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Publication# Étude du tomographe de haute résolution pour petits animaux ClearPET par la méthode de Monte Carlo

Résumé

The Lausanne ClearPET demonstrator is one of the new generation of high resolution small animal PET scanners. A high resolution PET scanner aims to maximize the signal-to-noise ratio measured in pixels for a given time without compromising spatial resolution. In order to achieve it, ClearPET scanners are based on phoswich technology : two different crystals (LSO and LuYAP) are aligned one behind the other and coupled to the same channel of a multichannel photo-detector. Depth-of-interaction is determined by a pulse shape analysis. To improve the prototype design, a Monte Carlo simulation toolkit dedicated to emission tomography – GATE – was created. The accurate description of time-dependent phenomena such as source or detector movement and source decay kinetics represents the most important feature of this software. The first part of this work presents the demonstrator built in Lausanne, mainly the DAQ process and the libraries for the data treatment, and the GATE functionality. In the second part, the measurements obtained with the ClearPET demonstrator combined with simulations are presented. The simulations allow estimation of the performance of a final scanner and refinement of the detector head design. Measurements as well as simulations give a spatial resolution of 1.3 mm on the scanner axis and 2.5 mm at 4 cm from the axis. Temporal resolution for two modules with the same sampling phase is about 4.3 ns for LSO and 4.9 ns for LuYAP. For a standard acquisition, the energy resolution at 511 keV is about 31 ± 4 % for LSO and 33 ± 8 % for LuYAP. The peaks at full energy before calibration are about 480 ± 50 keV1 for LSO and 470 ± 40 keV1 for LuYAP. These variations, coupled with hardware threshold, are one of the main reasons for the sensitivity and count rate performance limitations. A sensitivity of 4.37 ± 0.05 is estimated for a full ring design with four rings of detector modules. The large systematic errors are induced by the variability previously mentioned. ------------------------------ 1 As spectra are not normalized, keV unities are inappropriate. Channel numbers are more accurate.

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Charles Vivant Ignacio Meneveau, Marc Parlange

A scale-dependent dynamic subgrid model based on Lagrangian time averaging is proposed and tested in large eddy simulations sLESd of high-Reynolds number boundary layer flows over homogeneous and heterogeneous rough surfaces. The model is based on the Lagrangian dynamic Smagorinsky model in which required averages are accumulated in time, following fluid trajectories of the resolved velocity field. The model allows for scale dependence of the coefficient by including a second test-filtering operation to determine how the coefficient changes as a function of scale. The model also uses the empirical observation that when scale dependence occurs ssuch as when the filter scale approaches the limits of the inertial ranged, the classic dynamic model yields the coefficient value appropriate for the test-filter scale. Validation tests in LES of high Reynolds number, rough wall, boundary layer flow are performed at various resolutions. Results are compared with other eddy-viscosity subgrid-scale models. Unlike the Smagorinsky–Lilly model with wall-damping swhich is overdissipatived or the scale-invariant dynamic model swhich is underdissipatived, the scale-dependent Lagrangian dynamic model is shown to have good dissipation characteristics. The model is also tested against detailed atmospheric boundary layer data that include measurements of the response of the flow to abrupt transitions in wall roughness. For such flows over variable surfaces, the plane-averaged version of the dynamic model is not appropriate and the Lagrangian averaging is desirable. The simulated wall stress overshoot and relaxation after a jump in surface roughness and the velocity profiles at several downstream distances from the jump are compared to the experimental data. Results show that the dynamic Smagorinsky coefficient close to the wall is very sensitive to the underlying local surface roughness, thus justifying the use of the Lagrangian formulation. In addition, the Lagrangian formulation reproduces experimental data more accurately than the planar-averaged formulation in simulations over heterogeneous rough walls.

2005The ring cyclotron of the Paul Scherrer Institute (PSI) accelerates an intense proton beam from 72MeV up to 590MeV. This happens in four cavities of very high quality factor, oscillating in the fundamental mode. The beam can excite parasitic oscillation modes (HOMs), because of its time structure. Measurements showed that their field can leak out into the vacuum chamber. Until now, there is no tool available to predict the potentially harmful effect of these HOMs onto the beam operation of the cyclotron. It is foreseeable that these effects might play a role if even higher beam currents have to be accelerated. This dissertation therefore deals with the numerical analysis and measurement of beam-cavity interactions. First calculations for a single cavity, interacting with a proton bunch were performed with MAFIA's eigenmode- (E3), time domain- (T3) and particle-in-cell (TS3) solvers. However, the structured grid and the limited computing performance of MAFIA make realistic simulations impossible. A simplified computation method is developed in this dissertation since a self-consistent simulation is impossible on today's computers: The parallel eigensolver Omega3P of the Stanford Linear Accelerator Center (SLAC) allowed us to calculate eigenmodes of the entire ring cyclotron for the first time ever. The rf fields are expanded onto a superposition of these modes and the excitation is calculated in frequency domain. Trajectories of the particles in the static magnetic field, superposed with the space charge fields and the beam excited HOMs, are then simulated. However, the quantitative accuracy of this model is still limited. On the one hand, because of the simplification in the geometry of the simulated rf structure, which otherwise would lead to a problem size going beyond the available computing resources. On the other hand, because it is not yet possible to simulate strongly absorbing boundaries more accurately. The simulation results confirm that up to proton beam currents of 2mA, corresponding to the routinely accelerated beam intensities, only a small deformation of the charge distribution appears. This thesis leads to a new simulation tool for further studies of intensity increases in high power cyclotrons.

Alain Sapin, Jean-Jacques Simond, Mai Tu Xuan, Roland Wetter

The LCI (Load commutated Inverter) is still today a suitable solution for supplying high power variable speed drive systems. The following main topics are described in this contribution: The modeling of a 20 MW LCI synchronous drive system for a compression train. The numerical simulation of the complete drive system. An original real time measurement of the air gap torque. A comparison between simulation results and measurement performed on the test shop of the manufacturer.

2002