LASER CALIBRATION SYSTEM for STAR TPC Alexei Lebedev (BNL) for STAR collaboration · Design and description · Performance · Goals and results · Future developments
Calibration goals : Sector alignment 2. Electronics performance 3. Drift velocity measurement 4. Ex. B effects correction 5. Others STAR LASER SYSTEM 1. Laser beam is straight line 2. Origin of each laser beam is known with ~200 microns 3. Angles between beams are known with ~0. 1 mrad Results: Year 2000 1. Drift velocity measurement 2. Sector alignment data 3. Preliminary Ex. B correction Year 2001 1. Online drift velocity measurement 2. More precise sector alignment
STAR TPC LASER SYSTEM * 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
Optical scheme for one TPC half
Designed laser beams position in the TPC volume
Laser event image as seen from pad plane view
Side view of laser event. Laser tracks with known Z-position used to measure drift velocity
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.
First correction in sagitta measurement based on laser’s straightness
Y, cm X, cm Example of inner and outer sectors misalignment
Drift velocity measurement over extended period of time Data 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 different TPC 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 Ex. B correction -sector boundaries -space charge corrections
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