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

Davide Pinci, Cagliari University  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.

Davide Pinci, Cagliari University 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

Davide Pinci, Cagliari University The beam at  M1: the rate At low beam intensity, the rate has been measured by using a two scintillator finger coincidence (2x2 cm 2 ). 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 cm 2 FWHM; The total rate was 300 MHz.  85 MHz on 2x2 cm 2

Davide Pinci, Cagliari University 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 V gem ; A GEM recharge then occurs;

Davide Pinci, Cagliari University The currents on the detector electrodes GEM 1 GEM 2 GEM 3 Pad Beam Current Single GEM discharges discharge propagates Pad current drop due to discharge

Davide Pinci, Cagliari University The diffusion effect When the number of electrons in a hole becomes larger than the Raether limit (10 8 ) 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 cm 2 Standard GEM: A: 3/1/2/1 the classical geometry; B: 3/1/7/1 big transfer gap before the 3 rd 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

Davide Pinci, Cagliari University The gas mixtures studied We studied 3 different gas mixtures: Ar/CO 2 /CF 4 60/20/20 : the classical one; Ar/CF 4 /C 4 H 10 65/28/7: very good for time resolution (measured); Ar/CO 2 /CF 4 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/CO 2 /CF 4 45/15/40 gas mixture should give the same time performance as the Ar/CO 2 /C 4 H 10 65/28/7. Drift field 3 kV/cm

Davide Pinci, Cagliari University 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.

Davide Pinci, Cagliari University Discharges in LHCb The area of GEM foils used in the final chambers in LHCb will be 20 x 24 cm 2, but in that case the GEM foils will be segmented in 6 sectors of area  100 cm 2 ; 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/cm 2 ) in order to have less than 5000 discharges/sector in 10 years  discharge probability per incident particle <

Davide Pinci, Cagliari University Discharges: Ar/CO 2 /CF 4 60/20/20 Start of efficiency plateau: 99% in 25 ns per station. Narrow working region (10  20) Volts Discharge probability < Inefficiency 1% due to recharge dead time 1/nv = 2.25 ns  the gain needed at the knee is 2.0 x 10 4

Davide Pinci, Cagliari University Start of efficiency plateau: 99% in 25 ns per station. Discharge probability < Discharges: Ar/CF 4 /C 4 H 10 65/28/7  60 V wide working region 1/nv = 1.7 ns  the gain needed at the knee is 7.0 x 10 3 Inefficiency 1% due to recharge dead time

Davide Pinci, Cagliari University Discharges: Ar/CO 2 /CF 4 45/15/40 Discharge probability <  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 10 3  V tot = 1250 V; Start of efficiency plateau: 99% in 25 ns per station Inefficiency 1% due to recharge dead time

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 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/CO 2 /CF 4 60/20/20  narrow working region 10  20 V; Ar/CF 4 /C 4 H 10 65/28/7  wide working region  60 V; Ar/CO 2 /CF 4 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.

Davide Pinci, Cagliari University 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 1 st 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.  