1. Triple-GEM detector operation for high-rate particle triggering W. Bonivento, G. Bencivenni, A. Cardini, C. Deplano, P. de Simone, F. Murtas, D. Pinci, M. Poli-Lener and D. Raspino

2. Davide Pinci Università di Cagliari The Gas Electron Multiplier A Gas Electron Multiplier is made by 50 m thick kapton foil, copper clad on each side and perforated by an high surface-density of bi-conical channels; By applying a potential difference between the two copper sides an electric field as high as 100 kV/cm is produced in the holes acting as multiplication channels; The gain of a single GEM is of the order 20 100.

3. Davide Pinci Università di Cagliari The triple-GEM detector Multiple GEM structures allow to reach high gain in safe operating conditions, resulting very useful for ionizing particle detention; Several 3-GEM detector prototypes have been built and tested in last two years by our group; 3-GEM detector layout together with the labeling of different geometrical parameters We propose this kind of detector for equipping the central region (R1) of the first station (M1) of the LHCb system.

4. Davide Pinci Università di Cagliari Detector construction 10x10 cm 2 GEM stretched and glued on frames Prototype before closing

5. Davide Pinci Università di Cagliari Rate capability The rate capability was measured with an X-ray (5.9 keV) tube; The detector was supplied with an Ar/CO 2 /CF 4 (60/20/20) mixture resulting in a gain of about 2x10 4 ; A very good gain stability was found up to a photon flux of about 5x10 7 Hz/cm 2 LHCb R1M1 maximum rate

6. Davide Pinci Università di Cagliari Ageing test With a gain of 2x10 4 a total integrated charge of 13 C/cm 2 is expected in 10 years of operation in R1M1; By irradiating a 3-GEM chamber with an flux of 50 MHz/cm 2 X-rays, in 10 days a total charge of 20 C/cm 2 was integrated; Less than 5% change in the chamber behavior LHCb R1M1 maximum integrated charge P and T variations were monitored by using a low irradiated 3-GEM chamber

7. Davide Pinci Università di Cagliari The gas mixtures In the beam tests we studied 4 different gas mixtures: 1.Ar/CO 2 70/30; 2.Ar/CO 2 /CF 4 60/20/20; 3.Ar/CF 4 /C 4 H 10 65/28/7; 4.Ar/CO 2 /CF 4 45/15/40; Drift field 3 kV/cm Given n: the number of clusters per unit length; v: the electron drift velocity in the drift gap; The 1/nv term is the main contribution to the time resolution of this kind of detector. 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.

8. Davide Pinci Università di Cagliari The gas mixture gains The triple-GEM detector gain was measured by using X-rays for the different gas mixtures; The detector gain resulted essentially an exponential function of the sum of the 3 GEM voltage supplies: G = A e (Vgem1+Vgem2+Vgem3) A and depend on the gas mixture.

9. Davide Pinci Università di Cagliari The time performance Best TDC spectra for the gas mixtures used RMS 9.7 nsRMS 5.3 ns RMS 4.5 ns

10. Davide Pinci Università di Cagliari The efficiency in 25 ns A very important requirement for triggering in LHC experiments is to ensure an high efficiency in a 25 ns time window for a correct bunch crossing identification; Fast mixtures give an 25 of 98% also at moderate gain values Slow mixture 25 less than 88% also for high gain values Chamber efficiency in 25 ns

11. Davide Pinci Università di Cagliari Discharge studies In this case a discharge occurs; A discharge is seen: On the GEM electrodes as a sudden increase of the current needed for the recharge; On the pads as a momentary drop of current because of the drop of the detector gain. Because of the Landaus distribution tail, sometime a large amount of pairs is created in the gas; After the multiplication the charge can exceed the Raether limit giving raise to a streamer formation in the GEM holes;

12. Davide Pinci Università di Cagliari Discharge probability The discharge probability per incident particle was measured by testing 3 chambers at the Paul Sherrer Institute with a particle rate of about 300 MHz; No ageing or other damages observed on the 3 detectors after about 5000 discharges integrated; Up to this value the detector operation seems completely safe; Because of the particle rate in R1M1 (0.5 MHz/cm 2 ) in order to have less than 5000 discharges/detector in 10 years discharge probability per incident particle < 10 -12

13. Davide Pinci Università di Cagliari Combined results The results of the discharge probability per incident particle are compared with the efficiency in 25 ns; In LHCb R1M1 the 3-GEM chambers have to ensure a wide working region where: The discharge probability is less than 10 -12 ; The efficiency in 25 ns time window of 2 OR-ed chambers is greater than 99%; The pad cluster size is less than 1.2 with 1x1.25 cm 2 pad;

14. Davide Pinci Università di Cagliari Results: Ar/CO 2 /CF 4 60/20/20 Discharge probability < 10 -12 25 ns time window of two chambers in OR > 99% Very narrow working region (if any…)

15. Davide Pinci Università di Cagliari Results: Ar/CF 4 /C 4 H 10 65/28/7 Discharge probability < 10 -12 1030 V1075 V 45 V wide working region 25 ns time window of two chambers in OR > 99%

16. Davide Pinci Università di Cagliari Results: Ar/CO 2 /CF 4 45/15/40 Discharge probability < 10 -12 1250 V1315 V 65 V wide working region 25 ns time window of two chambers in OR > 99%

17. Davide Pinci Università di Cagliari 1.221.161.17 The pad cluster size The pad cluster size values at the end of the working regions are reported in plot;

18. Davide Pinci Università di Cagliari Summary After two years and several prototypes built and tested we may conclude: Very high rate capability (up to 5x10 7 Hz/cm 2 ); Very good ageing performance (good stability up to 20 C/cm 2 ); Ar/CO 2 /CF 4 60/20/20 narrow working region 10 V; Ar/CF 4 /C 4 H 10 65/28/7 wide working region 45 V; Ar/CO 2 /CF 4 45/15/40 wide working region 65 V with a cluster size lower than 1.17; The triple-GEM detector with Ar/CO 2 /CF 4 (45/15/40) mixture fulfills all the requirements to be used in R1M1