC

Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new high-efficiency $γ$-ray spectrometer designed for use in decay spectroscopy experiments with low-energy radioactive ion beams provided by TRIUMF's Isotope Separator and Accelerator (ISAC-I) facility. GRIFFIN is composed of sixteen Compton-suppressed large-volume clover-type high-purity germanium (HPGe) $γ$-ray detectors combined with a suite of ancillary detection systems and coupled to a custom digital data acquisition system. The infrastructure and detectors of the spectrometer as well as the performance characteristics and the analysis techniques applied to the experimental data are described.

physics.ins-det↗
arXiv2018

Three-level mixing model for nuclear chiral rotation: Role of planar component

Three- and two-level mixing models are proposed to understand the doubling of states at the same spin and parity in triaxially-deformed atomic nuclei with odd numbers of protons and neutrons. The Particle-Rotor Model for such nuclei is solved using the newly proposed basis which couples angular momenta of two valence nucleons and the rotating triaxial mean-field into left-handed $|\mathcal{L}\rangle$, right-handed $|\mathcal{R}\rangle$, and planar $|\mathcal{P}\rangle$ configurations. The presence and the impact of the planar component is investigated as a function of the total spin for mass A$\approx$130 nuclei with the valence h$_{11/2}$ proton particle, valence h$_{11/2}$ neutron hole and the maximum difference between principle axes allowed by the quadrupole deformation of the mean field. It is concluded that at each spin value the higher-energy member of a doublet of states is built on the anti-symmetric combination of $|\mathcal{L}\rangle$ and $|\mathcal{R}\rangle$ and is free of the $|\mathcal{P}\rangle$ component, indicating that it is of pure chiral geometry. For the lower-energy member of the doublet, the contribution of the $|\mathcal{P}\rangle$ component to the eigenfunction first decreases and then increases as a function of the total spin. This trend as well as the energy splitting between the doublet states are both determined by the Hamiltonian matrix elements between the planar ($|\mathcal{P}\rangle$) and non-planar ($|\mathcal{L}\rangle$ and $|\mathcal{R}\rangle$) subspaces of the full Hilbert space.

nucl-th↗
arXiv2018

Lifetimes of low-lying excited states in $^{86}_{36}$Kr$_{50}$

The evolution of nuclear magic numbers at extremes of isospin is a topic at the forefront of contemporary nuclear physics. $N=50$ is a prime example, with increasing experimental data coming to light on potentially doubly-magic $^{100}$Sn and $^{78}$Ni at the proton-rich and proton-deficient extremes, respectively. Experimental discrepancies exist in the data for less exotic systems. In $^{86}$Kr the $B(E2;2^+_1\rightarrow0^+_1)$ value - a key indicator of shell evolution - has been experimentally determined by two different methodologies, with the results deviating by $3σ$. Here, we report on a new high-precision measurement of this value, as well as the first measured lifetimes and hence transition strengths for the $2^+_2$ and $3^-_{(2)}$ states in the nucleus. The Doppler-shift attenuation method was implemented using the TIGRESS gamma-ray spectrometer and TIGRESS integrated plunger (TIP) device. High-statistics Monte-Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Lifetimes of $τ(2^+_1)=336\pm4\text{(stat.)}\pm20\text{(sys.)}$ fs, $τ(2^+_2)=263\pm9\text{(stat.)}\pm19\text{(sys.)}$ fs and $τ(3^-_{(2)})=73\pm6\text{(stat.)}\pm32\text{(sys.)}$ fs were extracted. This yields a transition strength for the first-excited state of $B(E2;2^+_1\rightarrow0^+)=259\pm3\text{(stat.)}\pm16\text{(sys.)}$ e$^2$fm$^4$. The measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than $3σ$, while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted $B(E2;2^+_1\rightarrow0^+_1)$ value indicates a more sudden reduction in collectivity in the $N=50$ isotones approaching $Z=40$.

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arXiv2017

Characteristics of GRIFFIN high-purity germanium clover detectors

The Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new experimental facility for radioactive decay studies at the TRIUMF-ISAC laboratory. The performance of the 16 high-purity germanium (HPGe) clover detectors that will make up the GRIFFIN spectrometer is reported. The energy resolution, efficiency, timing resolution, crosstalk and preamplifier properties of each crystal were measured using a combination of analogue and digital data acquisition techniques. The absolute efficiency and add-back factors are determined for the energy range of 80 - 3450 keV. The detectors show excellent performance with an average over all 64 crystals of a FWHM energy resolution of 1.89(6) keV and relative efficiency with respect to a 3"x3" NaI detector of 41(1)% at 1.3 MeV.

physics.ins-det↗
arXiv2014

Multiple chiral doublet bands of identical configuration in 103Rh

Three sets of chiral doublet band structures have been identified in the 103Rh nucleus. The properties of the observed chiral doublet bands are in good agreement with theoretical results obtained using constrained covariant density functional theory and particle rotor model calculations. Two of them belong to an identical configuration, and provide the first experimental evidence for a novel type of multiple chiral doublets, where an "excited" chiral doublet of a configuration is seen together with the "yrast" one. This observation shows that the chiral geometry in nuclei can be robust against the increase of the intrinsic excitation energy.

nucl-ex↗
arXiv2010

Signatures of chirality in the core-particle-hole systems

An odd-odd nucleus is treated as the core-particle-hole system. The core is described by the Bohr Hamiltonian. Different collective potentials of the core are investigated. The odd particle and hole are assumed to be in the symmetric [proton h11/2]x[neutron hole h11/2] configuration. Signatures of chirality in the odd-odd nucleus spectra are observed. The sufficient condition for the appearance of signatures of chirality in the core-particle-hole system is the alpha-symmetry of the core provided the particle-hole configuration of the odd valence particles is symmetric.

nucl-th↗
arXiv2010

Observation of isotonic symmetry for enhanced quadrupole collectivity in neutron-rich 62,64,66Fe isotopes at N=40

The transition rates for the 2_{1}^{+} states in 62,64,66Fe were studied using the Recoil Distance Doppler-Shift technique applied to projectile Coulomb excitation reactions. The deduced E2 strengths illustrate the enhanced collectivity of the neutron-rich Fe isotopes up to N=40. The results are interpreted by the generalized concept of valence proton symmetry which describes the evolution of nuclear structure around N=40 as governed by the number of valence protons with respect to Z~30. The deformation suggested by the experimental data is reproduced by state-of-the-art shell calculations with a new effective interaction developed for the fpgd valence space.

nucl-ex↗
arXiv2007

Linear polarization sensitivity of SeGA detectors

Parity is a key observable in nuclear spectroscopy. Linear polarization measurements of $γ$-rays are a probe to access the parities of energy levels. Utilizing the segmentation of detectors in the Segmented Germanium Array (SeGA) at the NSCL and analyzing the positions of interaction therein allows the detectors to be used as Compton polarimeters. Unlike other segmented detectors, SeGA detectors are irradiated from the side to utilize the transversal segmentation for better Doppler corrections. Sensitivity in such an orientation has previously been untested. A linear polarization sensitivity $Q \approx 0.14$ has been measured in the 350-keV energy range for SeGA detectors using $α$-$γ$ correlations from a \nuc{249}{Cf} source.

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