ACTAR: the new generation of active targets

ACTAR is the name of the new generation of active targets, to be used in reaction and decay studies with beams of exotic nuclei. These gaseous detectors work 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 allows for the use of a large target thickness, preserving the precision in the determination of the reaction kinematics; it ensures a high efficiency, coupled with a 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], the Maya active target [2-4] and IKAR at GSI [5]. 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 [6] and through experimental tests: 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 recent technologies such as GEM [7] and Micromegas [8]; and especially, the development of a new, modular electronics system, capable of recording, evaluating, transmitting and storing signals from a number of channels exceeding ten thousands. As a portable instruments, ACTAR will allow to fully exploit the opportunities offered by the upcoming radioactive ion beams facilities, such as, in Europe, SPIRAL2, REX-ISOLDE and FAIR.(author)