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 d-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 lt1 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 sensitivity10 ppb, CH4 content0.1 atm
best possible resolution
maximum possible D3/w
The best performer was a low pressure mixture,
with minimum content of all the components