High Pressure, Ultra-high Purity Hydrogen TPCs for Muon Capture Experiments*

Over the past two decades, the MuHe experiment, the MuCap and later the MuSun collaboration have developed a novel active target method based on high pressure time projection chambers (TPCs) filled with pure ³He, ultra-pure hydrogen (1% of liquid hydrogen (LH₂) density) or cryogenic deuterium gas (6% of LH₂ density). The TPCs were optimized for the specific requirements of measuring muon capture on the proton and light nuclei at the high intensity muon beams of PSI. The successful program reduced experimental uncertainties on the capture rates by typically one order of magnitude. The MuCap chamber employed gas amplification with anode and cathode at 4 mm pitch. The high density MuSun TPC was operated as an ionization chamber with anode pads read out with custom cyrogenic preamplifiers. Even with high drift fields of 10 kV/cm initial recombination is significant for nuclear recoils and shows a pronounced dependence on the track angle. In both chambers chemical purity at the ppb level was achieved with a continuous circulation and filter systems.