preliminary results of a detailed study on the discharge probability for a triple-gem detector at...
TRANSCRIPT
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
David
e P
inci
, C
aglia
ri U
niv
ers
ity
17 cm
The beam at M1 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.
David
e P
inci
, C
aglia
ri U
niv
ers
ity
The beam at M1: protons contamination
A little contamination of protons was present at the + peak;
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
David
e P
inci
, C
aglia
ri U
niv
ers
ity
The beam at M1: the rate 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.
Low beam-intensity High beam-intensity
The beam cross-section was 3x5 cm2 FWHM; The total rate was 300 MHz.
85 MHzon 2x2 cm2
David
e P
inci
, C
aglia
ri U
niv
ers
ity
Discharges studies
The time for a GEM recharge is given by:
total charge on the GEM ( 5 C)
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.
= 100 ms
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;
David
e P
inci
, C
aglia
ri U
niv
ers
ity
The currents on the detector electrodes
GEM 1
GEM 2
GEM 3
Pad
Beam Current
Single GEMdischarges discharge
propagates
Pad current drop due to discharge
David
e P
inci
, C
aglia
ri U
niv
ers
ity
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;
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 more the transfer gap is wide the more the cloud is spread
David
e P
inci
, C
aglia
ri U
niv
ers
ity
The gas mixtures studied 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
David
e P
inci
, C
aglia
ri U
niv
ers
ity
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);
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.
David
e P
inci
, C
aglia
ri U
niv
ers
ity
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;
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
David
e P
inci
, C
aglia
ri U
niv
ers
ity
Discharges: Ar/CO2/CF4 60/20/20
Start of efficiency plateau:
99% in 25 ns per station.
Narrow working region (10 20) Volts
Discharge probability < 10-
12
Inefficiency 1% due to recharge dead time
1/nv = 2.25 ns the gain needed at the knee is
2.0 x 104
David
e P
inci
, C
aglia
ri U
niv
ers
ity
Start of efficiency plateau:
99% in 25 ns per station.
Discharge probability < 10-
12
Discharges: Ar/CF4/C4H10 65/28/7
60 V wide working region
1/nv = 1.7 ns the gain needed at the knee is
7.0 x 103
Inefficiency 1% due to recharge dead time
David
e P
inci
, C
aglia
ri U
niv
ers
ity
Discharges: Ar/CO2/CF4 45/15/40
Discharge probability < 10-12
60 V wide working region
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;
Start of efficiency plateau: 99% in 25 ns per station
Inefficiency 1% due to recharge dead time
David
e P
inci
, C
aglia
ri U
niv
ers
ity
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.
David
e P
inci
, C
aglia
ri U
niv
ers
ity
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…
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.