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

LCTPC photoelectronLCTPC photoelectroncalibration systemcalibration system

D. Karlen / U. Victoria & TRIUMFD. Karlen / U. Victoria & TRIUMF

LCTPC subsystems meetingLCTPC subsystems meetingDESY, February 25-26, 2008DESY, February 25-26, 2008

Feb 25-26, 2008Feb 25-26, 2008 Photoelectron calibration systemPhotoelectron calibration system 22

Reminder of conceptReminder of concept photoelectrons produced in a well defined photoelectrons produced in a well defined

pattern on the cathode can monitor all pattern on the cathode can monitor all aspects of electron transport in the TPC: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-static electric and magnetic field non-uniformitiesuniformities

positive ion effectspositive ion effects

See: LCWS 07 presentation and write-upSee: LCWS 07 presentation and write-up


Producing photoelectonsProducing photoelectons fix aluminum targets (mostly dots) on the fix aluminum targets (mostly dots) on the

copper central cathode surfacecopper central cathode surface survey the locations of the targetssurvey the locations of the targets illuminate a large part of central cathode illuminate a large part of central cathode

with a short pulse of 266 nm UV lightwith a short pulse of 266 nm UV light• electrons are only emitted from the aluminum electrons are only emitted from the aluminum

surfacessurfaces• bring UV light from a laser to the endplate with bring UV light from a laser to the endplate with

fibresfibres

concept proven in prototype T2K TPCconcept proven in prototype T2K TPC


Fibre access through endplateFibre access through endplate


Fibre connectionFibre connection Concept modelled on T2K prototypeConcept modelled on T2K prototype

• for LCTPC: at 20 degree anglefor LCTPC: at 20 degree angle


Reducing the hole sizeReducing the hole size Original scheme had aOriginal scheme had a

diverging lens as the last diverging lens as the last elementelement

Now propose a converging Now propose a converging lens – allows for smaller lens – allows for smaller hole in the field cage hole in the field cage terminationtermination


Intensity on cathode (lower)Intensity on cathode (lower)


Intensity on cathode (both)Intensity on cathode (both)


Powering two fibresPowering two fibres Use a beam splitterUse a beam splitter

• would like to be able to selectively power one would like to be able to selectively power one or the other to determine systematics arising or the other to determine systematics arising from non uniformity of light profilefrom non uniformity of light profile

• normally run with both powerednormally run with both powered

Laser


Laser optics: 2003-4 setup at DESYLaser optics: 2003-4 setup at DESYSandblastedquartz reflector

UV Laser

Sandblastedquartz reflector

Photodiodefor trigger

Movablemirror

Splitter Blocker

Mirror

FocusingelementsSplitter

Blocker

Re-usesome ofthese parts?


Cathode targetsCathode targets precision aluminum circular dotsprecision aluminum circular dots

• 2 mm diameter2 mm diameter thin straight stipsthin straight stips

• 2 mm wide: to monitor diffusion2 mm wide: to monitor diffusion to be surveyed to be surveyed

with camera on with camera on router:router:


Pattern placement and surveyPattern placement and survey Router can scribe marks on the cathode to Router can scribe marks on the cathode to

place dots and stripsplace dots and strips• imaging camera mounted on router will surveyimaging camera mounted on router will survey

• T2K tests: dot placement RMS = 0.2 mm, T2K tests: dot placement RMS = 0.2 mm, survey precision of router: < 0.1 mmsurvey precision of router: < 0.1 mm

Casey Bojechko / UVic


Suggested pattern for LCTPCSuggested pattern for LCTPC Place aluminum pattern on a second Place aluminum pattern on a second

cathode, to ensure no delay or cathode, to ensure no delay or degradation in performance?degradation in performance?

Best resolution when targets project to Best resolution when targets project to corner locations of padscorner locations of pads• most important for large pad designsmost important for large pad designs• follow SACLAY large pad design with 24 rows follow SACLAY large pad design with 24 rows

and 72 columns (7mm high rows)and 72 columns (7mm high rows)


Possible patternPossible pattern

21 m m


Studies underway at UVicStudies underway at UVic optimizing optics designsoptimizing optics designs

• input focusinginput focusing• fibre-fibre couplingfibre-fibre coupling• output defocusingoutput defocusing

studying fibre damagestudying fibre damage• must be careful to not focus light on too small must be careful to not focus light on too small

a point on fibre enda point on fibre end burn marks appearburn marks appear

• doing a systematic study of power, and doing a systematic study of power, and repetition rate – to minimize damagerepetition rate – to minimize damage

• early tests – 300K pulses at 50 Hz result inearly tests – 300K pulses at 50 Hz result in20-60% loss20-60% loss


Fibre damage measurementsFibre damage measurements

All curves: 300K pulses


SummarySummary To demonstrate that the TPC is a viable To demonstrate that the TPC is a viable

tracker of the ILC, it is critical to tracker of the ILC, it is critical to demonstrate that distortions can be demonstrate that distortions can be measured and corrected, to a much higher measured and corrected, to a much higher precision than achieved beforeprecision than achieved before• the photoelectron calibration system provides the photoelectron calibration system provides

a new way to undertake these studiesa new way to undertake these studies• very useful to incorporate this system very useful to incorporate this system

relatively early in the LCTPC test schedulerelatively early in the LCTPC test schedule

Documents

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