arXiv subjects

M. Vandebrouck

Publications and source records attributed to M. Vandebrouck.

At least 19 recordsLinked to original sources

arXiv2026

Quenching of the proton $\pi0p_{3/2}$-$\pi0p_{1/2}$ spin-orbit splitting in $^{20}$O and the effect of the tensor force

We present the first direct measurement of the Z=6 shell gap in the neutron-rich 20O nucleus. The one-proton removal transfer reaction 2H(20O,3He)19N has been studied using the ACTAR TPC setup at GANIL. The use of ACTAR TPC enabled the measurement of low-cross section proton-removal reactions while preserving resolution. Eight p-hole states with l=1 were identified in 19N accounting for total strengths of 86% and 72% of the 0p3/2 and 0p1/2 single-particle orbitals, respectively. The energies and spectroscopic factors of the measured states allowed to determine the proton spin-orbit splitting 0p3/2 - 0p1/2 in 20O. The Z=6 shell gap has been established to be 5.30(14) MeV. These findings indicate a reduction of the Z=6 shell gap while adding neutrons to the sd-valence orbitals, consistent with the effects of the tensor force predicted by state-of-the-art shell model interaction SFO-tls while at variance with the emergence of a large Z=6 gap observed in other studies.

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arXiv2025

Applying Gaussian Mixture Models to Track Reconstruction in Inelastic Scattering Experiments with Active Targets

Active targets such as ACTAR TPC are well suited for studying giant resonances in unstable nuclei via inelastic scattering in inverse kinematics. A key challenge in such measurements is the detection of low-energy ejectiles emitted at small angles relative to the beam direction. Accurate reconstruction of these tracks is essential for disentangling different resonance modes. Probabilistic models such as the Gaussian Mixture Model (GMM) are particularly effective in capturing the complex covariance structures characteristic of the beam-recoil interface in narrow-angle events. In this work, we present a track reconstruction approach based on the GMM, specifically designed for inelastic scattering experiments with active targets. Special emphasis is placed on the treatment of low-energy tracks. The proposed method is demonstrated on simulated data of the $^{58}\mathrm{Ni}(\alpha,\alpha')^{58}\mathrm{Ni}$ reaction at an incident energy of $E=49$~MeV/nucleon, generated under conditions representative of the experiment carried out at GANIL for the same reaction.

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arXiv2025

Unbound neutron $\nu0d_{3/2}$ strength in $^{17}$C and the N=16 shell gap

Significant continuum strength has been observed to be populated in $^{17}$C produced in the d($^{16}$C,p) reaction at a beam energy of 17.2~MeV/nucleon. The strength appears at greater than $\sim$2~MeV above the single-neutron decay threshold and has been identified as arising from transfer into the neutron $0d_{3/2}$ orbital. Guided by shell model predictions the greater majority of the strength is associated with a 3/2$^+$ state at an excitation energy of 4.40$_{-0.14}^{+0.33}$ MeV and a much weaker 3/2$^+$ level at 5.60$_{-0.45}^{+1.35}$ MeV. The corresponding total widths were determined to be 3.45$_{-0.78}^{+1.82}$ and 1.6$_{-1.4}^{+4.6}$ MeV, respectively. From the backward angle proton differential cross sections and the branching ratios for neutron decay to the $^{16}$C(2$_{1}^{+}$) level, the corresponding spectroscopic factors to the ground state were deduced to be 0.47$\pm{10}$ and $<$0.09. Shell-model calculations employing the phenomenological SFO-tls interaction as well as Gamow Shell-Model calculations including continuum effects are in reasonable agreement with experiment, although the predicted strength lies at somewhat lower energy. The size of the N=16 shell gap ($\varepsilon_{ \nu0d_{3/2}}-\varepsilon _{\nu 1s_{1/2}}$) was estimated to be 5.08$_{-0.33}^{+0.43}$~MeV - some 1.3~MeV larger than found in the SFO-tls shell model calculation.

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arXiv2025

Two-proton correlations in the decay of 48Ni and 45Fe

The main observables of the rare two-proton emission process - half-life, total energy of the decay as well as energy and angular correlations between the emitted protons - have been measured for 48Ni and 45Fe in a recent experiment performed at the GANIL/LISE3 facility. The results, together with previous experimental work, are compared for the first time with calculations performed in the recently developed Gamow Coupled-Channel (GCC) framework and to 3-body predictions from literature. The comparison of the 48Ni and the 45Fe two-proton angular distributions with the GCC calculations confirms the adopted strength of the proton-proton interaction for both nuclei and the predominant small-angle emission. A comparison with 3-body model angular distributions indicates the shell closure of the f_7/2 orbital for 48Ni and a substantial occupancy of the p-orbital for 45Fe. Discrepancies between experimental data and theoretical predictions are found when studying the other observables: half-lives, total energy of the decay and energy correlations, showing the complexity of the description of the two-proton emission process.

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arXiv2024

Shape evolution in even-mass $^{98-104}$Zr isotopes via lifetime measurements using the $\gamma\gamma$-coincidence technique

The Zirconium (Z = 40) isotopic chain has attracted interest for more than four decades. The abrupt lowering of the energy of the first $2^+$ state and the increase in the transition strength B(E2; $2_1^\rightarrow 0_1^+$ going from $^{98}$Zr to $^{100}$Zr has been the first example of "quantum phase transition" in nuclear shapes, which has few equivalents in the nuclear chart. Although a multitude of experiments have been performed to measure nuclear properties related to nuclear shapes and collectivity in the region, none of the measured lifetimes were obtained using the Recoil Distance Doppler Shift method in the $\gamma\gamma$-coincidence mode where a gate on the direct feeding transition of the state of interest allows a strict control of systematical errors. This work reports the results of lifetime measurements for the first yrast excited states in $^{98-104}$Zr carried out to extract reduced transition probabilities. The new lifetime values in $\gamma\gamma$-coincidence and $\gamma$-single mode are compared with the results of former experiments. Recent predictions of the Interacting Boson Model with Configuration Mixing, the Symmetry Conserving Configuration Mixing model based on the Hartree-Fock-Bogoliubov approach and the Monte Carlo Shell Model are presented and compared with the experimental data.

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arXiv2023

Cross-shell states in $^{15}$C: a test for p-sd interactions

The low-lying structure of $^{15}$C has been investigated via the neutron-removal $^{16}$C$(d,t)$ reaction. Along with bound neutron sd-shell hole states, unbound p-shell hole states have been firmly confirmed. The excitation energies and the deduced spectroscopic factors of the cross-shell states are an important measure of the $[(p)^{-1}(sd)^{2}]$ neutron configurations in $^{15}$C. Our results show a very good agreement with shell-model calculations using the SFO-tls interaction for $^{15}$C. However, a modification of the $p$-$sd$ and $sd$-$sd$ monopole terms was applied in order to reproduce the $N=9$ isotone $^{17}$O. In addition, the excitation energies and spectroscopic factors have been compared to the first calculations of $^{15}$C with the $ab~ initio$ self-consistent Green's function method employing the NNLO$_{sat}$ interaction. The results show the sensitivity to the size of the $N=8$ shell gap and highlight the need of going beyond the current truncation scheme in the theory.

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arXiv2022

Experimental investigation of ground-state properties of $^7$H with transfer reactions

The properties of nuclei with extreme neutron-to-proton ratios, far from those naturally occurring on Earth, are key to understand nuclear forces and how nucleons hold together to form nuclei. $^7$H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron-to-proton ratio known so far. However, its sheer existence and properties are still a challenge for experimental efforts and theoretical models. Here we report experimental evidences on the formation of $^7$H as a resonance, detected with independent observables, and the first measurement of the structure of its ground state. The resonance is found at $\sim$0.7 MeV above the $^3$H+4n mass, with a narrow width of $\sim$0.2 MeV and a $1/2^+$ spin and parity. These data are consistent with a $^7$H as a $^3$H core surrounded by an extended four-neutron halo, with a unique four-neutron decay and a relatively long half-life thanks to neutron pairing; a prime example of new phenomena occurring in what would be the most pure-neutron nuclear matter we can access in the laboratory.

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arXiv2021

First observation of high-$K$ isomeric states in $^{249}$Md and $^{251}$Md

Decay spectroscopy of the odd-proton nuclei $^{249}$Md and $^{251}$Md 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 $\geq 910$ keV was found in $^{249}$Md. In $^{251}$Md, an isomeric state with a half-life of $1.4(3)$ s and an excitation energy $\geq 844$ keV was found. Similarly to the neighbouring $^{255}$Lr, 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.

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arXiv2021

Coulomb and nuclear excitations of $^{70}$Zn and $^{68}$Ni at intermediate energy

The reduced transition probabilities $B(E2; 0^+_{g.s.}\rightarrow2_1^+,2^+_2)$ in $^{70}$Zn and the full $B(E2; 0^+_{g.s.}\rightarrow2^+)$ strength up to S$_n$=7.79 MeV in $^{68}$Ni have been determined at the LISE/GANIL facility using the Coulomb-excitation technique at intermediate beam energy on a $^{208}$Pb target. The $γ$ rays emitted in-flight were detected with an array of 46 BaF$_2$ crystals. The angles of the deflected nuclei were determined in order to disentangle and extract the Coulomb and nuclear contributions to the excitation of the 2$^+$ states. The measured $B(E2; 0^+_{g.s.}\rightarrow2_1^+)$ of 1432(124) e$^2$fm$^4$ for $^{70}$Zn falls in the lower part of the published values which clustered either around 1600 or above 2000 e$^2$fm$^4$, while the $B(E2; 0^+_{g.s.}\rightarrow2^+_2)$ of 53(7) e$^2$fm$^4$ agrees very well with the two published values. The relatively low $B(E2; 0^+_{g.s.}\rightarrow2_1^+)$ of 301(38) e$^2$fm$^4$ for $^{68}$Ni agrees with previous studies and confirms a local magicity at $Z=28, N=40$. Combining the results of the low-energy spectra of $^{68}$Ni and $^{70}$Zn and their shell-model interpretations, it is interesting to notice that four different shapes (spherical, oblate, prolate and triaxial) are present. Finally, a summed $E2$ strength of only about 150 e$^2$fm$^4$ has been found experimentally at high excitation energy, likely due to proton excitations across the $Z=28$ gap. The experimental distribution of this high-energy $E2$ excitation agrees with SM calculations, but its strength is about two times weaker.

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arXiv2021

First beams at Neutrons For Science

The neutrons for science facility (NFS), the first operational experimental area of the new GANIL/SPIRAL-2 facility, received its first beams in December 2019. Proton-induced reaction cross-sections as well as neutron beam characteristics were measured during the first commissioning phases. The first results, showing the features of the facility, are presented here and compared with previously published data. The physics cases and the first accepted experiments are presented as well.

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arXiv2021

Neutron-proton pairing in the N=Z radioactive fp-shell nuclei 56Ni and 52Fe probed by pair transfer

The isovector and isoscalar components of neutron-proton pairing are investigated in the N=Z unstable nuclei of the \textit{fp}-shell through the two-nucleon transfer reaction (p,$^3$He) in inverse kinematics. The combination of particle and gamma-ray detection with radioactive beams of $^{56}$Ni and $^{52}$Fe, produced by fragmentation at the GANIL/LISE facility, made it possible to carry out this study for the first time in a closed and an open-shell nucleus in the \textit{fp}-shell. The transfer cross-sections for ground-state to ground-state (J=0$^+$,T=1) and to the first (J=1$^+$,T=0) state were extracted for both cases together with the transfer cross-section ratios $σ$(0$^+$,T=1) /$σ$(1$^+$,T=0). They are compared with second-order distorted-wave born approximation (DWBA) calculations. The enhancement of the ground-state to ground-state pair transfer cross-section close to mid-shell, in $^{52}$Fe, points towards a superfluid phase in the isovector channel. For the "deuteron-like" transfer, very low cross-sections to the first (J=1$^+$,T=0) state were observed both for \Ni\phe\, and \Fe\phe\, and are related to a strong hindrance of this channel due to spin-orbit effect. No evidence for an isoscalar deuteron-like condensate is observed.

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arXiv2020

Low-lying single-particle structure of 17C and the N = 14 sub-shell closure

The first investigation of the single-particle structure of the bound states of 17C, via the d(16C, p) transfer reaction, has been undertaken. The measured angular distributions confirm the spin-parity assignments of 1/2+ and 5/2+ for the excited states located at 217 and 335 keV, respectively. The spectroscopic factors deduced for these states exhibit a marked single-particle character, in agreement with shell model and particle-core model calculations, and combined with their near degeneracy in energy provide clear evidence for the absence of the N = 14 sub-shell closure. The very small spectroscopic factor found for the 3/2+ ground state is consistent with theoretical predictions and indicates that the ν1d3/2 strength is carried by unbound states. With a dominant l = 0 valence neutron configuration and a very low separation energy, the 1/2+ excited state is a one-neutron halo candidate.

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arXiv2020

Signature of a possible $α$-cluster state in $N=Z$ doubly-magic $^{56}$Ni

An inelastic $α$-scattering experiment on the unstable $N=Z$, doubly-magic $^{56}$Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for $α$-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at $kT$ = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of $α$ particles corresponds to an excitation energy around 96 MeV. The method of distributed $mα$-decay ensembles is in agreement with the experimental results if we assume that the $α$-gas state in $^{56}$Ni exists at around $113^{+15}_{-17}$ MeV. These results suggest that there may exist an exotic state consisting of many $α$ particles at the excitation energy of $113^{+15}_{-17}$ MeV.

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arXiv2020

Probing the Z = 6 spin-orbit shell gap with (p,2p) quasi-free scattering reactions

The evolution of the traditional nuclear magic numbers away from the valley of stability is an active field of research. Experimental efforts focus on providing key spectroscopic information that will shed light into the structure of exotic nuclei and understanding the driving mechanism behind the shell evolution. In this work, we investigate the Z = 6 spin-orbit shell gap towards the neutron dripline. To do so, we employed $^{A}$N(p,2p)$^{A-1}$C quasi-free scattering reactions to measure the proton component of the 2$^+_1$ state of $^{16,18,20}$C. The experimental findings support the notion of a moderate reduction of the proton 1p$_{1/2}$-1p$_{3/2}$ spin-orbit splitting, at variance to recent claims for a prevalent Z = 6 magic number towards the neutron dripline.

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arXiv2020

The Structure of $^{33}$Si and the magicity of the N=20 gap at Z=14

The structure of $^{33}$Si was studied by a one-neutron knockout reaction from a $^{34}$Si beam at 98.5 MeV/u incident on a $^{9}$Be target. The prompt $γ$-rays following the de-excitation of $^{33}$Si were detected using the GRETINA $γ$-ray tracking array while the reaction residues were identified on an event-by-event basis in the focal plane of the S800 spectrometer at NSCL (National Superconducting Cyclotron Laboratory). The presently derived spectroscopic factor values, $C^2S$, for the 3/2$^+$ and 1/2$^+$ states, corresponding to a neutron removal from the $0d_{3/2}$ and $1s_{1/2}$ orbitals, agree with shell model calculations and point to a strong $N=20$ shell closure. Three states arising from the more bound $0d_{5/2}$ orbital are proposed, one of which is unbound by about 930 keV. The sensitivity of this experiment has also confirmed a weak population of 9/2$^-$ and 11/2$_{1,2}^-$ final states, which originate from a higher-order process. This mechanism may also have populated, to some fraction, the 3/2$^-$ and 7/2$^-$ negative-parity states, which hinders a determination of the $C^2S$ values for knockout from the normally unoccupied $1p_{3/2}$ and $0f_{7/2}$ orbits.

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arXiv2020

In-beam gamma-ray and electron spectroscopy of $^{249,251}$Md

The odd-$Z$ $^{251}$Md 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 $\simeq$ 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 $g_K = 0.69^{+0.19}_{-0.16}$ for the ground-state band. The odd-$Z$ $^{249}$Md 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 $^{249,251}$Md. A surprising similarity of the $^{251}$Md ground-state band transition energies with those of the excited band of $^{255}$Lr 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.

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arXiv2019

Production cross section and decay study of $^{243}$Es and $^{249}$Md

In the study of the odd-$Z$, even-$N$ nuclei $^{243}$Es and $^{249}$Md, performed at the University of Jyväskylä, the fusion-evaporation reactions $^{197}$Au($^{48}$Ca,2$n$)$^{243}$Es and $^{203}$Tl($^{48}$Ca,2$n$)$^{249}$Md have been used for the first time. Fusion-evaporation residues were selected and detected using the RITU gas-filled separator coupled with the focal-plane spectrometer GREAT. For $^{243}$Es, the recoil decay correlation analysis yielded a half-life of $24 \pm 3$s, and a maximum production cross section of $37 \pm 10$ nb. In the same way, a half-life of $26 \pm 1$ s, an $α$ branching ratio of 75 $\pm$ 5%, and a maximum production cross section of 300 $\pm$ 80 nb were determined for $^{249}$Md. The decay properties of $^{245}$Es, the daughter of $^{249}$Md, were also measured: an $α$ branching ratio of 54 $\pm$ 7% and a half-life of 65 $\pm$ 6 s. Experimental cross sections were compared to the results of calculations performed using the KEWPIE2 statistical fusion-evaporation code.

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arXiv2019

First observation of 20B and 21B

The most neutron-rich boron isotopes 20B and 21B have been observed for the first time following proton removal from 22N and 22C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into 19B and one or two neutrons. Two-proton removal from 22N populated a prominent resonance-like structure in 20B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1-,2- ground-state doublet predicted by the shell model. In the case of proton removal from 22C, the 19B plus one- and two-neutron channels were consistent with the population of a resonance in 21B 2.47+-0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for 20,21B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

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