LASER CALIBRATION SYSTEM for STAR TPC  Alexei Lebedev (BNL) for STAR collaboration                  Design and description                 Performance                 Goals and results                 Future developments 

Calibration goals :

Sector alignment2.   Electronics performance3.   Drift velocity measurement4.   ExB effects correction5.   Others  STAR LASER SYSTEM 1.   Laser beam is straight line2.   Origin of each laser beam is known with ~200 microns3.   Angles between beams are known with ~0.1 mrad  Results: Year 20001.   Drift velocity measurement2.   Sector alignment data3.   Preliminary ExB correction Year 2001 1.   Online drift velocity measurement2.   More precise sector alignment 

* Narrow beams formed with

small mirrors intercepting

wide beams

* Fixed small mirrors provide

stable geometric origins

for each beam

* Drift velocity measurement

with 0.02% accuracy

* Alignment and distortions

correction

* Wide beams

also illuminate

central membrane

STAR TPC LASER SYSTEM

    

Optical scheme for one TPC half

Designed laser beams position in the TPC volume

Laser event image as seen from pad plane view

ΔΖLaser tracks used for velocity measurement

ΔV/V~.02%

Side view of laser event. Laser tracks with known Z-position used to measure drift velocity

Typical laser beam trackTypical image from membrane pattern

Single laser event in the TPC ~450 laser tracks

Precise measurement of angles between laser beams in bundle. 1-3 reference marks, 4-UV laser, 5- telescope, 6-theodolite, 7 -mirror, 8-vertical stage with quadrant, 9-bundle.

Event by event Z position of membrane, illuminated

by laser flash. Precise relative drift velocity monitor.

Stability of laser beam in TPC. Position of laser in pad row

shows accuracy ~ 70 microns.

B Straight laser beams measure momentum error due to electron drift distortion. For particle with momentum P(GeV/c), radius of curvature R=P/0.3B, R(m), B(T) sagitta S=L2/8R

Measured distortion (Slaser) at B=0.5T

Slaser = 168 μ m uncorrected Slaser = 16 μ m corrected

=10Distortion induced momentum error at P Gev/ c aftercorrection δP/ =0.8% ( P without correction δP/ =8.4%)P

, =10Much better than momentum error at P Gev/ , c which is :limited by hit reconstruction resolution δP/ = 10%P

P

R

L S

First correction in sagitta measurement based on laser’sstraightness

Y, cm

X, cm

Example of inner and outer sectors misalignment

Drift velocity measurement over extended period of timeData for RHIC 2000 year run.

Drift velocity measurement for RHIC 2001 year run. Dip at day near #74 caused by gas system failure and methane partial loss.

Remote control to view and align laser’s position in differentTPC points. Also this system used to monitor Forward TPC laser system.

Future developments:

Hardware-automated shutter for photodiode and warm laser-automated alignment with CCD picture correlation-better quality CCD readout for Forward TPC

Software-verify sector alignment, which was done from Au-Au central collisions-more in ExB correction-sector boundaries-space charge corrections