Samuel Duval

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.

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.

Mapping 238U decay chain equilibrium state in thin sections of geo-materials by digital autoradiography and microprobe analysis

A new approach is proposed in order to spatially localize and determine the equilibrium state of natural decay chains on hand-scale geological samples, thanks to a combination of three techniques: 1) Elementary chemical mapping by microprobe; 2) Alpha autoradiograph by gaseous detectors and 3) bulk alpha particle spectrometry. The quantitative nature of alpha autoradiograph and its comparison with U chemical maps allows to locate radioactive equilibrium state in four samples. This equilibrium state was confirmed by alpha spectrometry analysis.

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.

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.