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

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

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

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

Image on Chip processed Experimental Characterisation of B field effects Helmholz Coil Goals: verify sustainability of “image” distortions in HPDs at RICH B field values (and beyond!) Quantify/parameterize distortions Quantify the shielding effect of Mu-metal define calibration strategy LED + XY table HPD with shielding Image on Chip processed 160 points by LED Image on Chip 0 G

Distortions for Axial B fields (=0°) 10 G Unshielded 10 G Shielded 50 G Shielded 15 G Unshielded B(G) Ddmax (mm) 10 0.9 20 1.6 30 2.3 40 3.3 50 4.3 shielding needed (signal loss at 15 G!) shield reduces distortion effects by factor  4 at 50 G image still contained on chip On anode plane! Distortion correction needed!

Demagnification Law Rotation Law

Photon hit reconstruction Parameterization used to Reconstruct photon hit position from pixel hit position given the magnetic field estimate B// with a test pattern 30 G Applied B field [G] Average reconstruction bias [mm] 0.82 10 1.24 20 1.40 30 1.78 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!

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 0 G Larger central light spot Smaller spots at increasing distances from the central Applied B field [G] Estimated field [G] Bias [G] 10 9.0 ± 2.0 -1.0 20 20.5 ± 2.7 0.5 30 32.4 ± 2.7 2.4 40 40.8 ± 2.4 0.8 50 50.7 ± 1.9 0.7 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

Transverse fields ( =90°) B┴ 

Distortions for Transverse B fields (=90°) Unshielded Tube B(G) Ddmax (mm) 2.5 0.99 5.0 1.64 7.5 2.23 10 2.76 15 3.62 Shielded Tube B(G) Ddmax (mm) 30 0.45 50 0.73 50 G Shielded tube 2.5 G Unshielded tube B shielding needed  Distortion large @ 10 G shield reduces distortion effects by factor  20 Shielding is much more effective as for axial configuration with shielding distortions relatively small (@ 50 G maximum displacement ~ 1.5 LHCb pixels) no dilatation  no data loss

Diagonal fields ( =45°) Unshielded Tube Shielded Tube 42 G 15 G

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

Remnant field studies for bare tube 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

Conclusions Characterisation of and 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