HBD R&D: Update

1. HBD R&D: Update Itzhak Tserruya (for A. Kozlov, I. Ravinovich and L. Shekhtman) Weizmann Institute, Rehovot DC meeting Feb. 14, 2003

2. Outline • Introduction: a short reminder • HBD concept • R&D goals • R&D set-up • Results • Gain • Stability and sparking probability • Ion back-flow (feedback) • HBD feature: response to mip and electrons • Outlook

3. HBD in PHENIX inner coil HBD/TPC HBD/TPC • Compensate magnetic • field with inner coil (foreseen in • original design) • (B0 for r 50-60cm) • Compact HBD in inner • region to be complemented by • a TPC. • Specifications • * Electron efficiency 90% • * Double hit recognition 90% • * Modest  rejection ~ 200

4. Concept: • Windowless Cherenkov detector • Radiator and detector gas: CF4 • Large bandwidth and large Npe • Reflective CsI photocathode • No photon feedback • Proximity focus  detect blob • Low granularity • Detector element: multi - GEM • High gain • Reduced ion feedback Detector

5. Detector R&D Goals • Gain and stability: demonstrate that the detector can operate at a gain of 104. demonstrate stability at 104. operate at 104 in presence of highly ionizing particles. • Aging effects aging of GEM.  aging of CsI. • Ion back-flow (feed-back) • Response to mip and electrons * demonstrate hadron blindness. * optimize detector operation. • Other issues * CsI quantum efficiency and bandwidth. * CF4 scintillation. • “Prototype” in-beam test

6. Set-up for gain study Set-up for study with CsI photocathode Powering scheme Hg lamp Absorber Independent powering of the mesh for the study of signal as function of drift field Fe55 Am241 40 Mesh Mesh HV 3 (1.5)mm 3 (1.5) mm CsI R / 0 HV GEM1 GEM1 1.5mm R 1.5mm GEM2 GEM2 R 1.5mm 1.5mm GEM3 GEM3 R 2mm 2mm R R PCB PCB 2R R = 10MW Independent powering of the top of GEM1 for the study of ion back-flow pA Detector set-up

7. P/T  Strong gain variations with gas density: factor of 2 change in gain for 2% change in gas density. Gain variations with gas density

8. Pulse-height spectra in CF4 and Ar-CO2, gains above 104 CF4 Ar-CO2 ADC number ADC number

9. Fe55 Am241 40 Mesh 3 (1.5)mm GEM1 1.5mm GEM2 1.5mm GEM3 2mm PCB Q = Np {D (G11 G22 G3I)} Gain Triple-GEM: Gain Curves in Ar-CO2 and CF4 measured with Fe55 ED EI For a gain of 104 CF4 needs 130-150 V more than Ar/CO2

10. Total charge in avalanche in Ar-CO2 and CF4 measured with Am241 Charge saturation in CF4 !!!

11. Discharge probability vs. VGEM

12. Discharge probability vs. gain It seems that in Ar-CO2, the discharge threshold is close to the Raether limit (at 108), whereas in CF4 the discharge threshold seems to depend on GEM quality and occurs at voltages VGEM 560-600V

13. Hg lamp Absorber Current to the PCB as function of VGGEM . Mesh E=0 CsI GEM1 1.5mm GEM2 1.5mm GEM3 2mm PCB pA IPCB = I0 {’D (G11 G22 G3I)} Gain VGEM [V] Triple-GEM with CsI photo-cathode Thickness: 2000A.

14. I0 = Current measured at the mesh ( Gain=1) . (HV is applied to GEM1 with respect to the mesh.) Adopted procedure to determine I0 : apply Vmesh = 1kV, wait 20 min, read current to the mesh.

15. Gain(Fe55) = {D (G11 G22 G3I)} Gain(UV) = {’D (G11 G22 G3I)} Pretty good agreement between gain measured with Fe55 and UV illumination Triple GEM and CsI: Gain Curves measured with UV illumination + Dependence of gain on gas density

16. Hg lamp Absorber Mesh E=0 CsI GEM1 1.5mm GEM2 1.5mm GEM3 2mm PCB pA Ion back-flow (I) for different gases and induction field Ein Fraction of ion back-flow defined here as: Iphc / IPCB This is an upper limit. The exact definition should be: Iphc / (IPCB + IGEM3-bottom) The total ion back-flow is properly measured by Iphc. WIS, 2.01.2003

17. Ion back-flow (II) for different fields between GEM1&GEM3 Ions seem to follow the electric field lines At standard operating conditions, the upper limit of ion back-flow is 0.7.

18. CsI photocathode: stability and aging (I) UV lamp intensity: ~108 photons/cm2/s. In the 1st and 2nd runs UV and HV were turned-on simultaneously. In the 3rd run UV was turned-on 2h later than HV.

19. CsI photocathode: stability and aging (II) Same as previous slide (run 1), but horizontal scale in units of accumulated charge

20. CsI Aging History of the 2nd run with CsI photo-cathode. Each point is the measurement of the current I0 to the mesh (current at gain=1). Medium-term stability measurements were performed during day 3 (4 mC/cm2 ), day 4 (3 mC/cm2 ), day 5 (2 mC/cm2 ). Total charge in 10 years of PHENIX operation conservatively estimated to 1 mC/cm2

21. Aging: (I) CsI Realistic Pessimistic Min bias (in PHENIX acceptance) e 3 x 40 6 x 40 h 80 x 4 160 x 4 Gain 104 5 104 Ion feedback 0.5 1 Interaction rate at design L / 4L (s-1) 1400 5600 Operation per year (s) 1 107 3 107 Years of operation 10 10 Detector area R=70cm (cm2) 1.3 104 Total charge in 10 y of operation 3.8 C/cm2 0.9 mC/cm2 Photon and ion induced aging studies from literature: 20% QE loss after 100 – 10 000 C/cm2

22. Summary • Triple-GEM provides stable operation in CF4 at gain 104 ( several weeks experience). • Strong non-linearity is observed when total avalanche charge • exceeds ~3*106 electrons. • The discharge threshold in CF4 in the presence of heavily ionizing particles seems to be determined by the GEM quality rather than the avalanche charge or the gain. • Triple-GEM with CsI reflective photo-cathode shows stable performance at gain of 104 (few weeks experience). • Ion back-flow can be optimized down to the level of 0.7. It does not depend on gas and electric fields inside GEM package. • No deterioration of CsI observed after delivering a total charge up to 9mC/cm2 So far so good !

23. Outlook Our TDL: • Last milestone: demonstrate HBD properties of the detector

24. HBD test set-up (I) S1.S2  MIP C.S1.S2  “electron” Cosmic trigger C • pth 3.8 GeV • 1.30 m long • rate  1/min C: CO2 radiator S2 • can be pumped to 2 10-6 • directly coupled to detector CF4 Radiator S1 • can be pumped to 2 10-6 • several GEMs + MWPC • test with Fe55, UV lamp, ,  Detector Box

25. HBD Test Set-up (II) • Measurements are underway.

26. Outlook Our TDL: • Last milestone: demonstrate HBD properties of the detector • Start detector design • Repeat all measurements under much better controlled conditions (monitor gas density, monitor oxygen and water content of gas). • Measure the QE of CsI • Measure CF4 scintillation • Endurance tests • Study gas mixtures: CF4 – Ne or CF4 – Ar ?

27. The most attractive option: • Transmissive photocathode • Relatively high quantum efficiency Very large bandwidth: 6 – 11.5 eV Very large N0 940: 40 pe in a 50cm radiator electron efficiency > 90% CsI photocathode QE

28. Scintillation of CF4 • CF4 scintillates at 160nm. • Two measurements in the literature: • * NIM A371, 300 (1996):  110 ph/MeV • * NIM A354, 262 (1995):  200 ph/MeV • Planned to be measured at BNL 2/2003 • We may need shades to reduce it

29. Outlook Our TDL: • Last milestone: demonstrate HBD properties of the detector • Start detector design • Repeat all measurements under much better controlled conditions (monitor gas density, monitor oxygen and water content of gas). • Measure the QE of CsI • Measure CF4 scintillation • Endurance tests • Study gas mixtures: CF4 – Ne or CF4 – Ar ? The original goal of completing the detector R&D before the end of 2003 is well within reach.