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Détection par velocity map IMAGing des Electrons émis par les Radiosensibilisants suite à la collision avec des Ions

Contact : Violaine Vizcaino
Doctorant : Nicolas Sens

- IMAGERI (velocity map IMAGing of emitted Electrons from Radiosensitizers upon Ion collision, ANR JCJC) led by V. Vizcaino :
The aim of the experimental set-up IMAGERI is to measure absolute singly or doubly differential cross section for ion induced electron emission from atoms, molecules or NPs using a velocity map imaging spectrometer. A schematic representation of the set-up is shown in Figure 1.

The projectile ion beam (keV to MeV ions) is shaped by a series of slits before entering the collision chamber. To compensate the deviation of the ion beam due the extraction field in the VMI spectrometer an electrostatic deflector is placed at the entrance of the spectrometer. The ion beam current and position are monitored by a Faraday cup and an ion beam profiler respectively situated in the chamber following the collision chamber. The target beam crosses orthogonally the projectile ion beam. The setup allows us to produce a wide range of target beams. A supersonic jet source allows for measuring electron emission from noble gas atoms, e.g. the well-defined electrons from auto-ionizing states of Ar+ emitted in the range from 1 eV to 20 eV are, for instance, used for calibration purposes. With the use of an effusive cell, we were also able to produce in the gas phase a steady effusive beam of biomolecules (e.g. uracil) as well as metallic atoms (e.g. silver). Moreover, we have developed a nebulizer – aerodynamic lens system dedicated to experiments on metallic NPs of various sizes (from nm to few tenths of nm). The target beam is shaped by two consecutive skimmers before entering the VMI spectrometer. A pneumatic push-pull shutter is placed between the two skimmers to block the target beam in order to perform, alternately, background (B) and signal + background (S+B) measurement. After crossing the VMI spectrometer, a water-cooled quartz µbalance allows monitoring the mass flux of the target beam and latter deduce the beam density. The electrons created in the collision region are extracted by a homemade multi-electrodes VMI spectrometer (see Figure 2). The electrons are detected by a 80mm position sensitive detector constituted of two microchannel plates (MCPs) and a phosphor screen. The images are recorded with a 2.35 megapixel CMOS camera. The images represent the 2D projection of the electron cloud produced by the ionization of the target and by applying an inverse Abel transform we can reconstruct the full momentum (energy and emission angle) of the emitted electrons in order to get the electron yield as a function of angle, energy or both.

In the past 3 years we have designed, built and characterised the experimental set-up, in particular the VMI spectrometer by determining the energy calibration as well as energy resolution. We also have validated our methodology by measuring the first absolute cross section for electron emission from uracil and adenine upon C4+ @1 MeV/u collision with this set-up . The NPs source is currently beeing tested and characterised independently and should be mounted on the set-up in a near future.