Exotic, very neutron-rich nuclei: a laboratory for nuclear interactions

For the first time, an experiment has provided key observations on the spectroscopy of the neutron-rich unbound oxygen nuclei (proton number Z = 8), oxygen 28 (N = 20) and its neighboring isotope at N = 19, oxygen 27.  They were produced in high-energy reactions and observed by direct detection of their decay products, 24O and three or four neutrons. The study shows that it is possible to constrain the parameters of ab initio interactions from the energy differences of the observed states with respect to the last bound isotope - 24O (N = 16). These groundbreaking results were published in the journal Nature [Nat23].

Given the complexity of studying unbound nuclei, an exceptional detection system was implemented at the world's most powerful radioactive ion beam facility: RIBF in Japan. The data were obtained by an international collaboration (Samurai21) of around a hundred physicists (from 36 laboratories), including a team* of physicists from Irfu who were responsible for operating a key detector for the measurements, Minos. The experiment, carried out on the Samurai area of the RIBF (Radioactive Ion Beam Factory) facility at RIKEN in Japan, was piloted by groups of physicists from Titech (Tokyo Institute of Technology) and by the RIKEN-RIBF teams.

A new window into the deformation of nuclei has been recently opened with the realisation that nuclear collision experiments performed at high-energy colliders, such as the BNL Relativistic Heavy Ion Collider (RHIC) or the CERN Large Hadron Collider (LHC), give access to the shape of the colliding isotopes. A collaboration between high- and low-energy theorists, including researchers from IRFU, has demonstrated that quantitative information on nuclear deformations can be obtained and has shown that the isotope 129Xe appears as triaxial, i.e. a spheroid with three unequal axes. This result represents the first evidence of triaxiality in nuclear ground states attained in high-energy experiments. Moreover, it opens the way to exciting future investigations at the interface of low- and high-energy nuclear physics.

The pygmy dipole resonance (PDR) is a vibrational mode of the nucleus that occurs in neutron-rich nuclei. It is described as the oscillation of a neutron skin against a core symmetric in number of protons and neutrons (Figure 1). The 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 (a process that could explain the synthesis of heavy nuclei) via the increase of the neutron capture rate. However, despite numerous experiments dedicated to the study of the PDR, using charged particle or gamma-ray beams, a consistent description could not be extracted. Thus, new experimental approaches are needed to better characterize this vibrational mode of the nucleus.

The first measurement of Short-Range Correlations (SRC) in an exotic nucleus took place in May 2022 with the Cocotier instrument at the GSI facility in Darmstadt, Germany. This experiment is a milestone in the program that was started in 2017 with a grant from the French Research Agency that allowed physicists to build a liquid hydrogen target (see previous highlight). The goal of this experiment is to test the hypothesis that nucleon can form compact pair, the so-called SRC pair. This measurement campaign allowed us to gathered experimental data for about 60 hours with 16C beam and with a 12C beam for approximately 40 more hours in order to have a reference measurement with a well-studied stable beam. The IRFU team took a major role in preparing and running of this experiment, and is now in charge of the data analysis together with MIT, TU Darmstadt and LIP Lisbon team.

An experiment performed at GANIL with the new detection device ACTAR TPC (ACTive TARget Time Projection Chamber) allowed for the development of a new technique for the direct reconstruction of the 3D trajectories of the protons emitted by the isomeric state of 54mNi nucleus and simultaneously visualize their decay time (the 4th dimension). This observation is an unprecedented test of theoretical models since it probes some extremely weak components of the wave functions describing the structure of this nucleus. This technique is already foreseen to be used for the measurement of other similar decays. The result of this experiment was recently published in Nature Communications1.

DPhN physicists have played a decisive role in the first measurement of pairs of nucleons using a new method, that will pave the way to the study of short range interactions in radioactive nuclei.

Nucleons are social particles. Not only do they enjoy living in communities inside nuclei, but they also form couples within these communities. Indeed, one can observe protons and neutrons forming pairs inside nuclei. DPhN physicists have played a decisive role in the first measurement of such pairs of nucleons using a new method, that will pave the way to the study of these close (or short range) interactions in radioactive nuclei. The results have recently been published in Nature Physics [Pat21]. The study of these nucleon pairs in radioactive nuclei is the goal of the ANR project COCOTIER led by IRFU.

Understanding how the nuclear interaction emerges from the basic constituents of matter is one of the challenges of contemporary physics. The nuclear interaction between nucleons (proton or neutron) is seen as a manifestation of the strong force between quarks, which is mediated by the exchange of gluons and holds the nucleon together. In spite of the longstanding efforts, a unified nuclear interaction that allows predicting the properties of all nuclei does not exist yet.

Since 2010 the question of the size of the proton is at the heart of a controversy between atomic physicists and hadronic physicists. Indeed, very precise measurements of atomic physics have concluded that the size of the proton is much smaller than expected, in very strong disagreement with the experiments of elastic scattering. In collaboration with the University of Perugia, a physicist from IRFU has investigated to find the reason for such a difference. The results have been published in European Journal of Physics A [3].

IRFU engineers and physicists and their collaborators have just completed the development of a modern Sirius, a key element of the super spectrometer separator (S3) under construction at GANIL.

The ancients understood that heroes, like Orion with Sirius, need their faithful companion. IRFU engineers and physicists and their collaborators are no exception to the rule and have just completed the development of a modern Sirius, a key element of the super spectrometer separator (S3) under construction at GANIL. The tests having been successful and the system has been moved to GANIL for its final installation.

In Greek mythology, Sirius, Orion's faithful four-legged companion, an outstanding hunter, was transformed into a constellation and placed at his side. This famous canid also gave its name to the brightest star in the night sky. IRFU physicists have just honoured him in their own way, this time in the world of detectors.

Two “mirror” nuclei, in which the numbers of neutrons and protons are interchanged, have markedly different shapes—a finding that defies current nuclear theories. This striking result has been obtained by Irfu researchers in collaboration with an international team and has been recently published in Physics Review Letter [1] and highlighted as editor’s suggestion [2].

In December 2019, the NFS (Neutrons For Science) facility received its first proton beams, delivered by the linear accelerator of the new Spiral2 facility at the GANIL. On the fringes of the progressive commissioning of the accelerator in 2020, short beam periods were used to successfully test several NFS components. The first experiments are planned at the facility in the fall of 2021.

A first beam of protons accelerated up to 33 MeV was sent in December 2019 to the NFS irradiation station (Figure 1), coupled to a pneumatic transfer system aiming at transporting the irradiated samples to a measurement station. The production cross-sections of different nuclei (obtained through irradiation of iron and copper samples) were thus measured. The results of this test are in agreement with previously published data. The irradiation and measuring device, built and operated by physicists from the NPI laboratory in Rez (Czech Republic), will be used in the future for novel measurements of reaction cross sections by activation.

The spectroscopy of a mendelevium isotope, 251Md composed of 101 protons and 150 neutrons, reveals a surprise: when it rotates, it behaves exactly like a lawrencium isotope made of 103 protons and 152 neutrons. The experiment carried out at the University of Jyväskylä in Finland required the most advanced tools to study these rare and ephemeral nuclei: filtering and identification of the nuclei, gamma ray and electron detection. Is this completely unexpected similarity the result of chance, or is it related to the properties of strong interaction? The investigation continued with the theoreticians to try to understand this singularity. The results have just been published in the journal Physical Review C.

The combination of the AGATA multi-detector [right]
and the VAMOS spectrometer [left] showed that the
balance between the two contributions was more
complex than previously envisaged.                                           

The complexity of the atomic nucleus reflects a multi-component character of the « nuclear force » that holds protons and neutrons together. Proper separation and characterization of each of these components represents a challenge for both theoretical and experimental nuclear structure studies. The tin isotopes (nuclei with Z=50 protons and a number of neutrons depending on the isotope) provide ideal opportunities to study the competition between two of the nuclear force components: the so-called pairing, related to the marked tendency of protons and neutrons to form pairs in the nuclear matter, and the so-called quadrupole interaction term, describing the natural susceptibility of nuclei to adopt deformed shapes. Though of a different nature, these two interaction terms contribute to the goal of achieving an optimum organization of nucleons in the atomic nucleus that will minimize its energy.  Previous works have demonstrated that a shift of balance between these two components takes place when approaching tin-100, and this observation provided important constraints for theoretical descriptions of this so-called “doubly-magic” nucleus. Having the same numbers of protons and neutrons (Z=N=50), 100Sn is a key nucleus to validate model descriptions of exotic nuclei.

The simple question “Where does the Periodic Table end?” has excited scientific interest for a long time. In this context, understanding the structure of the heaviest nuclei, and through it their stability, is of major importance. A decade ago, there was no promising path to tackle this scientific quest. And yet, in the past few years, a collaboration composed of physicists coming from Irfu/DPhN, Jyvaskyla (Finland), GSI (Germany) and Argonne (USA) applied a newly developed technique relying on high performance accelerators and state-of-the-art detectors to investigate the isomeric (long-lived) states in heavy nuclei[KS1] [2] . It has triggered a renaissance in heavy ion elements science. New focal plane detectors equipped with digital electronics has been for the first time extended to short-lived states[KS3]  in heavy nuclei, allowing the detection of very close events and very short half-life (few µs). This opens new perspectives as  to date, the heaviest element found is Oganesson with a half-life of 0.58ms.The results have been published in the Physical review C [1].


For the first time, a team of researchers was able to measure and accurately identify daughter nuclei produced by the fission of uranium-239 fission. This was made possible by the unique combination of GANIL equipment and beams. It is published in the Physical Review Letter*.

Predicting properties of, e.g., molecules or atomic nuclei from first principles requires to solve the Schrödinger equation with high accuracy. The computing cost to find exact solutions of the Schrödinger equation scales exponentially with the number of particles constituting the system. Thus, with nuclei composed of tens or hundreds of nucleons, it necessitates accurate approximate methods of lower computing cost. However, such methods can be applied to a limited number of systems: the weakly correlated ones. Consequently, a universally applicable method is still missing. Employing a novel formalism recently developed at Irfu/DPhN [1], highly accurate solutions of the Schrödinger equation – in the context of the exactly solvable Richardson model - have been obtained, independently of the weakly- to strongly-correlated character of the system. This work has been performed in collaboration with ab initio quantum chemists from Rice University. This exciting new achievement, paving the way for precise ab initio computations of molecular or nuclear properties of a large number of systems, was recently published in Physical Review C [2] and highlighted as the Editor’s suggestion.

Pairing is ubiquitous in physics. From superconductivity to quantum shell structure, coupling particles into pairs is one of nature's preferred ways to lower the energy of a system. New results obtained at the Radioactive Isotope Beam Factory (RIBF, Japan) with the MINOS device, which was conceived and constructed at Irfu, show for the first time that pairing also plays an important role in single-proton removal reactions from neutron-rich nuclei. These results show that proton-removal cross sections can be used as a tool to investigate pairing correlations for very neutron rich nuclei not accessible via spectroscopy. Indeed, the latter are produced in too small quantities to consider spectroscopy, studying the gammas emitted during de-excitation for example. This study was recently published in Physical Review Letters [1].

An international collaboration led by the institutes of CEA-IRFU and of RIKEN (Japan) demonstrates, for the first time, the exceptional stability of the very-neutron rich nickel-78 nucleus and its doubly-magic character. The experiment at RIKEN was made possible by the unique combination of the MINOS device developed at CEA-Irfu and the very exotic beams produced by the RIBF facility of the Japanese accelerator.These results are published in Nature [Nat19].


Prediction of nuclear properties based on a realistic description of the strong interaction is at the heart of the ab initio endeavor in low-energy nuclear theory. Ab initio calculations have long been limited to light nuclei or to nuclei with specific proton and neutron numbers. Theoreticians from Irfu/DPhN have developed a new ab initio method from which properties of many more nuclei than before can be predicted while drastically decreasing the computational cost. This has been made possible by allowing symmetries of the nuclear Hamiltonian to spontaneously break in the calculation. This exciting new development, paving the way for precise computations of heavier nuclei within a reasonable time-frame, has recently been published in Physics Letter B [1].

Theoretical work involving researchers from Ganil, the University of Huelva in Spain and the Racah Institute of Physics in Israel, confirms the vibrational nature of cadmium-110, reproducing experimental observations that called this nature into question. Since the 1970s experiments have revealed the existence of non-vibrational states called "intruder" states. The new theoretical results, which are being extended to all even isotopes of cadmium, therefore solve a problem that is several decades old and has been highlighted by the publisher of the journal Physical Review C.

During an experiment carried out at GANIL in Caen (France), an international team, led by researchers from Irfu and the University of Oslo, studied the shape of the Zirconium-98 nucleus. The shape of a nucleus corresponds to the area where its protons and neutrons can be found. Understanding it means mastering the behaviour of each proton/neutron and their arrangement related to the nuclear force. The objective was to determine the shape of the nucleus in different excited states. The results give a complex scenario, for which three different shapes - spherical, slightly deformed and strongly deformed - coexist within the same nucleus depending on whether it is in its ground or excited state. In addition, its neighbouring nucleus, Zirconium-100 with only two more neutrons, behaves in the opposite way. This sudden change in the shape of these two isotopes is a rare phenomenon that strongly constrains nuclear structure models. These results have been published recently in Physical Review Letters [1].

The first triplet of superconducting multipoles of the S3 Super Separator Spectrometer arrived at Ganil on August 29, 2018. S3 is one of the experiment rooms of the Spiral2 facility. The magnet, with a mass of 2.8 tonnes, is 1.8 m long and almost as high. This innovative type of magnet is very compact despite the number of optical functions it can generate (quadrupole, sextupole, octupole and dipole). It is the first of a series of seven to be delivered to the Ganil.


The magnetic field is generated by a niobium-titanium alloy (NbTi) conductor arranged in an epoxy/glass fibre matrix and operate at the temperature of the liquid helium (4.2 kelvins). The power supply leads are composed of two types of high-temperature superconductors and nitrogen-cooled.
It’s a unique design resulting from a collaboration between Ganil, CEA/Irfu, the American laboratory in Argonne Nat. Lab. and the two manufacturers in charge of prototyping and series (Advanced Magnet Lab. for superconducting coils, Cryomagnetics Inc. for cryostats and integration).
This element was financed by EQUIPEX n° 10-EQPX-0046, awarded to S3 by the National Research Agency in 2011.

Contacts: Antoine Drouart, Myriam Grar (Ganil) et Hervé Savajols (Ganil)

During an experiment carried out at the accelerator of the Australian National University (Canberra, Australia), a French-Australian collaboration (GANIL Caen, IPN Orsay, IRFU/DPhN Saclay, ANU Canberra) first identified the fragments created in quasi-fission reactions with atomic numbers Z up to plutonium (Z=94) and mass A. For this study, near-fission reactions were induced during collisions between 48Ti projectile ions, accelerated to 276 MeV, and target atoms of 238U. The atomic numbers were deduced from the characteristic fluorescence X-ray emissions and the masses from the angular correlations and velocities of the emerging fragments. The data collected highlights shell effects which increase the production of nuclei around the magic number Z=82 (lead) in near-fission reactions. These results, which will make it possible to optimize experiments aimed at creating heavier elements by fusion, as well as the prospects opened up by this original experimental approach in the field of nuclear fission and fusion, have led to a publication in the journal Physical Review Letters (M. Morjean et al., Phys. Rev. Lett. 119, 222502).

To ascertain the "strangeness" property of the proton, an international collaboration including IRFU has produced, starting from protons, other particles containing a "strange" quark and characterized them. These high-precision measurements carried out at CERN in Geneva will lead to a better understanding of the contribution of strange quarks to the nucleon's spin. 

An international team has performed the first spectroscopy of the very neutron-rich isotopes 98,100 Kr. The collaboration, led by scientists from the CEA Irfu and RIKEN (Japan)  included several European groups and physicists from IPN-Orsay. This experiment showed that there are two coexisting quantum configurations at low excitation energy in the 98Kr nucleus. The competition between these two configurations, represented by different shapes, has been previously evidenced in the isotopic chains Rb, Sr and Zr  by an abrupt transition from one shape to another, starting from the 60th neutron. This experiment observes however that in the Kr isotopic chain there is a much more gradual transition between these two configurations as a function of neutron number. This study marks a decisive step towards an understanding of the limits of this quantum phase transition regionIt was published in Physical Review Letters [PRL 118, 242501 (2017)].

Take a very elongated nucleus of strontium-98, remove two protons... and it becomes a sphere. Although this abrupt shape transition, observed for the first time at GANIL, remains unexplained, a deeper understanding of this phenomenon will allow the physicists to know a little more about the complex organization of nucleons in the nucleus.  

An international team led by IRFU and the Japanese research institute RIKEN was able to study the structure of a neutron-rich zirconium nucleus (110Zr)—a first, calling certain models into question. Produced by an accelerator at RIKEN and measured by the MINOS detector, this heavy nucleus proves to be more deformed than expected. 

A comparison between experiment and theory on the ground state observables of the oxygen nuclei

The comparison of ab initio calculations and experimental data was the subject of original work initiated by physicists from the Nuclear Physics Department of the CEA-Saclay: for the first time, this study combines two types of fundamental observables of the nucleus: mass (binding energy of the fundamental state) and size in terms of nuclear radius, in the form of the mean square radius of the density of matter (all nucleons, protons and neutrons), and the proposed comparison is extended to the most neutron-rich isotopes. This work, published on July 27, 2016 in Physical Review Letters, was presented as a "highlight from the editor", and the results highlight the key observables on which to build our general understanding of the description of atomic nuclei, by linking them to ab initio forces. The comparison with several computational techniques was carried out in collaboration with theorists from the University of Surrey, Triumf, and MSU.

The "exotic" nuclei pose the challenge of a universal description of the nuclear structure and raise the question of  the evolution of the shell structure. An IRFU team has developed the Magic Number Off Stability (MINOS) project to answer these questions. A physics program has been established in collaboration with Japanese teams of the RIKEN institute, where RIBF (Radioactive Isotope Beam Factory) is the world's most efficient accelerator for producing neutron-rich nuclei at intermediate energies of several hundreds of MeV. Experiments with the MINOS detector began in 2014 and their first results have just been published in Physical Review Letters crowning five years of effort and paving the way for a harvest of exciting results in the years to come.

At a meeting in Brussels of the NUPECC Committee(1) on December 9, the researchers presented their long term plan for maintaining the leading position currently enjoyed by European institutions in the field of nuclear physics. The Spiral2 project in Caen, a collaboration between the CNRS/IN2P3(2) and the CEA/DSM(3), is one of the projects already contributing to this European strategy.


The long term plan for nuclear physics may be found on the NUPECC site in a number of forms, including the full 200 page report, a 20 page summary and a 20 minute video. 







Philippe CHOMAZ, chef de l'Institut





The instrument known as MUSETT1 detected its first heavy nuclei during a commissioning experiment that took place in early April 2010 at the GANIL2 accelerator in Caen. MUSETT was built for identifying very heavy elements: transfermium, which are the elements beyond fermium (Z=100).  Nuclear physicists are interested in these extreme state of matter for testing the theoretical models that describe the nuclei. Initial results obtained with MUSETT are highly satisfactory, providing very good identification of the produced isotopes, thanks to an original method called ‘genetic correlations’. This method can tag nuclei by detecting its decay. MUSETT provides a preview of the detection for the future Super Separator Spectrometer S3, dedicated to the hyper-intense SPIRAL23 beams, which will allow scientists to explore the heaviest nuclei.



The CHyMENE project (Cible d'Hydrogène Mince pour l'Etude des Noyaux Exotiques -Thin hydrogen target for the study of exotic nuclei) has the ambitious goal of producing a thin target of pure hydrogen, without using a container, suitable for experiments using the low-energy heavy ion beam planned for SPIRAL2.



A team from IRFU (SPhN and SACM) and from l'Inac/SBT have recently applied cryogenic techniques to successfully produce a ribbon of solid hydrogen 100 μm thick. The target will soon be tested in the beam. This will be a world first.


Below: Interview with Alain GILLIBERT, who is working on the CHyMENE project with Alexandre OBERTELLI and Emmanuel POLLACO




Start image: a solid hydrogen ribbon of extruded H2 (width 10 mm, thickness 100 μm), viewed through the porthole of the vacuum chamber (Photo V. Lapoux).






The high energy part of the SPIRAL2 linear accelerator (new GANIL1 accelerator scheduled for implementation in 2012) uses two types of superconducting cavities. IRFU's Accelerator, Cryogenics and Magnetism Department is responsible for the design and development of 12 cryomodules2 of the first type, to be installed at the injector output. 

On December 8, 2008, the qualification prototype cryomodule was successfully tested at full power. The superconducting cavity attained an accelerating gradient of 10.3 MV/m (million volts per meter), far greater than the specified value of 6.5 MV/m.

The shape of an atomic nucleus reflects the shell structure of the protons and neutrons of which it is formed. If the shells are completely filled, we speak of a "magic" nucleus, which is spherical in shape. Most nuclei, however, tend to be deformed because their shells are only partially filled. The most commonly encountered shapes are elongated (prolate) or flattened (oblate); these shapes can change from on nucleus to its neighbour by adding or removing a proton or neutron. In some cases it is sufficient to rearrange the protons or neutrons within the same nucleus to change its shape. The same nucleus can therefore assume different shapes corresponding to states of different energy. If such states come close in energy (one thousandth of the binding energy of the nucleus or so), the different shapes can mix. According to the laws of quantum mechanics, the nucleus can coexist in different shapes (e.g. elongated and flattened) at the same time. Such shape coexistence was observed in light krypton and selenium isotopes in a series of experiments performed by a team of researchers from the Nuclear Physics Department (SPhN) of IRFU .


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