Setup & tests of prototype bars

1. p Optical Properties of Bars for the PANDA Barrel DIRC Particle Track FocusingOptics Fused SilicaRadiator Grzegorz KalicyGSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany Roland Hohler, Dorothee Lehmann, Klaus Peters, Georg Schepers, Carsten Schwarz, Jochen Schwiening DetectorSurface Cherenkov Photon Trajectories PID for PANDA AT FAIR DIRC Principle DIRC PANDA: antiProton ANnihilation at DArmstadt DIRC: Detection of Internally Reflected Cherenkov light A charged particle traversing a radiator with refractive index n(l) withb = v/c> 1/n(l) emits Cherenkov photons on a cone with half opening angle If n > 2 some photons are alwaystotally internally reflected for b  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 timeof Cherenkov photons, definingqc, fc,tpropagationof photon. • Particle identification (PID) for PANDA will be performed by several specialized detectors. • For target spectrometer: • Barrel DIRC (3σπ/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. The PANDA Experiment at FAIR The PANDA Experiment at FAIR Expansion volume Barrel DIRC(22o –140o) Endcap Disk DIRC (5o –22o) Mirror DIRC PANDA Barrel DIRC • 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. Number of reflections for track perpendicular to the bar Counts Photon detectors and electronics PANDA Barrel DIRC DESIGN Geant Cherenkov photon tracking in event display Cherenkov photons Photon detectors and electronics Number of internal reflections Radiator bars PANDA Barrel DIRC DESIGN Bar boxes • 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. Particle track Radiator bars Surface Roughness 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. Map of transmitted intensity T Bar with 532 nm laser beam Bar with 532nm laser beam Setup & tests of prototype bars 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 Measured coefficient of total internal reflection for prototype bar for three wavelengths compared to expectation from scalar theory of scattering Reflection Coefficient 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) Setup & tests of prototype bars Wavelength [nm] • 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. 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.