Construction

For the construction of the TPC we relied much on the experience gained in 1998-2000 with the prototype TPC. The special feature of our chamber operation is the 10 bar pure hydrogen gas environment which makes the amplification and treatment of signals much more difficult than using conventional chamber gases. For a reasonable gain, it is necessary to apply typical voltages of $\sim$7 kV between anodes and cathodes.

In one respect, the setup with the prototype TPC and the chambers was not good enough for a real measurement of $\mu p$ capture: The required gas purity of 10$^{-8}$ could not be made using classical chamber materials and oil pumping. Typical impurity levels of 10$^{-5}$ to 10$^{-6}$ were measured leading to large transfer rates of muons to impurities and producing a significant distortion of the time slopes.

To overcome the impurity problem, we have totally redesigned the final chambers and apparatus applying the following measures:

• the pressure tank including all interior must be bakeable to 150$^o$C. The expansion coefficients for chambers must be chosen small and compatible with the tungsten wires
• only materials tested for low outgassing are used
• ultra-clean, oil free vacuum pumping of the TPC vessel with pressures reaching 10$^{-7}$ to 10$^{-8}$ mbar must be applied
• a vacuum pressure of 10$^{-5}$ to 10$^{-6}$ mbar must be maintained for a prolonged period, even after closing the high vacuum pumping

To reach these goals, we selected after extensive testing the following materials:

• quarz-glass frames for the TPC and the hydrogen chambers
• pointwelding of gold-tungsten wires onto metallized surfaces of the frames
• ceramic holding structures for TPC and wire chambers
• special clean soldering techniques for wires connecting the signal ports to the plugs mounted in the main flanges of the TPC vessel