arXiv subjects

R. Lozeva

Publications and source records attributed to R. Lozeva.

13 recordsLinked to original sources

arXiv2026

Precision Tests of Isospin Symmetry through Coulomb excitation of A = 62 Nuclei

Isospin symmetry in the $A=62$ mass system was investigated through Coulomb excitation reactions at the RIKEN Radioactive Isotope Beam Factory. Beams of $^{62}$Zn, $^{62}$Ga, and $^{62}$Ge were studied using the BigRIPS-ZeroDegree-DALI2$^+$ setup under identical experimental conditions, allowing for cancellation of systematic uncertainties. Inelastic scattering cross sections measured with two different targets were used to extract nuclear deformation lengths and $E2$ matrix elements. The isospin symmetry of the $A=62$ system was rigorously tested by examining the linearity of the proton matrix elements within the triplet with high precision. The observed linear relationship between the reduced proton matrix elements for the three nuclei holds within experimental uncertainties, providing a stringent test of isospin symmetry. This experiment provides the most accurate test, to date, of isospin symmetry rules using transition matrix elements. These results were interpreted using large-scale shell-model calculations, offering valuable insights into isospin symmetry behavior in this region of the nuclear chart.

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arXiv2024

High-resolution spectroscopy of neutron-rich Br isotopes and signatures for a prolate-to-oblate shape transition at N=56

The first systematic experimental study of the neutron-rich Br isotopes with two complementary state-of-the-art techniques is presented. These isotopes have been populated in the fission process at two different facilities, GANIL and ILL. New spectroscopic information has been obtained for odd-even $^{87-93}$Br isotopes and the experimental results have been compared with state-of-the-art Large-Scale Shell-Model and DNO Shell-Model calculations. As a result of such theoretical approaches, a transition from prolate ($^{87,89}$Br) to oblate ($^{91,93}$Br) shapes is obtained from the subtle balance between proton and neutron quadrupole deformations, as a clear signature of pseudo-SU3 quadrupole regime.

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arXiv2023

Isospin symmetry in the $T = 1, A = 62$ triplet

Excited states in the $T_z = 0, -1$ nuclei $^{62}$Ga and $^{62}$Ge were populated in direct reactions of relativistic radioactive ion beams at the RIBF. Coincident \grays were measured with the DALI2$^+$ array and uniquely assigned to the $A=62$ isobars. In addition, $^{62}$Ge was also studied independently at JYFL-ACCLAB using the ${}^{24}$Mg(${}^{40}$Ca,$2n$)${}^{62}$Ge fusion-evaporation reaction. The first excited $T=1, J^π=2^+$ states in $^{62}$Ga and $^{62}$Ge were identified at $979(1)$ and $965(1)$~keV, respectively, resolving discrepant interpretations in the literature. States beyond the first $2^+$ state in $^{62}$Ge were also identified for the first time in the present work. The results are compared with shell-model calculations in the $fp$ model space. Mirror and triplet energy differences are analyzed in terms of individual charge-symmetry and charge-independence breaking contributions. The MED results confirm the shrinkage of the $p$-orbits' radii when they are occupied by at least one nucleon on average.

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arXiv2023

133In: A Rosetta Stone for decays of r-process nuclei

The $β$ decays from both the ground state and a long-lived isomer of $^{133}$In were studied at the ISOLDE Decay Station (IDS). With a hybrid detection system sensitive to $β$, $γ$, and neutron spectroscopy, the comparative partial half-lives (logft) have been measured for all their dominant $β$-decay channels for the first time, including a low-energy Gamow-Teller transition and several First-Forbidden (FF) transitions. Uniquely for such a heavy neutron-rich nucleus, their $β$ decays selectively populate only a few isolated neutron unbound states in $^{133}$Sn. Precise energy and branching-ratio measurements of those resonances allow us to benchmark $β$-decay theories at an unprecedented level in this region of the nuclear chart. The results show good agreement with the newly developed large-scale shell model (LSSM) calculations. The experimental findings establish an archetype for the $β$ decay of neutron-rich nuclei southeast of $^{132}$Sn and will serve as a guide for future theoretical development aiming to describe accurately the key $β$ decays in the rapid-neutron capture (r-) process.

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arXiv2023

Beta-delayed neutron spectroscopy of $^{133}$In

The decay properties of $^{133}$In were studied in detail at the ISOLDE Decay Station (IDS). The implementation of the Resonance Ionization Laser Ion Source (RILIS) allowed separate measurements of its $9/2^+$ ground state ($^{133g}$In) and $1/2^-$ isomer ($^{133m}$In). With the use of $β$-delayed neutron and $γ$ spectroscopy, the decay strengths above the neutron separation energy were quantified in this neutron-rich nucleus for the first time. The allowed Gamow-Teller transition $9/2^+\rightarrow7/2^+$ was located at 5.92 MeV in the $^{133g}$In decay with a logft = 4.7(1). In addition, several neutron-unbound states were populated at lower excitation energies by the First-Forbidden decays of $^{133g,m}$In. We assigned spins and parities to those neutron-unbound states based on the $β$-decay selection rules, the logft values, and systematics.

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arXiv2022

Structure of $^{83}$As, $^{85}$As and $^{87}$As: from semi-magicity to $γ$-softness

The structure of $^{83}$As, $^{85}$As and $^{87}$As have been studied in fusion-fission reaction $^{238}$U+$^9$Be. Fission fragments were identified in mass and atomic number using the VAMOS++ spectrometer and the coincident $γ$-rays were detected in the $γ$-ray tracking array AGATA. New transitions in $^{83}$As and $^{85}$As are reported and placed in the level schemes. A level scheme of the excited states in $^{87}$As is proposed for the first time. The data are interpreted in frame of Large-Scale Shell-Model calculations, SU3 symmetries and Beyond Mean-Field frameworks. A spherical regime at magic number $N$=50 is predicted and the location of the proton $g_{9/2}$ orbital is proposed for the first time. Development of collectivity in a prolate deformed, $γ$-soft regime in the open shell cases $^{85}$As and $^{87}$As, most neutron-rich isotopes beyond $N$=50, is concluded. Data and theoretical calculations give confidence to a relatively high extrapolated excitation energy about 4 MeV of the $9/2^+$ state in $^{79}$Cu, one proton above $^{78}$Ni.

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arXiv2020

$β$-Decay Half-Lives of 55 Neutron-Rich Isotopes beyond the N=82 Shell Gap

The $β$-decay half-lives of 55 neutron-rich nuclei $^{134-139}$Sn, $^{134-142}$Sb, $^{137-144}$Te, $^{140-146}$I, $^{142-148}$Xe, $^{145-151}$Cs, $^{148-153}$Ba, $^{151-155}$La were measured at the Radioactive Isotope Beam Factory (RIBF) employing the projectile fission fragments of $^{238}$U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly-measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of $r$ process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second ($A$ $\approx$ 130) peak.

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arXiv2020

Fission of relativistic nuclei with fragment excitation and reorientation

Experimental studies of fission induced in relativistic nuclear collisions show a systematic enhancement of the excitation energy of the primary fragments by a factor of ~ 2, before their decay by fission and other secondary fragments. Although it is widely accepted that by doubling the energies of the single-particle states may yield a better agreement with fission data, it does not prove fully successful, since it is not able to explain yields for light and intermediate mass fragments. State-of-the-art calculations are successful to describe the overall shape of the mass distribution of fragments, but fail within a factor of 2-10 for a large number of individual yields. Here, we present a novel approach that provides an account of the additional excitation of primary fragments due to final state interaction with the target. Our method is applied to the 238U + 208Pb reaction at 1 GeV/nucleon (and is applicable to other energies), an archetype case of fission studies with relativistic heavy ions, where we find that the large probability of energy absorption through final state excitation of giant resonances in the fragments can substantially modify the isotopic distribution of final fragments in a better agreement with data. Finally, we demonstrate that large angular momentum transfers to the projectile and to the primary fragments via the same mechanism imply the need of more elaborate theoretical methods than the presently existing ones.

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arXiv2019

$^{78}$Ni revealed as a doubly magic stronghold against nuclear deformation

Nuclear magic numbers, which emerge from the strong nuclear force based on quantum chromodynamics, correspond to fully occupied energy shells of protons, or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and neutrons, are spherical and extremely rare across the nuclear landscape. While the sequence of magic numbers is well established for stable nuclei, evidence reveals modifications for nuclei with a large proton-to-neutron asymmetry. Here, we provide the first spectroscopic study of the doubly magic nucleus $^{78}$Ni, fourteen neutrons beyond the last stable nickel isotope. We provide direct evidence for its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective field theory interactions and the quasi-particle random-phase approximation. However, our results also provide the first indication of the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting further a rapid transition from spherical to deformed ground states with $^{78}$Ni as turning point.

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arXiv2016

The mutable nature of particle-core excitations with spin in the one-valence-proton nucleus 133Sb

The gamma-ray decay of excited states of the one-valence-proton nucleus 133Sb has been studied using cold-neutron induced fission of 235U and 241Pu targets, during the EXILL campaign at the ILL reactor in Grenoble. By using a highly efficient HPGe array, coincidences between gamma-rays prompt with the fission event and those delayed up to several tens of microseconds were investigated, allowing to observe, for the first time, high-spin excited states above the 16.6 micros isomer. Lifetimes analysis, performed by fast-timing techniques with LaBr3(Ce) scintillators, reveals a difference of almost two orders of magnitude in B(M1) strength for transitions between positive-parity medium-spin yrast states. The data are interpreted by a newly developed microscopic model which takes into account couplings between core excitations (both collective and non-collective) of the doubly magic nucleus 132Sn and the valence proton, using the Skyrme effective interaction in a consistent way. The results point to a fast change in the nature of particle-core excitations with increasing spin.

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arXiv2015

Nuclear structure of 140Te with N = 88: Structural symmetry and asymmetry in Te isotopes with respect to the double-shell closure Z = 50 and N = 82

We study for the first time the internal structure of 140Te through the beta-delayed gamma-ray spectroscopy of 140Sb. The very neutron-rich 140Sb, Z = 51 and N = 89, ions were produced by the in-flight fission of 238U beam on a 9Be target at 345 MeV per nucleon at the Radioactive Ion Beam Factory, RIKEN. The half-life and spin-parity of 140Sb are reported as 124(30) ms and (4-), respectively. In addition to the excited states of 140Te produced by the beta-decay branch, the beta-delayed one-neutron and two-neutron emission branches were also established. By identifying the first 2+ and 4+ excited states of 140Te, we found that Te isotopes persist their vibrator character with E(4+)/E(2+) = 2. We discuss the distinctive features manifest in this region, such as valence neutron symmetry and asymmetry, revealed in pairs of isotopes with the same neutron holes and particles with respect to N = 82.

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arXiv2012

First direct lifetime measurement of the 2+1 state in 72,74Zn: new evidence for shape transition between N = 40 and N = 42 close to Z = 28

We report here the first direct lifetime measurement of the 2+1 state in 72,74Zn. The neutron-rich beam was produced by in-flight fragmentation of 76Ge at the Grand Accélérateur National d'Ions Lourds and separated with the LISE spectrometer. The 2+1 state was excited by inelastic scattering and knock-out reaction on a CD2 target and its lifetime was measured by the recoil-distance Dopplershift method with the Köln plunger device combined with the EXOGAM detectors. The lifetimes of the 2+1 states in 72,74Zn were determined to be 17.9(18) and 27.0(24) ps, which correspond to reduced transition probabilities B(E2; 2+1 \rightarrow 0+) = 385(39) and 370(33) e2fm4, respectively. These values support the idea of a systematic maximum of collectivity at N = 42 for Zn, Ge and Se nuclei. In addition, the available systematics in the neighboring nuclei point towards a transition from a spherical oscillator at N = 40 to complete γ-softness at N = 42.

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arXiv2011

In-beam fast-timing measurements in 103,105,107Cd

Fast-timing measurements were performed recently in the region of the medium-mass 103,105,107Cd isotopes, produced in fusion evaporation reactions. Emitted gamma-rays were detected by eight HPGe and five LaBr3:Ce detectors working in coincidence. Results on new and re-evaluated half-lives are discussed within a systematic of transition rates. The $7/2_1^+$ states in 103,105,107Cd are interpreted as arising from a single-particle excitation. The half-life analysis of the $11/2_1^-$ states in 103,105,107Cd shows no change in the single-particle transition strength as a function of the neutron number.

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