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