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A Hadron Blind Detector for the PHENIX Experiment at RHIC

A Hadron Blind Detector for the PHENIX Experiment at RHIC. Babak Azmoun for the PHENIX Collaboration Brookhaven National Laboratory. October 13, 2004. The PHENIX HBD Team. Weizmann Institute of Science; Rehovot, Israel

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A Hadron Blind Detector for the PHENIX Experiment at RHIC

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  1. A Hadron Blind Detector for the PHENIX Experiment at RHIC Babak Azmoun for the PHENIX Collaboration Brookhaven National Laboratory October 13, 2004

  2. The PHENIX HBD Team • Weizmann Institute of Science; Rehovot, Israel • I.Tserruya, Z. Fraenkel, A. Kozlov, M. Naglis, I. Ravinovich, L. Shekhtman • Brookhaven National Lab; NY, USA • B.Azmoun, C.Woody • Stony Brook University; NY, USA • A. Milov, A. Drees, T. Hemmick, B. Jacak, A. Sickles • University of Tokyo; Tokyo, Japan • T.Gunji, H. Hamagaki, M. Inuzuka, T. Isobe, Y. Morino, S. Oda, K. Ozawa, S. Saito, T. Sakaguchi • Waseda University; Tokyo, Japan • Y.Yamaguchi • Riken; Wako, Japan • S. Yokkaichi • KEK; Tskuba-shi, Japan • S. Sawada Babak Azmoun, BNL

  3. Outline • Motivation (Physics case) • System specifications and detector concept • GEM +CF4+CsI R&D • Hadron blindness • Hadron rejection (alpha particles - in lab) • Pion rejection (KEK) • Detection efficiency • CsI quantum efficiency and expected N0 • Optical Properties of CF4 / Effects of gas impurities • Beam test @ PHENIX & KEK • Summary Babak Azmoun, BNL

  4. Motivation • In ordinary matter, theStandard Model of particle physics states that Chiral symmetry, which is a symmetry between light quark flavors, is normally broken due to constituent quark masses. However,at high temperatures and/or high baryon densities, such as those produced in relativistic heavy ion collisions at RHIC, this symmetry may be restored. • Low mass e+e- pairs are the best probe for Chiral Symmetry Restoration • Effects of CSR may have been seen in CERN SPS results • The same effects are predicted to occur at RHIC, but will be difficult to detect • Nevertheless, the RHIC program would be incomplete without a low mass lepton pair measurement • PHENIX is the only experiment at RHIC that could perform this measurement Babak Azmoun, BNL

  5. Low Mass Electron Pairs at RHIC • Strong enhancement of low-mass pairs persists at RHIC • Contribution from open charm becomes significant R. Rapp nucl-th/0204003 • Possibility to observe in- medium modification of the intermediate vector mesons (r,f,w): • Dropping of r invariant mass (Rapp & Wambach) • Broadening of vector meson (r, w, f) invariant masses (Brown et.al) • Thermal radiation from hadron gas (small…) pp rg*  e+e- Babak Azmoun, BNL

  6.  e+ e - po   e+ e- “combinatorial pairs” total background S/B ~ 1/500 Irreducible charm background all signal charm signal Experimental Challenges at RHIC • Both members of the electron pair are needed to reconstruct the Dalitz pair or conversion. • Single electron pair members contribute to background: • Limited by: • Low pT acceptance of outer PHENIX detector: • ( pT > 200MeV) • Limited geometrical acceptanceof present PHENIX • configuration Large combinatorial pair background due to copiously produced photon conversions and Dalitz decays : Need rejection factor > 90% of  e+ e - andp0  e+ e - Would like to improve S/B by ~ 100 - 200 Babak Azmoun, BNL

  7. Upgrade Concept: Utilize HBD to Identify and Reject background Hardware * Compensate magnetic field with inner coil to preserve (e+e-) pair opening angle (foreseen in original design  B0 for r  50-60cm) * Compact HBDin inner region (possibly to be complemented by a TPC or other tracking detector in the future). Strategy * Identify signal electrons (low mass pairs) with p>200 MeV in outer PHENIX detectors * Identify low-momentum electrons (p<200 MeV) in HBD * Reject pair if opening angle < 200 mrad (for a 90% rejection). Specifications * Electron efficiency  90% * Double hit recognition  90% * Modest  rejection ~ 200 Babak Azmoun, BNL

  8. Electron Pairs produce Cherenkov light, but Hadrons with P < 4 GeV/c do not: • b(Dileptons)>1/n • b(Hadrons) <1/n HBD Layout: Windowless Cherenkov Detector HBD Gas Volume: Filled with CF4 Radiator (nCF4=1.000620, rRADIATOR =50cm) Cherenkov blobs on image plane (Qmax = cos-1(1/n)~36 mradrBLOB~3.6cm) e+ Radiator gas = Working Gas e- q Pair Opening Angle Triple-GEM micropattern readout detector, (8 panels per side) Space allocated for services Dilepton pair Beam Pipe Babak Azmoun, BNL

  9. Reflective CsI photocathode (CsI coating on top surface of upper-most GEM foil) • No photon feedback • Proximity focus  detect blob • Low granularity (pad size~10cm2) • Detector element: multi - GEM • High gain (CF4~104) • Reduced ion feedback • Concept: • Windowless Cherenkov detector. • Same radiator and detector gas. • preferred option: CF4 (~50cm) • Large bandwidth and large Npe • (Bandwidth not limited by a window) The HBD Babak Azmoun, BNL

  10. R&D Program TOPICS • GEM Systematics (using CF4): gain Vs voltage, energy resolution, gain stability, etc. • Optical properties of CF4: transmittance, effects of gas contaminants • CsI: quantum efficiency, compatibility with CF4, aging, etc. • Simulation studies • Prototype/Beam Test: N0 measurement (Npe), hadron blindness (photoelectron efficiency & p-rejection), performance evaluation of GEM detector in high multiplicity environment. Babak Azmoun, BNL

  11. Fe55 x-ray UV lamp Gain Curve: Triple GEM with CsI and CF4:measured with Fe55 and with UV lamp • GEMs work with CsI and CF4! • Pretty good agreement • between gain measured • with Fe55 and UV lamp. • Gains in excess of 104 are • easily attainable. • Voltage for CF4 is ~140 V • higher than for Ar/CO2 but • slopes are similar for both • gases. • Gain increases by factor ~3 • for ΔV = 20V Babak Azmoun, BNL

  12. Aging CsI photocathode: * In spite of large ion back-flow there is no dramatic deterioration of the CsI QE. * For a total irradiation of ~10mC/cm2 , the QE drops by only 20%. (The total charge in 10 years of PHENIX operation is conservatively estimated to 1mC/cm2.) Stability measurements performed during day 3 (4 mC/cm2 ), day 4 (3 mC/cm2 ), day 5 (2 mC/cm2 ). GEM foils: * During the whole R&D period we never observed aging effects (e.g. loss of gain) on the GEM foils. Total irradiation was well in excess of 10mC/cm2 . Babak Azmoun, BNL

  13. At ED = 0: - signal drops dramatically as anticipated. - rate also drops dramatically large hadron suppression Charge Collection in Drift Gap : Mean Amplitude Rate Babak Azmoun, BNL

  14. D ED (+) G ET T G pA T ET G I EI ED = 0 Single Photoelectron Detection Efficiencymeasure detector response vs ED at fixed gain IPE Very efficient detection of photoelectrons even at negative drift fields !! Babak Azmoun, BNL

  15. Hadron Blindness (I): UV photons vs.  particles At slightly negative ED, photoelectron detection efficiency is preserved whereas charge collection is largely suppressed. Babak Azmoun, BNL

  16. CsI absolute QE: set-up Rotatable UV mirror Bandwidth: 6.2 – 10.3 eV PMT-2 and CsI have same solid angle C1 optical transparency of mesh (81%) C2 opacity of GEM foil (83.3%) All currents are normalized to I(PMT-1) QE(CsI) = QE(PMT-2) x I(CsI) / I(PMT-2) x C1 x C2 Babak Azmoun, BNL

  17. Cherenkov Spectrum: Output~1/l2 CsI QE tracks Cherenkov Spectrum into the deep UV-VUV regime • QE measurement is limited by dynamic range of Spectrometer (due to LiF windows): • Spectrometer lcutoff ~114nm ~10.8eV So… • Folding the measured CsI QE into the Cherenkov Spectrum gives an effective N0: • Measured in range 6.2-10 eV = 414 cm-1 in CF4 But… • N0 extrapolated to 11.5 eV (CF4 cutoff) = 915 cm-1 • Optimum expected value ~ 940 cm-1 (Original PHENIX HBD proposal) CsI QE: results Babak Azmoun, BNL

  18. VUV Transmission in the presence of gas Impurities • Transmittance: T = Is/Ivac(PMT Current Ratio: Sample-gas scan/Vacuum-Ref. Scan) Data in prevailing literature agrees with our data: • Clear correspondence between water and oxygen levels and the degree of absorbance • Tolerance Level: • [H2O]max ~ 15-20ppm • [O2]max ~ 5ppm Attenuation Coefficient in O2 Interaction Cross Sect. in H2O Babak Azmoun, BNL

  19. HBD Beam Test @ KEK PbGl Cal S3 50x45mm HBD S22 10x20mm C2 C1 S1 100x45mm 50cm CF4 Radiator p-(~98%), e-(~2%) [947] [845] [15.3] [cm] GEM D2 lamp Retractable Fe55 source Additional detectors including TPC, CNS-HBD, and an array of Silicon strips detectors ED = +1 KV/cm ED = -0.3 KV/cm Electrons Electrons 35 e=26e(1.38*19e) + 9pe 6 pe (efficiency ≠100%) • Low number of pe’s is due to absorption within the gas (presumably because of O2 and H20 impurities) • It was therefore not possible to make a definitive measurement of N0 Pions Pions ~3e ~19 e Babak Azmoun, BNL

  20. HBD Response Simulation Includes 20 cm absorption length in CF4, lamp shadowing, realistic losses and conservative N0 = 840 cm-1 Normal case, no absorption in CF4, no lamp shadowing, realistic losses and conservative N0 = 840 cm-1 Total signal: 38 e = 29 (dE/dx) + 9 (Cherenkov ) Blob size: single pad response =78%  very similar to data Total signal: 62 e = 29 dE/dx + 33 Cherenkov Blob size: single pad 12% more than one pad 88% Babak Azmoun, BNL

  21. Pion Rejection Applying a pion discriminator threshold @ ch.~60, greatly minimizes the number of pions detected, while maintaining ~100% of the pe signal Pion rejection factor will depend on the threshold that one can safely apply without the loss of the photolectron signal, and in turn depends on the number of photoelectrons detected. ED = -0.3 KV/cm Measured Pion spectrum Expected electron spectrum ~ 33 pe (qualitatively simulated: yield is not to scale) • blob signal is shared among 3-4 pads  ~10 pe’s per pad • Pion signal is contained within a single pad • The total pe signal will also receive a noise contribution from adding the noise from the 3-4 pads, the peak separation here may be a bit idealized in this respect Cut @ ADC ch.~60 Babak Azmoun, BNL

  22. Std. Conical, Segmented CERN Foils Active area of pads ~1.0x1.2cm2 Test of triple-GEM detector in PHENIX IR PHENIX IR ADC pulse height distr. (Ar/CO2) in Lab ADC Pulse height distr. (CF4) during Full Luminosity A-A Collisions in RHIC Gain Variation: +/-10%, as in the lab Added Background • The triple GEM detector performed smoothly within the PHENIX IR using both Ar/CO2 (70/30) and CF4 working gases and exhibited no sparking or excessive gain instabilities. • The operation of the GEM and the associated electronics were not hindered by the presence of the ambient magnetic field generated by the central magnet within the IR. • However, in close proximity to the beam pipe (50cm), the detector was sensitive to beam related background. • The fundamental implication of these tests is that the incorporation of a GEM detector among the inner PHENIX detectors is quite feasible when considering how stable the GEMs’ performance was in such a high multiplicity environment. Babak Azmoun, BNL

  23. Summary HBD: Exploits the concept that using GEMs, one can build a detector which is very efficient for detecting Cherenkov photons while being very insensitive to direct ionization from charged tracks. • GEM: • High gain/stable operation in pure CF4. • Applicable in the high multiplicity environment of PHENIX • Flexible design: provides surface for CsI evaporation, multi-stages are possible, and large active area with thin profile • CsI: • High Quantum efficiency in VUV region where the Cherenkov spectrum peaks • CF4: • High N0 (Cherenkov radiator)…to be confirmed • Transparent in the VUV • High gain capability (Detector working gas) • Compatible with CsI Future: N0 measurement, Measure CF4Scintillation, Test of realistic prototype in beam Babak Azmoun, BNL

  24. Additional Slides Babak Azmoun, BNL

  25. ~ ¾” honeycomb Readout Board and Preamps Preamp signals to shaper + ADC Hybrid Preamps with line drivers • Being developed by • BNL Instrumentation • Based on IO-535 • ± input signal • ± 2.5 V output wires Read pads ~ 3x3 cm2 GEMs • Need almost one full rack for the readout electronics Babak Azmoun, BNL

  26. Am241 D NP ED (+) G ET T G T ET G I EI Charge Collection in Drift Gap:(I) Am241 -spectra Babak Azmoun, BNL

  27. CF4 Transparency to UV Photons CF4 is transparent Babak Azmoun, BNL

  28. Micropattern Readout Detector: Gas Electron Multiplier (GEM) • General: • Invented by F. Sauli @ CERN in ~1995 • High precision micropattern readout w/ high/stable gain operation • Convenient geometry: possibility of multiple stages • Applicable in high rate environments • Electron avalanche takes place inside GEM holes: NO photon feedback to limit gain • Structure: • 50mm thick Kapton foil with 5mm copper cladding on both sides which act as electrodes. The foil is perforated with bi-conical shaped holes on the order of 50mm in diameter and a 120mm pitch distance. • Operation: • Amplification of charge from an incident electron is via an intense electric field inside GEM hole. • The field is generated by applying a potential between the electrodes of a few hundred volts. • The total charge collected from avalanche is on the order of thousands of times larger than charge of the incident electron, for a multi-stage GEM. • Geff ~ exp(DVFOIL) Charge transfer/Collection (1kV/cm) e- Avalanch (~40kV/cm) Babak Azmoun, BNL

  29. Summary of Measurements GEM R&D Set-Up • Gain measured in various gases: although the volt. potential across CF4 is higher for the same gain w.r.t. other gases, CF4 still produces sufficiently high gains (~103 -104). • Gain Vs Physical parameters: (e.g., temperature & pressure) vary the gain by up to ~100%. • FWHM energy resolution: for CF4~38% in a triple GEM structure, and ~20% for Ar/CO2 (70/30) • Gain Stability in Time: initial gain drift of ~50% is observed, but gain remains stable to within 10% after ~ 1hr. of continual operation (CERN GEM’s) • Charge collection and transfer efficiency through GEM stack: measured w.r.t. GEM field config.optimized field config. • Sparking probability in CF4 is low: discharge probability isn’t due to the gas, but to the quality of the GEM foils • Ion feedback: high, but harmless to CsI • Aging Studies: After accumulating 10mC/cm2, no significant change (~20%) in the CsI p.c. or the GEM’s behavior • Gain uniformity across GEM surface: ~25% variation in gain using 10x10cm GEM foil The GEM detector was extensively tested in the lab using both pure CF4 and Ar/CO2 gas mixtures for comparison. Detector Box GEM foils of 3x3 and 10x10 cm2 produced at CERN Babak Azmoun, BNL

  30. Hg lamp Absorber Independent of gas Mesh E=0 CsI GEM1 1.5mm GEM2 1.5mm Independent of Et GEM3 2mm PCB Depends only on EI (at low EI some charge is collected at the bottom face of GEM3) pA Ion Back Flow Fraction of ion back-flow defined here as: Iphc / IPCB Ions seem to follow the electric field lines. In all cases, ion back-flow is of order 1!!! Babak Azmoun, BNL

  31. Total Charge in Avalanche in Ar-CO2 and CF4 measured with Am241 Charge saturation in CF4 !!! When the total charge in CF4 exceeds 4 x 106 a deviation from exponential growth is observed leading to gain saturation when the total charge is ~2 x 107. Babak Azmoun, BNL

  32. 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 vs. ΔVGEM CF4 more robust against discharges than Ar/CO2 . HBD expected to operate at gains < 104 i.e. with very comfortable margin below the discharge threshold vs. Gain Discharge Probability alpha • Stability of operation and absence of • discharges in the presence of heavily ionizing • particles is crucial for the operation of the HBD. • Use Am241 to simulate heavily ionizing particles. alpha Babak Azmoun, BNL

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