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Many properties of the atomic nucleus, such as vibrations, rotations and incompressibility, can be interpreted as due to a two-component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a ‘bubble’ structure, has been discussed in a recurrent manner since the 1970s. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34 Si. The proton-to-neutron density asymmetry in 34 Si offers the possibility to place constraints on the density and isospin dependence of the spin–orbit force—on which nuclear models have disagreed for decades—and on its stabilizing effect towards limits of nuclear existence. The central densities of protons and neutrons in stable atomic nuclei are saturated. More exotic nuclei — with imbalanced proton and neutron numbers — may have depleted central densities. Experiments now suggest such depletion for the 34 Si nucleus.

Two experiments made with the MSP144 stepped pole magnetic spectrometer of FLNR-JINR Dubna measured the energy spectra of α particles emitted at zero degree (collinear kinematic) in the reactions 40Ar(220 MeV) + 232Th and 48Ca(270 MeV) + 238U. The study was pursued up to the maximum energy the alpha particles may have in a two-body reaction, without excitation of the reaction partners, the so-called kinematic limit. The observed cross sections in the vicinity of the kinematic limits were of the order of μb. In the indicated reactions, the heavy partners of the recorded alpha particles in the exit channel were respectively 268Sg and 282Db. At the kinematic limit, the heavy partners have excitation energies close to zero, therefore a high probability to survive.

The paper represents an overview of the measurements performed using GAINS at GELINA (JRC-Geel, Belgium). While undergoing continuous upgrades, the setup produced highly precise cross sections. Our measurements are primarily driven by technological needs with an emphasis on structural materials used in the development of nuclear facilities. However, most cases offered the opportunity to investigate various reaction mechanism and/or nuclear structure issues. We concentrate on several specific experiments describing the particular difficulties we met and the solutions we adopted to infer reliable data and to draw significant conclusions.

Many observations strongly support the hypothesis that nuclei may fission through several independent fission modes (multimodal fission) interpreted as different prescission shapes and fission paths in a multidimensional potential energy landscape where shell effects are dominant. Mass distributions of the fission fragments are sensitive to the potential energy landscape and appear to be single humped (symmetric) or double humped (asymmetric). In many cases a mixture of both modes is observed.We propose here our study on 180Hg. Binary fission fragments formed in the reaction 68Zn + 112Sn → 180Hg at different excitation energies around the Coulomb barrier were detected using the double-arm time-of-flight technique with the spectrometers CORSET. The experiment was performed at JYFL (Jyvaskyla, Finland). We will discuss an analysis of the mass distributions in terms of fission modes predicted by a five-dimensional fission model. We have found out that the mass distributions can be well reproduced by considering a symmetric fission mode and two asymmetric modes at (AL ≈ 80, AH ≈ 100) and (AL ≈ 70 and AH ≈ 110).

The accuracy of the predictions of the γ flux produced by a classical nova during the first hours after the outburst is limited by the uncertainties on several reaction rates, including the 18F(p,α)15O one. Better constraints on this reaction rate can be obtained by determining the spectroscopic properties of the compound nucleus 19Ne. This was achieved in a new inelastic scattering method using a 19Ne radioactive beam (produced by the GANIL-SPIRAL 1 facility) impinging onto a proton target. The experiment was performed at the VAMOS spectrometer. In this article the performances (excitation energy range covered and excitation energy resolution) and limitations of the new technique are discussed. Excitation energy resolution of σ = 33 keV and low background were obtained with this inverse kinematics method, which will allow extracting the spectroscopic properties of 19Ne.

We present the results of an experiment in which the structure of neutron-rich nuclei located in the vicinity of N=40 was studied. The importance of our results comes from the fact that knowing the behaviour of the neutron g9/2 orbital with increasing number of neutrons is one of the key points in defining the structure of these nuclei at low excitation energy. The nuclei of interest were produced by fragmentation of a 86Kr beam at 60MeV/u on a thick Be target at GANIL (France). Preliminary results on 75Cu and 78Ga isomers will be presented together with tentative spin and parity assignments.

The two-proton radioactivity has been observed experimentally in 2002, at projectile fragmentation facilities, more than 40 years after the first theoretical prediction of this process. First observations were indirect measurements, using standard silicon detector devices. Since then, a new generation of experiments allowed for a direct observation, and opened the field of more detailed studies, using tracking devices for the detection of the emitted protons.

The abundance calculations of the p-nuclei produced in explosive stellar sites rely on the Hauser-Feshbach (HF) theory with the alpha-article optical model potential (α-OMP) one of its major ingredients. To date, most of the (α, γ) cross sections measured show that HF calculations can be wrong by a factor of ten or more especially when phenomenological α-OMP are employed. To investigate the relevant uncertainties entering the HF calculations and furthermore develop global microscopic α-OMPs, systematic (α, γ) cross-section measurements are necessary. This led us to perform a feasibility study of (α, γ) measurements in inverse kinematics that will allow us to employ also radioactive beams in the future. Hence, the 4He(78Kr,γ)82Sr reaction was studied using the LISE3 spectrometer to separate the 82Sr recoils from the primary 78Kr beam. Although an excellent rejection factor > 1010 was achieved, the position of the ions of interest was unexpectedly masked by a secondary beam of high intensity. Given these, new setup improvements are proposed to remove the pollutant ions. Recently, many experiments were conducted in order to study the influence of the environment (especially in a metallic material) on the decay probability of radioactive nuclei. Additionally, hydrogen-like fusion reactions were performed indicating a change in the cross-section due to the influence of the Coulomb field screening induced by quasi-free electrons in metals. This was explained by the Debye screening model which treats metallic electrons within Maxwell-Boltzmann statistics. We measured the decay rate of 19O in metallic, insulating and superconducting environments whereas the electrons in the superconductors should obey the Bose-Einstein statistics. The decay rate measurement was supported by a branching ratios measurement. We found that the effect on the decay rate, if any, is less than the 0.1%, far below the theoretical predictions.

Rigid frameworks in some Euclidean space are embedded graphs having a unique local realization (up to Euclidean motions) for the given edge lengths, although globally they may have several. We study the number of distinct planar embeddings of minimally rigid graphs with $n$ vertices. We show that, modulo planar rigid motions, this number is at most ${{2n-4}\\choose {n-2}} \\approx 4^n$. We also exhibit several families which realize lower bounds of the order of $2^n$, $2.21^n$ and $2.28^n$. For the upper bound we use techniques from complex algebraic geometry, based on the (projective) Cayley--Menger variety ${\\it CM}^{2,n}(C)\\subset P_{{{n}\\choose {2}}-1}(C)$ over the complex numbers $C$. In this context, point configurations are represented by coordinates given by squared distances between all pairs of points. Sectioning the variety with $2n-4$ hyperplanes yields at most $deg({\\it CM}^{2,n})$ zero-dimensional components, and one finds this degree to be $D^{2,n}=\\frac{1}{2}{{2n-4}\\choose {n-2}}$. The lower bounds are related to inductive constructions of minimally rigid graphs via Henneberg sequences. The same approach works in higher dimensions. In particular, we show that it leads to an upper bound of $2 D^{3,n}= {({2^{n-3}}/({n-2}})){{2n-6}\\choose{n-3}}$ for the number of spatial embeddings with generic edge lengths of the $1$-skeleton of a simplicial polyhedron, up to rigid motions. Our technique can also be adapted to the non-Euclidean case. [PUBLICATION ABSTRACT]

Many properties of the atomic nucleus, such as vibrations, rotations and incompressibility, can be interpreted as due to a two component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a ‘bubble’ structure, has been discussed in a recurrent manner since the 1970s. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus $^{34}$Si. The proton-to-neutron density asymmetry in $^{34}$Si offers the possibility to place constraints on the density and isospin dependence of the spin–orbit force—on which nuclear models have disagreed for decades—and on its stabilizing effect towards limits of nuclear existence.