No description available.

No description available.

No description available.

No description available.

Th.--Sci. phys.--Grenoble 1, 1978. N°: 156. / Extr. en partie du Journal de physique, 38, 1977, 1051-1059.

Fusion reactions between 12C nuclei are among the most important in stellar evolution since they determine the destiny of massive stars (> 8 M). At thermonuclear energies (Ecm=1.5 ± 0.3 MeV), the 12C+12C reactions mainly proceed through 20Ne+α and 23Na+p channels. Since these energies are much lower than the height of the Coulomb barrier (Ecm= 6.1 MeV), the direct measurements of the 12C+12C reactions are very challenging because of the extremely small cross sections involved and the high beam-induced background originating from impurities in the targets. In addition, the 12C+12C reaction forms 24Mg at relatively high excitation energies (above the 12C 13.93 MeV thresholds) where molecular configurations are possible. Theoretical models fail to reproduce such structures and as a result, the extrapolation of high-energy cross section data towards the energy of astrophysical interest remain uncertain by 2-3 orders of magnitude. Further experimental efforts to measure at the lowest accessible energies are therefore in need. However, additionally to the extremely low cross sections and the resonant structure, the measurements at stellar energies of the 12C+12C reactions are troublesome due to natural hydrogen and deuterium contamination in the carbon targets. These target contaminants hamper the measurement of the 12C+12C process in all exit channels given that the 12C+1,2H reactions cross sections are considerably higher than that of the reaction of interest. In consequence, the use of ultra-low H content graphite targets and a study of the target behaviour under beam bombardment are necessary. This work focused on the experimental measurements of the 12C(12C,p)23Na and 12C(12C,α)20Ne reactions using charge particle detection. Although both channels were measured, only the proton channel was analysed and discussed in this thesis due to time constrains. The experiment was performed at the 3 MV pelletron tandem accelerator of the CIRCE (Centre for Isotopic Research on the Cultural and Environmental heritage) laboratory in Caserta, Italy. The experimental approach involved the development of optical calculations for optimal beam transportation (using the software COSY), the use of a four ΔE-Erest detectors system (a variable pressure CF4 ionization chamber used as the ΔE detector and a 300 mm2 Si detector used as the Erest) called GASTLY (GAs Silicon Two-Layer sYstem) and a study of the deuterium (hydrogen does not contribute to the beam-induced background at the detection angles and beam energies used here) contamination in graphite targets. The GASTLY detectors were placed at backward angles (121, 143 and 156o respect to the beam axis) and the 12C+12C reactions were investigated using carbon beams of Ecm=4.30 - 2.52 MeV with intensities of the order of μA. Highly Ordered Pyrolytic Graphite (HOPG) and highly pure (99.8%) natural graphite targets were used for the deuterium contamination study. A thermocamera was used to constantly monitor the target temperature during beam bombardment, allowing the investigation of target's deuterium content as a function of target temperature. Results showed a decrease in target's deuterium content of 53-80% in the target's temperature range of 200-1200 °C, depending on the type of target and detection angle. Furthermore, it was found that surrounding the scattering chamber with a nitrogen atmosphere while measuring low counting rate reactions (such as 12C+12C at low energies), the HOPG target's deuterium content decreases to about half its original value for a target temperature in the range between 800-1100 °C. For the 12C+12C reactions measurements, the HOPG target was used, maintaining high target temperatures. The p0-6 proton groups of the 12C(12C,p)23Na reaction were analysed and their yields, cross sections and astrophysical S-factors were obtained and are presented in this thesis. A comparison with previous data available in the literature is also presented, together with an indication for possible improvements in future investigations.

Les hydrocarbures aromatiques polycycliques (HAP) sont des composés organiques présents dans l’ensemble des compartiments environnementaux. Dans l’atmosphère, leurs sources sont à la fois naturelles (feux de biomasse, éruptions volcaniques) et anthropiques (industrie, transport, chauffage résidentiel). Une fois émis, sous forme gazeuse ou adsorbés à la surface de particules atmosphériques, les HAP sont susceptibles d’être impliqués dans des processus physico-chimiques tels que la photodégradation et/ou des réactions d’oxydation avec différentes espèces radicalaires. Du fait de leur toxicité avérée, ces composés font l’objet de différentes réglementations, législations françaises et européennes. Concernant le compartiment atmosphérique, seul le benzo(a)pyrène présente aujourd’hui des seuils d’émission à respecter. Afin de faire évoluer ces textes et de mettre en place des mesures de réduction d’émissions, diverses approches ont été développées dans le but de différencier leurs sources dans l’atmosphère. L’approche moléculaire, basée sur les profils moléculaires et les rapports de concentrations, permet d’apporter des informations quant à leurs origines. En revanche, elle souffre de biais induits par les conditions de formation des HAP (température, conditions environnementales…) et par les processus physico-chimiques dans lesquels ils sont impliqués. L’objectif principal de ce travail est de mettre en place une méthodologie de traçage de sources des HAP particulaires par une approche isotopique. Le développement du protocole analytique a été réalisé pour déterminer la composition isotopique moléculaire des HAP particulaires par GC/C/IRMS. Il a été montré que la réactivité des HAP sous l’action d’oxydants (O3, NO2, OH) et/ou de la lumière solaire n’induisait pas de variation significative de la composition isotopique moléculaire des HAP. Cette méthodologie a ainsi pu être appliquée sur des échantillons naturels, prélevés sur des sites caractérisés par des sources spécifiques. Il a été montré que les 13C/12C des HAP, en complément de données moléculaires, permettent de différencier les origines de ces composés. Par exemple, les caractéristiques moléculaires et isotopiques de HAP issus de la combustion de plusieurs espèces de bois d’origine méditerranéenne ont été déterminées en appliquant cette méthodologie à des échantillons collectés directement à l’émission. Enfin, dans le cadre de l’étude de la pollution et de ses impacts dans le Bassin d’Arcachon, les apports atmosphériques en HAP ont été mesurés par l’approche moléculaire couplée à d’autres outils (rétrotrajectoires, oxydants, roses des vents…) afin de compléter le diagnostic environnemental. / Polycyclic Aromatic Hydrocarbons (PAH) are carcinogenic compounds, present in all the compartments of the Environment. In the atmosphere, their sources are both from natural (biomass burning, volcanic emissions...) and anthropogenic (transport, industry, residential heating...) origins. Once emitted in the atmosphere, PAH are distributed between the gaseous or particulate phases and may be involved in different physico-chemical processes such as photodegradation, radical-initiated oxidations... Due to their carcinogenicity, PAH emissions are nowadays subjected to various regulations from France and more largely, European Union. In the atmosphere, benzo(a)pyrene has been selected as representative of the PAHs because of its high toxicity. In order to improve regulations involving emission reductions, several methodologies have been developed to perform source apportionment. The most commonly used in the literature is the molecular approach, based on molecular profiles and particular ratios. Nevertheless, conditions of PAH formation and physico-chemical processes affect these characteristic values. The main objective of this work was to develop a new methodology of particulate-PAH source tracking based on the molecular isotopic composition. The development of analytical procedure was performed to determine 13C/12C of PAHs by GC/C/IRMS. The study of the impact of PAH reactivity in the presence of O3, NO2, OH and/or solar radiations shows that no significant isotopic fractionation is induced on their isotopic compositions. Molecular isotopic approach was applied on natural particles, collected at different specific sites: 13C/12C of PAHs and molecular data allow differentiating particulate-PAH sources. Therefore, determinations of molecular and isotopic characteristics have been undertaken by applying this methodology on particulate-PAHs emitted during the combustion of fifteen Mediterranean woods. Finally, molecular approach coupled with different parameters (back-trajectories, oxidant concentrations, wind roses...) enables to measure the levels of PAH concentrations in the atmosphere in order to evaluate their impacts as a source of pollution in the Arcachon Bay.

<p>Neutron-carbon interactions have been studied with a focus on charged-particle production of relevance to radiation protection and medical applications, such as cancer therapy. The measurements have been performed using the particle-detection setup, MEDLEY, and the 96 MeV neutron beam at the The Svedberg Laboratory in Uppsala.</p><p>Double-differential cross sections of inclusive charged-particle production are compared with recent calculations from models based on the GNASH code including direct, preequilibrium and compound processes. For protons, the shapes of the cross-section spectra are reasonably well described by the calculations. For the other particles- <i>d</i>, <i>t</i>, ³He and α- there are important discrepancies, in particular for ³He-ions and α-particles, concerning both shape and magnitude of the spectra.</p><p>Using the new cross sections, partial as well as total KERMA coefficients have been determined. The coefficients have also been compared to previous experimental results and model calculations. The <i>p</i>, <i>d</i> and <i>t</i> KERMA coefficients are in good agreement with those from a previous measurement. For the helium isotopes, there are no previous measurements at this energy. The KERMA coefficients are considerably higher (by up to 30%) than those predicted by the calculations.</p><p>The KERMA results indicate that protons and α -particles are the main contributors to the dose. A 6x6x6 cm³ carbon phantom, exposed to a broad and a pencil-like beam, is used for the computation of the absorbed doses deposited by these two particles in spheres of 1 μm in diameter, located at various positions in the phantom. The maximum doses are deposited at ~3 cm from the surface of neutron impact for protons and within 1 cm for α-particles. For the pencil beam, deposited doses are spread over regions of ~1.5 cm and ~300 μm transverse to the beam for protons and α-particles, respectively. The results are consistent with previous integral measurements at lower energies.</p>

Neutron-carbon interactions have been studied with a focus on charged-particle production of relevance to radiation protection and medical applications, such as cancer therapy. The measurements have been performed using the particle-detection setup, MEDLEY, and the 96 MeV neutron beam at the The Svedberg Laboratory in Uppsala. Double-differential cross sections of inclusive charged-particle production are compared with recent calculations from models based on the GNASH code including direct, preequilibrium and compound processes. For protons, the shapes of the cross-section spectra are reasonably well described by the calculations. For the other particles- d, t, 3He and α- there are important discrepancies, in particular for 3He-ions and α-particles, concerning both shape and magnitude of the spectra. Using the new cross sections, partial as well as total KERMA coefficients have been determined. The coefficients have also been compared to previous experimental results and model calculations. The p, d and t KERMA coefficients are in good agreement with those from a previous measurement. For the helium isotopes, there are no previous measurements at this energy. The KERMA coefficients are considerably higher (by up to 30%) than those predicted by the calculations. The KERMA results indicate that protons and α -particles are the main contributors to the dose. A 6x6x6 cm3 carbon phantom, exposed to a broad and a pencil-like beam, is used for the computation of the absorbed doses deposited by these two particles in spheres of 1 μm in diameter, located at various positions in the phantom. The maximum doses are deposited at ~3 cm from the surface of neutron impact for protons and within 1 cm for α-particles. For the pencil beam, deposited doses are spread over regions of ~1.5 cm and ~300 μm transverse to the beam for protons and α-particles, respectively. The results are consistent with previous integral measurements at lower energies.

Th. 3e cycle--Phys. nucl.--Grenoble 1, 1981. N°: 17.