GEM Discharge studies

The occurrence of discharges in gas detectors is correlated with the transition from avalanche to streamer.
The transition depends on the voltage and ionization density.
In this case the total charge created by the multiplication processes could exceed the threshold value (Raether limit, 10^7-10^8 e- I+ pairs) for the transition from avalanche to streamer.
Due to the very small anode-cathode distance in GEM detectors, the transition from avalanche to streamer is most of the time followed by discharges.

Performance with X-Ray.
We didn't observe discharges
with X-Ray also with high fluxes
(50 MHz/cm2).

Discharge measurement at PSI.

GEM detectors have been tested at PSI with a hadron beam of 50 MHz
on the active area.
Discharges have been counted by monitoring current spikes on pad readout.

Discharge probability with
Ar-CO2-CF4: 60-20-20.

We have measured discharge probability with different flux at different chamber gain.
Some differences are observed between chambers with transfer gap of 2 and 7 mm.

Discharge probability with
Ar-CO2-CF4: 45-15-40.

At the working point we reach a discharge probablity of 10^-13.

With a discharge probability of 10^-9 the inefficiency due to the recharge time is 1%.

Discharge probability with

With this gas mixture we have reach 65Volt working plateau between 95% efficiency in 25 ns and discharge probability of 10^-12.

During the PSI test each detector integrated without any damage or aging effect about 5000 discharges, corresponding to a discharge probability per incident particle of
10^-12 in 10 years at LHCb.


F.M.  Last Updated on 6-Jan-2003