Kalicy, G.

Krebs, M.

Peters, K.

Schwarz, C.

Schwiening, J.

GSI Repository

Optical properties of the PANDA barrel DIRC prototype radiator bars∗M. Krebs† 1,2, G. Kalicy1,2,3, K. Peters1,2, C. Schwarz2, and J. Schwiening21GSI, Darmstadt, Germany; 2Goethe-Universita¨t, Frankfurt, Germany; 3now at Old Dominion University, Norfolk,Virginia, USAThe PANDA experiment at the Facility for Antiprotonand Ion Research in Europe (FAIR) at GSI, Darmstadt,will study fundamental questions of hadron physics andQCD. The Barrel DIRC (Detection of Internally ReflectedCherenkov light) is a fast Ring Imaging Cherenkovcounter, which will provide hadronic particle identification(PID) in the central region of the PANDA target spectrome-ter. In order to meet the PID requirements, the Barrel DIRChas to provide precise measurements of the Cherenkovangle, which is the crucial quantity for the identificationof traversed particles. The Cherenkov photons propagatethrough the radiators by total internal reflection until theyreach the detection plane, which is equipped with opticalsensors. The radiators, long rectangular bars made fromsynthetic fused silica, have to meet very strict optical andmechanical requirements. Properties like squareness andparallelism of the sides, very smooth polished surfaces, andsharp corners are of critical importance to ensure that theCherenkov photons reach the optical sensors without angu-lar distortions.A motion-controlled system using lasers with three dif-ferent wavelengths (an additional UV laser will be addedlater), shown in Fig. 1, was set up in the new DIRC opticslab to measure important radiator quantities like transmit-tance and reflectivity.Lasers405 nm532 nm635 nmBeam polarizer MirrorMirrorBeam splitter Reference diode (stationary)Brewster angleValue diode (moveable)radiator bar(moveable) Brewster mirror (moveable)xyzFigure 1: Schematic drawing of the current setup in theDIRC optical lab.The laser beam enters the radiator at Brewster angleto minimize the reflected intensities of the polarized laserlight at the bar ends. Depending on length and orientationof the bar the beam is internally reflected up to 31 timesuntil it exits the bar and hits the (value) photodiode. The∗Work supported by EU FP7 grant, contract no. 227431, Hadron-Physics2, and HGS-HIRe† m.krebs@gsi.desecond (reference) diode is used to correct for laser inten-sity fluctuations. The transmission T is given as the doubleratio of the intensities recorded by the value and referencediodes with and without the laser passing through the bar.The coefficient of total internal reflection R is calculatedas T = RN · exp (− LΛ), where N is the number of inter-nal reflections, Λ the attenuation length of fused silica atthis wavelength (determined in a separate measurement inthe same setup), and L the optical pathlength inside the bar.The calculated transmission values are shown as a matrixplot in Fig. 2.Figure 2: Transmission matrix for a radiator bar made bySchott-Lithotec. The black crosses mark measurements forlaser locations that deviate more than 3 σ from the meantransmission value. The values on the X- and Y-axis rep-resent the positions of the linear motors, which move theradiator.The areas with smaller transmission values are the resultof small imperfections of the bar and surface pollution. Thebeveled edges of this particular bar can lead to a diffusionof the laser beam when it exits the radiator bar near theedge, causing an intensity loss on the photodiode, leadingto drop of the calculated transmission. The measured meantransmission value is T = 0.975± 0.005.The setup, based on a smaller system designed for the testsof shorter bars [1, 2], was installed and commissioned inthe fall of 2014, including a detailed study of the systematicuncertainties. The system is now ready for the evaluationof a number of prototype radiator bars, made by differentmanufacturers, to determine if their fabrication processesmeet the PANDA Barrel DIRC requirements.References[1] R. Hohler, “Prototyp-Radiatoren eines Barrel-DIRC fu¨r dasPANDA-Experiment”, PhD thesis, Frankfurt, Germany, 2011[2] G. Kalicy, “Optical Properties of Radiators for the PANDABarrel DIRC”, N14-156, IEEE, Anaheim, USA, October2012GSI SCIENTIFIC REPORT 2014 MU-HADRONS-15DOI:10.15120/GR-2015-1-MU-HADRONS-15 123

Optical properties of the PANDA barrel DIRC prototype radiator bars

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Optical properties of the PANDA barrel DIRC prototype radiator bars

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