Characterization 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

RICH 2004

The LHCb Detector

RICH1 aerogel:2~10 GeV/c

C4F10 :10~60 GeV/c

RICH2

CF4 :16~100GeV/c

Magnet

pp interaction point

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

B field in RICH1

8 tonnes ARMCO shielding boxes

B field in RICH2

Shielding boxes

~6 tonnes ARMCO

Max 20 G

Mainly axial

< 10 G

Mainly

transverse

|B| measurements in situ confirm!

Photon Detectors

plane

Hybrid Photon Detector

B║

B┴

80mm

120m

m2.5x2.5 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

25

0.9

140

83

86

140mm

Mu-metal shielding

Experimental Characterisation of B field effects

Helmholz Coil

HPD with shielding

LED + XY table

0 G

160 points by LED Image on Chip

Image on Chip processed

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

Distortions for Axial B fields (=0°)

B(G) dmax (mm)

10 0.9

20 1.6

30 2.3

40 3.3

50 4.3 Distortion correction needed!

On anode plane!

shielding needed (signal loss at 15 G!)shield reduces distortion effects by factor 4

at 50 G image still contained on chip

50 G Shielded

15 G Unshielded

10 G Unshielded

10 G Shielded

Demagnification Law Rotation Law

j

jiji Baa

221 )()(' rBarBar 3

62

53 )()()( rBarBaBa

50 G

30 G

0 G

50 G

30 G

Photon hit reconstruction

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!

Applied B field [G] Average reconstruction bias

[mm]

0 0.82

10 1.24

20 1.40

30 1.78

Parameterization used to

Reconstruct photon hit position from pixel hit position given the magnetic field

estimate B// with a test pattern

30 G

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 GLarger 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 0.7Calibration 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) dmax (mm)

2.5 0.99

5.0 1.64

7.5 2.23

10 2.76

15 3.62

Shielded Tube

B(G) dmax (mm)

30 0.45

50 0.73

B

50 G Shielded tube

2.5 G Unshielded tube

shielding needed Distortion large @ 10 Gshield reduces of distortion effects by factor 20Shielding is much more effective as for axial configurationwith shielding distortions relatively small (@ 50 G maximum displacement ~ 1.5 LHCb pixels)no dilatation no data loss

Diagonal fields ( =45°)

//45BBB T

//452/ BBB T

Unshielded Tube Shielded Tube

10.6 G 2.5 G 42 G 15 G

Dependence from 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

I

B

I

B

+10G

Measurement point

Reference

I

B

-10G

Measurement point

Reference

Remnant field studies

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

Conclusions Characterization of and parameterization of ExB distortions of the image

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