Heures thésardes: Scintillation signal study of XEMIS, a liquid xenon Compton Camera for 3γ imaging technique

YUWEI ZHU

Subatech (groupe Xénon)

jeudi 19 avril 2018

The XEMIS project (Xenon Medical Imaging System) which makes use of 3γ imaging technique and liquid xenon Compton camera, aims to make a precise 3D localization of a specific radioactive emitter and reducing drastically (100 times less) the injected activity to the patient in cancer diagnosis. The 3γ imaging is characterized by the simultaneous detection of 3 γ-rays emitted by 44Sc. The first prototype of a single-phase liquid xenon Compton camera, XEMIS1, has been successfully developed demonstrating the experimental feasibility of 3γ imaging technique.
XEMIS2, which is a larger scale liquid xenon cylindrical camera for small animal imaging, has been designed for preclinical application. The XEMIS2 camera contains two LXe time projection chamber (TPC). The principle of XEMIS2 is based on the measurement of both scintillation and ionization signals, which permits to get the position and deposited energy of the interaction between the ionizing particle and the liquid xenon.
My thesis mainly concentrates on scintillation signal study of XEMIS2. The importance of scintillation signal in XEMIS2 is embodied in several aspects: in the first place, it can provide the γ-rays interaction time, and by combining PMT signals with the additional information provided by the ionization signal, it is possible to reconstruct the Z position with 100 microns accuracy of each interaction point inside the detector. Furthermore, in order to reduce the occupancy of the TPC, the scintillation signals can be used for spatial pre-localization of γ-ray interactions allowing to handle a 10-fold higher activity in the active zone. To detect the scintillation signals, the active volume of XEMIS2 is surrounded by a set of UV-sensitive Hamamatsu photomultipliers. There is no added global trigger system for physic event selection in XEMIS2, the self-triggered PMT channels will be utilized. We are developing a scintillation signal detection chain which consists of a pulse-shaping amplifier and a constant fraction discriminator (CFD). This detection chain which provides the CFD time and the duration of the pulse has been tested and characterized in the prototype XEMIS1.
To validate the geometry of the field rings configuration and light collection with PMTs in XEMIS2, a prototyping has been done in XEMIS1. A simulation of both the electric field in the drift area and the light collection map leads to a compromise for the definition of the field rings spacing. XEMIS2 is now under qualification and the first image of a living small animal is foreseen at the CIMA center of the Nantes Hospital.