Physics and technology of time projection chambers as active targets

Abstract

Many nuclear structure studies are presently performed with secondary radioactive beams to explore the nuclear chart far from stability. This entails a large decrease of available intensities, as compared to stable beam experiments, even with the highest power primary beam accelerators. To compensate for this intensity loss, thick targets and high detection efficiency are needed. Using secondary beams implies a change from direct kinematics to inverse kinematics: the target becomes the light reaction partner and the beam the heavy reaction partner. In quasi-elastic processes, the light reaction partner carrying most of the information emerges with very low recoil energy. In these conditions Time Projections Chambers (TPCs) filled with a gas that serves as both target and tracking medium (Active Target) have very unique properties that mitigate these difficulties. By construction, a TPC has a solid angle coverage of $ 4\pi$ , and even short tracks of low energy particles can be precisely measured if the readout has a high enough granularity. Due to the reaction characteristics large dynamics of the electronics and gas multipliers are needed, and specific data analysis methods of the 3-dimensional tracks have to be developed.