Final setup of the experiment

An overview of the proposed final setup outlined in this document is shown in Figs. 3 and 4.

Figure 3: Stereoview of the final experimental setup (scintillator array, wire chambers and TPC vessel mounting pipe)

Figure 4: Inner part of the final experimental setup (flanges of pressure tank, entrance chambers and TPC) in stereoview from the back side

The central part of our detector is a time projection chamber (TPC) embedded in a pressure vessel filled with 10 bar of ultra-pure deuterium-depleted hydrogen (protium). The TPC which was specially developed for this experiment has a sensitive volume of 15 x 12 x 30 cm$^3$ and acts as active muon target monitoring all muon stops and electrons from muon decay. In front of the TPC, there are two planes of wire chambers to be used for tracking the incoming muons.

The pressure chamber has cylindrical walls made of thin Aluminum (d$\sim$2 mm) to keep effects from multiple Coulomb scattering of through-going decay electrons low. The vessel is horizontally supported from the back side by a strong tube that acts also as high vacuum pumping pipe. For servicing and preparation work, it can be removed into a back side parking position and baked out there at temperatures up to 150C. Ultra clean protium is filled via a specially developed gas system using chemical purifying methods. The gas can be circulated and purified during the measurements.

Surrounding the pressure tank, two existing Sindrum chambers (nr. 3 and 5) and a big array of plastic detectors are mounted. This assembly will be used for tracking the electrons back to the muon stop location. The electron detector employs cylindrical symmetry around the beam axis and exhibits an effective solid angle $\Omega /4\pi\sim$75%.

Precise and independent timing is provided for muons by two thin scintillators in front of the TPC vessel, and for electrons by the scintillator hodoscope surrounding the Sindrum chambers.

In comparison with our previous prototype setup [3] (see chapter 4), used in 1998-2000 to develop and test our prototype TPC with electronics and data acquisition, our final setup represents a significant improvement in many respects as follows:

• The hydrogen tank and its full interior (TPC and 2 entrance wire chambers) is made of clean materials (metals, ceramics, quarz-glass frames, etc) that can be baked out up to 150$^o$C and evacuated down to $10^{-7} - 10^{-8}$ mbar. This level should be sufficient to maintain a hydrogen purity below a fraction of 10$^{-8}$ over weeks.
• A high-purity gas handling system was developed in 1999/2000 and is now ready for protium production at impurity levels at or below 10$^{-8}$ (cf. chapter 3.3).
• The walls of the pressure tank will be thin Aluminum of thickness 2 mm to keep the effects of multiple Coulomb scattering low. This will allow to detect and track the electrons outside the pressure tank, thereby avoiding any electronic bias and cross-talk effects from the TPC signals.
• The electron detector consisting of two Sindrum chambers and a plastic array provides the necessary big solid angle ($\sim$75%) to produce a statistics of $\geq 10^{10}$ events required for achieving the proposed high precision results.
• A small coil will be installed outside the TPC vessel to generate a magnetic field of $\sim$70 Gauss perpendicular to the beam and muon spin axis. With this field, spin effects from polarized $\mu{^+}$ decays can be kept well under control. Spin asymmetries get also strongly reduced by the cylindrical symmetry of our setup.