LCTPC photoelectron calibration system D. Karlen / U. Victoria & TRIUMF LCTPC subsystems meeting DESY, February 25-26, 2008

Feb 25-26, 2008 Photoelectron calibration system 2 Reminder of concept photoelectrons produced in a well defined pattern on the cathode can monitor all aspects of electron transport in the TPC: photoelectrons produced in a well defined pattern on the cathode can monitor all aspects of electron transport in the TPC: drift velocitydrift velocity diffusiondiffusion gas and electronics gaingas and electronics gain deviations from the ideal drift lines:deviations from the ideal drift lines: static electric and magnetic field non- uniformities static electric and magnetic field non- uniformities positive ion effects positive ion effects See: LCWS 07 presentation and write-up See: LCWS 07 presentation and write-up

Feb 25-26, 2008 Photoelectron calibration system 3 Producing photoelectons fix aluminum targets (mostly dots) on the copper central cathode surface fix aluminum targets (mostly dots) on the copper central cathode surface survey the locations of the targets survey the locations of the targets illuminate a large part of central cathode with a short pulse of 266 nm UV light illuminate a large part of central cathode with a short pulse of 266 nm UV light electrons are only emitted from the aluminum surfaceselectrons are only emitted from the aluminum surfaces bring UV light from a laser to the endplate with fibresbring UV light from a laser to the endplate with fibres concept proven in prototype T2K TPC concept proven in prototype T2K TPC

Feb 25-26, 2008 Photoelectron calibration system 4 Fibre access through endplate

Feb 25-26, 2008 Photoelectron calibration system 5 Fibre connection Concept modelled on T2K prototype Concept modelled on T2K prototype for LCTPC: at 20 degree anglefor LCTPC: at 20 degree angle

Feb 25-26, 2008 Photoelectron calibration system 6 Reducing the hole size Original scheme had a diverging lens as the last element Original scheme had a diverging lens as the last element Now propose a converging lens – allows for smaller hole in the field cage termination Now propose a converging lens – allows for smaller hole in the field cage termination

Feb 25-26, 2008 Photoelectron calibration system 7 Intensity on cathode (lower)

Feb 25-26, 2008 Photoelectron calibration system 8 Intensity on cathode (both)

Feb 25-26, 2008 Photoelectron calibration system 9 Powering two fibres Use a beam splitter Use a beam splitter would like to be able to selectively power one or the other to determine systematics arising from non uniformity of light profilewould like to be able to selectively power one or the other to determine systematics arising from non uniformity of light profile normally run with both powerednormally run with both powered

Feb 25-26, 2008 Photoelectron calibration system 10 Laser optics: setup at DESY Sandblasted quartz reflector UV Laser Sandblasted quartz reflector Photodiode for trigger Movable mirror Splitter Blocker Mirror Focusing elements Splitter Blocker Re-use some of these parts?

Feb 25-26, 2008 Photoelectron calibration system 11 Cathode targets precision aluminum circular dots precision aluminum circular dots 2 mm diameter2 mm diameter thin straight stips thin straight stips 2 mm wide: to monitor diffusion2 mm wide: to monitor diffusion to be surveyed with camera on router: to be surveyed with camera on router:

Feb 25-26, 2008 Photoelectron calibration system 12 Pattern placement and survey Router can scribe marks on the cathode to place dots and strips Router can scribe marks on the cathode to place dots and strips imaging camera mounted on router will surveyimaging camera mounted on router will survey T2K tests: dot placement RMS = 0.2 mm, survey precision of router: < 0.1 mmT2K tests: dot placement RMS = 0.2 mm, survey precision of router: < 0.1 mm Casey Bojechko / UVic

Feb 25-26, 2008 Photoelectron calibration system 13 Suggested pattern for LCTPC Place aluminum pattern on a second cathode, to ensure no delay or degradation in performance? Place aluminum pattern on a second cathode, to ensure no delay or degradation in performance? Best resolution when targets project to corner locations of pads Best resolution when targets project to corner locations of pads most important for large pad designsmost important for large pad designs follow SACLAY large pad design with 24 rows and 72 columns (7mm high rows)follow SACLAY large pad design with 24 rows and 72 columns (7mm high rows)

Feb 25-26, 2008 Photoelectron calibration system 14 Possible pattern

Feb 25-26, 2008 Photoelectron calibration system 15 Studies underway at UVic optimizing optics designs optimizing optics designs input focusinginput focusing fibre-fibre couplingfibre-fibre coupling output defocusingoutput defocusing studying fibre damage studying fibre damage must be careful to not focus light on too small a point on fibre endmust be careful to not focus light on too small a point on fibre end burn marks appear burn marks appear doing a systematic study of power, and repetition rate – to minimize damagedoing a systematic study of power, and repetition rate – to minimize damage early tests – 300K pulses at 50 Hz result in 20-60% lossearly tests – 300K pulses at 50 Hz result in 20-60% loss

Feb 25-26, 2008 Photoelectron calibration system 16 Fibre damage measurements All curves: 300K pulses

Feb 25-26, 2008 Photoelectron calibration system 17 Summary To demonstrate that the TPC is a viable tracker of the ILC, it is critical to demonstrate that distortions can be measured and corrected, to a much higher precision than achieved before To demonstrate that the TPC is a viable tracker of the ILC, it is critical to demonstrate that distortions can be measured and corrected, to a much higher precision than achieved before the photoelectron calibration system provides a new way to undertake these studiesthe photoelectron calibration system provides a new way to undertake these studies very useful to incorporate this system relatively early in the LCTPC test schedulevery useful to incorporate this system relatively early in the LCTPC test schedule