Contributors: H. Abele, N. Alahari, W. Barth, D. Bemmerer, K. Blaum, F. Bossi A. Bracco, M. Chiossi, A. Denig, M. Doser, S. Freeman, M. Gazdzicki, F. Gélis, H. Goutte, M. Grecco, M. Harakeh, M. Hori, G. Imbriani, E. Khan, K. Kirch, W. Korten, A. Laird, J. P. Lansberg, D. Lunney, F. Maas, G. Martinez-Pinedo, S. Masciocchi, A. Mengoni, O. Navillat-Cuncic, D. Rifuggiato, P. Rossi, E. Scomparin, J. Simpson, H. Schmieden, O. Schneider, N. Severijns, Th. Stöhlker, J. Stroth, H. Ströher, U. Thoma, S. Ulmer, C. A. Ur, Ch. Weinheimer, U. Wiedner, H. Wittig

Caen, in Normandy, France, is famous as the home of William the Conqueror. Equally well known in the field of nuclear physics is the Grand Accélérateur National d’Ions Lourds (GANIL) laboratory and its major upgrade of the existing infrastructure, Système de Production d’Ions RAdioactifs en Ligne de 2e génération (SPIRAL2) facility. GANIL is primarily focused on cutting edge research in fundamental nuclear physics using ion beams and is supplemented by strong programs in acceleratorbased atomic physics, condensed matter physics, radiobiology, and industrial applications. For many decades, GANIL has provided Highintensity stable beams (12C to 238U), beams of short-lived nuclei (Radioactive Ion Beams), produced both by in-flight separation (lifetimes ~ μs) and isotope separation on-line (ISOL) technique (lifetimes ~ ms). The five-cyclotron complex delivers stable beams from energies ~1 MeV to 95 MeV per mass unit with currents up to 10 μA, fragmentation beams up to ~ 50 MeV/A, and reaccelerated beams (SPIRAL1) from 1.2 MeV/A to 25 MeV/A (~40 isotopes). The intensities of the radioactive beams range from a few particles/s to ~107 p/s. The new superconducting linear accelerator (LINAC), in addition to very highintensity light beams, also provides a fourth type of beam, namely neutrons, to the already available arsenal of beams. These numerous types of beams are coupled to versatile detection facilities that allow the exploration of the behavior of nuclei in the phase space of excitation energy, angular momentum, and isospin. The first volume of Nuclear News (1991) portrayed the nuclear physics activities at GANIL, followed by the SPIRAL1 project (1995) and interdisciplinary physics (2000). In this article we present the evolution of the facility starting with the cyclotrons, the various associated detectors, and the current status of SPIRAL2. Figures 1 and 2 illustrate the cyclotron and LINAC complexes and their associated experimental halls. These complexes will be connected through a planned future project. GANIL, a multibeam facility, has been delivering a wide spectrum of stable and radioactive ion beams since 1983. Between 1983 and 1990, the facility relied on a cascade of three warm cyclotrons (KC0 = 30, KCSS1 = 380, KCSS2 = 380). Subsequently, a second injector was added. Various techniques were developed to increase the beam intensities, the number of isotopes, and the reliability of these beams. A major upgrade, in 2001, was the availability of reaccelerated radioactive ions from the SPIRAL1 facility. The cyclotrons serve as the driver for the production of radioactive atoms in a thick carbon target that can be reaccelerated by the new Cyclotron pour Ions de Moyenne Energie (CIME) (K = 265) up to a maximum energy of 25MeV/u (the highest in the world today). A review of the work done using SPIRAL1 beams till 2010 can be found in Ref. [1]. An upgrade, started in 2014, for increasing the number of reaccelerated beams using a Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source coupled with a charge breeder, extends those available with the existing Electron Cyclotron Resonance (ECR) ion source. This, added to the already available secondary beams by using the inflight method and stable beams, makes GANIL the only facility with this variety of beams. A continuous development of new and more intense stable beams, post-accelerated radioactive beams where GANIL has a niche, is ongoing. In parallel, there has been a continuous evolution in the detection systems, including the addition of various new detectors (discussed below). The functioning of the cyclotrons that has decreased in the last few years as a result of sharing the resources for construction of SPIRAL2, is being ramped up.

Neutron-rich 96 ; 98 Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N ¼ 60 .

The structure of neutron-rich Sr-96,Sr-98 nuclei was investigated by low-energy safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility, CERN, with the MINIBALL spectrometer. A rich set of transitional and diagonal E2 matrix elements, including those for non-yrast structures, has been extracted from the differential Coulomb-excitation cross sections. The results support the scenario of a shape transition at N = 60, giving rise to the coexistence of a highly deformed prolate and a spherical configuration in Sr-98, and are compared to predictions from several theoretical calculations. The experimental data suggest a significant contribution of the triaxal degree of freedom in the ground state of both isotopes. In addition, experimental information on low-lying states in Rb-98 has been obtained.

This corrects the article DOI: 10.1103/PhysRevLett.116.022701.

This corrects the article DOI: 10.1103/PhysRevLett.116.022701.

Received 1 August 2016DOI:https://doi.org/10.1103/PhysRevLett.117.099902This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsLow & intermediate energy heavy-ion reactionsNuclear Physics

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

Fission studies have been very active these recent years, both from an experimental point of view and from a theoretical one.This compels us to address the relevant questions concerning the fission studies main issues.In fundamental physics: do phenomena exist that are yet still not understood or unexpected in this domain?What are the experimental locks that we need to open to reach them?What is the necessary progress to be made in present theories to get a full reliable description of the process?As far as applications are concerned: what are the missing data?To which level of accuracy should they be measured or calculated?In this proceeding we will try to take on a few challenges we are facing and present some of the major recent achievements (subjective choice).

Until now, the mass asymmetry in the nuclear fission process has been understood in terms of the strong influence of the nuclear structure of the nascent fragments. Recently, a surprising asymmetric fission has been discovered in the light mercury region and has been interpreted as the result of the influence of the nuclear structure of the parent nucleus, totally discarding the influence of the fragments' structure. To assess the role of the fragment shell effects in the mass asymmetry in this particular region, a scission-point model, based on a full energy balance between the two nascent fragments, has been developed using one of the best theoretical descriptions of microscopic nuclear structure. As for actinides, this approach shows that the asymmetric splitting of the Hg-180 nucleus and the symmetric one of Hg-198 can be understood on the basis of only the microscopic nuclear structure of the fragments at scission.

A new approach called the Schr\"odinger Collective Intrinsic Model (SCIM) has been developed to achieve a microscopic description of the coupling between collective and intrinsic excitations. The derivation of the SCIM proceeds in two steps. The first step is based on a generalization of the symmetric moment expansion of the equations derived in the framework of the Generator Coordinate Method (GCM), when both Hartree-Fock-Bogoliubov (HFB) states and two-quasi-particle excitations are taken into account as basis states. The second step consists in reducing the generalized Hill and Wheeler equation to a simpler form to extract a Schr\"odinger-like equation. The validity of the approach is discussed by means of results obtained for the overlap kernel between HFB states and two-quasi-particle excitations at different deformations.

The light krypton isotopes were studied in a series of Coulomb excitation experiments using radioactive beams at GANIL. The static quadrupole moments found in these experiments give firm experimental evidence for the shape coexistence scenario that is based on theoretical calculations and on the systematics of low-lying excited 0 + states. The experimental results are interpreted within a phenomenological two-band mixing model. Configuration mixing calculations based on triaxial Hartree-Fock-Bogolyubov calculations with the Gogny D1S effective interaction have been performed and compared to experimental data.

Fission is a complex process which highlights many nuclear properties. Among the different theoretical approaches able to describe fission, microscopic ones have the advantage of describing the nuclear structure and the dynamics in a consistent manner. Along this line, we are now developing a formalism able to treat on the same footing the collective dynamics and the intrinsic excitations. This approach is based on the non adiabatic time-dependent Generator Coordinate Method, where couplings between HFB states and 2 quasi-particle states are explicitly taken into account. Guidelines of the new formalism under development are presented and some preliminary results on overlaps between non excited and excited states are discussed.

A systematic study of low energy nuclear structure at normal deformation is carried out using the Hartree-FockBogoliubov theory extended by the generator coordinate method and mapped onto a five-dimensional collective quadrupole Hamiltonian. Results obtained with the Gogny D1S interaction are presented from drip line to drip line for even-even nuclei with proton numbers Z = 10 to Z = 110 and neutron numbers N 200. The properties calculated for the ground states are their charge radii, two-particle separation energies, correlation energies, and the intrinsic quadrupole shape parameters. For the excited spectroscopy, the observables calculated are the excitation energies and quadrupole as well as monopole transition matrix elements. We examine in this work the yrast levels up to J = 6, the lowest excited 0 + states, and the two next yrare 2 + states. The theory is applicable to more than 90% of the nuclei that have tabulated measurements. We assess its accuracy by comparison with experiments on all applicable nuclei where the systematic tabulations of the data are available. We find that the predicted radii have an accuracy of 0.6%, much better than can be achieved with a smooth phenomenological description. The correlation energy obtained from the collective Hamiltonian gives a significant improvement to the accuracy of the two-particle separation energies and to their differences, the two-particle gaps. Many of the properties depend strongly on the intrinsic deformation and we find that the theory is especially reliable for strongly deformed nuclei. The distribution of values of the collective structure indicator R42 = E(4 + )/E(2 + ) has a very sharp peak at the value 10/3, in agreement with the existing data. On average, the predicted excitation energy and transition strength of the first 2 + excitation are 12% and 22% higher than experiment, respectively, with variances of the order of 40–50%. The theory gives a good qualitative account of the range of variation of the excitation energy of the first excited 0 + state, but the predicted energies are systematically 50% high. The calculated yrare 2 + states show a clear separation between γ and β excitations, and the energies of the 2 + γ vibrations accord well with experiment. The character of the 0 + state is interpreted as shape coexistence or β-vibrational excitations on the basis of relative quadrupole transition strengths. Bands are predicted with the properties of β vibrations for many nuclei having R42 values corresponding to axial rotors, but the shape coexistence phenomenon is more prevalent. The data set of the calculated properties of 1712 even-even nuclei, including spectroscopic properties for 1693 of them, are provided in CEA Web site and EPAPS repository with this article [1].

A systematic study of low energy nuclear structure at normal deformation has been carried out using the Generator Coordinate Method mapped onto a 5-Dimensional Collective quadrupole Hamiltonian (5DCH) by using the Gaussian Overlap Approximation (GCM-GOA). The collective space is spanned by Hartree Fock Bogoliubov (HFB) states under axial and triaxial quadrupole constraints deduced with the D1S Gogny force. In addition, our 5DCH includes the Thouless-Valatin dynamical corrections to its rotational kinetic terms. The work is described in detail elsewhere together with the corresponding comparisons with experimental data when it is available. Many properties show a satisfactory agreement with experiment, but there is an almost systematic overestimation of vibrational band head energies, which is the subject of the present paper. We show here the performance of the theory on related observables, and propose improvements of the theory to address the problem of the vibrational band heads. An important reason for the deficient is the treatment of vibrational inertial parameters. In particular, the theory needs to include the dynamical Thouless-Valatin corrections to the vibrational terms in the 5DCH. In present work, on the aim of a simple formula grounded by known symmetry rules within the 5DCH, these dynamical TV corrections are roughly estimated, allowing us to handle their possible effects in term of spectroscopic properties, to present a guess of the next model improvment, and to isolate some areas in the chart where states Inπ = 0+2, 2+2 or 2+3 seem to have important components out of the scope of a pure collective quadrupole approach.

Among the different theoretical approaches able to describe fission, microscopic ones can help us in the understanding of this process, as they have the advantage of describing the nuclear structure and the dynamics in a consistent manner. The sole input of the calculations is the nucleon-nucleon interaction. Such a microscopic time-dependent and quantum mechanical formalism has already been used, based on the Gaussian Overlap Approximation of the Generator Coordinate Method with the adiabatic approximation, to analyze the collective dynamics of low-energy fission in 238U [1]. However, at higher energies, a few MeV above the barrier, the adiabatic approximation doesn’t seem valid anymore. Indeed, manifestations of proton pair breaking have been observed in 238U and 239U for an excitation energy of 2.3 MeV above the barrier [2–4]. Taking the intrinsic excitations into account during the fission process will enable us to determine the coupling between collective and intrinsic degrees of freedom, in particular from saddle to scission. Guidelines of the new formalism under development are presented and some preliminary results on overlaps between non excited and excited states are discussed.