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

Åsmund Skjæveland Åsmund Skjæveland

Magnetic Magnetic Distortion Distortion of

• f

HPD Images HPD Images

The The LHCb LHCb experiment experiment

The The LHCb LHCb experiment experiment

• Observe CP violation in decays of heavy

Observe CP violation in decays of heavy hadrons produced in hadrons produced in p p-

• p

p collisions collisions

– – 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

Magnet deflects track ➨ ➨ momentum momentum and and charge charge

• RICH

RICH measure measure velocity velocity ➨

➨ combine

combine with with momentum momentum to to find find mass mass

The The RICH RICH detectors detectors in in LHCb LHCb

RICH detectors RICH detectors

• Measures velocities via

Measures velocities via Čerenkov Čerenkov radiation radiation

– – Uses Hybrid Photon Detectors Uses Hybrid Photon Detectors ➨ ➨ high resolution high resolution – – Combine images from many Combine images from many HPDs HPDs to find to find Čerenkov Čerenkov circles circles

RICH detectors RICH detectors

• Measures velocities via

Measures velocities via Cherenkov Cherenkov radiation radiation

– – Uses Hybrid Photon Detectors Uses Hybrid Photon Detectors ➨ ➨ high resolution high resolution – – Combine images from many Combine images from many HPDs HPDs to find to find Cherenkov Cherenkov circles circles

Hybrid Hybrid Photon Photon Detector Detector

• 37.5 mm radius

37.5 mm radius

• 110 mm

110 mm long long

• Quartz

Quartz optical

• ptical window

window

• Resolution

Resolution 256x 256x32 32 pixels pixels

• Photoelectron

Photoelectron optics

• ptics

➨ ➨ sensitive to sensitive to magnetic magnetic fields fields

Pixel Pixel Hybrid Hybrid Photon Photon Detector Detector

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 magnetic magnetic fields fields ➨ ➨ image is image is distorted distorted

– – Reconstruction Reconstruction of

• f 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

• ff image chip
• ff image chip ➨

➨ uncorrectable uncorrectable data loss data loss

Magnetic Shielding Magnetic Shielding

• RICHes

RICHes enclosed in enclosed in iron box iron box— —internal internal fi field eld reduced to reduced to

• 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 magnetic fi field eld

MuMetal-shield

Experiment setup Experiment setup

Projector Filter HPD Coils

Setup Setup

• Standard projector connected to laptop

Standard projector connected to laptop

• Dense optical

Dense optical filters filters

• HPD tube

HPD tube

• Read

Read-

• out electronics
• ut 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 defined defined pattern pattern

• Vary magnetic

Vary magnetic field field

• Record HPD image

Record HPD image

Observations for axial Observations for axial fields fields

0 G 30 G 20 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