jt: 1 the berkeley lab star tpc distortions in the transverse plane: an update jim thomas

JT: 1The Berkeley Lab

STAR

TPC Distortions in the Transverse Plane:

An Update

Jim Thomas


STARA Mnemonic for Listing the Potential Distortions

ZCal

Barrel EM Calorimeter

Endcap Calorimeter

Magnet

Coils

TPC Endcap & MWPC

ZCal

FTPCs

Vertex Position Detectors

Central Trigger Barrel or TOF

Time Projection Chamber

Silicon Vertex Tracker

RICH

Central Membrane Endwheel and PadplaneOuter Field Cage

Inner Field Cage


STARThe List of Distortions in the Transverse Plane

The list can be enumerated by surfaces:

• Outer field cage corrections • Inner field cage corrections• Central membrane corrections• End-wheel and pad-plane corrections • Pad Row 13 corrections and other local electrostatic defects • Rotation and miss-alignment of sectors with respect to their ideal locations• Rotation of either TPC end-wheel with respect to its ideal location

and by volume:

• Space Charge corrections due to charge in the volume of the TPC • Magnetic field corrections due to B fields in the volume of the TPC• Twist of the TPC with respect to the magnetic field axis and/or the measured map• General coordinate transformations

A few additional items are listed for completeness. (These items affect the drift of the electrons in the Z direction but do not strongly affect the distortions in the transverse plane.)

• Gas composition and variations in the drift velocity• Barometric pressure changes and variations in the drift velocity• Pressure variations as a function of height in the TPC• Temperature gradients in the TPC


STARWe think of our events like this …

Data Taken June 25, 2000.


STARMost of our events look more like this …


STARSpaceCharge from (all) events cause distortion

Z / 5 cm

Rad

ius

/ 5 c

m

Z

Radius

Dis

tort

ion


STARModel for the distortions

• Old Model– Beam gas events leave a uniform deposition of

charge in the TPC

– The charge from the collisions is not significant

• New Model– Beam gas events leave a 1/R2 distribution of

charge in the TPC

– The charge from the collisions is not significant in the 2001 data, but will be in the future (?)


STARUniform .vs. 1/R2 Space Charge Distribution

RadialDistortions

Z Z

Radius

RadiusD

isto

rtio

n

Dis

tort

ion


STARTwo sources of SpaceCharge

• Beam gas and other up stream events– not synchronous with our trigger

– Scales with beam intensity (not Luminosity)

– 1/R2 distribution of charge (?)

• The collisions at STAR– synchronous with our trigger

– Scales with Luminosity

– A/R + B/R2 distribution of charge (?)

• In the future, the average Luminosity will go up a factor of 40 but the beam intensity will only go up a factor of 2 to 4

– We have to prepare for a significant increase in space charge due to the collisions in the detector.

We have to be able to distinguish the two sources of distortion


STARA Wide Range of Charge Distributions

1/R3 1/R2 Wieman’s HiJet

1/R Linear 2:1 Linear

These (and other) Distributions are Available in StMagUtilities

Radius

Vo

ltag

e


STAR2 Equations, 2 Unknowns

• We can simultaneously fit the DCAs and match the steering at the RICH due to the beam gas induced space charge

– Choose the right charge distribution 1/R, 1/R2, HiJet, etc.

– Choose the RICH scaler normalization constant

Variations in Charge Shape

-1500.0

-1000.0

-500.0

0.0

500.0

1000.0

1500.0

1 3 4 51/

R 6 10 16

Wiem

an

1/R**2

1/R**3

Shape Parameter

Dis

pla

cem

ent

at R

ICH

in

mic

ron

s

delta Rich

with the DCA held constant


STARThese Items on the List Are Ready to Go

The list can be enumerated by surfaces:

Outer field cage corrections Inner field cage corrections• Central membrane corrections• End-wheel and pad-plane corrections Pad Row 13 corrections and other local electrostatic defects Rotation and miss-alignment of sectors with respect to their ideal locations Rotation of either TPC end-wheel with respect to its ideal location

and by volume:

Space Charge corrections due to charge in the volume of the TPC Magnetic field corrections due to B fields in the volume of the TPC Twist of the TPC with respect to the magnetic field axis and/or the measured map General coordinate transformations

A few additional items are listed for completeness. (And these I don’t know about.)

• Gas composition and variations in the drift velocity• Barometric pressure changes and variations in the drift velocity• Pressure variations as a function of height in the TPC• Temperature gradients in the TPC


STARConclusions

• Tools are available to calculate all known distortions – in the transverse plane

– gain, t0, and drift velocity corrections have not been discussed

• SpaceCharge corrections are significant – especially at 40x <L>

• We need to track the beam current and/or the distribution of charge in the TPC for untriggered events. We will need scalers and diagnostics for each source of charge

– Monitor L– Monitor beam current

• The RICH Mult Scaler is gone and we need a replacement

• Recent progress with the laser cluster finder means we might be able to use this data …


STARDCA, Steering at the RICH, and p

• The RICH Scaler has an arbitrary normilization

• The DCA’s have been tuned to be the same in both cases

• p is different by a factor of 2

• Steering at the Rich changes sign under these conditions

Uniform Charge Distribution (0.004)

-2500.0

-2000.0

-1500.0

-1000.0

-500.0

0.0

500.0

0 50 100 150 200 250

Z axis

Dis

pla

cem

ent

(mic

ron

s o

r M

eV)

DCA

delta Rich

delta p (x10)

Event Charge Distribution (0.0021)

-2500.0

-2000.0

-1500.0

-1000.0

-500.0

0.0

500.0

1000.0

0 50 100 150 200 250

Z axis

Dis

pla

cem

ent

(mic

ron

s o

r M

eV)

DCA

delta Rich

delta p (x10)

jt: 1 the berkeley lab star tpc distortions in the transverse plane: an update jim thomas

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