Level structure of <math><mmultiscripts><mi>Ac</mi><mprescripts/><none/><mn>221</mn></mmultiscripts></math> and <math><mmultiscripts><mi>Fr</mi><mprescripts/><none/><mn>217</mn></mmultiscripts></math> from decay spectroscopy, and reflection asymmetry in <math><mmultiscripts><mi>Ac</mi><mprescripts/><none/><mn>221</mn></mmultiscripts></math>

International audiencePa225 and Ac221 were produced at the IGISOL facility through proton-induced fusion-evaporation reactions and have been studied using α-particle spectroscopy, as well as α-γ and α-electron coincidence spectroscopy. The level scheme of Ac221, daughter of Pa225, and of Fr217, daughter of Ac221 were reconstructed. An interpretation of Ac221 levels as K=5/2± and K=3/2± parity-doublet bands is proposed. Such bands appear in reflection-asymmetric models and would be an indication of a static reflection asymmetric shape for Ac221

VADER: A novel decay station for actinide spectroscopy

A research programme focused on the study of the nuclear structure of actinide isotopes has recently been implemented at the IGISOL facility, University of Jyväskylä. Within this scope, a new decay station named VADER (Versatile Actinides DEcay spectRoscopy setup) has been developed and commissioned. The system consists of a compact array of silicon detectors, a liquid-nitrogen-cooled silicon lithium (Si(Li)) detector and three broad energy germanium detectors (BEGe), placed around a thin implantation carbon foil. The combined use of different detectors allows the measurement of particles, conversion electrons and de-excitation rays in coincidence, enabling a full reconstruction of nuclear decay schemes. The measurement of basic nuclear decay observables provides a picture of the nuclear shell evolution in neutron-deficient actinides, and highlights the possible emergence of reflection-asymmetric shapes in the region.nonPeerReviewe

Multinucleon Transfer Reactions in the 238U+238^{238}\mathrm {U}+{^{238}}U System Studied with the VAMOS+AGATA+ID-Fix

International audienceThe measurement of the production cross sections of exotic neutronrich heavy nuclei, in the uranium region, in the vicinity of the N = 152 deformed shell gap was carried out via multinucleon transfer reactions of 238U + 238U at 7.193 and 6.765 MeV/A using the VAMOS++ magnetic spectrometer coupled to the AGATA and ID-Fix photon detection arrays. This article reports on the status of the VAMOS++ data analysis and results on the population of the strongest (±1n) transfer channels observed from the decay of long-lived products after irradiation

Opportunities and limitations of in-gas-cell laser spectroscopy of the heaviest elements with RADRIS

International audienceThe radiation detection resonance ionization spectroscopy (RADRIS) technique enables laser spectroscopic investigations of the heaviest elements which are produced in atom-at-a-time quantities from fusion-evaporation reactions. To achieve a high efficiency, laser spectroscopy is performed in a buffer-gas environment used to thermalize and stop the high-energy evaporation residues behind the velocity filter SHIP. The required cyclic measurement procedure in combination with the applied filament collection for neutralization as well as confinement of the stopped ions and subsequent pulse-heat desorption constrains the applicability of the technique. Here, some of these limitations and also opportunities that arise from this unique measurement setup will be evaluated

In-beam γ-ray and electron spectroscopy of Md249,251

The odd-Z 251Md nucleus was studied using combined γ-ray and conversion-electron in-beam spectroscopy. Besides the previously observed rotational band based on the [521]1/2− configuration, another rotational structure has been identified using γ−γ coincidences. The use of electron spectroscopy allowed the rotational bands to be observed over a larger rotational frequency range. Using the transition intensities that depend on the gyromagnetic factor, a [514]7/2− single-particle configuration has been inferred for this band, i.e., the ground-state band. A physical background that dominates the electron spectrum with an intensity of ≃60% was well reproduced by simulating a set of unresolved excited bands. Moreover, a detailed analysis of the intensity profile as a function of the angular momentum provided a method for deriving the orbital gyromagnetic factor, namely gK=0.69+0.19−0.16 for the ground-state band. The odd-Z 249Md was studied using γ-ray in-beam spectroscopy. Evidence for octupole correlations resulting from the mixing of the Δl=Δj=3 [521]3/2− and [633]7/2+ Nilsson orbitals were found in both 249,251Md. A surprising similarity of the 251Md ground-state band transition energies with those of the excited band of 255Lr has been discussed in terms of identical bands. Skyrme-Hartree-Fock-Bogoliubov calculations were performed to investigate the origin of the similarities between these bands.peerReviewe

First observation of high-K isomeric states in 249Md and 251Md

Decay spectroscopy of the odd-proton nuclei 249Md and 251Md has been performed. High-K isomeric states were identified for the first time in these two nuclei through the measurement of their electromagnetic decay. An isomeric state with a half-life of 2.8(5) ms and an excitation energy ≥910 keV was found in 249Md. In 251Md, an isomeric state with a half-life of 1.4(3) s and an excitation energy ≥844 keV was found. Similarly to the neighbouring 255Lr, these two isomeric states are interpreted as 3 quasi-particle high-K states and compared to new theoretical calculations. Excited nuclear configurations were calculated within two scenarios: via blocking nuclear states located in proximity to the Fermi surface or/and using the quasiparticle Bardeen–Cooper–Schrieffer method. Relevant states were selected on the basis of the microscopic-macroscopic model with a deformed Woods–Saxon potential. The most probable candidates for the configurations of K-isomeric states in Md nuclei are proposed.peerReviewe

Study of the pygmy dipole resonance using neutron inelastic scattering at GANIL-SPIRAL2/NFS

International audienceThe pygmy dipole resonance (PDR) has been the subject of numerous studies, both experimental and theoretical. Indeed, the study of the PDR has been and still is of great interest since it allows to constrain the symmetry energy, an important ingredient of the equation of state of nuclear matter that describes the matter within neutron stars. Moreover, the PDR is predicted to play a key role in the r-process via the increase of the neutron capture rate. However, despite numerous experiments dedicated to the study of the PDR, a consistent description is still missing. In this context, we have proposed to study the PDR using a new probe: the neutron inelastic scattering reaction (n,n’γ). An experiment to study the pygmy resonance in 140Ce using the (n,n’γ) reaction has been performed in September 2022. This experiment has been made possible thanks to the high-intensity proton beam of the new accelerator SPIRAL2 at GANIL and the NFS (Neutron For Science) facility. The experimental setup was composed of the new generation multi-detectors PARIS, for the detection of γ-rays coming from the de-excitation of the PDR, and MONSTER, for the detection of scattered neutrons. In this article, the experiment motivation and description are presented