arXiv subjects

J. -C. Thomas

Publications and source records attributed to J. -C. Thomas.

18 recordsLinked to original sources

arXiv2026

Studies of $\rm ^{144,148}Sm+α$ potential for the $p$-process nucleosynthesis

Nucleosynthesis reaction networks leading to $p$-nuclei involve a combination of different types of photodisintegration and capture reactions, as well as $β^+$ decays or electron captures. Photodisintegration reactions involving $α$ particles present a particular interest as they serve as branching points of the reaction networks. The cross sections of these reactions depend crucially on the $α$-nucleus interaction. The $α$ optical model potential (AOMP) is determined mostly by means of experimental differential elastic scattering distributions. Several previous studies have focused on the case of $\rm ^{144}Sm$, an intriguing $p$-nucleus that is semi-magic with 82 neutrons. This work presents new experimental data on $α$ elastic and inelastic scattering on $\rm ^{148}Sm$, its closest stable isotope. Isotopic effects on the description of the AOMP are studied, as well as their consequences on the prediction of $α$-induced reaction cross sections at astrophysical energies. It is shown that the isotopic ratio for $(α,γ)$ cross sections can be multiplied up to a factor of two when these effects are included.

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arXiv2026

Direct transfer to $^{46,48}$K as a survey of the $π(s_{1/2})$$-ν(sdpf)$ interaction

The collapse of the canonical $N=28$ magic number in nuclei with $Z<20$ has drawn significant interest as it relates to the emergence of an island of inversion centered on $^{42}$Si and $^{44}$S. In particular, interactions between the $πs_{1/2}$ orbital -- empty in $^{42}$Si and full in $^{44}$S -- and the neutron orbitals just above and below the $N=28$ gap are expected to be critical in this region, but remain relatively unexplored. In this paper, we expand upon the results of our previous study of the direct transfer reaction $^{47}$K(d,p$γ$)$^{48}$K [C.\,J.~Paxman \textit{et al.}, Phys. Rev. Lett. 134, 162504 (2025)] with the results of the complementary $^{47}$K(d,t$γ$)$^{46}$K reaction. Through this study, we present a comprehensive scan of the interaction between the critical $πs_{1/2}$ orbital and a broad range of neutron orbitals spanning nearly two full shells. We identify several discrepancies between the experimental results and state-of-the-art shell model calculations, which suggest a deficiency of the shell model to fully capture the complex proton configuration mixing in this region, highlighting a significant challenge for single-particle descriptions of the island of inversion.

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arXiv2025

Superallowed $0^+ \rightarrow 0^+$ $β$ decay studies at GANIL and upcoming opportunities with DESIR and S$^3$-LEB

Corrected transition rates ($\mathcal{F}t^{0^+ \rightarrow 0^+}$) of $0^+ \rightarrow 0^+$ superallowed $β$ decays currently give the most precise value of $V_{ud}$, the dominant term of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. By setting stringent constrains on the CKM unitarity, these decays allow probing physics beyond the Standard Model in the electroweak sector. A recent global reevaluation of the $\mathcal{F}t^{0^+ \rightarrow 0^+}$ values has indicated a violation of CKM unitarity prompting reassessment of the theoretical radiative and isospin symmetry breaking corrections applied on the experimental transition rates $ft$. In this article we briefly discuss this current situation and the experimental program at GANIL geared towards constraining isospin symmetry breaking corrections. We conclude by presenting the opportunities that will be available at DESIR and S$^3$-LEB, the upcoming low-energy radioactive ion beam facilities at GANIL.

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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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arXiv2025

Probing exotic cross-shell interactions at N=28 with single-neutron transfer on 47K

We present the first measurement of the $^{47}$K($d,pγ$)$^{48}$K transfer reaction, performed in inverse kinematics using a reaccelerated beam of $^{47}$K. The level scheme of $^{48}$K has been greatly extended with nine new bound excited states identified and spectroscopic factors deduced. Detailed comparisons with SDPF-U and SDPF-MU shell-model calculations reveal a number of discrepancies with these results, and a preference for SDPF-MU is found. Intriguingly, an apparent systematic overestimation of spectroscopic factors and a poor reproduction of the energies for 1$^-$ states suggests that the mixing between the $πs^{\,\,\,1}_{1/2} d^{\,\,\,4}_{3/2}$ and $πs^{\,\,\,2}_{1/2} d^{\,\,\,3}_{3/2}$ proton configurations in $^{48}$K is not correctly described using current interactions, challenging our descriptions of light $N=28$ nuclei.

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arXiv2018

Electron shakeoff following the \b{eta}+ decay of 19Ne+ and 35Ar+ trapped ions

The electron shakeoff of 19F and 35Cl atoms resulting from the \b{eta}+ decay of 19Ne+ and 35Ar+ ions has been investigated using a Paul trap coupled to a time of flight recoil-ion spectrometer. The charge-state distributions of the recoiling daughter nuclei were compared to theoretical calculations based on the sudden approximation and accounting for subsequent Auger processes. The excellent agreement obtained for 35Cl is not reproduced in 19F. The shortcoming is attributed to the inaccuracy of the independent particle model employed to calculate the primary shakeoff probabilities in systems with rather low atomic numbers. This calls for more elaborate calculations, including explicitly the electron-electron correlations.

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arXiv2014

Sizeable beta-strength in 31Ar(beta-3p) decay

We present for the first time precise spectroscopic information on the recently discovered decay mode beta-delayed 3p-emission. The detection of the 3p events gives an increased sensitivity to the high energy part of the Gamow-Teller strength distribution from the decay of 31Ar revealing that as much as 30% of the strength resides in the beta-3p decay mode. A simplified description of how the main decay modes evolve as the excitation energy increases in 31Cl is provided.

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arXiv2014

Multi-particle emission in the decay of 31Ar

A multi-hit capacity setup was used to study the decay of the dripline nucleus 31Ar, produced at the ISOLDE facility at CERN. A spectroscopic analysis of the beta-delayed three-proton decay of 31Ar is presented for the first time together with a quantitative analysis of the beta-delayed two-proton-gamma-decay. A new method for determination of the spin of low-lying levels in the beta-proton-daughter 30S using proton-proton angular correlations is presented and used for the level at 5.2 MeV, which is found to be either a 3+ or 4+ level, with the data pointing towards the 3+. The half-life of 31Ar is found to be 15.1(3) ms. An improved analysis of the Fermi beta-strength gives a total measured branching for the beta-3p-decay of 3.60(44) %, which is lower than the theoretical value found to be 4.24(43) %. Finally the strongest gamma-transitions in the decay of 33Ar are shown including a line at 4734(3) keV associated to the decay of the IAS, which has not previously been identified.

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arXiv2013

Relative proton and gamma widths of astrophysically important states in 30S studied in the beta-decay of 31Ar

Resonances just above the proton threshold in 30S affect the 29P(p,gamma)30S reaction under astrophysical conditions. The (p,gamma)-reaction rate is currently determined indirectly and depends on the properties of the relevant resonances. We present here a method for finding the ratio between the proton and gamma partial widths of resonances in 30S. The widths are determined from the beta-2p and beta-p-gamma decay of 31Ar, which is produced at the ISOLDE facility at the European research organization CERN. Experimental limits on the ratio between the proton and gamma partial widths for astrophysical relevant levels in 30S have been found for the first time. A level at 4688(5) keV is identified in the gamma spectrum, and an upper limit on the proton to gamma width of 0.26 (95 % C.L.) is found. In the two-proton spectrum two levels at 5227(3) keV and 5847(4) keV are identified. These levels are previously seen to gamma decay and upper limits on the gamma to proton width of 0.5 and 9, respectively, (95 % C.L.) are found, where the latter differs from previous calculations.

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arXiv2012

Isotopic fission fragment distributions as a deep probe to fusion-fission dynamics

During the fission process, the nucleus deforms and elongates up to the two fragments inception and their final separation at scission deformation. The evolution of the nucleus energy with deformation is determined by the macroscopic properties of the nucleus, and is also strongly influenced by the single-particle structure of the nucleus. The fission fragment distribution is a direct consequence of the deformation path the nucleus has encountered, and therefore is the most genuine experimental observation of the potential energy landscape of the deforming nucleus. Very asymmetric fusion-fission reactions at energy close to the Coulomb barrier, produce well-defined conditions of the compound nucleus formation, where processes such as quasi-fission, pre-equilibrium emission and incomplete fusion are negligible. In the same time, the excitation energy is sufficient to reduce significantly structural effects, and mostly the macroscopic part of the potential is responsible for the formation of the fission fragments. We use inverse kinematics combined with spectrometers to select and identify the fission fragments produced in $^{238}$U+$^{12}$C at a bombarding energy close to and well-above the Coulomb barrier. For the first time, the isotopic yields are measured over the complete atomic-number distribution, between Z=30 and Z=63. The experimental set-up also allows to identify transfer-induced reactions, which lead to low-energy fission where the nuclear shell structure shows a strong influence on the fission-fragment distributions. The resulting set of data gives the possibility to observe the fission fragment properties over a wide range of excitation energy, and they reveal the vanishing of the shell effects in the potential energy of the fissioning nucleus, as well as the influence of fission dynamics.

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arXiv2009

beta-decay study of Cu-77

A beta-decay study of Cu-77 has been performed at the ISOLDE mass separator with the aim to deduce its beta-decay properties and to obtain spectroscopic information on Zn-77. Neutron-rich copper isotopes were produced by means of proton- or neutron-induced fission reactions on U-238. After the production, Cu-77 was selectively laser ionized, mass separated and sent to different detection systems where beta-gamma and beta-n coincidence data were collected. We report on the deduced half-live, decay scheme, and possible spin assignment of 77Cu.

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arXiv2008

Nuclear charge radius of $^8$He

The root-mean-square (rms) nuclear charge radius of ^8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of ^6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from ^6He to ^8He is an indication of the change in the correlations of the excess neutrons and is consistent with the ^8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.

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arXiv2007

First direct observation of two protons in the decay of $^{45}$Fe with a TPC

The decay of the ground-state two-proton emitter 45Fe was studied with a time-projection chamber and the emission of two protons was unambiguously identified. The total decay energy and the half-life measured in this work agree with the results from previous experiments. The present result constitutes the first direct observation of the individual protons in the two-proton decay of a long-lived ground-state emitter. In parallel, we identified for the first time directly two-proton emission from 43Cr, a known beta-delayed two-proton emitter. The technique developped in the present work opens the way to a detailed study of the mechanism of ground-state as well as beta-delayed two-proton radioactivity.

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arXiv2004

Beta-decay properties of $^{25}$Si and $^{26}$P

The $β$-decay properties of the neutron-deficient nuclei $^{25}$Si and $^{26}$P have been investigated at the GANIL/LISE3 facility by means of charged-particle and $γ$-ray spectroscopy. The decay schemes obtained and the Gamow-Teller strength distributions are compared to shell-model calculations based on the USD interaction. B(GT) values derived from the absolute measurement of the $β$-decay branching ratios give rise to a quenching factor of the Gamow-Teller strength of 0.6. A precise half-life of 43.7 (6) ms was determined for $^{26}$P, the $β$- (2)p decay mode of which is described.

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arXiv2003

Torsion primes in loop space homology

We construct a finite 1-connected CW complex X such that $H_*(ΩX; Z)$ has p-torsion for the infinitely many primes satisfying p ~ 5,7,17,19 mod 24, but no p-torsion for the infinitely many primes satisfying p ~ 13 or 23 mod 24.

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arXiv2003

Graded Lie algebras with finite polydepth

If A is a graded connected algebra then we define a new invariant, polydepth A, which is finite if $Ext_A^*(M,A) \neq 0$ for some A-module M of at most polynomial growth. Theorem 1: If f : X \to Y is a continuous map of finite category, and if the orbits of H_*(ΩY) acting in the homology of the homotopy fibre grow at most polynomially, then H_*(ΩY) has finite polydepth. Theorem 2: If L is a graded Lie algebra and polydepth UL is finite then either L is solvable and UL grows at most polynomially or else for some integer d and all r, $\sum_{i=k+1}^{k+d} {dim} L_i \geq k^r$, $k\geq$ some $k(r)$.

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arXiv2002

On the two-proton emission for Fe45 - a new type of radioactivity

In an experiment at the SISSI-LISE3 facility of GANIL, the decay of the proton drip-line nucleus Fe45 has been studied after projectile fragmentation of a Ni58 primary beam at 75 MeV/nucleon impinging on a natural nickel target. Fragment-implantation events have been correlated with radioactive decay events in a 16x16 pixel silicon strip detector on an event-by-event basis. The decay-energy spectrum of Fe45 implants shows a distinct peak at (1.06+/-0.04)MeV with a half-life of T1/2 = (4.7+3.4-1.4)ms. None of the events in this peak is in coincidence with beta particles which were searched for in a detector next to the implantation detector. For a longer correlation interval, daughter decays of the two-proton daughter Cr43 can be observed after Fe45 implantation. The decay energy for Fe45 agrees nicely with several theoretical predictions for two-proton emission. Barrier-penetration calculations slightly favour a di-proton emission picture over an emission of two individual protons and point thus to a He2 emission mode.

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arXiv1999

On the discovery of doubly-magic $^{48}$Ni

The paper reports on the first observation of doubly-magic Nickel-48 in an experimental at the SISSI/LISE3 facility of GANIL. Four Nickel-48 isotopes were identified. In addition, roughly 100 Nickel-49, 50 Iron-45, and 290 Chromium-42 isotopes were observed. This opens the possibility to search for two-proton emission from these nuclei.

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