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Preliminary results of a detailed study on
the discharge probability for a triple-GEM
detector at PSI
G. Bencivenni, A. Cardini,
P. de Simone, F. Murtas and D. Pinci
The beam at M1
Davide Pinci, Cagliari University
 The positive beam was composed by protons and pions.
 By inserting 1 mm of aluminum on the beam line, protons
loose energy more than pions and it’s possible to separate
the two components of the beam after a magnetic dipole;
 By using the
coincidence of two
scintillator fingers we
scanned the beam
profile in order to find
the pion and proton
peak positions.
 In this configuration we
centered our chambers
on the pion peak.
 17 cm
The beam at M1: protons contamination
 A little contamination of
protons was present at the +
peak;
Davide Pinci, Cagliari University
 By studying counting rate of a
scintillator finger as a function
of the discriminator threshold
we estimate the ratio:
p/tot=50 kHz/720 kHz  7%
 Protons with momentum of
350 MeV/c loose, by ionization,
a mean energy 5 times higher
than pions.
Total rate
Proton rate
The beam at M1: the rate
Davide Pinci, Cagliari University
 At low beam intensity, the rate has been measured by using
a two scintillator finger coincidence (2x2 cm2).
 At high beam intensity we extrapolated the rate by using the
GEM detector currents.
 85 MHz
on 2x2 cm2
Low beam-intensity
 The beam cross-section was 3x5 cm2 FWHM;
 The total rate was 300 MHz.
High beam-intensity
Discharges studies
 A discharge is mainly due to a
streamer formation in a GEM hole
which acts as a conductive channel
between the two sides of the GEM
causing a drop in the Vgem;
 A GEM recharge then occurs;
Davide Pinci, Cagliari University
 The time for a GEM recharge is given by:
total charge on the GEM (  5 C)
= 100 ms
the current provided by the HV supply (50 A)
 The HV supply gives the average values of the monitored
currents every 500 ms;
 A discharge is seen as an increase of the monitored current for
a GEM electrode;
 On the pads a discharge in a GEM is seen as a drop of current
because of the drop of the detector gain.
The currents on the detector electrodes
GEM 1
Single GEM
discharges
Davide Pinci, Cagliari University
GEM 2
GEM 3
Pad current
drop due to
discharge
Pad
Beam Current
discharge
propagates
The diffusion effect
 When the number of electrons in a hole becomes larger than
the Raether limit (108) a streamer can occur;
 The electron diffusion in the transfer gaps can help to reduce
the discharge probability by spreading the electron cloud;
Davide Pinci, Cagliari University
The more the transfer gap
is wide the more the cloud
is spread
 We built 3 detectors with different geometries using 10x10 cm2
Standard GEM:
A: 3/1/2/1 the classical geometry;
B: 3/1/7/1 big transfer gap before the 3rd GEM;
C: 2/2/2/1 the same gap before any GEM;
 Lab test with alpha particles have shown a reduction by a
factor 100 in discharge probability between chamber A and B.
The gas mixtures studied
Davide Pinci, Cagliari University
 We studied 3 different gas mixtures:
 Ar/CO2/CF4 60/20/20 : the classical one;
 Ar/CF4/C4H10 65/28/7: very good for time resolution
(measured);
 Ar/CO2/CF4 45/15/40 : very promising for the time resolution
(test beam is going on);
Since the 1/nv term is the main
contribution to the time resolution
the Ar/CO2/CF4 45/15/40 gas
mixture should give the same time
performance as the Ar/CO2/C4H10
65/28/7.
Drift field 3 kV/cm
Results from the PSI test
 We performed a very high statistics study on the discharge
probability;
 Each detector has integrated a total number of discharges as
high as 5000;
 No apparent ageing or other damages have been observed on
the 3 detectors (test is going on);
Davide Pinci, Cagliari University
Run 6
Run 43
Run 75
At the end of the test
beam, after about
5000 discharges (also
in very “hard” runs)
the detectors work as
in the first runs.
Discharges in LHCb
 The area of GEM foils used in the final chambers in LHCb will be
20 x 24 cm2, but in that case the GEM foils will be segmented in
6 sectors of area  100 cm2;
 The sectors will be supplied through a resistor chain;
Davide Pinci, Cagliari University
 Any damage in a sector won’t have effect on the other ones;
 Because of the particle rate in R1M1 (0.5 MHz/cm2) in order to
have less than 5000 discharges/sector in 10 years 
discharge probability per incident particle < 10-12
Discharges: Ar/CO2/CF4 60/20/20
Inefficiency 1% due to
recharge dead time
Davide Pinci, Cagliari University
Discharge
probability < 10-12
Start of efficiency plateau:
99% in 25 ns per station.
1/nv = 2.25 ns  the gain
needed at the knee is
2.0 x 104
Narrow working region (10  20) Volts
Discharges: Ar/CF4/C4H10 65/28/7
Inefficiency 1% due to
recharge dead time
Davide Pinci, Cagliari University
Discharge
probability < 10-12
Start of efficiency plateau:
99% in 25 ns per station.
1/nv = 1.7 ns  the gain
needed at the knee is
7.0 x 103
 60 V wide working region
Discharges: Ar/CO2/CF4 45/15/40
 Since the 1/nv term for this gas mixture is the same of
Isobutane-based one the efficiency knee is expected to be at
the same gain value: 7 x 103  Vtot = 1250 V;
Davide Pinci, Cagliari University
Inefficiency 1% due to
recharge dead time
Discharge
probability < 10-12
Start of efficiency
plateau: 99% in 25 ns
per station
 60 V wide working region
Davide Pinci, Cagliari University
Conclusions
 3 triple-GEM detectors have been tested with very high intensity
hadron-beam (up to 300 MHz + with 7% of protons);
 About 5000 discharges have been integrated on each chamber
without any damage or ageing effect;
 A discharge probability less than 10-12 per incident particle ensures
safe operation for a GEM detector in R1M1;
 3 set of data have been taken with 3 different gas mixtures:
 Ar/CO2/CF4 60/20/20  narrow working region 10  20 V;
 Ar/CF4/C4H10 65/28/7  wide working region  60 V;
 Ar/CO2/CF4 45/15/40  low discharge probability and very
good time performance expected (test beam is going on);
 The new geometries with wide gap have shown a discharge
probability of about one order of magnitude smaller.
Wide gap chamber: alpha vs. pions
 The discharge probability suppression found in the wide-gap
chamber with alpha particles (2 order of magnitude) has not
been found also with penetrating particles (less than 1 order of
magnitude). Why? We have an idea…

Davide Pinci, Cagliari University
 Alpha particles don't penetrate
behind the 1st GEM.
 The electron cloud is then amplified
and diffused.

A penetrating particle ionizes the gas
all along the track.
 The statistical fluctuation of the
ionization in a wide gap could increase
significatively the charge density and a
streamer can occur.