ALTO (Accélérateur Linéaire et Tandem d’Orsay) est une installation équipée de deux accélérateurs pour la recherche et les applications industrielles (un tandem de 15 MV et une accélérateur linéaire). Mon travail de thèse consiste à l’instrumentation pour la recherche fondamentale de la partie accélérateur linéaire d’ALTO qui fournir des faisceaux de noyaux radioactifs. Les faisceaux de noyaux radioactifs riches en neutrons sont produits par la technique de séparation isotopique en ligne (ISOL). Cette méthode de production permet trois types d’expérience : la mesure de masse, l’orientation nucléaire et les expériences de décroissances radioactives. Parmi ces trois types d’expériences, j’ai participé aux développements de deux nouvelles plateformes expérimentales dans le cadre du projet de l’instrumentation de l’installation ISOL d’ALTO. Le premier, BEDO (BEta Decay studies in Orsay) est un ensemble de détecteurs dédié à la spectroscopie β-γ des noyaux décroissants par désintégration β produits par ALTO. Je présente ici, la mise en fonctionnement de cette plateforme expérimentale, ses caractéristiques techniques et les développements d’outils permettant d’aboutir aux premiers résultats. Pour cette expérience un faisceau de la masse 82 a été produit, saisissant cette opportunité, une ré-investigation de la décroissance de ⁸²Ge vers ⁸²As a permis d’établir un nouveau schéma de niveaux pour ⁸²As et de donner les premières indications de la présence d’états issus de configurations intruses dans les isotones impair-impair N=49. Le second projet développé est POLAREX (POLARization of EXotic nuclei), il s’agit d’une plateforme expérimentale dédiée aux expériences d’orientation nucléaire. Mon travail traite ici de l’entière réhabilitation du cryostat à dilution ³He-⁴He (élément principal et le plus complexe de l’installation) et des développements techniques et R&D apportés à l’ensemble de la plateforme. L’ensemble de ces contributions a permis la validation du fonctionnement de l’installation avec les premières mesures physiques sur les noyaux de ⁵⁴Mn, ⁵⁶Co, ⁵⁷Co créés par activation d’une feuille de Fer avec des deutons produits par le Tandem. / ALTO (Accélérateur Linéaire et Tandem d’Orsay) is a facility composed of two accelerators dedicated to research and industrial applications. There is a 15 MV tandem and a linear accelerator. My PhD work was to develop the instrumentation of the linear accelerator part of ALTO which provides radioactive beams for fundamental research. These radioactive beams are produced using the Isotope Separation On-Line method (ISOL). This technique allows three kinds of experiments: mass measurement, nuclear orientation and radioactivity experiments. Among those three types of experiments, I worked on the development of two new experimental platforms for the ALTO instrumentation. The first one, BEDO (BEta Decay studies in Orsay) is an ensemble of detectors dedicated to β-γ spectroscopy of β-decaying nuclei produced by ALTO. I present in this thesis, the commissioning of this new experimental set-up, its technical characteristics and the tools development leading to the first results. For this commissioning experiment a mass 82 radioactive beam was produced, taking this opportunity the ⁸²Ge vers ⁸²As decay was re-investigated allowing to establish a new level scheme for ⁸²As and giving the first evidences for the presence of intruder states in the N=49 odd-odd isotones. The second project, which is developed, is POLAREX (POLARization of EXotic nuclei), a new facility for nuclear orientation experiments. My thesis deals with the entire reconditioning of a ³He-⁴He dilution refrigerator (major and most complex element of the facility) and R&D and technical developments of the platform. These contributions allowed the successful commissioning of the new experimental platform with the first physical measurements on ⁵⁴Mn, ⁵⁶Co, ⁵⁷Co created by activation of an iron foil with deuterons produced by the Tandem.

L’émission de paires de neutrons par les noyaux riches en neutrons 18C et 20O (isotones N = 12) est étudié par réactions de knock-out d’un nucléon des faisceaux secondaires 19N et 21O, peuplant ainsi des états non liés jusqu’à 15 MeV au-dessus de leur seuil d’émission deux neutrons. L’analyse des corrélations des triples coïncidences fragment+n+n montre que la décroissance 19N(−1p)→18C → 16C+n+n est clairement dominée par l’émission directe de paires. Les corrélations n-n, les plus grandes jamais observées, suggèrent la prédominance d’un coeur de 14C entouré de quatre neutrons arrangés en paires très corrélées. De plus, une importante compétition du mode de décroissance séquentiel est observée dans la décroissance 21O(−1n) → 20O → 18O+n+n, interprétée par la déformation causée par le knock-out d’un neutron très lié ayant pour effet de casser le cœur de 16O et ainsi de réduire le nombre de paires.De plus, les états non liés du 26F et 28F sont étudiés. Les deux systèmes étant peuplés par knock-out d’un nucléon du 27F dans le cas du 26F et du 29Ne ou du 29F pour 28F. Cinq états ont été observés pour 26F avec en particulier l’état de plus basse énergie (0.39 MeV) identifié comme l’état 3+ résultant du couplage d5/2 ⊗ d3/2 . Pour 28F, cinq états ont aussi été observés et l’état fondamental (200 keV) a été identifié comme étant de parité négative, plaçant ainsi 28F dans l’îlot d’inversion. / The emission of neutron pairs from the neutron-rich N = 12 isotones 18C and 20O has been studied by high-energy nucleon knockout from 19N and 21O secondary beams, populating unbound states of the two isotones up to 15 MeV above their two-neutron emission thresholds. The analysis of triple fragment-n-n correlations shows that the decay 19N(−1p) → 18C → 16C+n+n is clearly dominated by direct pair emission. The two-neutron correlation strength, the largest ever observed, suggests the predominance of a 14C core surrounded by four neutrons arranged in strongly correlated pairs. On the other hand, a significant competition of a sequential branch is found in the decay 21O(−1n) → 20O → 18O+n+n, attributed to its formation through the knockout of a deeply-bound neutron that breaks the 16O core and reduces the number of pairs.Moreover, unbound states in 26F and 28F have been studied. The two systems were probed using single-nucleon knockout reaction from secondary beams of 27F respectively in the case of 26F, and 29Ne and 29F for 28F. Five possible states have been identified in 26F, with in particular the lowest energy one (0.39 MeV) being identified as the 3+ state resulting from the d5/2 ⊗ d3/2 coupling. In the case of 28F, five unbound state have also been observed and in particular its ground state (200 keV) has been identified as a negative parity state, meaning that 28F is located inside the island of inversion.

L’étude de la spectroscopie et de la structure des noyaux exotiques a permis ces dernières années de nombreuses découvertes par rapport à ce qu’on connaissait sur les noyaux stables : les noyaux peuvent être déformés et certains nombres magiques ne sont pas persistants lorsque l’on s’approche des drip line. Autour du 68Ni, qui présente des signes de fermeture de sous couche à N=40 (énergie d’excitation du 2+1 élevée et B(E2 ;2+->0+) faible), les noyaux exotiques tels les isotopes de Cr, Fe, Zn et Ge, présentent des déformations et les expériences récemment menées indiquent un développement rapide de la collectivité. L’expérience menée au LNL (Laboratori Nazionali di Legnaro) sur les isotopes 70,72,74Zn a conclu à la mesure d’une durée de vie étonnamment longue pour l’état 4+ du 74Zn, menant à un rapport entre B(E2; 4+ -> 2 +) et B(E2; 2+ -> 0+) très faible, non expliqué par les calculs au-delà du champ moyen ou de modèle en couche. Cette expérience a été l’une des premières de la campagne du démonstrateur AGATA.Deux types de réactions directes sont utilisées pour extraire les facteurs spectroscopiques, quantités reliées au nombre d’occupation des couches. Les valeurs trouvées expérimentalement ne sont pas consistantes entre elles dans le cas d’arrachage d’un nucléon très lié au noyau. Cette différence entre les deux mesures expérimentales pourrait provenir du fait qu’on ne modélise pas correctement le mécanisme de réaction des réactions d’arrachage d’un nucléon. Des calculs basés sur une approche d’une cascade intra-nucléaire suivie d’une phase d’évaporation tendent à montrer les insuffisances dans la modélisation actuelle des réactions. / The study of exotic nuclei spectroscopy and structure allowed, in the last years, numerous discoveries in respect to what is known from stable nuclei: nuclei can be deformed and the magic numbers are not persistent when moving toward the drip lines. Around 68Ni, which presents signs of shell closures at N=40 (high excitation energy of the 2+1 and low B(E2)), exotic nuclei, like Cr, Fr, Zn and Ge, present deformation and recent experiments point to rapid development of collectivity. The experiment done at LNL (Laboratori Nazionali di Legnaro) on 70,72,74Zn isotopes concluded a surprisingly long life time for the 4+ states of 74Zn, leading to a ratio of B(E2; 4+ -> 2 +) to B(E2; 2+ -> 0+) very low, not explicated by calculations beyond ,mean field or shell models. This experiment was one of the first of the AGATA demonstrator. Two types of direct reactions are used to extract spectroscopic factors, quantity linked to the shell occupation number. The experimental values are not consistent between the cases for the removal of one deeply bound nucleon in the nuclei. This difference between the two experimental measures could come from an incorrect modelization of the reaction mechanism or nucleon removal. Calculations based on intra-nuclear cascade followed by an evaporation phase show shortcomings in the current reaction models.

Analisamos de maneira qualitativa as propriedades da base de estados do modelo simplético SCM, através de um cálculo em seu submodelo Sp(l,R) que aplicamos à descrição das propriedades espectroscópicas do núcleo 12 ANTPOT C. Através de um cálculo em uma base deformada, mostramos que a necessidade de considerarmos estados de base altamente excitados para descrevermos os observáveis pelo SCM, se devia a uma falta de otimização da base. A base otimizada é dada pela projeção em momento angular de estados de fonons deformados, determinados pelo método da projeção antes da variação. Esses fonons estão relacionados com as ressonâncias gigantes monopolares e quadrupolares. Mostramos que para descrevermos adequadamente as transições eletromagnéticas entre estados pertencentes a representações irredutíveis distintas de Sp(3,R) , era necessário considerar a mistura de representações. Através de um cálculo exploratório, analisamos o efeito dessa mistura nas transições monopolares e quadrupolares entre o estado intruso O IND 2 POT + , que é associado a uma representação prolata de Sp(l,R) e os estados da banda rotacional fundamental, que são associados a uma representação oblata de Sp(l,R). / In this thesis, we present a qualitative analysis of an Sp(l,R) calculation, applied to the description of the spectroscopic properties of 12 ANTPOT C . Based on a calculation in a deformed harmonic oscillator basis, we show that, the need to include highly excited basis states to describe the properties of 12 ANTPOT C, is due to a lack of optimization of the spherical basis. The optimized basis is given by the angular momentum projection of deformed phonon states, determined by the method of variation after projection, where the deformed phonons are associated to giant monopole and quadrupole resonances. We have also shown that, for an adequate description of the electromagnetic transitions between states belonging to different irreducible representations of Sp(3,R), it is necessary to consider mixture of representations. In a preliminary calculation we discuss the effect of this mixture to the monopole and quadrupole transitions between the intruder state 0 IND 2 POT + and the states of the ground rotational band associated, respectively, to a prolate and oblate representations.

As part of an ongoing program to study fundamental symmetries in nuclear physics, a thorough investigation into shell-model isospin-symmetry-breaking (ISB) calculation theory has been conducted using direct reactions to observe detailed nuclear-structure information. The work presented in this Thesis focuses on the 62Ga superallowed beta-decay system, and consists of two primary experiments; 1) A 64Zn(d,t)63Zn single-neutron transfer reaction, aimed at observing spectroscopic strengths to help guide calculation model-space truncations for the beta-decay wave function radial-overlap component of ISB, and 2) A two-neutron 64Zn(p,t)62Zn transfer to search for excited 0+ states in the daughter nucleus of 62Ga. The experiments were performed at the Maier-Leibnitz-Laboratory, on the joint campuses of the Ludwig-Maximilians Universitat and the Technische Universitat Munchen, in Garching, Germany. In total, 162 states in 63Zn were populated from the 64Zn(d,t) reaction, up to an excitation energy of 4.8 MeV, including the observation of 125 new levels, and unique spin/parity assignments for 92 states. As a result, this work provides the most complete picture for low-spin states in 63Zn to date. A comparison of the extracted S values to the predicted shell-model spectroscopic factors shows an overall over-prediction of strength for the 2p3/2 orbital, and a large disagreement for the 1f7/2 orbital above ~3.5 MeV. No significant 1g9/2 strength was observed, leading to the conclusion that the importance of the 1g9/2 orbital for ISB is small. Additionally, 67 states were observed in 62Zn using the two-neutron pickup mechanism, including the observation of five 0+ states. More than 99% of the total 0+ (p,t) cross-section is observed in the ground-state reaction channel, implying a nearly maximal overlap of the wave functions with the two-nucleon transfer operator. The dominance of the ground-state-to-ground-state (p,t) cross section is strikingly similar to the dominance of the superallowed Fermi beta-decay between isobaric-analogue 0+ states. This suggests that the population of excited 0+ states in the (p,t) reaction may reflect the population in the Fermi decay process, and can be used to guide future experimental and theoretical work. Further discussion of these results as they relate to the ISB correction calculations, and the implications for future theoretical work are presented in this Thesis. / This work was supported in part by the Natural Sciences and Engineering Research Council of Canada, the Ontario Ministry of Economic Development and Innovation, the DFG Cluster of Excellence `Origin and Structure of the Universe', and NSF grant PHY-1068217.

Within this thesis, nuclear resonance fluorescence experiments were analyzed which have been performed at the gamma ELBE facility of the Helmholtz-Zentrum Dresden-Rossendorf and the HI gamma S facility of the Triangle Universities Nuclear Laboratory. The dipole strength up to the neutron separation energy, its distribution as well as its split into electric and magnetic strength were determined. The influence of crucial nuclear parameters, like deformation and neutron excess, on the data was investigated. For the first time a whole set of enriched gaseous targets was measured in the energy region close to the neutron separation threshold. At ELBE the scattering of photons on four different isotopes 124, 128, 132, 134 Xe was investigated by irradiating containers with enriched target material with a broad bremsstrahlung distribution. The endpoint energies were chosen to be 12MeV. This ensures excitations up to the neutron separation threshold. The two isotopes 128, 134 Xe were measured in an additional campaign at HI gamma S facility. The region below the threshold was explored in detail in these experiments. A second, more model-independent determination of the cross section was possible. The work shows, how the measured spectra taken with high-purity germanium detectors, have to be corrected for several, partly overlapping effects in order to determine the complete excitation strength. The calculation of different backgrounds, detector response functions and the influence of inelastic scattering constitute the main part of the presented work. With the help of GEANT4 simulations the amount of not-nuclear scattered photons was estimated. GEANT4 was also used to test the influence of the extended targets on the detection efficiency and response. The code gamma DEX, which calculates deexcitation schemes based on statistical assumptions, was updated and finally used for the unfolding of the spectrum. The measured data is compared to different strength function models and a theoretical prediction based on a QRPA calculation. The summed strength is also set into comparison to other experimental data sets and a global trend for low-lying strength was found. This shows, that the nuclear deformation which has a large influence on the dipole strength above the threshold is only of minor impact for the strength at lower energies. Instead of this, the neutron excess seems to be the dominating factor for the strength in the investigated energy region. This work was supported by the German Research Foundation (DFG), Project No. SCHW883/1-1.

Le travail présenté dans ce manuscrit regroupe quatre études par spectroscopie gamma de quatre noyaux produits par une réaction de fission induite par protons de 25 MeV, sur une cible épaisse d'238U. Cette expérience a été menée au JYFL à Jyväskylä auprès du multidétecteur JUROGAM II. Dans ce manuscrit l'étude spectroscopique détaillée des noyaux : 84Se, 86Se, 83As et 98Y a été menée. Les trois premiers noyaux sont des noyaux sphériques se situant au voisinage de la couche magique N = 50. Leurs études peuvent permettre de mieux aborder l'évolution du gap de cette couche, mais aussi d'améliorer les interactions utilisées dans les calculs de modèle en couches permettant de reproduire leurs comportements. Huit nouvelles transitions ont pu être assignées à ces noyaux, ces résultats expérimentaux sont relativement bien en accord avec les calculs de modèle en couches effectués. Le noyau 98Y est un noyau possédant une coexistence de formes nucléaires. Alors que les états d'énergies inférieures à 500keV ont une configuration sphérique, les états situés au dessus sont déformés. Cinq nouvelles transitions ont été déterminées et ajoutées au schéma de niveaux existants, et deux calculs utilisant deux modèles collectifs ont été réalisés. Le premier utilisant le modèle GICM reproduit le comportement des états au dessous de 500keV et le deuxième QPRM reproduit les états prolate déformés permettant l'assignation d'un comportement sphérique à l'isomère (10-). / The work presented in this thesis includes four gamma-ray spectroscopy studies of four nuclei produced by fission induced by 25 MeV protons, on a thick target of 238U. This experiment was realized at JYFL in Jyväskylä with the multi-detector JUROGAM II. In this manuscript a detailed spectroscopic study of nuclei : 84Se, 86 Se, 83 As and 98Y was performed. The first three nuclei are spherical and they are on or adjacent to the N =50 shell closure. Their studies provide a better understanding of the possible decrease in energy of this shell closure, and also of the interactions used in shell-model calculations. Eight new transitions have been assigned to these nuclei and these experimental results are in a relatively good agreement with the shell-model calculations. In the nucleus 98Y a coexistence of nuclear shapes is observed. While the low-energy states below 500keV have a spherical configuration, higher lying states are deformed. Five new transitions were discovered and added to the existing levels scheme, and two calculations using two collective models have been made. The first set of calculations has used the GICM model to reproduce the behavior of states below 500keV and the second set of calculations has used QPRM to reproduce the prolate deformed states, and permitted the assignment of a spherical shape to the isomer (10-).

Cette thèse de doctorat vise l’examen critique de certaines théories de champ moyen nucléaire phénoménologiques, en se focalisant sur la description fiable des niveaux de particules individuelles. L’approche suivie ici est nouvelle en ce sens que elle permet non seulement la prédiction des valeurs numériques obtenues avec ce formalisme, mais également une estimation des distributions de probabilités correspondant aux résultats expérimentaux. Nous introduisons le concept des ≪erreurs théoriques≫, visant estimer, dans un cadre mathématique bien établi, les incertitudes relatives aux modélisations théoriques. Il est également introduit une notion subjective de pouvoir prédictif des Hamiltoniens nucléaires, qui est analysé dans le contexte des spectres énergétiques de particules individuelles. Le concept mathématique du ≪Problème Inverse≫ est appliqué aux Hamiltoniens de champ moyen réalistes. Cette technique permet la prédiction de propriétés du système partir d’un nombre limité de données. Afin d'approfondir notre connaissance des Problèmes Inverses, nous focalisons notre attention sur un problème mathématique simple. Une fonction dépendant de quatre paramètres libres est introduite afin de reproduire des données ≪expérimentales≫. Nous étudions le comportement des paramètres ≪fittés≫, leur corrélation, ainsi que les erreurs associées. Cette étude nous aide comprendre la signification de la formulation correcte du problème en question. Il nous montre également l'importance d'inclure les erreurs expérimentales et théoriques dans la solution. / This thesis is a critical examination of phenomenological nuclear mean field theories, focusing on reliable description of levels of individual particles. The approach presented here is new in the sense that it not only allows to predict the numerical values obtained with this formalism, but also yields an estimate of the probability distributions corresponding to the experimental results. We introduce the concept of ‘theoretical errors’ to estimate uncertainties in theoreticalmodels. We also introduce a subjective notion of ‘Predictive Power’ of nuclear Hamiltonians, which is analyzed in the context of the energy spectra of individual particles. The mathematical concept of ‘Inverse Problem’ is applied to a realistic mean-field Hamiltonian. This technique allows to predict the properties of a system from a limited number of data. To deepen our understanding of Inverse Problems, we focus on a simple mathematical problem. A function dependent on four free parameters is introduced in order to reproduce ‘experimental’ data. We study the behavior of the ‘fitted’ parameters, their correlation and the associated errors. This study helps us understand the importance of the correct formulation of the problem. It also shows the importance of including theoretical and experimental errors in the solution.

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