1. Scan ~100 bar entry positions with laser diode measures transmitted intensity (relative to reference intensity) determine attenuation length (Λ) by aiming laser down length of bar (correct for Fresnel) determine reflection coefficient (R) by bouncing laser off bar surfaces for 80 cm long bar 31 internal reflections for bar faces, 15 for sides calculate R from mean transmitted intensity (T) at the Brewster angle calculate surface roughness (σ) using scalar theory of scattering Setup & tests of prototype bars PANDA Barrel DIRC Optical Properties of Bars for the PANDA Barrel DIRC Grzegorz Kalicy GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Roland Hohler, Dorothee Lehmann, Klaus Peters, Georg Schepers, Carsten Schwarz, Jochen Schwiening DIRC The PANDA Experiment at FAIR PANDA Barrel DIRC designed as a Fast Focusing DIRC. Basic approach similar to BABAR-DIRC. Important improvements: Focusing optics remove size of bar from Cherenkov angle resolution term. Faster timing (100 ps or better) allows partial correction of chromatic effects. Compact multi-pixel photon detectors allow smaller expansion region. Surface Roughness DIRC Principle DIRC: Detection of Internally Reflected Cherenkov light A charged particle traversing a radiator with refractive index n( ) with  v/c  > 1/n( ) emits Cherenkov photons on a cone with half opening angle If n >  2 some photons are always totally internally reflected for  1 tracks. Radiator and light guide: Long, rectangular Synthetic Fused Silica bars. Photons exit via focusing lens into expansion region. Imaging on MCP-PMT array DIRC is intrinsically a 3-D device, measuring: x, y and time of Cherenkov photons, defining  c  c  t propagation of photon. HK 53.28, DPG Spring Meeting, Mainz, May 2012 G.Kalicy@gsi.de Work supported by EU FP6 grant, contract number 515873, DIRACsecondary-Beams, and EU FP7 grant, contract number 227431, HadronPhysics2, and the Helmholtz Graduate School for Hadron and Ion Research HGS-HIRe. Setup & tests of prototype bars Bar with 532nm laser beam Example: scan of bar sides at 532nm Attenuation length Λ = (385 ± 204) m N = 15 reflections T = 0.9914 ± 0.0019a R = 0.99961 ± 0.00016 Surface roughness: σ = (15 ± 3) Å (all errors dominated by systematics) Detector Surface Fused Silica Radiator Particle Track Cherenkov Photon Trajectories Focusing Optics PANDA: antiProton ANnihilation at DArmstadt PANDA Barrel DIRC D ESIGN Measured coefficient of total internal reflection for prototype bar for three wavelengths compared to expectation from scalar theory of scattering Radiator bars Photon detectors and electronics Map of transmitted intensity T Number of reflections for track perpendicular to the bar Bar with 532 nm laser beam Motion - Controlled Setup Measurements of coefficient of total internal reflection (R) and bulk attenuation (Λ) of radiator bars at multiple laser wavelengths → determine quality of surface finish with few Å accuracy. Radiator bars Photon detectors and electronics Bar boxes Geant Cherenkov photon tracking in event display More than 200 internal reflections possible before photon will exit the bar. To transport 90% of internally reflected photons down the bar reflectivity at the level of 0.9995 is needed. Cherenkov photons Particle track Particle identification (PID) for PANDA will be performed by several specialized detectors. For target spectrometer: Barrel DIRC (3σ π/K separation for momentum range 0.5 - 3.5 GeV/c) Endcap Disk DIRC Time-of-Flight system dE/dx of tracking system Momentum distribution in barrel region is an excellent match to DIRC range. Endcap Disk DIRC (5 o – 22 o ) Barrel DIRC (22 o –140 o ) p Next steps: Expand wavelength range using a UV laser (266 nm). Measure prototype bars from additional vendors. Qualify the production and polishing processes of the different bar manufacturers. Mirror PID for PANDA AT FAIR Expansion volume Wavelength [nm] Reflection Coefficient Number of internal reflections Counts