**DISCHARGE
STUDIES WITH MULTI-GEM DETECTORS**

**Systematic measurement
confirm that in multiple GEM structures higher gains can be sustained before
discharges in presence of heavily ionizing background. Both gain and sustainable
gain are increased by about one order of magnitude at each addition of a GEM.
The discharge probability depends on the experiment-dependent source of background,
and has to be verified in realistic running conditions. For systematic studies
in the laboratory, the gain is measured with soft X-rays sources or X-ray
generators, while the heavily ionizing background is emulated with an ^{241}Am
alpha source; very convenient also the use of an alpha emitter, Radon 220
(generated by natural Thorium), introduced in the gas flow**.

Gain
as a function of voltage in single, double and triple GEM; the ordinate
provides the value of the potential applied to each GEM: |
Discharge
probability on exposure to alpha particles as a function of voltage applied
to each GEM: |

Discharge
probability on alphas as a function of gain for single, double and triple
GEM: |
Discharge
probability as a function of voltage asymmetry in the double GEM: |

Discharge
probability as a function of voltage asymmetry in the triple GEM: |
Discharge
propagation probability from the last GEM to the readout electrode, as
a function of induction field, for several connections of sectors. The
curve "4 sectors up" corresponds to a full GEM of 10x10 cm2;
"1 sector up" correspond to 1/4 of the area: |

**The discharge
in one GEM (usually the last in a cascade) may remain confined to the multiplier,
or propagate to the next structure. It has been found that the energy of the
primary discharge (determined by voltage and capacitance) strongly affects
the propagation probability. This leads to sectorization, i.e. separation
of GEM electrodes in segments, individually powered through high value resistors,
to reduce the discharge energy (see GEM POWERING).**

**To know more,
see the full study in:
S. Bachman et al, Discharge studies and prevention in the gas electron
multiplier (GEM), Nucl. Instrum. Methods A479(2002)294)**