Time Projection Chambers
The main tracking devices of the NA61/SHINE experiment are four large volume Time Projection Chambers (TPC). Two of them (Vertex TPCs: VTPC-1 and VTPC-2) are located in the magnetic field, two others (Main TPCs: MTPC-L and MTPC-R) are positioned downstream of the magnets symmetrically to the beamline. In addition, a smaller TPC (GAP-TPC) is mounted between the two VTPCs. It is centred on the beamline for measuring particles with the smallest production angles. The TPCs allow reconstruction of over 1000 tracks in a single Pb+Pb interaction. Up to 234 clusters and samples of energy loss per particle trajectory provide high statistics for precise measurements. The TPCs consist of a large gas volume in which particles leave a trail of ionization electrons. A uniform vertical electric field is established by a surrounding field cage made of aluminized Mylar strips that are kept at the appropriate electric potential by a voltage divider chain. The electrons drift with constant velocity under the influence of the field towards the top plate where their position, arrival time, and total number are measured with proportional wire chambers. In order to achieve high spatial resolution the chamber top plates are subdivided into pads of about one square centimetre area, a total of about 180'000 for all TPCs. From the recorded arrival times of the track signals and the known pixel positions one gets a sequence of 3-dimensional measured points along the particle trajectories.
Each Main TPC has a readout surface at the top of 3.9×3.9m² and a height of the field cage of 1.1m. It is filled with a gas mixture of Ar/CO2 in the proportion 95/5. The track signals are read out by 25 proportional chambers providing up to 90 measured points and ionization samples on each particle trajectory. The accuracy of the measurement of the average ionization energy loss for a particle is about 4%.
Each Vertex TPC consists of a gas box with 2.0×2.5m² top surface area and 0.67m depth. The inserted field-cage structures exclude the region of 0.12m on either side of the beamline in which the particle density in Pb+Pb reactions is so high that trajectories cannot be resolved. A gas mixture of Ar/CO2 in the proportion 90/10 is employed. The readout is performed by 6 proportional chambers on the top which provide up to 72 measurements and ionization samples on the particle trajectories. Between VTPC-1 and VTPC-2 an additional tracking device, the GAP-TPC, is located directly on the beamline. It covers the gap left for the beam between the sensitive volumes of the VTPCs and MTPCs. High momentum tracks can be better extrapolated back to the primary vertex using the additional points measured in the GAP-TPC. Particles originating from the primary vertex but measured only in the MTPCs can be better distinguished from conversion electrons faking high momentum tracks outside the magnetic field.