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
CERN st 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
RICH aerogel:2~10 GeV/c
C4F10 :10~60 GeV/c
pp interaction point
Magnet

3. 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!

4. 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

5. 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

6. 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!

7. Demagnification Law Rotation Law

8. 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!

9. 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

10. Transverse fields ( =90°)B┴ 

11. 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 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

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

15. 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