Åsmund Skjæveland Magnetic Distortion of HPD Images

The LHCb experiment

Observe CP violation in decays of heavy hadrons produced in p-p collisions Observe CP violation in decays of heavy hadrons produced in p-p collisions –Need accurate particle identification: Tracking stations track particles with high precision Tracking stations track particles with high precision Magnet deflects track ➨ momentum and charge Magnet deflects track ➨ momentum and charge RICH measure velocity ➨ combine with momentum to find mass RICH measure velocity ➨ combine with momentum to find mass

The RICH detectors in LHCb

RICH detectors Measures velocities via Čerenkov radiation Measures velocities via Čerenkov radiation –Uses Hybrid Photon Detectors ➨ high resolution –Combine images from many HPDs to find Čerenkov circles

RICH detectors Measures velocities via Cherenkov radiation Measures velocities via Cherenkov radiation –Uses Hybrid Photon Detectors ➨ high resolution –Combine images from many HPDs to find Cherenkov circles

Hybrid Photon Detector 37.5 mm radius 37.5 mm radius 110 mm long 110 mm long Quartz optical window Quartz optical window Resolution 256x32 pixels Resolution 256x32 pixels Photoelectron optics ➨ sensitive to magnetic fields Photoelectron optics ➨ sensitive to magnetic fields

Pixel Hybrid Photon Detector

Problem RICH1 is close to spectrometer magnet RICH1 is close to spectrometer magnet HPDs will be measurably affected by small magnetic fields ➨ image is distorted HPDs will be measurably affected by small magnetic fields ➨ image is distorted –Reconstruction of circle requires undisturbed images: Photon hits must be measured very accurately –If perturbation is large, data will be pushed off image chip ➨ uncorrectable data loss

Magnetic Shielding RICHes enclosed in iron box—internal field reduced to max. 25 G (RICH1) and 10 G (RICH2) RICHes enclosed in iron box—internal field reduced to max. 25 G (RICH1) and 10 G (RICH2) MuMetal shield around each HPD further reduces magnetic field MuMetal shield around each HPD further reduces magnetic field MuMetal-shield

Experiment setup Projector Filter HPD Coils

Setup Standard projector connected to laptop Standard projector connected to laptop Dense optical filters Dense optical filters HPD tube HPD tube Read-out electronics Read-out electronics

Calibration Projected image much larger than HPD Projected image much larger than HPD Find HPD by sweeping horizontal and vertical lines across the screen Find HPD by sweeping horizontal and vertical lines across the screen Fine-tune with line patterns Fine-tune with line patterns

Calibration

Calibration

Calibration

Data taking Illuminate HPD with a defined pattern Illuminate HPD with a defined pattern Vary magnetic field Vary magnetic field Record HPD image Record HPD image

Observations for axial fields 0 G 30 G 20 G 50 G

Observations Small but clear background Small but clear background –thermal electrons –ion feedback Image is rotated Image is rotated Image is enlarged Image is enlarged Image center is slightly displaced Image center is slightly displaced

Conclusion Entire image on chip: No data loss Entire image on chip: No data loss –Reconstruction of undisturbed image probably possible

To Be Done Make and test other test patterns Make and test other test patterns Measure other field configurations Measure other field configurations Make and test image correction algorithm Make and test image correction algorithm Publish Publish Have a beer Have a beer