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Hadron Blind Detector for PHENIX experiment at RHIC

This paper discusses the development and implementation of a Hadron Blind Detector (HBD) for the PHENIX experiment at RHIC. The HBD is designed to measure low mass di-electrons and investigate the properties of particles created in newly discovered matter. The paper outlines the physics, principles of operation, detector concept, and progress of the HBD.

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Hadron Blind Detector for PHENIX experiment at RHIC

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  1. Hadron Blind Detector for PHENIX experiment at RHIC Alexander Milov (for the PHENIX collaboration) Jun 02, 2006 Alexander Milov CIPANP06 June 02, 2006

  2. Outline • Physics: • Low mass pair measurements with di-electrons. • HBD principles of operation: • Original ideas • Their implementation • Detector concept • R&D • HBD progress • Full scale prototype in PHENIX • Detector under construction • Summary Alexander Milov CIPANP06 June 02, 2006

  3. Why low mass di-electrons? • A unique probe to investigate newly discovered matter: • Particles built with light quarks should have different properties in created media than in vacuum: mass, width, branching ratios. • Short living particles, decaying before freeze-out. • Decay products leave media undisturbed by strong interactions. • Decays we are interested in: • ρ (m = 770MeV τ ~ 1fm/c)  e+e- • ω (m = 782MeV τ ~20fm/c)  e+e- • φ (m =1020MeV τ ~40fm/c)  e+e- • Here is the signal! • With HUGE combinatorial background Alexander Milov CIPANP06 June 02, 2006

  4. Why low mass di-electrons? Signal quality we need • A unique probe to investigate newly discovered matter: • Particles built with light quarks should have different properties in created media than in vacuum: mass, width, branching ratios. • Short living particles, decaying before freeze-out. • Decay products leave media undisturbed by strong interactions. • Decays we are interested in: • ρ (m = 770MeV τ ~ 1fm/c)  e+e- • ω (m = 782MeV τ ~20fm/c)  e+e- • φ (m =1020MeV τ ~40fm/c)  e+e- NA60 (CERN) Lower energy • Here is the signal! • With HUGE combinatorial background Alexander Milov CIPANP06 June 02, 2006

  5. New RICH for PHENIX? • This volume is in the magnetic field • If we put mirrors inside, where do we send light to? • Let’s get rid of mirrors and put detector right in the beam • Possible, but… • it still must be thin • it has to detect a single UV photon and be blind to all ionizing particles passing through it!!! Alexander Milov CIPANP06 June 02, 2006

  6. The original idea • The original idea by Y.Giomataris and G.Charpak 1991 (NIMA 310) • Electrons from Cherenkov light are produced at the photocathode and amplified full way. Mesh Cherenkov photon HV • Electrons from primary ionization are produced at random and are amplified much less (exponential law). Ionizing particle Photocathode Alexander Milov CIPANP06 June 02, 2006

  7. Original idea modified • Problem: such setup requires a window. • Radiator gas must be transparent. • The avalanche in the gas contains as many photons as electrons • What happens if photons shine back on photocathode? • We need to separate the radiator and detector volume by a window HV • And get even more problems… • Windows are bulky • Window is a perfect source of the Cherenkov photons from any particle! • Let’s get rid of the window! Alexander Milov CIPANP06 June 02, 2006

  8. Original idea modified light conversion probability • Solution: Pull electron through! • Photomultipliers do exactly that. overall: product of two Photocathode • But at a significant cost of efficiency • Light conversion probability is the best at the upper surface • Electron extraction probability is the best on the lower surface • They work against each other resulting in small overall efficiency electron extraction probability HV • Much better way to get electron on the same side and then pull it through Alexander Milov CIPANP06 June 02, 2006

  9. Gas Electron Multiplier (GEM) 150μ • The original idea by F.Sauli (mid 90s) US Patent 6,011,265 • HV creates very strong field such that the avalanche develops inside the holes • The same field is sufficient to pull electrons from the surface into holes • Photon feedback is not there: GEMs screen the photocathode Alexander Milov CIPANP06 June 02, 2006

  10. The concept. HV • Take a GEM • Put a photocathode on top • Electron from Cherenkov light goes into the hole and multiplies • Use more GEMs for larger signal • Pick up the signal on pads • What about ionizing particle? • Mesh with a reverse bias drifts ionization away from multiplication area • Sensitive to UV and blind to traversing ionizing particles Alexander Milov CIPANP06 June 02, 2006

  11. Photocathode and gas. • Photocathode: • CsI is an obvious choice. • We are using INFN built evaporator, currently at Stony Brook to do this project. • High area, • High vacuum, • In-situ Q.E. control, • Zero exposure to open air. • Gas CF4 (well known): • Transparent up to 11.5 eV, makes perfect match to CsI • Is a good detector gas. Alexander Milov CIPANP06 June 02, 2006

  12. Photocathode and gas. • Photocathode: • CsI is an obvious choice. • We are using INFN built evaporator, currently at Stony Brook to do this project. • High area, • High vacuum, • In-situ Q.E. control, • Zero exposure to open air. • Gas CF4 (was not really known): • Has high electron extraction probability • Has avalanche self quenching mechanism • Gas CF4 (well known): • Transparent up to 11.5 eV, makes perfect match to CsI • Is a good detector gas. Alexander Milov CIPANP06 June 02, 2006

  13. Gas purity. • Even CF4 is transparent, oxygen and especially water can absorb UV light • Water blocks the UV in the region of highest CsI efficiency. • More water in presence of ionization can damage the detector physically. • Oxygen is a good photo absorber too. • Desired levels: • H20 < 10 ppm • 02 < 5 ppm. Transmission for high purity Ar and CF4 Alexander Milov CIPANP06 June 02, 2006

  14. The design. 72 36 Number of photoelectrons Made of 2 units with R~60cm, the volume is filled with CF4 magnetic field is turned off Electrons emit Cherenkov light Cherenkov light is registered by 12 photo-detectors in each unit Signal is read out by 94 pads in each unit, pad size ~ size of a circle Accumulating ~36 photoelectrons from each primary electron, while most other operational RICHes have ~15 or less. High statistics allows to separate 2 close electrons even if their signals overlay! Alexander Milov CIPANP06 June 02, 2006

  15. Final detector construction • Final HBD under construction. • Mechanical parts for the cage and GEM on frames are being produced at the Weizmann Institute in Israel. • First GEMs were shipped from WIS to SUNYSB on Sunday for CsI coating. • Final electronics (Nevis Columbia) is waiting test results. • McPherson gas transparency monitor to arrive at BNL this month. Inner side ~1.2m Detector panels glued on a jig Outer (copper) side Alexander Milov CIPANP06 June 02, 2006

  16. The full scale prototype. • Full scale prototype was moved into PHENIX last Wednesday. • 1 instrumented GEM sector. • Final electronics. • Integrated into PHENIX DAQ. • We expect first results very soon. Alexander Milov CIPANP06 June 02, 2006

  17. Summary: • PHENIX detector at RHIC can investigate a new class of probes to study newly invented sQGP. • It requires to built a revolutionary detector based on novel ideas and techniques. Figure of merit for such detector is 6 times more than any detector built so far. • The concept of a new detector has been developed over last several years backed up by extensive R&D and simulations. • The final detector is now in construction. The prototype installed in PHENIX right now and is taking data. • Full HBD will be installed during next physics run. Alexander Milov CIPANP06 June 02, 2006

  18. Highlights: • CsI quantum efficiency and CF4 transparency make a perfect match. • Amplification field is sufficient to pull all electrons from GEM surface into holes. • GEM acts as a semitransparent photocathode providing high Q.E. • GEM geometry eliminates photon feed back from amplification region onto the photocathode. • Self quenching mechanism works for GEMs operating in pure CF4 • High electron extraction efficiency is measured into pure CF4 • Given optical purity is observed CF4 chemical activity is not an issue Alexander Milov CIPANP06 June 02, 2006

  19. BACKUPS Alexander Milov CIPANP06 June 02, 2006

  20. Event display (simulation). Alexander Milov CIPANP06 June 02, 2006

  21. Background sources? • In the decays contributing to the background: • π0  e+ e- γ • π0  γγ  e+ e- γ • Only one electron is detected in PHENIX and another is lost • To cut the background we need a new detector such that: • It sees only electrons • Located at the origin • It does not produce its own background (is thin) • … • … • … ~12 m Alexander Milov CIPANP06 June 02, 2006

  22. “Classic” Cherenkov Detector Cherenkov light detector unit window ring primary particle gas with n~1.0006 mirror 1m+ • Classic RICH (Ring Imaging Cherenkov Counter) has following parts • gaseous radiator (n ~ 1.0004 – 1.0006) • VUV mirror • window CaF2 (cheaper) LiF (better) • photo-detector (gaseous or PMT) HADES @GSI Alexander Milov CIPANP06 June 02, 2006

  23. Some technical details. Number of photons (n-1)2/λ2 λ • Photocathode choice: • There are no many options for solid photocathode. • CsI evaporated onto surface is pretty much the only choice • Gas: • As transparent in UV as possible. • As high refraction index as possible. • But still usable as a detector working gas. • The best possible choice is CF4 • Chemists in the room should throw a flag! • What if moisture gets in the detector? • H20 + CF4 + e-  HF + X?  • Physicists in the room may stay calm… • Moisture in the detector kills UV transparency and CsI much before it kills the detector  • Monitoring gas transparency and humidity on PPM level is required Alexander Milov CIPANP06 June 02, 2006

  24. What to do with background? • Main difference between the signal and background is due to the mass of the primary particle. • Compare these two: φ 1020MeV π0 135MeV Same Momentum • Here is a solution: we need to eliminate close e+e- pairs which are due to background and use the rest for the analysis. Alexander Milov CIPANP06 June 02, 2006

  25. What are we looking at? • We want to distinguish between two different states of matter: • Normal nuclear matter where quarks are confined in triplets • sQGP where quark interact “freely” with other quarks around • OK, we need a probe! • Probe interacts in a differently way with different media • Oops! • If probe leaves the media it’s no different as if it never been there. • We need another probe • Better now! • But we still need to catch up what is left from the probe! Alexander Milov CIPANP06 June 02, 2006

  26. What are we looking at? • Let’s look at the probe closer. • We are interested what happens to the daughter particles of our probe • Decays shall leave the media. If during their journey the interaction continues we loose the information! • So we need a probe, whose decay products go out without interacting with a media Alexander Milov CIPANP06 June 02, 2006

  27. What does it look like • All raw materials (FR4 sheets, honeycomb, HV resistors, HV connectors) ordered and most of them in house • Detector box design fully completed • Jig design underway • Small parts (insert, pins, screws, HV holders..) in the shops • Detector construction to start Nov. 1st • PCB design almost complete • Detailed construction schedule foresees shipment of boxes to SUNY in January 2006. Alexander Milov CIPANP06 June 02, 2006

  28. Mechanical parts and PCB. PCB final design. Quick MC shows no difference with standard cells Entrance window frames are ready, the window itself to be tight between them Alexander Milov CIPANP06 June 02, 2006

  29. Gas monitoring system. Input and two outputs measured by switching gas Input and two outputs measured by moving mirror. More expensive but clearly better Alexander Milov CIPANP06 June 02, 2006

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