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

Itzhak Tserruya, BNL, DC upgrades Feb.14,20032Outline 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

Itzhak Tserruya, BNL, DC upgrades Feb.14,20033 HBD in PHENIX 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 HBD/TPC inner coil

Itzhak Tserruya, BNL, DC upgrades Feb.14,20034Detector Detector element: multi - GEM High gain Reduced ion feedback Concept: Windowless Cherenkov detector Radiator and detector gas: CF 4 Large bandwidth and large N pe Reflective CsI photocathode No photon feedback Proximity focus  detect blob Low granularity

5 Detector R&D Goals Gain and stability:  demonstrate that the detector can operate at a gain of  demonstrate stability at  operate at 10 4 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. * CF 4 scintillation.  “Prototype” in-beam test

Itzhak Tserruya, BNL, DC upgrades Feb.14,20036 Detector set-up Mesh GEM1 GEM2 GEM3 PCB Fe55 Am (1.5)mm 1.5mm 2mm Set-up for gain studySet-up for study with CsI photocathode Mesh GEM1 GEM2 GEM3 PCB 3 (1.5) mm 1.5mm 2mm Absorber CsI pA Hg lamp Powering scheme Independent powering of the mesh for the study of signal as function of drift field Independent powering of the top of GEM1 for the study of ion back-flow R R R R R / 0 R = 10M  HV R 2R

Itzhak Tserruya, BNL, DC upgrades Feb.14,20037 Gain variations with gas density P/T  Strong gain variations with gas density: factor of 2 change in gain for 2% change in gas density.

Itzhak Tserruya, BNL, DC upgrades Feb.14,20038 Pulse-height spectra in CF4 and Ar-CO2, gains above 10 4 CF 4 Ar-CO 2 ADC number

Itzhak Tserruya, BNL, DC upgrades Feb.14,20039 Triple-GEM: Gain Curves in Ar-CO 2 and CF 4 measured with Fe 55 For a gain of 10 4 CF 4 needs V more than Ar/CO 2 Mesh GEM1 GEM2 GEM3 PCB Fe55 Am (1.5)mm 1.5mm 2mm Q = N p {  D (G 1  1 G 2  2 G 3  I )} Gain EDED EIEI

Itzhak Tserruya, BNL, DC upgrades Feb.14, Total charge in avalanche in Ar-CO 2 and CF 4 measured with Am 241 Charge saturation in CF 4 !!!

Itzhak Tserruya, BNL, DC upgrades Feb.14, Discharge probability vs. V GEM

Itzhak Tserruya, BNL, DC upgrades Feb.14, Discharge probability vs. gain It seems that in Ar-CO 2, the discharge threshold is close to the Raether limit (at  10 8 ), whereas in CF 4 the discharge threshold seems to depend on GEM quality and occurs at voltages  V GEM  V

Itzhak Tserruya, BNL, DC upgrades Feb.14, Triple-GEM with CsI photo-cathode Thickness: 2000A.  V GEM [V] Current to the PCB as function of  VG GEM. I PCB = I 0 {  ’ D (G 1  1 G 2  2 G 3  I )} Gain Mesh GEM1 GEM2 GEM3 PCB 1.5mm 2mm Absorber CsI pA Hg lamp E=0

Itzhak Tserruya, BNL, DC upgrades Feb.14, I 0 = Current measured at the mesh ( Gain=1). (HV is applied to GEM1 with respect to the mesh.) Adopted procedure to determine I 0 : apply Vmesh = 1kV, wait 20 min, read current to the mesh.

15 Triple GEM and CsI: Gain Curves measured with UV illumination Gain(Fe 55 ) = {  D (G 1  1 G 2  2 G 3  I )} Gain(UV) = {  ’ D (G 1  1 G 2  2 G 3  I )} + Dependence of gain on gas density Pretty good agreement between gain measured with Fe 55 and UV illumination

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

Itzhak Tserruya, BNL, DC upgrades Feb.14, 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: ~10 8 photons/cm 2 /s. In the 1 st and 2 nd runs UV and HV were turned-on simultaneously. In the 3 rd run UV was turned-on 2h later than HV.

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

20 CsI Aging History of the 2 nd run with CsI photo-cathode. Each point is the measurement of the current I 0 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 ). Total charge in 10 years of PHENIX operation conservatively estimated to 1 mC/cm 2

Itzhak Tserruya, BNL, DC upgrades Feb.14, Aging: (I) CsI Min bias (in PHENIX acceptance) e 3 x 40 6 x 40 h 80 x x 4 Gain Ion feedback Interaction rate at design L / 4L (s -1 ) Operation per year (s) Years of operation Detector area R=70cm (cm 2 ) Total charge in 10 y of operation 3.8  C/cm mC/cm 2 Photon and ion induced aging studies from literature: 20% QE loss after 100 –  C/cm 2 Realistic Pessimistic

22  Triple-GEM provides stable operation in CF 4 at gain 10 4 ( several weeks experience).  Strong non-linearity is observed when total avalanche charge exceeds ~3*10 6 electrons.  The discharge threshold in CF 4 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 10 4 (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/cm 2Summary So far so good !

Itzhak Tserruya, BNL, DC upgrades Feb.14, Our TDL:  Last milestone: demonstrate HBD properties of the detectorOutlook

24 can be pumped to  several GEMs + MWPC test with Fe 55, UV lamp, ,  can be pumped to  directly coupled to detector HBD test set-up (I) Detector Box CF 4 Radiator Cosmic trigger S1 S2 C S1.S2  MIP C.S1.S2  “electron” C: CO 2 radiator p th  3.8 GeV 1.30 m long rate  1/min

Itzhak Tserruya, BNL, DC upgrades Feb.14, Measurements are underway. HBD Test Set-up (II)

Itzhak Tserruya, BNL, DC upgrades Feb.14, Our TDL:  Last milestone: demonstrate HBD properties of the detectorOutlook  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 CF 4 scintillation  Endurance tests  Study gas mixtures: CF 4 – Ne or CF 4 – Ar ?

Itzhak Tserruya, BNL, DC upgrades Feb.14, CsI photocathode QE Transmissive photocathode – Relatively high quantum efficiency The most attractive option: Very large bandwidth: 6 – 11.5 eV Very large N 0  940: 40 pe in a 50cm radiator electron efficiency > 90%

Itzhak Tserruya, BNL, DC upgrades Feb.14, Scintillation of CF 4  CF 4 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

29Outlook The original goal of completing the detector R&D before the end of 2003 is well within reach. 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 CF 4 scintillation  Endurance tests  Study gas mixtures: CF 4 – Ne or CF 4 – Ar ?

Itzhak Tserruya, BNL, DC upgrades Feb.14,200330