Time projection chamber (TPC)

Fig. 4 in chapter 1 shows a stereo design drawing of our new time projection chamber. The TPC is mounted on the back flange of a cylindrical pressure tank (inner free length 56 cm, free diameter 28 cm) which will be filled with 10 bar hydrogen gas. The TPC, with sensitive volume $\Delta x\cdot\Delta y\cdot\Delta z$ = 15 x 12 x 30 cm$^3$, represents the hart of our detector system. It can reconstruct in three dimensions the ionisation tracks from charged particles, including tracks from minimum ionizing particles. This task is accomplished by a vertical homogeneous electrical field of $\sim$2.3 kV/cm over the entire TPC volume causing the negative charges to drift along the field lines toward multi wire proportional planes at the bottom. There, the charges are amplified by a typical factor of 5000 and read out by 74 anode wires in x-direction giving the z-coordinate and by 37 strip cathode wires in z-direction giving the x-coordinate. The y-coordinate, defining the height in the TPC, is determined by the drift time which ranges approximately from 0 to 24 $\mu s$.

The TPC will be operated at room temperature with ultra pure "protium" gas, i.e. hydrogen depleted of deuterium. Before the gas filling, the entire high pressure tank must be baked out at 150$^o$C and high vacuum pumped to ensure that the high gas purity can be reached and maintained over a prolonged run period. In the TPC, the exact stop location of each individual muon can be firmly determined, avoiding any wall stops. This is one of the essential requirements of our $\mu p$ capture measurement. The absolute time of the muons is measured with scintillation counter $\mu$SC in front of the TPC vessel. Muon tracking by the external and internal wire chambers ensures that the correct time can be unambiguously assigned to the muon track seen by the TPC.

The dynamical range of signal treatment is large enough, that the minimum ionizing electrons from muon decay can also be seen in the TPC. This feature is important for precise studies of diffusion processes which constitute a systematic problem (see section 4.3). It will be used to determine the residual deuterium content in the protium.

For the electron time determination, the measurements will rely entirely on the detectors outside the hydrogen pressure tank, i.e. on the two Sindrum chambers for directional back tracking and on the plastic hodoscope for the absolute time measurement. The separation of detector functions for electrons from those for muons ensures independent absolute time measurements without the dangers of electronic cross-talks.