O. Lemaire

Simulation of liquid xenon time projection chambers and 3γ camera with GATE

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. In order to demonstrate experimentally the feasibility of 3γ imaging, a first prototype, XEMISI (XEnon Medical Imaging System) was developed. The last published results of XEMISI, including the energy resolution obtained in the LXe and the very low electronic noise are very promising. The next step is to construct new prototype XEMIS2, which is a full liquid xenon cylindrical camera, dedicated to small animal imaging. To assess the performances of this camera for 3-gamma imaging, we developed a simulation of such a system using GATE (Geant4 Application for Emission Tomography). The simulation of XEMISI shows good data-simulation agreement. Based on this study, we are confident in the simulation of XEMIS2 and we expect a spatial resolution of ~500 J.lm and an energy resolution better than 5% depending on the electric field. Moreover, the full efficiency of the 3-gamma imaging technique has been estimated to ~5 % by simulating a rat phantom.

Measurement of the Transverse Diffusion Coefficient of Charge in Liquid Xenon

Liquid xenon (LXe) is a very attractive material as a detection medium for ionization detectors due to its high density, high atomic number, and low energy required to produce electron-ion pairs. Therefore it has been used in several applications, like γ detection or direct detection of dark matter. Now Subatech is working on the R & D of LXe Compton telescope for 3γ medical imaging, which can make precise tridimensional localization of a (β+, γ) radioisotope emitter. The diffusion of charge carriers will directly affect the spatial resolution of LXe ionization signal. We will report how we measure the transverse diffusion coefficient for different electric field (0.5 ~ 1.2 kV/cm) by observing the spray of charge carriers on drift length varying until 12 cm. With very-low-noise front-end electronics and complete Monte-Carlo simulation of the experiment, the values of transverse diffusion coefficient are measured precisely.

Scintillation Signal in XEMIS2, a Liquid Xenon Compton Camera with 3γ Imaging Technique

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 to reduce 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 which is a (β+, γ) emitter. The second prototype XEMIS2 is a liquid xenon cylindrical camera for small animal imaging. The active volume of XEMIS2 is surrounded by a set of VUV-sensitive Hamamatsu photomultipliers, for the scintillation signals detection. A pulse-shaping amplifier was tested in XEMIS1 for the readout of the scintillation signal of the PMT. The typical output pulse shows a relatively good performance of the pulse-shaping amplifier providing a possible solution for XEMIS2 scintillation DAQ. Meanwhile, the pulse-shaping amplifier and the constant fraction discriminator (CFD) have lay the foundation of the preliminary design of XEMIS2 scintillation signal detection chain.

XEMIS: Liquid Xenon Compton Camera for 3γ Imaging

An innovative liquid xenon Compton camera project, XEMIS (XEnon Medical Imaging System) has been proposed by SUBATECH laboratory, for a new functional medical 3γ imaging technique based on the detection in coincidence of 3 γ-rays. The purpose of this 3γ imaging modality is to obtain a 3D image using 100 times less activity than in current PET systems. The combination of a liquid xenon time projection chamber (LXe TPC) and a specific (β+, γ) radionuclide emitter 44Sc is investigated in this concept. In order to provide an experimental demonstration for the use of a LXe Compton camera for 3γ imaging, a succession of R&D programs, XEMIS1 and XEMIS2, have been carried out using innovative technologies. The first prototype XEMIS1 has been successfully validated showing very promising results for energy, spatial and angular resolutions with an ultra-low noise front-end electronics. The second phase dedicated to a 3D imaging of small animals, XEMIS2, is now under installation and qualification, while the characterizations of ionization signal using Monte Carlo simulation has shown preliminary good performances for energy measurement.

Development of a readout electronic for the measurement of ionization in liquid xenon compton telescope containing micro-patterns

We report on the electronic dedicated to the acquisition of the ionization current signal induced by the interactions of γ-rays inside a Compton telescope with Liquid Xenon. In order to achieve sub-millimeter resolution we successfully adapted an existing ASIC originally designed for semi-conductor detector with low input capacitance and low dark current called IDeF-x-Lxe, and fabricated in a standard 0.35 μm CMOS technology. With a copper pad area of 0.25 in2 of our first prototype, and a Micromegas micromesh located 50 μm above the anode used as Frisch grid, the noise measurement shows an ENC of ~100 erms- thanks to a meticulous setup. Each part of the coupling between the detector and the electronic has been specially tailored to cope with temperature constraints and with noise requirements.