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Åsmund SkjævelandÅsmund Skjæveland
Magnetic Distortion of Magnetic Distortion of HPD ImagesHPD Images
The LHCb experimentThe LHCb experiment
The LHCb experimentThe LHCb experiment
• Observe CP violation in decays of heavy Observe CP violation in decays of heavy hadrons produced in p-p collisionshadrons produced in p-p collisions– Need accurate particle identification:Need accurate particle identification:
•Tracking stations track particles with high Tracking stations track particles with high precisionprecision
•Magnet deflects track Magnet deflects track ➨ ➨ momentum and chargemomentum and charge
•RICH measure velocity RICH measure velocity ➨➨ combine with combine with momentum to find massmomentum to find mass
The RICH detectors in The RICH detectors in LHCbLHCb
RICH detectorsRICH detectors
• Measures velocities via Čerenkov Measures velocities via Čerenkov radiationradiation– Uses Hybrid Photon Detectors Uses Hybrid Photon Detectors ➨➨ high high
resolutionresolution– Combine images from many HPDs to find Combine images from many HPDs to find
Čerenkov circlesČerenkov circles
RICH detectorsRICH detectors
• Measures velocities via Cherenkov Measures velocities via Cherenkov radiationradiation– Uses Hybrid Photon Detectors Uses Hybrid Photon Detectors ➨➨ high high
resolutionresolution– Combine images from many HPDs to find Combine images from many HPDs to find
Cherenkov circlesCherenkov circles
Hybrid Photon DetectorHybrid Photon Detector
• 37.5 mm radius37.5 mm radius
• 110 mm long110 mm long
• Quartz optical Quartz optical windowwindow
• Resolution 256xResolution 256x32 32 pixels pixels
• Photoelectron Photoelectron optics optics ➨➨ sensitive sensitive to magnetic fieldsto magnetic fields
Pixel Hybrid Photon Pixel Hybrid Photon DetectorDetector
ProblemProblem
• RICH1 is close to spectrometer magnetRICH1 is close to spectrometer magnet
• HPDs will be measurably affected by HPDs will be measurably affected by small magnetic fields small magnetic fields ➨ image is ➨ image is distorteddistorted– Reconstruction of circle requires Reconstruction of circle requires
undisturbed images: Photon hits must be undisturbed images: Photon hits must be measured very accuratelymeasured very accurately
– If perturbation is large, data will be pushed If perturbation is large, data will be pushed off image chip off image chip ➨ uncorrectable data loss➨ uncorrectable data loss
Magnetic ShieldingMagnetic Shielding
• RICHes enclosed in RICHes enclosed in iron box—internal iron box—internal field reduced to field reduced to max. 25 G (RICH1) max. 25 G (RICH1) and 10 G (RICH2)and 10 G (RICH2)
• MuMetal shield MuMetal shield around each HPD around each HPD further reduces further reduces magnetic fieldmagnetic field
MuMetal-shield
Experiment setupExperiment setup
ProjectorFilterHPDCoils
SetupSetup
• Standard projector connected to Standard projector connected to laptoplaptop
• Dense optical filtersDense optical filters
• HPD tubeHPD tube
• Read-out electronicsRead-out electronics
CalibrationCalibration
• Projected image much larger than Projected image much larger than HPDHPD
• Find HPD by sweeping horizontal and Find HPD by sweeping horizontal and vertical lines across the screenvertical lines across the screen
• Fine-tune with line patternsFine-tune with line patterns
CalibrationCalibration
CalibrationCalibration
CalibrationCalibration
Data takingData taking
• Illuminate HPD with Illuminate HPD with a defined patterna defined pattern
• Vary magnetic fieldVary magnetic field
• Record HPD imageRecord HPD image
Observations for axial fieldsObservations for axial fields
0 G
30 G
20 G
50 G
ObservationsObservations
• Small but clear Small but clear backgroundbackground– thermal electronsthermal electrons– ion feedbackion feedback
• Image is rotatedImage is rotated
• Image is enlargedImage is enlarged
• Image center is Image center is slightly displacedslightly displaced
ConclusionConclusion
• Entire image on chip: Entire image on chip: No data lossNo data loss– Reconstruction of undisturbed image Reconstruction of undisturbed image
probably possibleprobably possible
To Be DoneTo Be Done
• Make and test other test patternsMake and test other test patterns
• Measure other field configurationsMeasure other field configurations
• Make and test image correction Make and test image correction algorithmalgorithm
• PublishPublish
• Have a beerHave a beer