Initial Performance of the PHENIX Hadron Blind Detector at RHIC Craig Woody Brookhaven National Lab For the PHENIX Collaboration N IEEE Nuclear Science Symposium and Medical Imaging Conference Orlando, FL October 27, 2009

C.Woody, NSS-N15-1, October 27, HBD Team n Weizmann Institute of Science A. Milov, M. Naglis, I. Ravinovich, D.Sharma, I.Tserruya n Stony Brook University Z.Citron, M.Connors, M. Durham, T.Hemmick, J.Kamin, B.Lewis, V. Pantuev, M.Prossl, J.Sun n Brookhaven National Lab B.Azmoun, R.Pisani, T.Sakaguchi, A.Sickles, S.Stoll, C.Woody n University of California Riverside A.Iordanova, S.Rolnick n Columbia University (Nevis Labs) C-Y. Chi

C.Woody, NSS-N15-1, October 27, Measurement of Low Mass Electron Pairs in Relativistic Heavy Ion Collisions Measurement of low mass pairs and vector mesons in heavy ion collisions is limited by large combinatorial background from  0 Dalitz decays and  conversions Would like to reduce this background by a factor of ~ 100 or better Need rejection factor > 90% for:  e + e - and    e + e – Requires single electron efficiency > 90% (pairs > 80%) Electrons pairs measured in PHENIX without HBD (200 GeV Au-Au) arXiv:

C.Woody, NSS-N15-1, October 27, The PHENIX Hadron Blind Detector Cherenkov blobs e+e+ e-e-  pair opening angle ~ 1 m Radiator gas = Working gas Gas volume filled with pure CF 4 radiator Proximity Focused Windowless Cherenkov Detector Electrons produce Cherenkov light Pions with P < 4 GeV/c do not However, we don’t want to detect the ionization signal from the hadrons Radiating tracks form “blobs” on an image plane  Key to the measurement is distinguishing one blob from two Operate PHENIX in a mode which produces essentially zero field in the region of the HBD Electron pairs do not open up ! Dalitz pairs & conversions make two overlapping blobs, single electrons from vector meson decays make only one

C.Woody, NSS-N15-1, October 27, The Hardon Blind Concept Primary ionization is drifted away from GEM and collected by a mesh UV photons produce photoelectrons on a CsI photocathode and are collected in the holes of the top GEM Triple GEM stack provides gain ~ few x10 3 Amplified signal is collected on pads and read out Mesh CsI layer Triple GEM Readout Pads e-e- Primary ionization g HV Primary ionization signal is greatly suppressed at slightly negative drift field while photoelectron collection efficiency is mostly preserved Collection efficiency for photoelectrons and ionization Z.Frankel et.el., NIM A546 (2005) A.Kozolov et.al. NIM A523 (2004)

C.Woody, NSS-N15-1, October 27, History  The HBD was originally installed into PHENIX in the fall of 2006 for Run 7 ( ) at RHIC  The detector encountered several severe high voltage problems which ultimately damaged many of the GEM foils Minor GEM sparks induced larger, more damaging mesh to GEM sparks due to large stored energy in filter capacitors Spark in one module would induce sparks in other modules due to photon propagation in CF 4 Problem found with LeCroy 1471N modules that allowed HV to be reapplied after a trip causing spurious HV spikes  Some very useful data was taken with the detector during Run 7, but the entire detector was subsequently rebuild and reinstalled for Run 9 (Feb - July 2009, 200 & 500 GeV pp)

C.Woody, NSS-N15-1, October 27, Improvements and Lessons Learned  We believe the main problem with GEM sparking was due to dust ! Previously, we were clean, but not clean enough !  New Rapid Method procedure was instituted to build the detector Most of the assembly is done near laminar flow table (clean) Only CsI coated GEMs were installed inside glove box (dusty) No testing of individual stacks inside glove box (only after assembly) Used only for the West Detector for Run 9. East Detector was rebuilt almost entirely inside the glove box and did not perform as well  New voltage divider Zeners installed to limit mesh to GEM voltage Increased field in 1 st transfer gap. Enabled us to achieve same gain (G ~ 10 3 ) at lower dV across GEMs  HV relay box installed between LeCroy HV PS and detector Discharges both mesh and GEM divider chain if either one trips  New sophisticated HV control program  Increased gas flow through detector to improve transparency

C.Woody, NSS-N15-1, October 27, Rebuilding the Detector Standard GEMs installed near laminar flow table (cleanest area inside tent) CsI coated GEMs installed inside glove box (minimize exposure time) Rapid Method – Used only for the West Detector for Run 9 East Detector was later rebuilt this way for Run 10

C.Woody, NSS-N15-1, October 27, CsI Photocathodes Lucite with Al/MgF 2 coating Monitor QE using CF 4 scintillation as a calibrated light source Scintillation Cube Installed in each half of HBD New photocathodes have good QE Photocathodes produced over one year ago have not deteriorated in terms of their QE QE Intact!

C.Woody, NSS-N15-1, October 27, Gain Determination Using Scintillation in CF 4 Scintillation Ionization FORWARD REVERSE 1 p.e. For low multiplicity (p-p) events scintillation produces essentially single photoelectrons on each pad Fit single p.e. distribution to an exponential Gain ~ 1/slope Allows pad by pad calibration ADC channel Used as a gain monitor throughout the run

C.Woody, NSS-N15-1, October 27, Optimizing Reverse Bias Conditions ES1 -10V ES3 -15V Scan voltage between top GEM and mesh to optimize the p.e. collection field for each module Requires setting ~ 4KV PS to ~ 5 V (0.1%) Operating Point 0 V– Red 5V – Black 10V – Green 15V - Blue 20 V - Yellow 25V - Magenta  Requires high precision and good stability in HV PS and control system

C.Woody, NSS-N15-1, October 27, Position Resolution Position resolution:   ≈  z ≈ 1 cm Consistent with resolution expected from pad size: a = 1.55 cm (hex) 2a/√12 = 0.9 cm Data taken with PHENIX Central Magnet at zero field HBD in Forward Bias in order to see MIPs Reconstruct tracks in PHENIX Central Arm and project to HBD.  zz I.Ravinovich

C.Woody, NSS-N15-1, October 27, Single vs Double Electron Clusters Use reconstructed Dalitz pairs (M ee < 150 MeV/c) in PHENIX Central Arms Match to single or double clusters in HBD ~ 40 p.e. per two electron track I.Ravinovich ~ 22 p.e. per single electron track 50% increase from Run 7 Agrees with our expected yield taking into account p.e. collection efficiency and transmission loss in the gas

C.Woody, NSS-N15-1, October 27, Hadron Rejection Hadron Rejection Factor Charge from dE/dx in the small (~ 100  m) region above the top GEM and in the first transfer gap is very small compared to the Cherenkov signal from electrons Hadron Response Measured using reconstructed tracks in the PHENIX Central Arm projected to the HBD Reverse Bias I.Ravinovich

C.Woody, NSS-N15-1, October 27, Electron Efficiency Measured using well identified electrons in low mass region (0.025 < m < GeV) measured in the PHENIX Central Arms and matched with hits in the HBD Single electron efficiency > 90% Pair efficiency ~ 80%. Meets requirements for achieving 90 % rejection of photon conversions and Dalitz decays I.Ravinovich Opening angle

C.Woody, NSS-N15-1, October 27, HBD East Refurbished Again HBD East performed less reliably than the West during Run 9 A a result, the East detector was removed and rebuilt using the Rapid Method at Stony Brook during September ‘09 Removal, rebuild and reinstallation was accomplished in record time (35 days!) The detector was reinstalled in PHENIX on Oct 1 st, tested, and is ready for operation HBD East in PHENIX 10/1/09 after refurbishing at Stony Brook

C.Woody, NSS-N15-1, October 27, Summary  The HBD had its first successful operation in Run 9  Improved p.e. yield per e-track from ~ 14 in Run 7 to ~ 22 in Run 9  Electron efficiency and hadron rejection shown to be very good and will meet our requirements for background suppression  The detector performed very well overall, but the West detector, (rebuilt using the Rapid Method) performed better than the East  East detector was removed and rebuilt in Aug - Sep ’09 using the Rapid Method and was reinstalled into PHENIX for Run 10  The entire HBD is now ready and fully operational to make the first dedicated low mass electron pair measurement in Au-Au collisions at RHIC during Run 10 starting in Dec 2009

C.Woody, NSS-N15-1, October 27, Backup Slides

C.Woody, NSS-N15-1, October 27, Measuring Low Mass Pairs in PHENIX ~12 m e+e+ e+e+ e-e- e-e- PHENIX with HBDPHENIX without HBD

C.Woody, NSS-N15-1, October 27, Detector Construction 24 Triple GEM Detectors (10 modules per side) Area = 23 x 27 cm 2 Mesh electrode Top gold plated GEM for CsI Two standard GEMS Kapton foil readout plane One continuous sheet per side Hexagonal pads (a = 15.6 mm) Honeycomb panels Mylar entrance window HV panel Pad readout plane HV panelTriple GEM module with mesh grid Very low mass (< 3% X 0 including gas) Detector designed and built at the Weizmann Institute

C.Woody, NSS-N15-1, October 27, HBD Detector Parameters Acceptance at nominal location (r=5cm) |  | ≤0.45,  =135 o GEM size ( ,z) 23 x 27 cm 2 Segmentation 26 strips (0.8 x 27 cm) Number of detector modules per arm 10 Frame 5 mm wide, 0.3mm cross Hexagonal pad size a = 15.6 mm Number of pads per arm 960 Dead area within central arm acceptance 6% Radiation length within central arm acceptance box: 0.92%, gas: 0.54%, preamps+sockets: 0.66% Total: 2.12% Weight per arm (including accessories) <10 kg

C.Woody, NSS-N15-1, October 27, GEM Foil Damage Run 7 Voltage Divider GEMs segmented on one side to limit stored energy Filter Caps Mesh-GEM Spark Spark Damage On Top GEM

C.Woody, NSS-N15-1, October 27, “Clean Tent” at Stony Brook High Vacuum GEM Storage Container CsI Evaporator and quantum efficiency measurement Large glove box O 2 < 5 ppm H 2 O < 10 ppm Laminar Flow Hood Class ( N < 0.5 m m particles/m 3 )

C.Woody, NSS-N15-1, October 27, Evaporator and QE Measurement Complete CsI evaporation station was given on loan to Stony Brook from INFN/ISS Rome (Thank you Franco Garibaldi !) Produces 4 photocathodes per evaporation Deposit 2400 – 4500 Å 2 nm /sec Vacuum ~ Torr Contaminants measured with RGA Measures photocathode quantum efficiency in situ from nm over entire area Photocathodes transported to glove box without exposure to air Virtually no water ! Small “chicklets” evaporated at same time for full QE measurement ( nm )

C.Woody, NSS-N15-1, October 27, New Voltage System LeCroy 1450 HV System 1471N HV Module Added Trip Detection Circuit Relay Board trips mesh + GEM whenever either trip M.Proissl

C.Woody, NSS-N15-1, October 27, Electronic Readout and Noise C-Y. Chi and T. Sakaguchi Readout system based on 8 channel, 12 bit, 65 MHz digitizer Full readout takes 12 samples  6 words Reduced readout takes sum of two pre samples, sum of two post samples, plus one sample in between  2 words Average sigma for pedestal noise ~ 1.5 ADC channels  0.3 p.e. 12 sample ADC readout

C.Woody, NSS-N15-1, October 27, Improved Gas Quality in Run 9 Output ~ 30 ppm H 2 O and < 5 ppm O 2 Scanning Monochrometer ( nm) Gas Transmission Monitor Input EastWest ~ 0 ppm H 2 O and O 2 Gained several photoelectrons per electron track relative to Run 7 High flow rate ( ~ 4 lpm) Recirculating gas system B.Azmoun and R. Pisani

C.Woody, NSS-N15-1, October 27, Gain Stability vs Pressure and Temperature Large intrinsic gain variation due to P/T P/T variation at BNL over Run 7 HV was adjusted whenever P/T changed outside a range of ~.03 (five P/T HV settings ) Gain generally remained stable to < 10% for all properly working modules

C.Woody, NSS-N15-1, October 27, Gain Stability EastWest West detector generally performed better than East

C.Woody, NSS-N15-1, October 27, Overall Background Rejection Require tracks match with clusters in HBD Reject double clusters (> 30 p.e.) Reject clusters with nearby hits Reject single pad clusters Rejection cuts applied : Preliminary results (in p-p) indicate large (~ factor 30) reduction in combinatorial background while preserving vector meson signal Compare electron pair mass spectrum with and without HBD cuts I.Ravinovich Electron pairs – Central Arms (200 GeV pp) 