Heures thésardes: Direct Measurement of Ionization from Electron Recoils for 3γ Imaging with a Liquid Xenon Compton Camera XEMIS

Yajing Xing

Subatech (groupe Xénon)

jeudi 26 avril 2018

The 3γ imaging is an innovative functional imaging modality, based on the detection in coincidence of three γ-rays by using on liquid xenon Compton camera and a (β+,γ) radionuclide emitter, for purpose of obtaining a 3D image with 100 times less activity than in current Positron-emission tomography (PET) systems. The ability to successfully obtain a high-quality image is greatly dependent on the spatial and energy resolution of the detector, the data analysis process and the image reconstruction method. A succession of R&D programs XEMIS1 and XEMIS2, designed to detect simultaneously the UV scintillation light and ionization signals from ionizing radiation, has been developed to explore the feasibility and advantage of this new camera.
The experimental results of the first prototype XEMIS1 shown a spatial resolution along the z-axis of roughly 100 μm for 511 keV photoelectric events with an ultra-low noise front-end electronics (below 100 e- RMS). XEMIS2, a larger single-phase liquid xenon cylindrical camera dedicated to 3D image of small animals, is now under assembly and qualification. More than 20000 pixels will be equipped for charge collection; in addition, a new self-trigger analog ASIC, XTRACT, has been developped for the sake of continuous read-out with negligible dead-time during 20 minutes.
The objective of this thesis work is to realize the direct measurement of ionization signal in order to obtain the deposited energy and location for each γ-ray interaction vertex in the TPC. The simulation of electrons recoils trajectory in LXe reproduced a shift of 100 μm for 511 keV, which may explain the spatial resolution of XEMIS1. Besides, a complete simulation and study with XEMIS1 is still ongoing for optimizing the ionization signal measurement. Characterizations of ionization signal using Monte Carlo simulation and data analysis have shown a good performance for energy measurement. At the present stage, the main work is to characterize the performance of the new electronic chain from front-end electronics to data acquisition in XEMIS1 and developing the XEMIS2 data analysis program before the operation of XEMIS2. The resultat of the first test at LXe temperature will be presented. The data analysis for resolution optimizing will become the core mission for resolution optimizing to reach the desired performances in times to come.