Bonus Radial Time Projection Chamber

Radial Time Projection Chamber

The RTPC planned for Bonus is shown in the figure above. The sensitive volume is a 20 cm long cylindrical section having inner diameter 8 cm and outer diameter 12 cm. The outer cylinder wall will actually be composed of a pair of concentric Gas Electron Multipliers (GEMs) inside an instrumented anode layer. The gaps between the GEMs and between the last GEM and the anode will be about 2 mm. The inner wall of the sensitive volume will serve as the drift electrode. A radial electric field between it and the first GEM will force the trail of ionization electrons to drift outward. These electrons will be amplified by the GEMs and collected by the patterned anode surface. The anode electrodes will be either pads (about 4mm x 5mm) or stereo strips. The readout electronics requirements are typical for a Time Projection Chamber (TPC): a high density of time-resolved analog sampling circuits. However, since the drift gap is only 2 cm deep, sample intervals as short as 100 ns or less may be required.

This short drift distance is one of the features that makes a Radial TPC a better choice than a conventional TPC in which ionization trails would drift to the ends of the cylinder. With a track latency time of only ~2 µs, the RTPC can handle the ~1 MHz rate of protons expected in the experiment without suffering pattern recognition problems. Radial drift is also compatible with the non-uniform character of the axial magnetic field that this detector must tolerate. Since the electric and magnetic field lines can be parallel over at most a tiny fraction of the drift volume, the Lorentz angle contribution to the electron drift paths will have to be accounted for during analysis of data from the TPC. This is true whether the electron drift is parallel or perpendicular to the cylinder axis. By using radial drift, thereby keeping the drift distances short, the Radial TPC will minimize this confusion.