J. S. Stutzmann

On the operation of a micropattern gaseous UV-photomultiplier in liquid-Xenon

Operation results are presented of a UV-sensitive gaseous photomultiplier (GPM) coupled through a MgF2 window to a liquid-xenon scintillator. It consisted of a reflective CsI photocathode deposited on top of a THick Gaseous Electron Multiplier (THGEM); further multiplication stages were either a second THGEM or a Parallel Ionization Multiplier (PIM) followed by a MICROMEsh GAseous Structure (MICROMEGAS). The GPM operated in gas-flow mode with non-condensable gas mixtures. Gains of 104 were measured with a CsI-coated double-THGEM detector in Ne/CH4(95:5), Ne/CF4(95:5) and Ne/CH4/CF4 (90:5:5), with soft X-rays at 173 K. Scintillation signals induced by alpha particles in liquid xenon were measured here for the first time with a double-THGEM GPM in He/CH4(92.5:7.5) and a triple-structure THGEM/PIM/MICROMEGAS GPM in Ne/CH4(90:10) with a fast-current preamplifier.

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.

Scintillation detection with a gaseous photomultiplier for Compton imaging with liquidxenon

We report on the development of a liquid-xenon time-projection chamber, designed to validate a new medical imaging concept named “3γ imaging”. The advantages of detection gamma-induced scintillation light with a gaseous photomultiplier (GPM) operating in cryogenic mode are discussed; first results are presented.