Active Target
Résumé
The ACTAR (Active TARget) collaboration is set out to develop and build new generation of active targets, to be used in reaction and decay studies with beams of exotic nuclei. The active targets are gaseous detectors that function as time-projection chambers (TPCs), recording the tracks of ionizing particles traversing the gas volume. Exotic particle-emission decay channels can be studied by stopping the radioactive nuclei in the gas; alternatively, by using the nuclei of the gas atoms as target nuclei, it becomes possible to study reactions induced by weak beams of exotic ions. The technique yields high efficiencies and allows for the use of a large target thickness with a very small target contribution to the energy resolution. Further, at the same time, the tracking preserves the precision in the determination of the reaction kinematics. Of particular importance is the low detection threshold. Applications of active targets are wide, ranging from direct reactions, such as inelastic scattering and nucleon transfer, to resonant reactions, and exotic decay modes. The validity of the method has been demonstrated by the recent results obtained with the TPC of the Bordeaux group [1] and with the Maya active target [2-4] The new generation of these detectors aims at improving significantly on the performance of the present devices. Several aspects are at study, both by using a specialized simulation package [5] and experimental techniques; an increase of the dynamic range of detected particles, obtained by magnetic confinement or through the use of ancillary detectors; electron amplification methods based on nouvelle technologies based on GEM [6] and Micromegas [7] to give high amplification and data rates; highly segmented pad structure (25 pads/cm²) to reach first-rate resolutions. The ACTAR project is attached to a program called GET (General Electronics for TPCs) where IRFU/Saclay, CENBG/Bordeaux, GANIL/Caen, MSU/Michigan and Riken-Kyoto are placing a common effort to build a generic, micro-electronic based, fully numeric and modular electronic system to cover the front-end to the data acquisition. Time-stamped pulse shape data will be collected on 12 or 14bit ADCs. The scale free system will be able to cover from 300 to 16,000 channels has a numeric multi-level trigger and on-line data treatment and reduction capabilities. Time-stamped data are presented to event-building software to give through-put at a maximum of 1000 events/sec. As a portable instruments, ACTAR or similar detectors will fully exploit the opportunities offered by the upcoming radioactive ion beams facilities, such at GANIL, NSCL ISOLDE, RIKEN and GSI.