1. Temporal uranium records of oak tree rings (Quercus petraea) by (LA)-HR-ICP-MS from a former uranium mining site (Rophin, France) 2. Scintillation signal study of XEMIS, a liquid xenon Compton Camera for 3γ imaging technique

jeudi 19 avr. 2018, 14:00 → 15:30 Europe/Paris

Amphi GALOIS (Subatech - IMT Atlantique)

Chloé Therreau

1 Temporal uranium records of oak tree rings (Quercus petraea) by (LA)-HR-ICP-MS from a former uranium mining site (Rophin, France)

Dendroanalysis (i.e., the determination of the element contents in tree rings) is widely used in studies of environment and climate change. Uranium concentration in annual growth rings of oak (Quercus Petraea) trees was measured using both Laser Ablation (LA) and solution mode High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS). Direct uranium analysis of solid samples was performed using a New Wave UP213 nano-second ND:YAG laser ablation device coupled to an Element-XR HR-ICP-MS. Three tree samples taken for the present study were located upstream and downstream of a uranium mine site at Rophin (Puy de Dôme, France), with a mining operation period between 1949 and 1958. About thirty tons of uranium were extracted and thirty thousand tons of wastes were stored afterwards. Five dated growth rings years 1900, 1940, 1952, 1958 and 1990 chosen for the study cover the mining history of the site. The aim of the study is to assess the use of oak tree rings as biomonitors of past and recent uranium contamination from mining areas in order to contribute to a better understanding of the transfer of uranium from mining activities and thus its impact on the environment. In parallel, performances of both techniques (laser ablation vs solution mode) for uranium determination in tree ring samples will be evaluate in order to identify the best analytical tool for the purpose of the study.
Using the LA-HR-ICP-MS technique, two-dimensional (2D) mapping of uranium has been developed to study its spatial distribution at the surface of tree rings. Pressed pellets of the standard reference material NIST1570a spinach leaves and doped cellulose powder at concentrations ranging from0 to 400 ng.g−1 were used as uranium standards for quantification. The UP213 laser was operated in pre-ablation mode before analysis to overcome sample contamination during handling. 2D maps of uranium concentrations on one mm2 surface of tree rings were carried out in two hours with the following laser ablation parameters: ablation line scan, 5 μm.min−1 scan speed, 10Hz laser frequency, 80μm spot diameter and 50% output energy. Time resolved uranium signals were normalized to 13C internal standard signals. For HR ICP-MS solution analysis, tree rings were cut and acid digested by oven with the mixture 15.5 M concentred nitric acid/30% hydrogen peroxide. Uranium data show the discrepancy of both analytical techniques explained by the sample heterogeneity. However, higher concentrations of uranium are observed for downstream trees in growth rings covering the mine operation period.
Keywords: HR-ICP-MS, laser ablation, oak tree rings, uranium, 2D elemental mapping, mining activities.

Orateur: Yahaya Hassan Loni (Radiochimie)

2 Scintillation signal study of XEMIS, a liquid xenon Compton Camera for 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 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.

Orateur: Yuwei Zhu (Xenon)