1. R&D on Hadron Blind detector, recent results 27.11.2002 - 15.01.2003 Issues addressed: - gain limits in CF 4 with heavily ionizing particles - operation with reflective CsI photo-cathode, gain and ion back-flow, optimization of the powering scheme - measurement of stability (first attempts)

2. Mesh GEM1 GEM2 GEM3 PCB Fe55 Am241 40 3 (1.5)mm 1.5mm 2mm Mesh GEM1 GEM2 GEM3 PCB 3 (1.5)mm 1.5mm 2mm Hg lamp Absorber R(0) R R R R R 2R2R CsI Experimental set-up pA R = 10M  Set-up for the study of gain limits Set-up for the study of operation with CsI photo-cathode Powering scheme HV Independent powering of the G1 top for the measurement of ion back-flow Independent powering of the mesh for the measurement of signal as a function of drift field

3. Gain of Triple-GEM in Ar-CO 2 and CF 4, irradiation with Fe 55

4. Pulse-height spectra in CF 4 and Ar-CO 2, gains above 10 4

5. Total charge of the avalanche from  -particle Saturation for Q>3*10 6

6. Discharge probability as a function of Vgem

7. Discharge probability as a function of gain

8. Triple-GEM with reflective CsI photo-cathode, thickness 2000A. Current to the PCB as a function of Vgem. Amplification Extraction

9. Current measured at the mesh ( Gain=1). HV is applied to GEM1 with respect to the mesh. Final procedure to determine current at gain=1: apply Vmesh = 1kV, wait 20 min, read current to the mesh.

10. Gain in Ar-CO 2 and CF 4 measured with UV illumination Gain measured with Fe 55 Factor 2 discrepancy is not understood yet

11. Ion back-flow for different gases and induction field

12. WIS, 2.01.2003 Ion back-flow for different values of induction field. Optimization of electrons collection efficiency.

13. WIS, 2.01.2003 Ion back-flow for different fields in the region between GEM1&GEM3

14. History of the 2nd run with CsI photo-cathode. Each point is the measurement of extraction to the mesh (current at gain=1). Medium-term stability measurements were performed during day 3 (4 mC/cm 2 ), day 4 (3 mC/cm 2 ), day 5 (2 mC/cm 2 ). Upper limit of the total charge in 10 years of PHENIX operation estimeted to 1 mC/cm 2

15. Stability measurements. UV intensity corresponds to ~10 8 photons/cm 2 /s. In the 1st and 2nd runs UV and HV were applied simultaneously, in the 3d run UV was applied 2h later than HV.

16. Same as previous slide (run 1), horizontal scale in units of accumulated charge

17. Summary. - Triple-GEM provides stable operation in CF 4 at gain 10 4 in short term (hours - days); - Strong non-linearity is observed when total avalanche charge exceeds ~3*10 6 electrons; - Discharges in the presence of HIPs occur in CF 4 at gain of ~10 6 (small GEM, no defects). In case of large GEM discharge threshold is determined by the GEM quality rather than avalanche charge or voltage. - Triple-GEM with CsI reflective photo-cathode shows stable performance at gain of 10 4 in short term (hours to days); - Ion back-flow can be optimized down to the level of 0.7, it does not depend on gas and electric fields inside GEMs package; - No deterioration of CsI observed after delivering charge up to 9mC/cm 2