[email protected] cern 1st december 2004
DESCRIPTION
RICH 2004. Characterization and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH - PowerPoint PPT PresentationTRANSCRIPT
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
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