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Ann.Van.Lysebetten@cern.ch CERN 1st December 2004

RICH 2004. Characterisation and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH

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Ann.Van.Lysebetten@cern.ch CERN 1st December 2004

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  1. RICH 2004 Characterisation and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH Gianluca Aglieri Rinella, T.F. Bellunato, C. D’Ambrosio, R. Forty, T. Gys, M. Patel, D. Piedigrossi, A. Van Lysebetten Ann.Van.Lysebetten@cern.ch CERN 1st December 2004

  2. The LHCb Detector • experiment at LHC collider dedicated to CP violation and rare phenomena in B meson decay • efficicient pion/kaon separation up to ~100 Gev/c RICH detectors RICH2 CF4 :16~100GeV/c RICH1 aerogel:2~10 GeV/c C4F10 :10~60 GeV/c pp interaction point Magnet

  3. < 10 G Max 20 G Mainly transverse Mainly axial Photon Detectors plane |B| measurements in situ confirm! B field in RICH1 B field in RICH2 8 tonnes ARMCO shielding boxes Shielding boxes ~6 tonnes ARMCO

  4. B┴  Hybrid Photon Detector 80mm • 2.5x2.5 mm2granularity => σ(θCH) = 0.6 mrad (RICH1) • S20 multi-alkali photocathodeon inner surface of the entrance quartz window • Cross-focusing electron optics (~20kV, demagnification factor of 5 • Photo-electrons focused on theanode assembly(16x16 mm2), fully encapsulated in the vacuum enclosure 120mm 0.9 86 25 Mu-metal shielding 140 83 B║ 140mm

  5. 0 G Experimental Characterisation of B field effects Helmholz Coil • Goals: • verifysustainability of “image” distortionsinHPDs at RICH B field values (and beyond!) • Quantify/parameterizedistortions • Quantify theshielding effectof Mu-metal • definecalibration strategy LED + XY table HPD with shielding Image on Chip processed 160 points by LED Image on Chip

  6. 10 G Shielded 10 G Unshielded Distortions for Axial B fields (=0°) 50 G Shielded 15 G Unshielded • shielding needed (signal loss at 15 G!) • shieldreducesdistortion effects byfactor 4 • at 50 G image still contained on chip On anode plane! Distortion correction needed!

  7. Demagnification Law Rotation Law 50 G 30 G 0 G 50 G 30 G

  8. Photon hit reconstruction • Parameterization used to • Reconstructphoton hit position from pixel hit position given the magnetic field • estimateB//withatest pattern 30 G • Reconstruction error at 25 G larger than intrinsic resolution of the HPD (2.5/(12) = 0.72 mm) • Uncertainty on centre of optical axis • preliminary results!

  9. 0 G Larger central light spot Smaller spots at increasing distances from the central Test Pattern • Allows easy automatic search and identification of clusters (correspondence problem) • Spots averaged centers as coordinates for the analysis • Sample rotation of the image at various radii • Estimate the B field axial component by best fit of the model • Calibration Strategy: • Projection of a static pattern on the detectors plane in the experiment • Automated calibration procedure on the full set of HPDs determining field in each tube

  10. Transverse fields ( =90°) B┴ 

  11. Distortions for Transverse B fields (=90°) 50 G Shielded tube 2.5 G Unshielded tube B • shielding needed Distortion large @ 10 G • shieldreducesdistortion effectsbyfactor  20 • Shielding is muchmore effectiveas for axial configuration • with shieldingdistortions relatively small(@ 50 G maximum displacement ~ 1.5 LHCb pixels) • no dilatationno data loss

  12. Diagonal fields ( =45°) Unshielded Tube Shielded Tube 42 G 15 G 10.6 G 2.5 G

  13. Dependence on HV Shielded HPD Operation at reduced HV 30 G Axial B field 50 G Transverse B field • No difference in demagnification law • Somewhat larger rotation • No difference observed

  14. Remnant field studies B B B I I I Measurement point Measurement point +10G Reference -10G Reference • for bare tube • maximum offset of 1.3 mm for BT 40 G on anode • no effect observed for axial configuration • for shielded tube • maximum offset of 0.15 mm for BT 40 G on anode  negligible • no effect observed for axial configuration

  15. Conclusions • Characterisation ofand parameterization of ExB image distortions • Distortion correction for RICH1 needed • No need for correction in RICH2 boxes? • Shielded HPDs are fully operational in the residual magnetic field (25 G) inside the shielding boxes • No coverage loss • Preliminary results on distortion correction for given B║ proposed • possible due to radial symmetry • recovering spatial resolution • B║ estimation with test pattern • No significant dependence from HV measured • Remnant field no problem for shielded HPD • Automated calibration procedure for the LHCb RICH to estimate the axial field applied on each of the 484 HPDs installed in the experiment has been proposed

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