Magnetic Distortion Distortion of of Magnetic HPD Images HPD - PowerPoint PPT Presentation

Magnetic Distortion Distortion of of Magnetic HPD Images HPD Images Åsmund Skjæveland Åsmund Skjæveland

The LHCb LHCb experiment experiment The

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

The RICH RICH detectors detectors in in LHCb LHCb The

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

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

Hybrid Photon Photon Detector Detector Hybrid • 37.5 mm radius • 37.5 mm radius • 110 mm • 110 mm long long • Quartz • Quartz optical optical window window • Resolution • Resolution 256x 256x32 32 pixels pixels • Photoelectron • Photoelectron optics optics ➨ sensitive to ➨ sensitive to magnetic fi elds fi elds magnetic

Pixel Hybrid Hybrid Photon Photon Detector Detector Pixel

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

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

Experiment setup Experiment setup Filter HPD Coils Projector

Setup Setup • Standard projector connected to laptop • Standard projector connected to laptop • Dense optical • Dense optical fi lters fi lters • HPD tube • HPD tube • Read • Read- -out electronics out electronics

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

Calibration Calibration

Calibration Calibration

Calibration Calibration

Data taking Data taking • Illuminate HPD with a • Illuminate HPD with a de fi ned pattern pattern de fi ned • Vary magnetic • Vary magnetic fi eld fi eld • Record HPD image • Record HPD image

Observations for axial fi elds fi elds Observations for axial 20 G 0 G 30 G 50 G

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

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

To Be Done 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