arXiv subjects

W. Urban

Publications and source records attributed to W. Urban.

8 recordsLinked to original sources

arXiv2026

Structure of even-even Zr isotopes with 52$\leq$N$\leq$58 neutrons

Excited levels in $^{92}$Zr have been studied in cold-neutron capture reaction using the EXILL Ge array at ILL Grenoble. Excited levels in $^{94}$Zr and $^{98}$Zr nuclei have been studied using high-statistics $\gamma$-coincidence data from neutron-induced fission of $^{235}$U measured with EXILL. The goal of was to search for new levels in $^{92,94,98}$Zr and to improve spin-parity assignments to excited levels. Total of 54 new levels, 180 new $\gamma$ transitions and 70 new or improved spin-parity assignments have been determined in the three isotopes in the present work. A precise neutron binding energy of 8634.81(2) keV has been determined in $^{92}$Zr. In $^{94}$Zr a Gamow-Teller transition is proposed. The (9$^-$), 3894.1-keV level in $^{98}$Zr is likely a few ns isomer. A new technique of estimating half-lives in a picosecond range, developed in this work, provided 61 half-lives and 60 B($\pi ,\lambda$) rates for transitions in $^{96,98}$Zr and $^{94,96}$Sr. New-type systematics, backed by Large Scale Shell Model calculations have been used to classify 2$^+$ excitations in Zn-Zr even-even nuclei of the 50$\le$N$\le$60 range. This information and new systematics of 0$^+$ excitations in Sr and Zr isotopes explain the evolution of collectivity in zirconium isotopes, showing the role of various single particle excitations in the phase transition and coexistence of the region. The role of the $\nu$9/2$^+$[404] extruder orbital as a catalyst in creating 0$^+$ excitations and the deformation change in the region has been discussed.

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arXiv2026

Emerging collectivity and phase transition in mass A$\approx$150 region. New information for Nd isotopes

Low and medium spin excitations in $^{146,148,150,152}$Nd isotopes, populated in $\beta^-$ decay of corresponding Pr isotopes or in prompt-$\gamma$ fission of $^{252}$Cf have been studied using Gammasphere array of Ge spectrometers. 159 new levels, including two new isomers, 305 new $\gamma$ transitions and 83 new spin-parity assignments were added in the four studied nuclei. The structure of excited levels in the studied Nd isotopes is discussed using phenomenological classifications and systematics and compared to calculations reported in other works. Particular attention is paid to $0^+$ and $2^+$ excitations related to the emerging quadrupole collectivity and to the role of the 11/2$^-$[505] neutron extruder in the process.

nucl-ex↗

arXiv2021

Measurement of relative isotopic yield distribution of even-even fission fragments from $^{235}$U($n_{th}$,$f$) following $γ$ ray spectroscopy

A detailed investigation on the relative isotopic distributions has been carried out for the first time in case of even-even correlated fission fragments for the $^{235}$U($n_{th}$,$f$) fission reaction. High-statistics data were obtained in a prompt $γ$ ray spectroscopy measurement during the EXILL campaign at ILL, Grenoble, France. The extensive off-line analysis of the coincidence data have been carried out using four different coincidence methods. Combining the results from 2-dimensional $γ-γ$ and 3-dimensional $γ-γ-γ$ coincidence analysis, a comprehensive picture of the relative isotopic yield distributions of the even-even neutron-rich fission fragments has emerged. The experimentally observed results have been substantiated by the theoretical calculations based on a novel approach of isospin conservation, and a reasonable agreement has been obtained. The calculations following the semi-empirical GEF model have also been carried out. The results from the GEF model calculations are found to be in fair agreement with the experimental results.

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arXiv2021

Medium-spin states of the neutron-rich nucleus $^{87}$Br

Medium-spin excited states of the neutron-rich nucleus $^{87}$Br were observed and studied for the first time. They were populated in fission of $^{235}$U induced by the cold-neutron beam of the PF1B facility of the Institut Laue-Langevin, Grenoble. The measurement of $γ$ radiation following fission has been performed using the EXILL array of Ge detectors. The observed level scheme was compared with results of large valence space shell model calculations. The medium-spin level scheme consists of three band-like structures, which can be understood as bands built on the $πf_{5/2}$, $π(p_{3/2}+f_{5/2})$ and $πg_{9/2}$ configurations. The behavior of the observed $πg_{9/2}$ band at high spins shows a considerable deviation from the shell model predictions. This deviation in this band is probably the result of an increased collectivity, which can be understood assuming that the $πg_{9/2}$ high-$\it j$ proton polarizes the core.

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arXiv2020

Low-spin particle/hole-core excitations in $^{41,47,49}$Ca isotopes studied by cold-neutron capture reactions

We present recent results on the structure of the one-valence-particle $^{41}$Ca and $^{49}$Ca, and one-valence-hole $^{47}$Ca, nuclei. The isotopes of interest were populated via the cold-neutron capture reactions $^{40}$Ca(n,$γ$), $^{48}$Ca(n,$γ$) and $^{46}$Ca(n,$γ$), respectively. The experiments were performed at the Institut Laue-Langevin, within the EXILL campaign, which employed a large array of HPGe detectors. The $γ$ decay and level schemes of these nuclei were investigated by $γ$-ray coincidence relationships, leading to the identification of 41, 10, and 6 new transitions in $^{41}$Ca, $^{47}$Ca, and $^{49}$Ca, respectively. Branching ratios and intensities were extracted for the $γ$ decay from each state, and $γ$-ray angular correlations were performed to establish a number of transition multipolarities and mixing ratios, thus helping in the spin assignment of the states. The experimental findings are discussed along with microscopic, self-consistent beyond-mean-field calculations performed with the Hybrid Configuration Mixing model, based on a Skyrme SkX Hamiltonian. The latter suggests that a fraction of the low-spin states of the $^{41}$Ca, $^{49}$Ca, and $^{47}$Ca nuclei is characterized by the coexistence of either 2p-1h and 1p-2h excitations, or couplings between single-particle/hole degrees of freedom and collective vibrations (phonons) of the doubly-magic "core".

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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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arXiv2012

The Refractive Index of Silicon at Gamma Ray Energies

The index of refraction n(E_γ)=1+δ(E_γ)+iβ(E_γ) is split into a real part δand an absorptive part β. The absorptive part has the three well-known contributions to the cross section σ_{abs}: the photo effect, the Compton effect and the pair creation, but there is also the inelastic Delbrück scattering. Second-order elastic scattering cross sections σ_{sca} with Rayleigh scattering (virtual photo effect), virtual Compton effect and Delbrück scattering (virtual pair creation) can be calculated by integrals of the Kramers-Kronig dispersion relations from the cross section σ_{abs}. The real elastic scattering amplitudes are proportional to the refractive indices δ_{photo}, δ_{Compton} and δ_{pair}. While for X-rays the negative δ_{photo} dominates, we show for the first time experimentally and theoretically that the positive δ_{pair} dominates for γrays, opening a new era of γoptics applications, i.e. of nuclear photonics.

physics.atom-ph↗

arXiv2001

Fast nuclear rotation and octupole deformation

The 150Sm nucleus has been studied to high spins in a measurement of gamma radiation following the 136Xe(18O,4n)150Sm, compound-nucleus reaction at beam energy of 76 MeV. The measurement was performed at NBI Riso using the NORDBALL array. Alternating parity, s=+1 band in 150Sm has been observed up to spin I=22. This band is crossed by two aligned bands, corresponding to a reflection-symmetric shape. After the second crossing the s=+1 band ends abruptly, suggesting that the octupole shape vanishes in 150Sm above spin I=22, as predicted by calculations. Other explanations, assuming continuation of the s=+1 band past the two alignments are also discussed.

nucl-th↗