Download dbd

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.
Problem: production of the δ- electron number per 2D-read-out cell
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.
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.
3
best possible resolution
maximum possible D3/w
2
D3/w
100
2.5
1.5
10
1
Xe/He(40%)/CH4
(1atm)
Xe/He(65%)/CH4
(1atm)
Xe/CH4
(0.3atm)
Xe/CH4
(0.17atm)
0.5
energy reolution (%)
1000
O2 sensitivity=10 ppb, CH4 content=0.1 atm
1
0
The best performer was a low pressure mixture, with minimum content of all the components