L’imagerie 3γ est une nouvelle technique d’imagerie médicale nucléaire qui a été proposée au laboratoire Subatech. Cette technique consiste à localiser tridimensionellement la position de la ...désintégration d’un radioisotope innovant émetteur (β+, γ), le 44Sc. Il s’agit pour cela d’associer la détection des deux photons gamma de 511 keV issus de la désintégration d’un positon, assurée par une couronne de détecteurs de tomogaphie à émission de positon, à la détection dutroisième photon par un télescope Compton au xénon liquide. La position de l’interaction entre le photon et le xénon, ainsi que l’énergie déposée, sont relevées grâce à la mesure du signal d’ionisation à l’aide d’un chambre MICROMEGAS (MICROMEsh GAseousStructure), tandis que le déclenchement de l’acquisition et la mesure du temps de l’interaction sont assurés parla détection du signal de scintillation. Le principe de la TPC est ainsi utilisé pour l’imagerie Compton.Afin de faire la preuve expérimentale de faisabilité de l’imagerie 3γ, un prototype de petite dimension, XEMIS (XEnon Medical Imaging System), a été développé.Cette thèse constitue une étape importante vers cette preuve de faisabilité. Les travaux qui y sont exposés portent sur la caractérisation de la réponse du détecteur pour un faisceau de gammas de 511 keV et sur l’analyse des données qui en sont issues. La mesure des résolutions en énergie et temporelle seront exposés, ainsi que celle de la pureté du xénon liquide.
3γ imaging is a new nuclear medical imaging technique which has been suggested by Subatech laboratory. This technique involves locating three-dimensional position of the decay of an innovative radioisotope (β+, γ) emitter the 44Sc. The principle consist in the detection of two photons of 511 keV gamma rays from the decay of the positron, provided by a PET ring detector, associated to the detection of the third photon by a Liquid xenon Compton telescope. The energy deposited in the interaction between the photon and xenon and its position are identified by measuring the ionization signal with a MICROMEGAS chamber (MicroMesh Gaseous Structure), while the trigger and time measurement of the interaction are provided by the detection of thescintillation signal. The principle of the TPC is thus usedto Compton imaging.In order to demonstrate experimentally the feasibility of imaging 3γ, a small prototype, XEMIS (Xenon MedicalImaging System) was developed. This thesis is an important step towards the proof of feasibility. In this work are exposed the characterization of the detector response for a beam of 511 keV gamma rays and the analysis of data derived from it. The measurement of energy and time resolutions will be presented, as well as the purity of the liquid xenon.
Searches for the pair production of supersymmetric particles under the assumption that R-parity is violated via a single dominant LLEbar, LQDbar or UbarDbarDbar coupling are performed using the data ...collected by the ALEPH detector at LEP at centre-of-mass energies from 189 to 209Gev. The numbers of observed candidate events in the data are in agreement with the Standard Model expectation, and limits on the production cross sections and on the masses of charginos, sleptons, squarks and sneutrinos are derived.
Events containing only energetic photons are analysed in a sample of 628 pb -1 of data recorded from e+e- collisions at centre-of-mass energies between 189 and 209 GeV by the ALEPH detector at LEP. ...The $\reenunug$ and $\reeggg$ cross sections are measured and found to be in agreement with the Standard Model predictions. The number of light neutrino generations is determined to be N_\nu = 2.86 \pm 0.09. Upper limits are derived on the cross sections for photon production in the context of several supersymmetric models. Limits are also set on the parameters of models with extra spatial dimensions with contact interactions and with excited electrons.
An improved search for Bs oscillations is performed in the ALEPH data sample collected during the first phase of LEP, and reprocessed in 1998. Three analyses based on complementary event selections ...are presented. First, decays of Bs mesons into hadronic flavour eigenstates are fully reconstructed. This selection yields a small sample of candidates with excellent decay length and momentum resolution and high average Bs purity. Semileptonic decays with a reconstructed Ds- meson provide a second sample with larger statistics, high average Bs purity, but a poorer momentum and decay length resolution due to the partial decay reconstruction. Finally, semileptonic b-hadron decays are inclusively selected and yield the data sample with the highest sensitivity to Bs oscillations, as the much higher statistics compensate for the low average Bs purity and poorer time resolution. A lower limit is set at 10.9 ps-1 at 95% CL, significantly lower than the expected limit of 15.7 ps-1.
The production rates of the orbitally excited Ds** mesons, Ds1 and Ds2*, are measured with the 4.1 million hadronic Z decays recorded by the ALEPH detector during 1991--1995. The Ds** mesons are ...reconstructed in the decay modes Ds1+ -> D*+ K0, Ds1+ -> D*0 K+ and Ds2*+ -> D0 K+. The production rate of the Ds1 is measured to be n(Z->Ds1+-)=(0.52+-0.09+-0.06)%, under the assumption that the two considered decay modes of the Ds1 saturate the branching ratio. The production rate of the Ds2* is determined to be n(Z->Ds2*+-)=(0.83+-0.29+0.07-0.13)%, assuming that the branching fraction of the decay Ds2*+ -> D0 K+ is 45%. The production rates in Z->cc and Z->bbar decays are measured separately.
A search for events with a photon pair arising from the decay of a Higgs boson produced in association with a fermion pair, is performed in 893pb^-1 of data recorded by the ALEPH detector at LEP at ...centre-of-mass energies up to 209GeV. No excess of such events is found over the expected background. An upper limit is derived on the product of the e^+e^- -> HZ cross section and the H -> gamma gamma branching fraction as a function of the Higgs boson mass. A fermiophobic Higgs boson produced with the Standard Model cross section is excluded at 95\% confidence level for all masses below 105.4GeV/c2
A search for the pseudoscalar meson eta_b is performed in two-photon interactions at LEP~2 with an integrated luminosity of 699 pb^-1 collected at e+e- centre-of-mass energies from 181 GeV to 209 ...GeV. One candidate event is found in the six-charged-particle final state and none in the four-charged-particle final state, in agreement with the total expected background of about one event. Upper limits of Gamma_gammagamma(eta_b) * BR(eta_b -> 4 charged particles) 48 eV Gamma_gammagamma(eta_b) * BR(eta_b -> 6 charged particles) 132 eV are obtained at 95\% confidence level, which correspond to 95\% confidence level upper limits of $9.0\%$ and $25\%$ on these branching ratios.
