Double beta decay TPC

1. Double beta decay TPC High energy resolution of less then 1% due to electron counting technique GEANT4 simulation: tracks of double beta decay from Xe-136 in mixture Xe/CH4 (70/30) at 0.3 atm. electron number per 2D-read-out cell Problem: production of the δ-rays makes high spikes in the electron density which leads to high wire and strip occupancy worsening the energy resolution: High diffusion and low drift velocity mixture is necessary to reduce occupancy per read-out cell.

2. Finding the mixture for best resolution • The energy resolution is mostly defined by D3/w, D - diffusion coefficient, w - drift velocity (Garfield sim.) • and <1% can only be achieved at very low electric fields (1-3V/cm) • At a very low field the sensitivity to electronegative gas impurities plays crucial role. The lower the electric field, the better the energy resolution but worse the O2 tolerance: the electric field was chosen so that the O2 sensitivity is 10 ppb. • O2 sensitivity is the O2 content when 1% of electrons get attached. • The dipole molecules are worst quenchers in this case, they strongly increase O2 sensitivity. The attachment increase from CH4 is lower than from many other quenchers according to "Electron-Molecular Interactions and Their Applications", edited by L.G. Christophorou, Academic Press (1984), vol.1, p. 583. • Reducing CH4 content improves the energy resolution but adversely affects the quenching: CH4 content was 0.1 atm, constant. O2 sensitivity=10 ppb, CH4 content=0.1 atm best possible resolution maximum possible D3/w The best performer was a low pressure mixture, with minimum content of all the components