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Recent heavy ion reaction studies aimed at elucidating the properties of low density matter produced in near Fermi-energies are discussed. At the lowest densities and temperatures large alpha mass fractions are observed. The symmetry free energies derived from isoscaling analyses are far above those obtained in common effective interaction calculations, reflecting cluster formation, primarily of alpha particles, not included in such calculations. Theoretical modeling of these results requires the consideration of cluster formation and medium modifications of cluster properties.

Experiments with 1-3 MeV He-4 beams and 2.2-3.2 MeV proton beams have been performed in order to develop a precise and highly sensitive method for the detection and analysis of light elements in threat or drug materials using simultaneously the RBS and ERDA techniques. Commercially available plastic films have been used as samples. The 1 MeV He-4 beam has been found to be inadequate owing to the rapid destruction of the targets. Best results have been obtained using proton beams with energies higher than 3 MeV. For 3.2 MeV proton beams, ERDA has been used for the analysis of the hydrogen constituent. (c) 2009 Elsevier B.V. All rights reserved.

Le rêve ultime du radiothérapeute a toujours été d?avoir à disposition des technologies capables de délivrer avec une parfaite précision des doses élevées aux volumes tumoraux sans irradier les tissus sains avoisinants. Ce rêve ne deviendra jamais réalité, mais tous les efforts des physiciens, médecins et industriels doivent être combinés pour que la réalité s?approche le plus possible de cette utopie. Depuis le début des années 60 et l?avènement des accélérateurs linéaires d?électrons montés sur des unités de traitement, la technologie en thérapie par photons a évolué énormément. Aujourd?hui, les traitements les plus à la pointe incluent la radiothérapie par modulation d?intensité (« Intensity-Modulated Radiation Therapy » ou IMRT) avec l?aide d?outils d?imagerie hautement sophistiqués. L?IMRT est une technique complexe qui requiert un excellent contrôle de la précision dans toutes les étapes du processus du traitement. Ces étapes peuvent être résumées en trois catégories qui sont 1) l?étalonnage et la stabilité de l?unité de traitement, 2) le positionnement et la qualité des données associées au patient et 3) la précision du calcul de dose effectué lors de la planification du traitement. Pour améliorer l?incertitude globale d?un traitement donné, des efforts de recherche sont nécessaires dans les trois catégories. Cette thèse se concentre sur le processus de calcul de dose. Durant les dernières décennies, la complexité et la précision des algorithmes de dose ont augmenté fortement grâce aux énormes progrès de l?informatique. Malgré cela, l?immense majorité des algorithmes utilisent des méthodes analytiques impliquant des approximations importantes au niveau de la physique de transport des particules. Il est cependant possible de concevoir des algorithmes qui évitent ces approximations en se basant sur des méthodes dites de Monte Carlo (MC) qui simulent fidèlement la réalité physique et qui sont consid????????reees aujourd??hui comme les plus précises de calcul de, The radiotherapy community's dream has always been to have available technologies being able to deliver a perfectly accurate high dose to tumor volumes without irradiating surrounding healthy tissues. This dream will never come true, but all the efforts of physicists, physicians and manufacturers must be combined to bring reality as close as possible to this utopia. Since the beginning of the 60s and the advent of electron linear accelerator mounted on treatment units, the technology in photon therapy has evolved tremendously. Nowadays, the state of art in photon treatments involves intensitymodulated radiotherapy (IMRT) with the help of highly sophisticated imaging devices. IMRT is a complex technique which requires a good control in the accuracy of all the steps involved in a radiotherapy treatment. Those may be summarized into 3 main categories: the calibration and stability of the treatment unit, the positioning and the quality of the data associated to the patient and the dose calculation process during treatment planning. To improve the overall uncertainty of a given treatment, research efforts are needed in the three categories. This thesis focuses on the dose calculation process. During the last decades, the complexity and accuracy of dose calculation algorithms increased with the huge improvements in computer technology. Despite this, the vast majority of the algorithms perform important approximations regarding the physics of the particles transport. However, it is possible to design algorithms with minor approximations based on Monte Carlo methods (MC), which faithfully simulate the physical reality and are considered as the most accurate to compute dose in human tissue. Unfortunately, until recently, the speed of MC simulations was too slow to be compatible with the time constraints associated with clinical routine. But the continuous improvements in computer power combined with the introduction of pertinent simplifications in the MC codes allowed for th, (SBIM 3) -- UCL, 2008

/sup 232/Th, /sup 237/Np, /sup 238/U, /sup 239/Pu, and /sup 241/Am targets were bombarded with proton beam of 26.5 and 62.9 MeV energies. In this contribution some preliminary results are presented and discussed. Comparison with the predictions of the nuclear-reaction model code TALYS is also shown., Anglais

Experimental analyses of moderate-temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon Zn-64 projectiles with Mo-92 and Au-197 target nuclei reveal a large degree of alpha particle clustering at low densities. For these gases, temperature- and density-dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A <= 4. At densities of 0.01 to 0.05 times the ground-state density of symmetric nuclear matter, the temperature- and density-dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean-field calculations and reflects the clusterization of low-density nuclear matter. The results are in quite reasonable agreement with calculated values obtained with a recently proposed virial equation of state calculation.

Le LHC est un accélérateur de hadrons conçu essentiellement pour la découverte du boson de Higgs supposé être à l'origine de la masse des particules élémentaires. Ce boson est cependant le maillon manquant au Modèle Standard. Au LHC, les protons seront produits avec une énergie de 7 TeV chacun et la luminosité totale atteindra 10^34 cm-2 s-1. CMS est l'une des quatre grandes expériences actuellement en construction autour du LHC. Sa partie centrale sera constituée de senseurs au silicium à micro-pistes. Ces détecteurs au silicium sont, à présent, les plus précis pour la reconstruction des trajectoires des particules chargées. Néanmoins, les conditions de fonctionnement du LHC créent un environnement très hostile qui pourrait affecter à long terme les performances du silicium ainsi que l'électronique qui lui est associée. De ce fait, il devient judicieux de contrôler les effets des radiations et de s'assurer du bon fonctionnement de ces détecteurs pour toute la durée de leur opération. Cette thèse s'inscrit dans le cadre des tests de résistance aux radiations des senseurs à micro-pistes de CMS. Une présentation générale de CMS, des senseurs au silicium à micro-pistes ainsi que des effets des dégâts radiatifs sur ces derniers sera donnée en premier lieu. Cette présentation sera suivie d'un descriptif des dispositifs d'irradiation et de caractérisation électrique développés spécialement pour tester ces senseurs. Enfin, les résultats obtenus seront présentés et discutés., (PHYS 3)--UCL, 2006

Neutron-induced fission cross-sections of Tl-205, Pb-204,Pb-206,Pb-207,Pb-208 and Bi-209 have been measured in the energy range from 30 to 180 MeV. The measurements were performed with quasi-monoenergetic neutron beams using a multi-section Frisch-gridded ionization chamber. The neutron-induced fission cross-sections of U-238 were used as reference data. The experimental techniques are described in detail as well as the data processing. The results are compared with existing experimental data. (c) 2006 Elsevier B.V. All rights reserved.

Measurements have been performed to determine inclusive cross sections (d2σddE, dσdE, dσd and σtot) for the production of neutral and light charged particles (LCP) induced by beams of protons of 26.5, 48.5 and 62.9 MeV and alpha's of 25.4, 45.5 and 57.8 MeV incident energies on natSi targets. The secondary LCPs (p, d, t, 3He, α, 6Li, 7Li and 7Be) were detected at angles from 10° to 165 ° in steps of ±10°. Neutrons emitted in these reactions were only recorded in coincidence with, at least, one detected LCP. Some comparisons between theoretical calculations based on the GNASH and TALYS nuclear reaction codes and the experimental results are presented and discussed. © 2005 American Institute of Physics., SCOPUS: cp.p, info:eu-repo/semantics/published

Cette dissertation présente les résultats des mesures des sections efficaces inclusives de production des particules chargées légères (protons, deutons, tritons, 3He, alpha, 6Li, 7Li et 7Be) ainsi que les multiplicités neutroniques associées, relevés lors des réactions nucléaires induites par protons et alpha sur une cible de natSi. Ces mesures ont été réalisées auprès du cyclotron CYCLONE de Louvain-la-Neuve aux énergies de 26.5, 48.5 et 62.9 MeV pour les réactions (p, (nat)Si) et de 25.4, 45.5 et 57.8 MeV pour les réactions (alpha, (nat)Si). Le système de détection, original dans sa conception, a été assuré par 15 télescopes constitués d'un assemblage de jonctions silicium et de scintillateurs inorganiques CsI assurant une couverture angulaire de 0 à 360°. Toutes les particules chargées étudiées dans ce travail ont été identifiées sur base de l'information de leurs pertes d'énergie dans les différentes jonctions et par la technique de l'analyse en forme des signaux lumineux issus des CsI. La détection des neutrons a été assurée par le multi-détecteur à neutrons DEMON constitué de 81 scintillateurs organiques liquides. Ces neutrons ont été identifiés par la technique de l'analyse en forme du signal lumineux. L'énergie des neutrons a été déduite par mesure de leur temps de vol. Les distributions angulaires des pics élastiques et inélastiques de la diffusion des protons et des alpha sur le 28Si ont été confrontées aux prédictions des modèles Optique, DWBA et Canaux Couplés, utilisant différents potentiels optiques. Une comparaison des résultats expérimentaux avec ceux déjà publiés sur des réactions similaires ou comparables a également fait l'objet de ce travail. De plus, les résultats ont été confrontés à ceux des prédictions des codes de simulation GNASH et TALYS. Une attention particulière a été portée aux prédictions du code TALYS. Enfin, ce travail fournit une très large et très complète base de nouvelles données expérimentales qui constitue un atout majeur et b, (PHYS 3)--UCL, 2005

The kinetic energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A and 47A MeV $^{64}$Zn with $^{58}$Ni, $^{92}$Mo and $^{197}$Au. For each system investigated, the double isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10-25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable to those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when AMD-V transport model calculations predict entry into the final evaporative or fragmentation stage of de-excitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed. Comment: 9 pages, 5 figures

This paper reports on the use of PIXE and RBS (resonant and non-resonant RBS) implemented with proton beams to simultaneously analyse light and heavy elements in materials of cultural heritage significance, as exemplified by Russian icons or in lead seals. It is shown that in spite of its poor mass resolution RBS with protons can provide useful information when combined with PIXE. In the case of Russian icons, it is possible to discriminate between an Au-Ag bilayer and an alloy of these metals in the gilds. However, when applied to lead seals RBS with protons encounters a significant limitation due to some deficiency in the available computer programs used for spectrum processing. (c) 2005 Elsevier B.V. All rights reserved.

La demande énergétique mondiale toujours en croissance, combinée aux préoccupations environnementales, constitue le moteur qui anime la recherche de nouvelles énergies. La fusion thermonucléaire est une des technologies qui offrent la possibilité d'une production d'énergie abondante, sûre et non polluante ; cependant son apprivoisement n'est pas encore au point ! La présence d'impuretés dans les plasmas est un des obstacles majeurs à vaincre pour atteindre les conditions nécessaires pour que la fusion thermonucléaire devienne suffisamment efficace. Leur densité dans le plasma dépend des matériaux de revêtement des parois et du contrôle des conditions aux limites, conditions qui définissent leur processus de génération et leur diffusion dans la région centrale. Les pertes énergétiques ont été considérablement réduites par l'utilisation de revêtements à basse charge nucléaire (Z), principalement ceux à base de carbone (comme le graphite). Ainsi, les chercheurs se sont intéressés aux effets des impuretés hydrocarbonées se dégageant des parois des réacteurs, à savoir leur structure et les voies de leur fragmentation, en particulier, les caractéristiques de leurs processus collisionnels avec les autres constituants du plasma. L'ion moléculaire est un composant essentiel de nombreux plasmas astrophysiques où l'ionisation et la dissociation se font principalement avec des électrons ; sa concentration est élevée dans l'ionosphère, région de l'atmosphère située à une altitude comprise entre 90 et 250 Km. Le rôle important joué par cet ion dans la modélisation ionosphérique nous a mené à réaliser une série de mesures des sections efficaces de ses différents processus collisionnels par impact d'électrons. La majorité des travaux expérimentaux antérieurs ne concernait que la molécule neutre. Ce travail de thèse a porté sur l'étude expérimentale de l'ionisation simple, de l'excitation dissociative et de l'ionisation dissociative par impact d'électrons des ions hydrocarbonés poly, Nous avons ainsi pu obtenir les sections efficaces absolues de plusieurs voies dissociatives. Nous avons identifié leur seuil d'apparition en tenant compte des structures de l'ion moléculaire primaire et des ions produits. Nous avons déterminé aussi les distributions d'énergie cinétique des fragments produits dans le référentiel du centre de masse à partir des distributions énergétiques observées dans le référentiel du laboratoire, nous avons ainsi pu estimer l'énergie maximale libérée lors de chacun des processus dissociatifs. Les résultats ont montré que la section efficace de l'ionisation simple est faible en comparaison avec les sections efficaces d'excitation et d'ionisation dissociative. Celle-ci dépend du nombre, généralement réduit, des états liés stables de l'ion doublement chargé. Ils ont montré aussi que les courbes des sections efficaces des processus d'excitation dissociative et d'ionisation ont une allure très différente. Dans le premier cas, elle augmente abruptement au voisinage du seuil alors que dans le second cas, elle augmente progressivement avec l'énergie des électrons. Pour les processus de dissociation, nous avons besoin d'une analyse angulaire et d'une détection en coïncidence des fragments produits, permettant ainsi de déterminer la nature exacte du mécanisme et d'identifier avec plus de précision l'état ou les états moléculaires impliqués dans chacune des réactions. Les résultats obtenus sont concluants et ont permis d'améliorer l'étude théorique semi-classique menée par Probst et Märk (2004) concernant l'ionisation de . Ils ont montré aussi que les prédictions empiriques de Janev et Reiter (2002) relatives aux familles d'hydrocarbures ne sont pas encore au point. Elles sont en désaccord avec nos résultats, montrant ainsi qu'il y a grand besoin d'autres études expérimentales des processus collisionnels des hydrocarbures et d'autres ions moléculaires avant de pouvoir formuler une quelconque loi d'échelle ou une théorie capable de prédire le, (PHYS 3)--UCL, 2004

An extensive experimental survey of the features of the disassembly of a small quasi-projectile system with $A \sim$ 36, produced in the reactions of 47 MeV/nucleon $^{40}$Ar + $^{27}$Al, $^{48}$Ti and $^{58}$Ni, has been carried out. Nuclei in the excitation energy range of 1-9 MeV/u have been investigated employing a new method to reconstruct the quasi-projectile source. At an excitation energy $\sim$ 5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the de-excitation processes. The fragment topological structure shows that the rank sorted fragments obey Zipf's law at the point of largest fluctuations providing another indication of a liquid gas phase transition. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is $8.3 \pm 0.5$ MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents $\beta$, $\gamma$ and $\sigma$ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point. Comment: Physical Review C, in press; some discussions about the validity of excitation energy in peripheral collisions have been added; 24 pages and 32 figures; longer abstract in the preprint

The critical behavior for the light nuclei with A$\sim 36$ has been investigated experimentally by the NIMROD multi-detectors. The wide variety of observables indicate the critical point has been reached in the disassembly of hot nuclei at an excitation energy of 5.6$\pm$0.5 MeV/u. Comment: 4 pages, 2 figures; Proceeding of 18th Nuclear Physics Division Conference of the Euro. Phys. Society (NPDC18) "Phase transitions in strongly interacting matter", Prague, 23.8.-29.8. 2004. To be published in Nuclear Physics A

The reaction systems, 64Zn + 58Ni, 64Zn + 92Mo, 64Zn + 197Au, at 26A, 35A and 47A MeV, have been studied both in experiments with a 4$\pi$ detector array, NIMROD, and with Antisymmetrized Molecular Dynamics model calculations employing effective interactions corresponding to soft and stiff equations of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained. The velocity distributions of $\alpha$ particles and fragments with Z >= 3 show distinct differences in calculations with the soft EOS and the stiff EOS. The velocity distributions of $\alpha$ particle and Intermediate Mass Fragments (IMF's) are best described by the stiff EOS. Neither of the above direct observables nor the strength of the elliptic flow are sensitive to changes in the in-medium nucleon-nucleon (NN) cross sections. A detailed analysis of the central collision events calculated with the stiff EOS revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with 197Au at 47A MeV a significant radial expansion takes place. For reactions with 58Ni and 92Mo at 47A MeV semi-transparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain. Comment: 34 pages, 43 figures

Thin Al foils of 50 mug/cm(2) thickness were implanted with He-4 ions with fluences ranging between 10(17) and 10(18) atoms/cm(2). The bulk He contents was measured by the Rutherford backscattering method. The He implantation profile versus depth was obtained by elastic recoil detection analysis. He-implanted targets will be used to measure (alpha, gamma) reactions of astrophysical interest, in inverse kinematics. (C) 2002 Elsevier Science B.V. All rights reserved.

Initial experiments at the ATLAS facility [Nucl. Instr. and Meth. B 92 (1994) 241] resulted in a clear detection of cosmogenic Ar-39 signal at the natural level. The present paper summarizes the recent developments of Ar-39 AMS measurements at ATLAS: the use of an electron cyclotron resonance (ECR) positive ion source equipped with a special quartz liner to reduce K-39 background, the development of a gas handling system for small volume argon samples, the acceleration of Ar-39(8+) ions to 232 MeV, and the final separation of Ar-39 from K-39 in a gas-filled spectrograph. The first successful AXIS measurements of Ar-39 in ocean water samples from the Southern Atlantic ventilation experiment (SAVE) are reported. Published by Elsevier B.V.

The L X-ray spectra of Ta, Au and Th induced by 0.6-4 MeV/nucleon Kr beams have been studied by means of a LiF (1 0 0)-equipped crystal spectrometer having an overall resolution of 30 eV at 8.146 keV. The mean energy shifts of the main L lines were measured and electron vacancy configurations deduced. At the highest beam energies L diagram lines have been observed and attributed to the internal conversion of transition energies between nuclear Coulomb excited levels. From the intensities of the main L lines, thick target yields and X-ray production cross sections are deduced. (C) 1998 Elsevier Science B.V.

The principal goal of this thesis is to highlight a possible "neutron skin" effect in the nuclear reaction mechanisms between heavy ions at energies around 6 MeV/nucleon. To this end, the reactions 58Ni+122Sn at 354 and 375.5 MeV and 64Ni+116Sn at 382.5 MeV were carried out at the CYCLONE accelerator in Louvain-la-Neuve. The projectile Ni and the target Sn were selected because of their magicity in proton. This characteristic implies the existence of a proton core more bound and of a neutron peripheral distribution in the isotopes 64Ni and 122Sn, rich in neutrons. The first goal was, to observe the possible influence of these neutrons on the fusion probability between the projectile and the target. In addition, the two types of reactions were selected in order to produce the same compound nucleus (180Pt) with the same excitation energy or with the same angular momentum. Thus, the effect of a neutron skin could be observed from the point of view of the production as from the point of view of the de-excitation of the compound nucleus. This de-excitation can proceed by evaporation of light particles or by fission. So, on the one hand, the fusion, fusion-evaporation and fusion-fission cross sections, and, on the other hand, the multiplicities of the light particles (neutrons, protons or alpha) emitted by the compound nucleus or by the fission fragments should have been compared. The experimental realization of this project required the use of the neutron detection system DEMON made up of 90 liquid scintillators laid out on a 4 m in diameter sphere which contain an aluminium chamber under vacuum. The chamber contained two multi-wire proportionnal counters XY intended for the detection of the fission fragments, two systems of Silicon junctions coupled with microchannels intended for the detection of evaporation residues and finally six triple Silicon telescope intended for the detection of the light charged particles (protons and alpha). At the end of the analysis, it was, L'objectif principal de cette thèse est de mettre en évidence un éventuel effet de "peau de neutrons" dans les mécanismes des réactions nucléaires entre ions lourds à des énergies de l'ordre de 6 MeV/nucléon. Dans ce but, les réactions 58Ni+122Sn à 354 et à 375.5 MeV et 64Ni+116Sn à 382.5 MeV ont été réalisées auprès de l'accélérateur CYCLONE de Louvain-la-Neuve. Le projectile Ni et la cible Sn ont été choisis en raison de leur magicité en proton. Cette caractéristique implique l'existence d'un coeur de protons plus liés et d'une distribution plus périphérique des neutrons dans les isotopes 64Ni et 122Sn, riche en neutrons. Il s'agissait donc, dans un premier temps, d'observer l'influence possible de ces neutrons excédentaires sur la probabilité de fusion du projectile et de la cible. D'autre part, les deux types de réactions ont été choisis afin de produire le même noyau composé 180Pt de même énergie d'excitation ou de même moment angulaire. Ainsi, l'effet de peau de neutrons pouvait être observé tant du point de vue de la production que du point de vue de la désexcitation du noyau composé. Cette désexcitation peut se faire par évaporation de particules légères ou par fission. Plus précisement, il s'agissait de comparer, d'une part, les sections efficaces de fusion, de fusion-évaporation et de fusion-fission, et, d'autre part, les multiplicités des particules légères (neutrons, protons ou alpha) émises par le noyau composé ou les fragments de fission. La réalisation expérimentale de ce projet a nécessité l'utilisation du système de détection de neutrons DEMON constitué de 90 scintillateurs liquides disposés sur une sphère de 4 m de diamètre au centre de laquelle une chambre en aluminium était mise sous vide. Celle-ci contenait deux chambres à gaz et à multi-fils XY destinés à la détection des fragments de fission, deux systèmes de jonctions au Silicium couplées à des galettes à micro-canaux destinés à la détection de résidus d'évaporation et enfin six triple-télésc, (PHYS 3)--UCL, 2003

Intermediate mass fragment (Z>2) emission in Sn-124,Xe-124+Sn-124,Sn-112 reactions at 28 MeV /nucleon were studied using neutron ion multidetector for reaction oriented dynamics, a 4pi charged particle detection system. A number of observables, such as isotopic yield distributions, energy spectra of light charged particles and intermediate mass fragments, isotopic and isobaric yield ratios, and average neutron to proton ratios are investigated. These observables show significant dependence on the isospin N/Z of the reacting system. It is observed that the formation of neutron-rich clusters are correlated with the excess neutrons in the composite system and depends on the temperature of the emitting source. The origin of light particles and fragments was studied through observations of rapidity distribution as a function of collision violence. With increasing centrality, the heavier He-6 isotope is found to be emitted closer to the midrapidity region than the lighter He-3 isotope. The emission of heavy fragments from the midrapidity region becomes increasingly favorable for fragments with higher charge Z . The results suggest that the midrapidity region is not only neutron rich but also a rich source of heavy fragment (cluster) formation.

Fission lifetime studies are performed using the "neutron-clock" technique associated with statistical and dynamical model calculations. In this framework we have undertaken, at the Louvain-la-Neuve cyclotron facility, the study of Ne-20+Tb-159 and Ne-20+Tm-169 induced fission reactions between E/A=8 and 16 MeV. In this work we have determined (a) fusion-evaporation and fusion-fission cross sections; (b) the prescission and postscission multiplicities of evaporated light particle (neutron, proton, and alpha) and their energy and angular distributions, and (c) the total energy balance of fission process after taking into account preequilibrium or incomplete-fusion processes. The comparison of the experimental data, multiplicities, and cross sections, with the statistical-model calculations incorporating simple aspects of the dynamics have enabled us to determine the ranges of fission lifetimes and the contribution of fast fission at different compound-nucleus excitation energies.

(PHYS 3)--UCL, 2002

Solid He-4 targets were prepared by implantation of He ions of various energies into thin and thick Al host foils. He concentrations and implantation profiles were analyzed through the proton backscattering technique taking advantage of the He-4(p,p)He-4 strong resonance around 2.2 MeV incident energy. The He retention was found to depend strongly on the Al foil thickness, but on the other hand it was found very stable versus the time after implantation. The consistency of the SIMNRA code was tested in different situations. (C) 2002 Elsevier Science B.V. All rights reserved.

Thin Al foils of 50 mu g/cm/sup 2/ thickness were implanted with /sup 4/He ions with fluences ranging between 10/sup 17/ and 10/sup 18/ atoms/cm/sup 2/. The bulk He contents was measured by the Rutherford backscattering method. The He implantation profile versus depth was obtained by elastic recoil detection analysis. He-implanted targets will be used to measure ( alpha , gamma ) reactions of astrophysical interest, in inverse kinematics., Anglais

The experimental setup of Louvain-1a-Neuve (UCL-Belgium) used to perform light-charged particle production experiment in fast neutron-induced reactions is presented. A short description of the neutron modular detector DEMON is also given. DEMON is a detector array for neutrons emitted in heavy ion induced reactions at low to intermediate energies.

Reaction mechanisms and multifragmentation processes have been studied for Zn-64 + Ni-58 collisions at intermediate energies with the help of antisymmetrized molecular dynamics (AMD-V) model calculations. Experimental energy spectra, angular distributions, charge distributions, and isotope distributions, classified by their associated charged particle multiplicities, are compared with the results of the AMD-V calculations. In general the experimental results are reasonably well reproduced by the calculations. The multifragmentation observed experimentally at all incident energies is also reproduced by the AMD-V calculations. A detailed study of AMD-V events reveals that, in nucleon transport, the reaction shows some transparency, whereas in energy transport the reaction is much less transparent at all incident energies studied here. The transparency in the nucleon transport indicates that. even for central collisions, about 75% of the projectile nucleons appear in the forward direction. In energy transport about 80% of the initial kinetic energy of the projectile in the center-of-mass frame is dissipated. The detailed study of AMD-V events also elucidates the: dynamics of the multifragmentation process. The study suggests that, at 35A MeV, the semitransparency and thermal expansion are the dominant mechanisms for the multifragmentation process, whereas at 49A MeV and higher incident energies a nuclear compression occurs at an early stage of the reaction and plays an important role in the multifragmentation process in addition to that of the thermal expansion and the semitransparency.

(PHYS 3)--UCL, 1999

ORION is a sectorized 4 pi neutron detector: part of it being used as a time-of-flight spectrometer. In a sector, six phototubes collect the light created during the neutron slowing down. The amount of charge integrated by each one strongly depends on the impact position of the neutron. Several tests have been performed with cosmic rays to study in detail the relation between the collected charge distribution and the particle impact position. The results are presented and compared with a simulation based on simple assumptions of light propagation in this particular medium. The detailed analysis has revealed a rather good resolution, better than 5 cm, and at the same time, regions of ambiguity where the correct position could not always be extracted. A new detector prototype, TOURNESOL, allowing one to overcome the present limitations is under construction. (C) 1998 Elsevier Science B.V. All rights reserved.

The neutron detection performances of the 4 pi ORION counter were investigated with monoenergetic neutron beams delivered by the Louvain-la-Neuve facility. Two responses are supplied by such a counter: a prompt response, arising from the slowing down of neutrons within the liquid scintillator contained in the detector, and a delayed response corresponding to the subsequent capture of the neutrons by Gd nuclei: with which the detector is loaded. Two experimental methods were used, allowing one to probe the responses for different neutron energies. Despite the large size of the detector, the prompt response is observed to be very similar to that exhibited by small cells. For both responses, a reasonable overall agreement is found between the data and the predictions of Monte-Carlo simulations. The timing resolution. of the order of 3 ns, makes this detector suitable for time-of-flight methods in applications requiring a high detection efficiency while accepting a moderate energy resolution. (C) 1998 Elsevier Science B.V. All rights reserved.

The L X-ray spectra of Ta, Au and Th induced by 0.6-4 MeV/nucleon Kr beams have been studied by means of a LiF (1 0 0)-equipped crystal spectrometer having an overall resolution of 30 eV at 8.146 keV. The mean energy shifts of the main L lines were measured and electron vacancy configurations deduced. At the highest beam energies L diagram lines have been observed and attributed to the internal conversion of transition energies between nuclear Coulomb excited levels. From the intensities of the main L lines, thick target yields and X-ray production cross sections are deduced. © 1998 Elsevier Science B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

The specific decay mode of hot nuclei surviving fission or fragmentation and leading in the final state to the production of a large residue has been studied in Ar+Au collisions at 60 A MeV. The temperature of these residues has been obtained by analysing the kinematical characteristics (angular and kinetic energy distributions) of the coincident neutrons detected by the DEMON neutron detector. A total cross section of 50 mb has been measured for the production of residues with an initial temperature of 6 MeV or more. This result is compared with the predictions of a microscopic transport model. © 1997 Elsevier Science B.V., SCOPUS: ar.j, info:eu-repo/semantics/published

The L X-ray spectra of Ta, Au and Th induced by 0.6 to 4 MeV/nucleon (MeV/n) 84Kr ions have been studied by means of a LiF(100) equipped crystal spectrometer with an overall resolution of 30 eV at 8.146 keV. The mean energy shifts of the main lines have been measured with accuracies ranging from 5 eV for well separated lines, to 30 eV for poorly resolved lines. Mean energy shifts per M and N spectator electron vacancies were deduced and compared with predictions of different models. The mean number of these vacancies at the various beam energies were estimated. Diagram lines were also observed and assigned to the internal conversion of transition energies between nuclear Coulomb excited levels. In the case of the Lα lines of Ta and Au and at the lower beam energies (0.6 and 1.2 MeV/n) the full width at half maximum of these lines was used to deconvolute the shifted peaks in the individual corresponding satellite lines. © Springer-Verlag 1997., SCOPUS: ar.j, info:eu-repo/semantics/published

The L X-ray spectra of Ta, Au and Th induced by 0.6 to 4 MeV/nucleon (MeV/n) Kr-84 ions have been studied by means of a LiF(100) equipped crystal spectrometer with an overall resolution of 30 eV at 8.146 keV. The mean energy shifts of the main lines have been measured with accuracies ranging from 5 eV for well separated lines, to 30 eV for poorly resolved lines. Mean energy shifts per M and N spectator electron vacancies were deduced and compared with predictions of different models. The mean number of these vacancies at the various beam energies were estimated. Diagram lines were also observed and assigned to the internal conversion of transition energies between nuclear Coulomb excited levels. In the case of the L alpha lines of Ta and Au and at the lower beam energies (0.6 and 1.2 MeV/n) the full width at half maximum of these lines was used to deconvolute the shifted peaks in the individual corresponding satellite lines.

The specific decay mode of hot nuclei surviving fission or fragmentation and leading in the final state to the production of a large residue has been studied in Ar+Au collisions at 60 A MeV. The temperature of these residues has been obtained by analysing the kinematical characteristics (angular and kinetic energy distributions) of the coincident neutrons detected by the DEMON neutron detector, A total cross section of 50 mb has been measured for the production of residues with an initial temperature of 6 MeV or more. This result is compared with the predictions of a microscopic transport model. (C) 1997 Elsevier Science B.V.

A 4 pi charged particle detector array with a low velocity threshold has been used to detect the products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Two methods were used for sorting these events with respect to their impact parameters and three methods were compared to determine the reaction plane. The transverse momentum analysis has been found to be the best method to extract the direction of the reaction plane for this system and for the experimental set-up used here. The energy of vanishing flow for central collisions has been found to be around 90-95 MeV/u. The azimuthal distributions of mid-rapidity particles exhibit a preferential in-plane emission and no squeeze-out effect.

The excitation energy sharing between the two partners of binary dissipative collisions was derived for the Ar-40 + Ag-nat system at 27 and 44 MeV/u. From the eventual equilibration of energy between the two partners, and their separation time obtained from Landau-Vlasov simulations, the energy relaxation time was extracted, and compared with theoretical predictions.

Doctorat en sciences physiques -- UCL, 1997

In Ar-36 + Al-27 reactions at bombarding energies E/A from 55 to 95 MeV, the mass and excitation energy of the quasi-projectile have been reconstructed at all impact parameters. Up to excitation energies E*/A of similar to 5-6 MeV, the mean kinetic energies of Z = 1 to Z = 9 fragments are in agreement with values predicted by sequential statistical decay, Above, a collective expansion energy is needed that grows with E*/A, reaching 1 to 2.2 MeV/u, depending on the de-excitation process, at E*/A similar to 10 MeV. It decreases with the fragment charge, The dominant role of excitation energy supports the scenario of thermal expansion.

Formation and decay of hot nuclei have been studied in Zn-64 + Ti-nat collisions between 35 and 79 MeV/nucleon. The mass and excitation energy of excited quasiprojectiles are reconstructed from the kinematical characteristics of their decay products. In central collisions, excitation energies larger than 10 MeV/nucleon are reached. Comparisons with theoretical predictions indicate that a fraction of the excitation energy is associated with an isotropic radial flow.

Light charged particles emitted from the reactions Zn-64+Al-27 at beam energies 49, 57, 69 and 79 MeV/u have been measured with a 4 pi multidetector array. The collective transverse momentum in the reaction plane (in-plane flow) is observed to vary strongly with the impact parameter and the emitted fragments. The flow parameter decreases monotonically from 49 to 79 MeV/u and falls to zero at a beam energy about 79 MeV/u in central collisions. This energy of vanishing flow increases with impact parameter and may be the same for different fragments. The disappearance of flow is also studied with the Boltzmann-Uehling-Uhlenbeck equation and it is demonstrated that the flow parameter and E(VF) are very sensitive to the in-medium cross section sigma(NN) and weakly sensitive to the nuclear compressibility in central collisions. Comparisons with experimental data indicate that sigma(NN) is around 35 to 45 mb in central collisions.

DEMON is a detector array for neutrons emitted in heavy ion induced reactions at low to intermediate energies. The neutron detection efficiency of some single DEMON modules has been measured in the energy range 8-21 MeV, with detection thresholds down to 0.1 MeV equivalent-electron (MeVee). The influence on the efficiency of cross talk, neutron-gamma and neutron-charged particle discrimination techniques was found to be negligibly small. Monte Carlo simulations have been performed and are compared with the experimental data. The relative weight of the interaction channel 12C(n,n′γ) in the organic liquid scintillator was determined experimentally. Taking into account observed saturation effects in the detector output, a calibration method is proposed for high light outputs, based on the energy loss of cosmic ray muons. © 1995., SCOPUS: ar.j, info:eu-repo/semantics/published

DEMON is a detector array for neutrons emitted in heavy ion induced reactions at low to intermediate energies. The neutron detection efficiency of some single DEMON modules has been measured in the energy range 8-21 MeV, with detection thresholds down to 0.1 MeV equivalent-electron (MeVee). The influence on the efficiency of cross talk, neutron-gamma and neutron-charged particle discrimination techniques was found to be negligibly small. Monte Carlo simulations have been performed and are compared with the experimental data. The relative weight of the interaction channel C-12(n,n'gamma) in the organic liquid scintillator was determined experimentally. Taking into account observed saturation effects in the detector output, a calibration method is proposed for high light outputs, based on the energy loss of cosmic ray muons.

A 4 pi multidetector array with a high efficiency has been used ia detect the charged products from reactions induced by Ar-36 on Al-27 at energies ranging from 55 to 95 MeV/u. Well characterized events were selected and sorted as a function of the impact parameter. Fusion or full stopping events have a very small cross section. Dissipative binary collisions accompanied by pre-equilibrium (mid-rapidity) emission, and followed by de-excitation of the quasi-projectile and quasi-target, were found to dominate for all impact parameters. The mass of the quasi-projectile has been reconstructed from its products and found to be slightly below the projectile mass. Isotropic emission in its rest frame has been observed at all energies and impact parameters, indicating that equilibrated nuclei may have been formed. The excitation energy of the quasi-projectile, determined via calorimetry, and the slope of kinetic energy spectra of alpha-particles were found to increase with incident energy and with decreasing impact parameter. Excitation energies per nucleon above II MeV are reached in a sizable cross section (70 mb). The mass distribution and kinetic energy spectra of emitted particles and fragments are essentially governed by the excitation energy, with a weak dependence on the impact parameter. The evolution of dissipative processes from deep inelastic collisions to the participant-spectator regime is discussed.

A method is proposed for estimating the energy of vanishing flow (also called balance energy, E(bal)) without reconstructing the reaction plane. This method is based on the shapes of experimental particle-particle azimuthal correlation functions. It is used for determining E(bal) for three systems: Ar + Al, Zn + Ti, Zn + Ni. The results are compared with estimations using flow-parameter analysis and with theoretical expectations.