arXiv subjects

A. Volya

Publications and source records attributed to A. Volya.

At least 19 recordsLinked to original sources

arXiv2026

A detailed view at magnetic dipole strengths: The case of semi-magic $^{50}$Ti

Magnetic dipole, $M1$, strengths were studied in semi-magic $^{50}$Ti up to the neutron-separation threshold by combining data from $(d,p)$ one-neutron transfer, $(\gamma,\gamma')$ real-photon scattering, $(e,e')$ inelastic scattering at extreme backward angles, and $(p,p')$ at $E_p = 210$ MeV and extreme forward angles. The combination of all probes provided unique access to the neutron spin-flip contribution and the possibility to evaluate its role in generating the spin-flip $M1$ strengths. The small contribution of the neutron $(1f_{7/2})^{-1}(1f_{5/2})^{+1}$ spin-flip transitions, which were probed with the $(d,p)$ reaction, to the overall strength in $^{50}$Ti questions the standard picture for the microscopic origin of spin-flip strength in the $fp$ shell. For $^{50}$Ti, this letter shows that $J^{\pi} = 1^+$ states with larger neutron $(1f_{7/2})^{-1}(1f_{5/2})^{+1}$ spectroscopic factors do not correspond to the ones with the largest $B(M1;0^+_1 \rightarrow 1^+_i)$ strengths.

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arXiv2026

Detailed experimental study of excited states in $^{50}$Ti via the $(d,p)$ and $(d,p\gamma)$ reactions

Excited states of semi-magic $^{50}$Ti were studied up to the neutron-separation energy via the $(d,p)$ and $(d,p\gamma)$ reactions. In total, 82 excited states were identified based on the measurement of angular distributions with the Super-Enge Split-Pole Spectrograph (SE-SPS) at Florida State University. From the experimental data, sum rules related to vacancies were calculated for the $2p_{3/2}$, $2p_{1/2}$, $1f_{5/2}$, $1g_{9/2}$, and $2d_{5/2}$ neutron single-particle orbitals and compared to predictions obtained with the time-dependent continuum shell model (TDCSM), the quasiparticle-phonon model (QPM), and the relativistic equation of motion theory (REOM$^3$). A comparison for the $(d,p)$ data obtained for $^{51}$Ti and $^{50}$Ti is also presented, focusing on differences of the single-particle strength fragmentation in even-$A$ and odd-$A$ nuclei.

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arXiv2026

Unfolding of exotic near-threshold structure and decay dynamics in $^{17}$B

Neutron-rich boron isotopes provide a valuable testing ground for threshold-driven structure and reaction phenomena, including halo formation and exotic decay modes. In particular, the structure of $^{17}\mathrm{B}$ and its relation to unbound $^{16}\mathrm{B}$ are of special interest. The $^{16}\mathrm{B}$ nucleus is slightly unbound by approximately $50~\mathrm{keV}$, while $^{17}\mathrm{B}$ is bound with a neutron separation energy of about $1.4-1.6~\mathrm{MeV}$. The observation of a $1640~\mathrm{keV}$ $\gamma$ ray in $^{17}\mathrm{B}$, which we argue originates from a $1/2^-$ excited state, points to a remarkable situation in which $\gamma$ decay and two-neutron decay can compete. We analyze and identify the main reasons for this competition: $L=2$ emission of the neutron pair, and structural realignment driven by the proximity of the one-body threshold, in particular the nearby $s$-wave neutron decay channel. The decay is a unique near-threshold $L=2$ process in which multiple structural components contribute, each with coexisting direct and virtual sequential amplitudes whose interference governs the observables. Because threshold dynamics, continuum coupling, and interference of multiple quantum pathways are universal, closely related scenarios arise in ultracold atoms near Feshbach resonances, few-body atomic and molecular breakups, mesoscopic and photonic open systems, and other areas where open-quantum-system effects impact observables. We employ advanced theoretical models to study this first-of-its-kind case and provide a coherent theoretical perspective based on configuration interaction and complex-energy formalisms that incorporate both reaction continuum and structural effects near threshold.

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arXiv2025

Elucidating the role of the surface energy in density functional theory

The saturation of symmetric nuclear matter -- reflected in the nearly constant interior density of heavy nuclei -- is a defining property of nuclear matter. Modern relativistic energy density functionals (EDFs) calibrated exclusively to the properties of finite nuclei, make robust predictions with quantified uncertainties about the bulk properties of symmetric nuclear matter in the vicinity of the saturation density. Following the same fitting protocol, nonrelativistic Skyrme EDFs systematically predict higher saturation densities than their relativistic counterparts. To investigate this tension in the bulk limit, we study the ground-state properties of hypothetical symmetric macroscopic nuclei containing thousands of nucleons. Using both relativistic and non-relativistic EDF frameworks, we extract the corresponding liquid-drop parameters. We find a clear correlation between the volume and surface energy coefficients: Skyrme models, which saturate at higher densities, develop softer and more diffuse surfaces with lower surface energies, whereas relativistic EDFs, which saturate at lower densities, produce more defined and less diffuse surfaces with higher surface energies. This compensating behavior allows both classes of models to reproduce empirical nuclear radii despite their distinct saturation properties. Our analysis suggests that the apparent disparity in saturation densities arises from the intrinsic balance among saturation density, bulk binding energy, and surface tension, rather than from the fitting protocol.

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arXiv2025

Determination of proton and neutron contributions to the $0_{g.s.}^+ \rightarrow 2_1^+$ excitations in $^{42}$Si and $^{44}$S using inelastic proton scattering in inverse kinematics and intermediate energy Coulomb excitation

We have measured the $0_{g.s.}^+ \rightarrow 2_1^+$ transition in the neutron rich $N=28$ isotope $^{42}$Si using the probes of intermediate energy Coulomb excitation and inelastic proton scattering in inverse kinematics at the Facility for Rare Isotope Beams with beam particle rates of $\approx 5$ particles/s. The results of these two measurements allowed us to determine $M_n/M_p$, the ratio of the neutron and proton transition matrix elements for the $0_{g.s.}^+ \rightarrow 2_1^+$ transition. In addition, we have measured the $0_{\mathrm{g.s.}}^+ \rightarrow 2_1^+$ transition in the isotone $^{44}$S using inverse kinematics inelastic proton scattering. By comparing the $^{44}$S proton scattering result with a recent intermediate energy Coulomb excitation result on the same transition, we were able to determine $M_n/M_p$ for the $0_{g.s.}^+ \rightarrow 2_1^+$ transition in this nucleus as well. This work strengthens the evidence that $^{42}$Si has a stable quadrupole deformation in its ground state and that $^{44}$S does not. Both conclusions are further supported by shell model calculations carried out with the FSU interaction.

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arXiv2025

Quantifying alpha clustering in the ground states of 16-O and 20-Ne

Understanding the role of multi-nucleon correlations in the structure of light nuclei is at the forefront of modern nuclear science. In this letter, we present a quantitative benchmark study of alpha-cluster correlations in the ground states of 16-O and 20-Ne. Experimental data provide direct evidence that the wave functions of the ground states of 16-O and 20-Ne are dominated by alpha-cluster correlations, in agreement with the predictions of sophisticated nuclear structure models. We also provide a new model-independent constraint for the alpha asymptotic normalization coefficient of the 16-O ground state and discuss the implications of these findings on the 12-C(alpha,gamma)16-O reaction, which is of critical importance for nuclear astrophysics.

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arXiv2025

Exploring $\beta$ decay and $\beta$-delayed neutron emission in exotic $^{46,47}$Cl isotopes

In this paper, $\beta^-$ and $\beta$-delayed neutron decays of $^{46,47}$Cl are reported from an experiment carried out at the National Superconducting Cyclotron Laboratory using the Beta Counting System. The half-lives of both $^{46}$Cl and $^{47}$Cl were extracted. Based on the delayed $\gamma$-ray transitions observed, the level structure of $N = 28$ $^{46}$Ar was determined. Completely different sets of excited states above the first $2^+$ state in $^{46}$Ar were populated in the $^{46}$Cl $\beta0n$ and $^{47}$Cl $\beta1n$ decay channels. Two new $\gamma$-ray transitions in $^{47}$Ar were identified from the very weak $^{47}$Cl $\beta0n$ decay. Furthermore, $^{46}$Cl $\beta1n$ and $^{47}$Cl $\beta2n$ were also observed to yield different population patterns for levels in $^{45}$Ar, including states of different parities. The experimental results allow us to address some of the open questions related to the delayed neutron emission process. For isotopes with large neutron excess and high $Q_{\beta}$ values, delayed neutron emission remains an important decay mode and can be utilized as a powerful spectroscopic tool. Experimental results were compared with shell-model calculations using the FSU and $V_{MU}$ effective interactions.

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arXiv2024

Cross-shell excited configurations in the structure of 34Si

The cross-shell excited states of $^{34}$Si have been investigated via $\beta$-decays of the $4^-$ ground state and the $1^+$ isomeric state of $^{34}$Al. Since the valence protons and valence neutrons occupy different major shells in the ground state as well as the intruder $1^+$ isomeric state of $^{34}$Al, intruder levels of $^{34}$Si are populated via allowed $\beta$ decays. Spin assignments to such intruder levels of $^{34}$Si were established through $\gamma$-$\gamma$ angular correlation analysis for the negative parity states with dominant configurations $(\nu d_{3/2})^{-1} \otimes (\nu f_{7/2})^{1}$ as well as the positive parity states with dominant configurations $(\nu sd)^{-2} \otimes (\nu f_{7/2}p_{3/2})^2$. 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 $^{34}$Si. Shell model interaction derived from a more fundamental theory with the Valence Space In Medium Similarity Renormalization Group (VS-IMSRG) method was also employed to interpret the structure of $^{34}$Si.

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arXiv2023

$\beta^-$ decay of neutron-rich $^{45}$Cl at magic number N=28

Results from the study of $\beta^-$-decay of $^{45}$Cl, produced in the fragmentation of a 140-MeV/u $^{48}$Ca beam, are presented. The half-life for $^{45}$Cl $\beta$-decay is measured to be 513(36) ms. The $\beta^-$ and $\beta^- 1n$ decay of $^{45}$Cl populated excited states in $^{45,44}$Ar, respectively. On the basis of $\gamma$-ray singles and $\gamma$-$\gamma$ coincidence data, decay schemes for the two daughter nuclei have been established. They are compared with shell model calculations using the FSU interaction. The low-lying negative parity states for $^{45}$Ar are well described by a single particle (neutron) occupying orbitals near the Fermi surface, whereas neutron excitations across the $N = 20$ shell gap are needed to explain the positive-parity states which are expected to be populated in allowed Gamow-Teller $\beta$-decay of $^{45}$Cl. The highest $\beta$-feeding to the 5/2$^+$ state in $^{45}$Ar from the ground state of $^{45}$Cl points towards a 3/2$^+$ spin-parity assignment of the ground state of the parent over the other possibility of 1/2$^+$. The high Q$_{\beta^-}$ value of $^{45}$Cl decay allows for the population of $1p1h$ states above the neutron separation energy in $^{45}$Ar leading to positive parity states of $^{44}$Ar being populated by removal of one neutron from the $sd$ shell. The spin-parities of the excited levels in $^{44}$Ar are tentatively assigned for the first time by comparison with the shell model calculations. The 2978~keV level of $^{44}$Ar is identified as the excited 0$^+$ level which could correspond to a different configuration from the ground state.

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arXiv2022

Probing the Non-exponential Decay Regime in Open Quantum Systems

The most important law of radioactivity is that of the exponential decay. In the realm of quantum mechanics, however, this decay law is neither rigorous nor fundamental. The deviations from the exponential decay have been observed experimentally at the early stage of a decay process, but there is little evidence for non-exponential behavior at long times. Yet such long-term non-exponentiality is expected theoretically to probe the non-resonant background components of the initial wave function which preserve the structural interference and the memory of how the state was created. In this paper, we propose new observables that can be used for experimental investigations of the post-exponential decay regime, including the decay of threshold resonances, particle correlations in three-body decays, and interference between near-lying resonances. While the specific examples presented in this work pertain to atomic nuclei, the properties of non-exponential decay are generic, i.e., they apply to other many-body open quantum systems, such as hadrons, atoms, molecules, and nanostructures.

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arXiv2022

Eigenstate thermalization and disappearance of quantum many-body scar states in interacting fermion systems

The recent discovery of quantum many-body scar states has revealed the possibility of having states with low entanglement that violate the eigenstate thermalization hypothesis in nonintegrable systems. Such states with low entanglement entropy are rare but naturally exist in the integrable system of free fermions. Here, we demonstrate analytically that these atypical states would be always eliminated when an arbitrary weak interaction is introduced between the fermions. In particular, we show that the probability of having a many-body scar state with entanglement entropy satisfying a sub-volume scaling law decreases double exponentially as the system size. Thus, our results provide a quantitative argument for the disappearance of scar states in interacting fermion systems.

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arXiv2022

alpha-cluster structure of 18Ne

In this work we study alpha-clustering in 18Ne and compare it with what is known about clustering in the mirror nucleus 18O. The excitation function of 18Ne was measured in inverse kinematics from the resonant elastic scattering reaction of 14O on 4He in the excitation energy range from 8 to 17 MeV, using the active target TexAT. The analysis was performed using a multi-channel R-matrix approach. Detailed spectroscopic information is obtained from the R-matrix analysis: excitation energy of the states, spin and parity as well as partial alpha and total widths. This information is compared with theoretical models and previous data. Clustering structures appear to be robust and mostly isospin symmetric. A good correspondence was found between the levels in 18O and 18Ne. We carried out an extensive shell model analysis of the experimental data. This comparison suggests that strongly clustered states remain organized in relation to the corresponding reaction channel identified by the number of nodes in the relative alpha plus core wave function. The agreement between theory and experiment is very good and especially useful when it comes to understanding the clustering strength distribution. The comparison of the experimental data with theory shows that certain states, especially at high excitation energies, are significantly more clustered than predicted. This indicates that the structure of these states is collective and is aligned towards the corresponding alpha reaction channel.

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arXiv2022

Multiparticle-hole excitations in nuclei near N = Z = 20: $^{41}$K

This experimental study of high-spin structure near N = Z = 20 nuclei was focused on $^{41}$K, but will also mention three newly observed $\gamma$ transitions in $^{41}$Ca observed in the same reaction. High-spin states were populated using the $^{26}$Mg($^{18}$O, $p2n\gamma$)$^{41}$K and $^{26}$Mg($^{18}$O, $3n\gamma$)$^{41}$Ca reactions. The experiment was carried out at an incident beam energy of 50 MeV at the Florida State University (FSU) John D. Fox Superconducting Linear Accelerator Laboratory and used the FSU high-purity germanium detector array. The $^{41}$K level scheme was extended to 12325 keV, possibly with J$^{\pi}$ = 25/2$^-$ or 27/2$^+$, by means of 25 new transitions and that of $^{41}$Ca to 9916 keV. Linear polarization and a measure of angular distribution results are also reported and used to provide information on the spins and parities of several states in the $^{41}$K decay scheme. The results have been compared to the $spsdpf$ cross-shell FSU shell model interaction calculations. The theoretical results from configurations involving no or one additional nucleon promoted from the $sd$ to the $fp$ shell agree relatively well with the energies of known states, while those that involve multi-particle excitations paint an interesting and complex picture of interplay between single-particle excitations, collective pairing, and deformation. This presents an interesting challenge for future theory.

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arXiv2021

The Lowest Broad Alpha Cluster Resonances in $^{19}$F

There is a deep astrophysical interest in the structure of $^{19}$F states close to the alpha decay threshold. The nuclear structure of these states is important for understanding of the development of $α$ clustering in the $^{20}$Ne region. Emergence of clustered states and generally states that favor coupling to reaction channels near the corresponding decay thresholds is currently of special interest in theoretical physics. Excitation function for $^{15}$N($α$,$α$) elastic scattering was measured by the TTIK method. These new data together with old, high energy resolution data, were analyzed using the R matrix approach. $^{19}$F nuclear structure was calculated using configuration interaction methods with the recently developed effective interaction Hamiltonian. The parameters of broad low spin $\ell = 0$ and 1 relative partial wave resonances close to the $α$ decay threshold in $^{19}$F were identified. Detailed theoretical analysis was carried out identifying all states coupled to the $\ell = 0$ and 1 alpha cluster channels. Considering hierarchy of states with different harmonic oscillator shell excitations allows to evaluate coupling to the alpha channels with different number of nodes in the relative wave function and helps to explain the distribution of the clustering strength and emergence of broad clustering resonances. Comparison of clustering in $^{20}$Ne into $^{16}$O+$α$ and consideration of spin-orbit splitting of the $^{15}$N+$α$ channel provides additional evidence. Detailed analysis of new and old experimental data allows to identify a series of $α$ clustering resonances in $^{19}$F and to assess the distribution of the clustering strength which is of importance to questions of astrophysics and for theoretical understanding of many-body physics and emergence of clustering in loosely bound or unstable nuclei.

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arXiv2020

Evolution of the N=20 and 28 Shell Gaps and 2-particle-2-hole states in the FSU Interaction

The FSU $spsdfp$ cross-shell interaction for the shell model was successfully fitted to a wide range of mostly intruder negative parity states of the $sd$ shell nuclei. This paper reports the application of the FSU interaction to systematically trace out the relative positions of the effective single-particle energies of the $0f_{7/2}$ and $1p_{3/2}$ orbitals, the evolution from normally ordered low-lying states to the "Island of Inversion" (IoI), and the behavior of a wide range of excited states with a $0f_{7/2}$ proton and neutron coupled to maximum spin of $7 \hbar$. Above a proton number of about 13 the $0f_{7/2}$ orbital lies below that of $1p_{3/2}$, which is considered normal ordering, but systematically at $Z = 10$ to $12$ the orbitals cross. The calculations reproduce well the 2p2h - 0p0h inversion in the configurations of nuclei inside the IoI, they reproduce the absolute binding energies and the transition to normal ordering as the proton number approaches that of the neutrons. The important role of $1p_{3/2}$ neutron pairs in the IoI is also demonstrated. The calculations account well for the energies of the fully aligned states with 0, 1, or 2 individual $sd$ nucleon aligned in spin with the aligned $π0f_{7/2}$ - $ν0f_{7/2}$ pair and reproduce well their systematic variation with $A$ and number of aligned $sd$ nucleons. The results presented in this paper give hope for the predictive power of the FSU interaction for more exotic nuclei to be explored in near future.

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arXiv2020

Inverse-kinematics proton scattering from $^{42,44}$S, $^{41,43}$P and the collapse of the $N=28$ major shell closure

Excited states of the neutron-rich isotopes $^{42,44}$S and $^{41,43}$P have been studied via inverse-kinematics proton scattering from a liquid hydrogen target, using the GRETINA $γ$-ray tracking array to extract inelastic scattering cross sections. Deformation lengths of the $2^+_1$ excitations in $^{42,44}$S have been determined and, when combined with deformation lengths determined with electromagnetic probes, yield the ratio of neutron-to-proton matrix elements $M_n/M_p$ for the $2^+_1$ excitations in these nuclei. The present results for $^{41,43}$P$(p,p')$ are used to compare two shell model interactions, SDPF-U and SDPF-MU. As in a recent study of $^{42}$Si, the present results on $^{41,43}$P favor the SDPF-MU interaction.

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arXiv2019

Structure of $^{38}$Cl and the quest for a comprehensive shell model interaction

The higher-spin structure of $^{38}$Cl ($N = 21$) was investigated following the $^{26}$Mg($^{14}$C, $pn$) reaction at 30 and 37 MeV beam energies. The outgoing protons were detected in an $E- ΔE$ Si telescope placed at 0$^\circ$ close to the target with a Ta beam stopper between the target and telescope. Multiple $γ$ rays were detected in time coincidence with the protons using an enhanced version of the FSU $γ$ detection array. The level scheme was extended up to 8420 keV with a likely spin of 10 $\hbar$. A new multishell interaction was developed guided by the experimental information. This FSU interaction was built by fitting to the energies of 270 experimental states from $^{13}$C to $^{51}$Ti. Calculations using the FSU interaction reproduce observed properties of $^{38}$Cl rather well, including the spectroscopic factors. The interaction has been successfully used to interpret the $1p1h$ and $2p2h$ configurations in some nearby nuclei as well.

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arXiv2019

$^{21}$Ne level structure in the resonance $^{17}$O+$α$ elastic scattering

The first study of resonances in $^{17}$O+$α$ elastic scattering was carried out using the Thick Target Inverse Kinematics (TTIK) method. The data were analyzed in the framework of an $\textit{R}$-matrix approach. Many $α$-cluster states were found in the $^{21}$Ne excitation region of the 9-13 MeV excitation energy including the first observation of a broad $\textit{l}$=0 state in an odd-even nucleus, which is likely the analog of the broad 0$^+$ at 8 MeV in $^{20}$Ne. The observed structure in $^{21}$Ne appeared to be strikingly similar to that in $^{20}$Ne populated in the resonance $^{16}$O+$α$ scattering. The results are also useful for refinement of data on an $^{17}$O($α$,$\textit{n}$) reaction important for astrophysics.

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