arXiv subjects

F. C. L. Crespi

Publications and source records attributed to F. C. L. Crespi.

At least 19 recordsLinked to original sources

arXiv2026

Data sorting modes of phoswich detector array

The different data-sorting modes of the phoswich detector array PARIS used for detecting high-energy (4$-$10 MeV) $γ$ rays are investigated. The characteristics including time resolution, energy resolution and detection efficiency under various modes are studied. The present study shows that PARIS has capabilities of rejecting escape and pileup events when used for decay spectroscopy. Notably, the methods presented in this work refer specifically to the $β$-decay experiment of $^{80g+m}$Ga conducted with three PARIS clusters comprising 27 phoswich detectors, rather than to a general report on the PARIS array or its overall performance for in-beam spectroscopy. Compared with the 2"$\times$2"$\times$2" LaBr$_3$(Ce) detector (Ciemała et al., 2009), even in individual mode, PARIS provides significant suppression of single- and double-escape peaks and reduces background via vetoing function of the outer-volume NaI(Tl) crystals. In contrast to the common approach of adding back the energies in LaBr$_3$(Ce) and NaI(Tl) to increase the detection efficiency of the full-energy peak, using NaI(Tl) as a veto shield provides a superior trade-off for applications where spectral purity is essential. Employing add-back analysis within each cluster of nine phoswiches or between all phoswiches could enhance full-energy peak efficiency and further suppress escape peaks and background. Applying a multiplicity condition provides a further suppression but simultaneously lowers the statistics of full-energy peaks.

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arXiv2025

First evidence for the J$>$1 components of the pygmy dipole resonance in neutron-rich nuclei

Gamma ($γ$) decay shapes the synthesis of heavy elements in neutron-rich nuclear environments of neutron star mergers, supplying the Universe with heavy elements. The low-energy pygmy dipole resonance (PDR) influences nuclear reaction rates of the rapid nucleosynthesis through enhanced $γ$ transitions. However, since it is difficult to reproduce astrophysical conditions in laboratories, PDR was previously observed only in $J = 1$ spin states. Here we report the first experimental observation of $J > 1$ components of PDR, identified in the $β$-delayed $γ$ decay of the J$^π$ = 3$^{-}$ spin-parity isomer of $^{80}$Ga. The data analysis, combined with decay information and theoretical calculations allows the identification of resonant structures below the neutron emission threshold of the neutron-rich germanium $^{80}$Ge as J$^π = (2,3)^-$ components of the PDR built on the low-lying J$^π$ = 2$^+$ quadrupole state. Our findings extend the concept of PDR beyond dipole states, with implications for nuclear structure theory and experiment, as well as the element production in the cosmos.

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arXiv2025

Statistical properties and photon strength functions of the ${}^{112,114}$Sn isotopes below the neutron separation threshold

Here, we report on the measurements of the $γ$-ray strength functions and nuclear level densities of ${}^{112,114}$Sn performed for the first time at the 9~MV Tandem accelerator facilities at IFIN-HH using the Oslo method. We extract thermodynamic properties and gross and fine properties of the pygmy dipole resonance for systematic comparison in the chain of Sn isotopes. The results are compared with microscopic models implemented in the TALYS reaction code and the fully microscopic quasiparticle-phonon model for the underlying nuclear structure of the dipole strength in ${}^{112,114}$Sn. The quasiparticle-phonon model results show the importance of complex configurations to the low-energy dipole response in the pygmy dipole resonance energy region. The experimental data are further included in the cross-section and reaction rate calculations for the $(\mathrm{n},γ)$ reaction of the $p$-process nuclei ${}^{112,114}$Sn showing a significant increase in reaction rates at high temperatures compared to existing nuclear databases.

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arXiv2023

AGATA: Performance of $γ$-ray tracking and associated algorithms

AGATA is a modern $γ$-ray spectrometer for in-beam nuclear structure studies, based on $γ$-ray tracking. Since more than a decade, it has been operated performing experimental physics campaigns in different international laboratories (LNL, GSI, GANIL). This paper reviews the obtained results concerning the performances of $γ$-ray tracking in AGATA and associated algorithms. We discuss $γ$-ray tracking and algorithms developed for AGATA. Then, we present performance results in terms of efficiency and peak-to-total for AGATA. The importance of the high effective angular resolution of $γ$-ray tracking arrays is emphasised, e.g. with respect to Doppler correction. Finally, we briefly touch upon the subject of $γ$-ray imaging and its connection to $γ$-ray tracking.

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arXiv2022

PANDORA project: photo-nuclear reactions below $A=60$

Photo-nuclear reactions of light nuclei below a mass of $A=60$ are studied experimentally and theoretically by the PANDORA (Photo-Absorption of Nuclei and Decay Observation for Reactions in Astrophysics) project. Two experimental methods, virtual-photon excitation by proton scattering and real-photo absorption by a high-brilliance gamma-ray beam produced by laser Compton scattering, will be applied to measure the photo-absorption cross sections and the decay branching ratio of each decay channel as a function of the photon energy. Several nuclear models, e.g. anti-symmetrized molecular dynamics, mean-field type models, a large-scale shell model, and ab initio models, will be employed to predict the photo-nuclear reactions. The uncertainty in the model predictions will be evaluated from the discrepancies between the model predictions and the experimental data. The data and the predictions will be implemented in a general reaction calculation code TALYS . The results will be applied to the simulation of the photo-disintegration process of ultra-high-energy cosmic rays in inter-galactic propagation.

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arXiv2021

Narrow resonances in the continuum of the unbound nucleus $^{15}$F

The structure of the unbound $^{15}$F nucleus is investigated using the inverse kinematics resonant scattering of a radioactive $^{14}$O beam impinging on a CH$_2$ target. The analysis of $^{1}$H($^{14}$O,p)$^{14}$O and $^{1}$H($^{14}$O,2p)$^{13}$N reactions allowed the confirmation of the previously observed narrow $1/2^{-}$ resonance, near the two-proton decay threshold, and the identification of two new narrow 5/2$^{-}$ and 3/2$^{-}$ resonances. The newly observed levels decay by 1p emission to the ground of $^{14}$O, and by sequential 2p emission to the ground state (g.s.) of $^{13}$N via the $1^-$ resonance of $^{14}$O. Gamow shell model (GSM) analysis of the experimental data suggests that the wave functions of the 5/2$^{-}$ and 3/2$^{-}$ resonances may be collectivized by the continuum coupling to nearby 2p- and 1p- decay channels. The observed excitation function $^{1}$H($^{14}$O,p)$^{14}$O and resonance spectrum in $^{15}$F are well reproduced in the unified framework of the GSM.

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arXiv2021

Lifetime Measurements in the Even-Even $^{102-108}$Cd Isotopes

The heaviest N=Z doubly-magic nucleus, $^{100}$Sn, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction in extreme conditions. In particular, the Cd isotopes are expected to present features similar to those found in the Sn isotopic chain, since they have only two proton holes in the Z=50 shell. In this manuscript, the lifetime measurements of low-lying states in the even-mass $^{102-108}$Cd is presented. Thanks to the powerful detection capabilities of AGATA array and VAMOS++ spectrometer, the unusual employment of multi-nucleon transfer reactions permitted to investigate the first 2$^+$ and 4$^+$ states in all these nuclei, together with various deformed bands in $^{106}$Cd. The results were interpreted in the context of new state-of-the-art beyond-mean-field calculations, using the symmetry-conserving configuration-mixing approach. Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z=48 and Z=50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with $β_2 \approx 0.2$. At this deformation Z=48 becomes a closed-shell configuration, which is favored with respect to the spherical one.

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arXiv2021

Evidence for enhanced neutron-proton correlations from the level structure of the $N=Z+1$ nucleus $^{87}_{43}$Tc$^{\ }_{44}$

The low-lying excited states in the neutron-deficient $N=Z+1$ nucleus $^{87}_{43}$Tc$^{\ }_{44}$ have been studied via the fusion-evaporation reaction $^{54}$Fe($^{36}$Ar, $2n1p$)$^{87}$Tc at the Grand Accélérateur National d'Ions Lourds (GANIL), France. The AGATA spectrometer was used in conjunction with the auxiliary NEDA, Neutron Wall, and DIAMANT detector arrays to measure coincident prompt $γ$-rays, neutrons, and charged particles emitted in the reaction. A level scheme of $^{87}$Tc from the (9/2$^{+}_{g.s.}$) state to the (33/2$^{+}_{1}$) state was established based on 6 mutually coincident $γ$-ray transitions. The constructed level structure exhibits a rotational behavior with a sharp backbending at $\hbarω\approx 0.50$ MeV. A decrease in alignment frequency and increase in alignment sharpness in the odd-mass isotonic chains around $N=44$ is proposed as an effect of the enhanced isoscalar neutron-proton interactions in odd-mass nuclei when approaching the $N=Z$ line.

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arXiv2021

Complete set of bound negative-parity states in the neutron-rich 18N nucleus

High-resolution gamma-ray spectroscopy of 18N is performed with the Advanced GAmma Tracking Array AGATA, following deep-inelastic processes induced by an 18O beam on a 181Ta target. Six states are newly identified, which together with the three known excitations exhaust all negative-parity excited states expected in 18N below the neutron threshold. Spin and parities are proposed for all located states on the basis of decay branchings and comparison with large-scale shell-model calculations performed in the p-sd space, with the YSOX interaction. Of particular interest is the location of the 0^-_1 and 1^-_2 excitations, which provide strong constrains for cross-shell p-sd matrix elements based on realistic interactions, and help to simultaneously reproduce the ground and first-excited states in 16N and 18N, for the first time. Understanding the 18N structure may also have significant impact on neutron-capture cross-section calculations in r-process modeling including light neutron-rich nuclei.

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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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arXiv2020

Accessing tens-to-hundreds femtoseconds nuclear state lifetimes with low-energy binary heavy-ion reactions

A novel Monte Carlo technique has been developed to determine lifetimes of excited states in the tens-to-hundreds femtoseconds range. The method is applied to low-energy heavy-ion binary reactions populating nuclei with complex velocity distributions. Its relevance is demonstrated in connection with the $^{18}$O(7.0 MeV/u) + $^{181}$Ta experiment, performed at GANIL with the AGATA+VAMOS+PARIS setup, to study neutron-rich O, C, N, ... nuclei. Excited states in $^{17}$O and $^{19}$O, with known lifetimes, are used to validate the method over the $\sim$20-400 fs lifetime-sensitivity range. Emphasis is given to the unprecedented position resolution provided by $γ$-tracking arrays, which turns out to be essential for reaching the required accuracy in Doppler-shift correction, at the basis of the detailed analysis of $γ$-ray lineshape and resulting state lifetime determination. The technique is anticipated to be an important tool for lifetime investigations in exotic neutron-rich nuclei, produced with intense ISOL-type beams.

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arXiv2020

Detailed low-spin spectroscopy of 65Ni via neutron capture reaction

An extended investigation of the low-spin structure of the $^{65}$Ni nucleus was performed at the Institut Laue-Langevin, Grenoble, via the neutron capture reaction $^{64}$Ni(n,$γ$)$^{65}$Ni, using the FIPPS HPGe array. The level scheme of $^{65}$Ni was significantly expanded, with 2 new levels and 87 newly found transitions. Angular correlation analyses were also performed, allowing us to assign spins and parities for a number of states, and to determine multipolarity mixing ratios for selected $γ$ transitions. The low-energy part of the experimental level scheme (up to about 1.4 MeV) was compared with Monte Carlo Shell Model calculations, which predict spherical shapes for all states, apart from the 9/2$^+$ and the second excited 1/2$^-$ states of oblate deformation.

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arXiv2020

Manifestation of the Berry phase in the atomic nucleus $^{213}$Pb

The neutron-rich $^{213}$Pb isotope was produced in the fragmentation of a primary 1 GeV $A$ $^{238}$U beam, separated in FRS in mass and atomic number, and then implanted for isomer decay $γ$-ray spectroscopy with the RISING setup at GSI. A newly observed isomer and its measured decay properties indicate that states in $^{213}$Pb are characterized by the seniority quantum number that counts the nucleons not in pairs coupled to angular momentum $J=0$. The conservation of seniority is a consequence of the Berry phase associated with particle-hole conjugation, which becomes gauge invariant and therefore observable in semi-magic nuclei where nucleons half-fill the valence shell. The $γ$-ray spectroscopic observables in $^{213}$Pb are thus found to be driven by two mechanisms, particle-hole conjugation and seniority conservation, which are intertwined through the Berry phase.

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arXiv2020

Shape Coexistence at Zero Spin in 64Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic 64Ni nucleus has been considerably expanded: combining four experiments, several 0+ and 2+ excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0+ excitation is located at a surprisingly high energy (3463 keV), with a collective 2+ state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N = 40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.

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arXiv2020

Testing ab initio nuclear structure in neutron-rich nuclei: lifetime measurements of second 2+ states in 16C and 20O

To test the predictive power of ab initio nuclear structure theory, the lifetime of the second 2+ state in neutron-rich 20O, tau(2+_2 ) = 150(+80-30) fs, and an estimate for the lifetime of the second 2+ state in 16C have been obtained, for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens-to-hundreds femtoseconds range, by analyzing the Doppler-shifted gamma-transition line shapes of products of low-energy transfer and deep-inelastic processes in the reaction 18O (7.0 MeV/u) + 181Ta. The requested sensitivity could only be reached owing to the excellent performances of the AGATA gamma-tracking array, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The experimental lifetimes agree with predictions of ab initio calculations using two- and three-nucleon interactions, obtained with the valence-space in-medium similarity renormalization group for 20O, and with the no-core shell model for 16C. The present measurement shows the power of electromagnetic observables, determined with high-precision gamma spectroscopy, to assess the quality of first-principles nuclear structure calculations, complementing common benchmarks based on nuclear energies. The proposed experimental approach will be essential for short lifetimes measurements in unexplored regions of the nuclear chart, including r-process nuclei, when intense ISOL-type beams become available.

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arXiv2020

Experimental study of the 2n-transfer reaction $^{138}$Ba($^{18}$O,$^{16}$O)$^{140}$Ba in the projectile energy range 61-67 MeV

Two-neutron transfer reactions serve as an important tool for nuclear structure studies in the neutron rich part of the nuclear chart. In this article, we report on the first experimental attempt to populate the excited states of $^{140}$Ba employing the 2n-neutron transfer reaction $^{138}$Ba($^{18}$O,$^{16}$O)$^{140}$Ba. $^{140}$Ba is highly important, as it is placed on the onset of octupole correlations and the lifetimes of its excited states are completely unknown, with the sole exception of the first 2$^+$ state. The experiment was carried out at the Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN-HH) in Magurele, Romania. Lower limits on the lifetimes of ground state band up to the 8$^+$ state are reported. Furthermore, relative cross sections regarding the 2n-transfer reaction with respect to the fusion and the total inelastic reaction channels have been deduced. Further investigation directions of the nuclear structure of $^{140}$Ba are also discussed.

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arXiv2019

Pairing-quadrupole interplay in the neutron-deficient tin nuclei: first lifetime measurements of low-lying states in $^{106,108}$Sn

The lifetimes of the low-lying excited states $2^+$ and $4^+$ have been directly measured in the neutron-deficient $^{106,108}$Sn isotopes. The nuclei were populated via a deep-inelastic reaction and the lifetime measurement was performed employing a differential plunger device. The emitted $γ$ rays were detected by the AGATA array, while the reaction products were uniquely identified by the VAMOS++ magnetic spectrometer. Large-Scale Shell-Model calculations with realistic forces indicate that, independently of the pairing content of the interaction, the quadrupole force is dominant in the $B(E2; 2_1^+ \to 0_{g.s.}^+)$ values and it describes well the experimental pattern for $^{104-114}$Sn; the $B(E2; 4_1^+ \to 2_1^+)$ values, measured here for the first time, depend critically on a delicate pairing-quadrupole balance, disclosed by the very precise results in $^{108}$Sn. This result provides insight in the hitherto unexplained $B(E2; 4_1^+ \to 2_1^+)/B(E2; 2_1^+ \to 0_{g.s.}^+) < 1$ anomaly.

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arXiv2018

Is Seniority a Partial Dynamic Symmetry in the First $νg_{9/2}$ Shell?

The low-lying structures of the midshell $νg_{9/2}$ Ni isotopes $^{72}$Ni and $^{74}$Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following $β$ decay of the mother nuclei $^{72}$Co and $^{74}$Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first $(8^+)$ levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron $g_{9/2}$ shell. We find that the disappearance of the seniority isomerism in the $(8^+_1)$ states can be explained by a lowering of the seniority-four $(6^+)$ levels as predicted years ago. For $^{74}$Ni, the internal de-excitation pattern of the newly observed $(6^+_2)$ state supports a restoration of the normal seniority ordering up to spin $J=4$. This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near $^{78}$Ni.

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