Claudia Höhne - GSI Darmstadt, Germany CBM collaboration

1. Concept for a RICH detector for the CBM experiment at the future accelerator facility FAIR at GSI in Darmstadt Claudia Höhne - GSI Darmstadt, Germany CBM collaboration

2. Outline • context of the RICH detector • CBM @ FAIR • CBM physics • requirements for RICH detector • design of RICH detector • mirrors • photodetector • radiator • simulations • summary - outlook - future plans

3. SIS 100 Tm SIS 300 Tm U: 35 AGeV p: 90 GeV CBM @ FAIR • Facility for Antiproton and Ion Research • „next generation“ accelerator facility: • double-ring synchrotron • simultanous, high quality, intense primary and secondary beams • cooler/ storage rings (CR, NESR, HESR) Cooled antiproton beam: hadron spectroscopy Ion and Laser induced plasmas: High energy density in matter Structure of nuclei far from stability Compressed Baryonic Matter

4. RHIC SPS SIS300 hadronic phase nuclei CBM physics Investigation of the phase diagram of strongly interacting matter lattice QCD : Fodor / Katz, Nucl. Phys. A 715 (2003) 319 • high T, low mB •  top SPS, RHIC, LHC • low T, high mB •  SIS • intermediate range ?  low energy runs SPS, AGS  SIS 300 @ GSI ! Critical point? Deconfinement? • Highest baryon densities • → in medium properties of hadrons(r, w, f), restoration of chiral symmetry? dense baryonic medium dilute hadron gas

5. CBM experiment Compressed Baryonic Matter experiment • tracking, vertex reconstruction: radiation hard silicon pixel/strip detectors (STS) in a magnetic dipole field • electron ID: RICH & TRD (& ECAL)  p suppression  104 • hadron ID: TOF (& RICH) • photons, p0, m: ECAL • high speed DAQ and trigger

6. RICH detector in CBM task of RICH detector • precise measurement of e+e- pairs from the decay of r, w, f mesons within a large acceptance (p-suppression ~ 10-4–10-3) • improve p/K separation at higher momenta • (kaon ID by TOF quickly deteriorates above 4 GeV/c) detector requirements • radiator with high threshold (gth > 40) • → pp,th ~ 5-6 GeV/c, 90% of qe reached at 12-13 GeV/c • sufficient radiator length for generation of Cherenkov photons (Ng >> 10), small radiation length, good UV transparency of radiator gas • low material budget (holds for all detector parts) to minimize secondary interactions and in particular e+e- pairs from g-conversion • large, continuous mirror-surface with excellent optical properties • fast photodetectors (107 Hz) with wide detection range, high qe, high granularity

7. RICH design • 2.2m long gas radiator • gas vessel with beam pipe in the center • 2 mirror and 2 photo-detector planes (vertically separated) • mirror: Be+glass, R=450cm • 2 x (450cm x 175cm) • photo-detector: PMT plane • shielded by magnet yoke • 2 x (280cm x 140cm) • support structures preferably from side

8. heater High temperature (~600C) glass beryllium RICH mirror (IHEP Protvino, Russia) • spherical mirror, R=450cm • Be hexagons (3mm thick, maximum diameter 60cm, 1.3kg) covered with 0.5mm glass • → 1.25% of X0 • glass polishing, Al covering, SiO2 coating → 92% total reflectivity in wide wavelength range • excellent optics, no degradation and radiator gas pollution due to long exposition in a radiation hard environment expected production: assembly:

9. RICH mirror • prototype available: • optical surface roughness sh = 1.6nm (after glass polishing, Al covering and SiO2 coating) • → diffuse reflection of only 12% of total for l = 150nm • image diameter of a point source D0 = 0.4mm (contains 95% of reflected light) • → angular deviation from nominal curvature sq = 0.03mrad Be plate for LHCb Be-mirror prototype

10. PMTs (IHEP Protvino + Moscow Electrolamp) • PMT FEU-Hive • K2CsSb photo-cathode, 25% quantum efficiency at l = 410nm • to be covered with transparanet WLF film (p-theraphenyl) → 22% qe for wide range • ~90% geometrical efficiency

11. PMT FEU-Hive • external PMT diameter 6mm • photo-cathode diameter 5mm • → ~105 channels per detector plane • length 6cm • high voltage ~ 2kV • effective number of dynodes 12 • amplification 106 • → effective operation in one-photoelectron regime • power dissipation 40mW • noise current ~ 3000 e-/s • capacitance 10-15 pF • dynamical range of signal charge • Q = (0.25-25) 106 e- • average signal time ~ 1ns

12. radiator • no window between radiator and photo-detectors: He, N2, CH4 • fluorescence? CH4 as quenching gas in mixture? • gth > 40, UV transmittance, radiation length! • ideal would be an easy handling (gas system) n gthqc pp,thlth X0 He 1.000035 119.5 0.48° 16.7 GeV/c ~ 50nm 5300m N2 1.000298 41 1.4° 5.72 GeV/c ~ 80nm 304m CH4 1.000444 33.6 1.7° 4.68 GeV/c 145nm 650m 60%N2 + 40%CH4 1.000356 37.5 1.53° 5.25 GeV/c 145nm 386m 40%He + 60%CH4 1.0002804 42.2 1.36° 5.9 GeV/c 145nm 999m

13. 3.3m 4.7m 2m Simulation (GEANT3) • CBM detector simulation framework • GEANT 3, GEANT 4 • implementation of RICH detector • Cherenkov properties of materials from HADES, literature beam • study basic properties of current detector concept • prove feasibility of desired p-suppression • optimize geometrical design, optical layout Gasbox: 250 mm aluminum

14. one quarter of mirror/ photodetector: f = 80o 60o 40o q = 5o 10o 15o 20o 25o 30o 35o 20o Imaging properties of mirror • rings(q,f) - q polar angle, • f azimuth angle • no diffusion at reflection • no magnetic field, no multiple scattering • → eccentricity for large q,f optimize optical design of detector!

15. Single particles • e/p separation (depending on radiator) up to 11-14 GeV/c • p identification from 5-7 GeV/c to 11-14 GeV/c • diameter of ring 10.6-12 cm ≈ 17-20 PMTs • wide acceptance covered single e- acceptance radiator 40%He+60%CH4

16. figure of merit Cherenkov spectrum for N2 40%He + 60%CH4 lmin Ng N0 [cm-1] NPMT 120nm 33 292 25 200nm 23 204 18 250nm 15 138 11 300nm 11 93 8 Ng= 1.3 NPMT Importance of continuation of the development of PMTs with large qe in the UV range!

17. UrQMD event 35 AGeV central Au+Au • about 40 rings/event: • 33 electrons (~13 from primary vertex) • 7 pions • 0.1 muons

18. UrQMD event 35 AGeV central Au+Au • z-coordinates of track vertices for particles detected in RICH: • target (z = 0cm) - 125mm Au • 7 STS – 2x100mm, 5x200mm Si • (z = 5,10,20,40,60,80,100 cm) • beampipe (z ~ 20-30 cm) • magnet yoke (z ~110-140 cm) optimize CBM detector layout to further suppress e+e- pairs from g-conversion!

19. p misidentification • first estimation • assume 100% ring finding, match closest track to a certain ring • large number of charged tracks per event, additional information available from TRD, TOF ideal tracking 1% momentum resolution

20. summary - outlook - future plans concept of RICH detector for the CBM experiment introduced optimize detector layout (optics system!) continue RICH R&D (radiator, mirror, PMT) 2006/2007 RICH prototype 2006/2007 2007/2008 beam tests of RICH prototype 2007/2008 2008/2009 final RICH design 2009/2010 RICH production - 2012 installation, commissioning, beam!