Laser Calibration System for - PowerPoint Presentation

Slide1

Laser Calibration System for sPHENIX TPC

B.Azmoun

, BNLSlide2

Laser beams as a Calibration tool

Objective: Use 266nm laser (4.66eV) to liberate charge along beam trajectory to simulate controlled particle tracks in TPC (long history for using laser beams in gaseous detectors)

Features

2-photon process excites trace organic molecules (~ppb) in gas with ~5-8eV ionization potential (working gas ionization potentials are typically >10eV)1-20mJ/mm2 required to liberate an amount of charge equivalent to mipIonization clusters are distributed randomly along the track left by the laser beam as they are for an ionizing particle Ionization yield ~square of laser intensityExample: 1mm beam s ~400mminduced ionization trail s =400mm/sqrt(2)=280mm.Smearing in z direction due to length of pulse: for 5ns pulse with uniform distr.  s =5ns*vd/sqrt(12) ~ tens of mmMetals have work functions comparable to 4eV, so most metal surfaces will also liberate electrons if illuminatedCharge is always deposited in a perfectly straight lineIonization trail from laser simulates infinite momentum particle with zero bend in B-fieldPosition of beam can be determined beforehand to high precisionPredictable energy deposit, no multiple scatteringLaser can be triggered (and interspersed between physics events)

Ref. H.J.

Hilke, “Detector Calibration with Lasers”

B.Azmoun, BNL

2Slide3

Challenges of operating a TPC

High multiplicity environments produce space charge from the primary energy deposit and due to IBF from charge multiplication at the readout (which typically generate orders of magnitude more ions)

Temporal and spatial variations of the drift velocity can be due to variations of ambient conditions: temperature, pressure, E-field distortions from space charge, gas impurities, but gas mixture variations at the level of several percent usually have the largest impact on drift velocity

Intrinsic E-field non-uniformity of field cage (usually controlled to a high level)Mechanical misalignment of TPC field cage in B-field, resulting in ExB effectEndcap wheel displacement/inclination wrt to drift fieldImpact on TPC measurementsThere are many modes for distorting the normal path of an electron in a uniform field, which can severely affect track reconstruction  the challenge is to quantify the magnitude of these distortions and remove them Example of how laser might help:Momentum resolution is defined by accuracy in sagitta measurement and multiple scatteringFor high momentum particles (where MS is negligible), the error in the sagitta is the width of the inverse momentum distribution for radial laser tracks (ALICE)pid: not so much for sPHENIXTPC related challengesB.Azmoun, BNL3Slide4

Calibration Tools

Requirements

Laser beams should sample enough of the drift volume to be able to adequately map out the uniformity in the drift velocity, the distortions in the drift field and check

ExB corrections, etc. Need enough beams to cross all sectors (for sector to sector alignment), with an adequately long track segment over each sector, and with enough time hits along drift (z) Electron density along laser beam should be higher than the surrounding charge due to the passage of high energy particlesThe stability, accuracy, and precision of the laser beam position must be below the required resolution for track reconstruction Benefits of laser beam dataMap out E-field non-uniformitiesMap drift velocities throughout TPC volumeAllow corrections for space charge distortions (devise a method that exploits the “straightness” of laser tracks, whose position is known to high precision)Allow for ExB correctionsOffline correction for mechanical alignment (field cage wrt r/o plane, sector-to-sector alignment)Debug errors in tracking codeIdentify wrong cabling/mapping and non-functioning electronicsGain monitoringMonitor transverse (maybe longitudinal) diffusion Check clock frequency by matching membrane image for both TPC halvesB.Azmoun, BNL4Slide5

Schedule of Calibration measurements

Condition

Calibration step

E=OFFB=OFFCollisions=OFFSurvey of beam positions (how?)E=ONB=OFFCollisions=OFFSector-to-sector alignment, Drift field alignment to endcap, MPGD Gain, Meas. E-field uniformity meas., ExB baseline,Directly determine systematic errors in global reconstruction E=ONB=ONCollisions=OFFExB effect (need magnetic field map  Hall probes?), Directly determine systematic errors in global reconstruction Baseline drift vel., Baseline for distortions due to space chargeE=ONB=OFFCollisions=ONBaseline for distortions due to space charge (?)E=ONB=ONCollisions=ONCorrection for space charge distortion (laser data interspersed with physics data), drift velocity mapB.Azmoun, BNL5Slide6

STAR laser calibration system

Use 4

th

harmonic of 1064nm laser (266nm), 130mJ/3-5ns pulse Nd-YAG at 10Hz repetition rate, Q-switched (transition from square to Gaussian profile at 20-40m)Laser head placed outside of magnetic field, use intricate conduit system with optical elements to steer beam to TPC volume Telescope used to expand beam size to ~25mmWide beam split by bundles of 7 micro-mirrors ~1mm diam.Mirror bundle: quartz fibers with ends cut at 45deg. polished and coated for optimal reflectivity at 266nm (6 fibers arranged hexagonally + 1 in middle) 20-40mJ/pulse (up to 40mJ/mm2)Planes of 42 laser beam patterns along Z, 30cm apart 6 planes per TPC half (252 laser beams) used to flood TPC volume to map distortions and measure drift velocityProduce Poisson “beam” for alignment procedureDrift velocity measurements to within ~0.02% accuracyRequire 200mm res. in sagitta measurementRequire under 1mm resolution for Z-position 200mm accuracy for mirror positionsSurvey position of mirrors used to define laser beam trackSystematic error checked by comparing identical data from two TPC halves: R=0.99999134+/-0.000225Laser beam track stability: ~64mm(10 mip), ~150-200mm(1-2mip)B.Azmoun, BNL6Slide7

ALICE laser calibration system

Laser beam distribution system is similar to STAR

336 laser beams, 1mm diam. each (6 laser rods x 4 mirror bundles/rod x 7 beams/bundle x 2 TPC halves)

 42 beams per laser beam planeOrganize laser beams into planes at const. z since it is better to measure drift velocity using tracks with ~constant drift times and perpendicular to wires (overall smallest clusters for providing best single point resolution) [this is less complicated than using inclined tracks, but not necessary for sPHENIX]Strategically placed beams cross over neighboring sectors, avoid beam crossings, flood all sectors and provide 4 discrete z samples  optimized for sector alignment and for measuring distortions in the drift velocityUse the end-plates for absolute referencePhotoelectric effect at central membrane and the readout plane define the absolute start and end point of the drift length B.Azmoun, BNL7Slide8

Requirements on precision of ALICE laser system

TPC drift field uniformity relative error: within 10

-4

TPC spatial res.: 150mm (r-phi), 200mm(z)Must ensure matching precision for calibration tracks: Position of laser must be known to a precision of <100mm (x,y,z) and <0.05-0.1mrad for polar and azimuthal angles (from survey of laser beams during construction)Reproducibility (stability of beam position): 100mmPlacement of micro-mirrors is the most important factor for determining the precision of the laser beam position 2nd most important alignment is the angle of the wide beam relative to the mirror bundlesB.Azmoun, BNL8Slide9

ILC Prototype TPC Laser calibration system

9

Array of Al dots

Reconstructed position after drift, whichintegrates all distortions and misalignmentsDiffuse laser lightRely on pattern of small Al dots (not lines) on membrane to carry information (integrated along full drift) about distortions on small volume elementNo Z-information, however this may be Ok for relatively small TPC’s with short drift lengths, where space charge is not as big an issueB.Azmoun, BNLSlide10

Proposed sPHENIX Laser System

The overall calibration strategy is similar to STAR and ALICE since we plan to shoot laser beams into the TPC drift volume, however there are constraints that must be taken into consideration

DESIGN CONSTRAINTS

No radial space available along barrel for laser beam distr. system  driven by TPC momentum spec. (ie, maximizing number of layers leaves little space between TPC and EMCal)Only input ports available are on endcap (12 outer + 12 inner)planes of laser tracks not possible (ala STAR and ALICE), but angled tracks areAlso, can diffusely illuminate central membrane from endcap ala STAR and ILC TPCSince there is also little room for a beam conduit system at the endcaps, multiple (relatively small) lasers must be used, each in close proximity to the laser ports at the endcapRelatively small Diode-Pumped Solid State, Nd:YAG (Q-switched) 266nm Lasers are available (1-6mJ/<1ns pulse)Beam splitter will require development: how many stable split beams can be established at each TPC port? DPSS Laser/Beam splitterB.Azmoun, BNL

10Slide11

Preliminary Laser System Concept

STRATEGY

Shine diffuse light onto membrane to liberate small clusters of charge

Pattern on central membrane shall be dots (ala ILC) not lines (ala STAR), as lines only allow space charge distortion determinations perpendicular to the lines  dots provide for more info.Shoot laser beams into TPC volume to mimic particle tracksStrategically place beams so that each sector will have at least one laser track above it and 2 time hit rangesAt least three varieties of laser sources:Diffuse laser targeting central membrane.Laser going from R=60cm to R=20cmLaser going from R=40cm to R=80cmCharge Cluster from gas ionizationCharge Cluster from photoelectric effect B.Azmoun, BNL11Slide12

Benefits of this constrained design

Instead of uniformly flooding the drift volume with laser beams, use guidance from simulations to shoot beams where they are needed most;

ie

, in areas where we expect high levels of distortion in drift velocity, in space charge, in E-field non-uniformities, and in areas where there is the greatest uncertainty in these parametersLaser and beam mechanics are directly mounted to endplate, which is effectively a good optical bench, resulting is minimal mechanical driftThe optical system at each laser port is small, relatively simple and decoupled from other ports, which simplifies the alignment procedure No complex, long chain of optical elements required to be kept in alignmentSince the required track resolution for the TPC is ~150mm, the beam position must be known to a precision better than ~150mm. Ultimately the envisioned beam mechanics for the sPHENIX TPC laser system should be more stable than that of STAR and ALICE, while the requirements for beam position accuracy and precision are comparable.Laser beams continuously sample all of Z at varying angles (more complicated analysis, but provides a rich data set)Possibly can introduce trace additives that have a negligible effect on TPC performance, but will increase rate of ionization at lower beam energy density (if laser power becomes an issue)Possibility to measure/monitor diffusion via photoelectric effect on small dots on membraneB.Azmoun, BNL12Slide13

Added benefits of this TPC design

IBF can be suppressed significantly in

sPHENIX

TPC Gating grid: optimized mesh structure or alternating potential wire plane interrupts path of ions (low diffusion) into drift volume by ~100%, while allowing electrons (high diffusion) through with ~100% transparency (R&D ongoing)Optimize gap field configurations of MPGD r/o for minimizing IBF while allowing ~100% electron transparencyNew hybrid MPGD: GEM+MM operation allows for more degrees of freedom to control flow of ionsDetailed simulation of expected charge build-up in high multiplicity environment was done at SBUSpace charge mostly builds up at surface of inner field cage (r=20cm), but the fiducial acceptance starts at 30cm  require few mm correction compared to 10-20cm correction!…More???B.Azmoun, BNL13Slide14

Naive concept for using laser for space point correction

Space charge correction in ALICE (in a nutshell)

The positions of the measured space points (blue) in the TPC are shifted away from the path of the high energy particle due to space charge and electric field distortions

The ITS+TRD+TOF is used to extrapolate a curved reference track (red) crossing the TPC volumeSkipping over the details, the position of the blue points is corrected by an amount (Dx, Dy) to match the reference track.

sPHENIX

TPC

Laser track

MVTX

TPC

Use prior knowledge

of laser track

Apply real-life boundary conditions to simulations

Extrap

. MVTX to

corrected TPC data

From a quick discussion with Carlos…

For

sPHENIX

, since there are no space points outside of the TPC



cannot extrapolate reference track

Therefore, must rely on another reference: straight laser track

Laser tracks can provide a

D

x

,

D

y

correction map by constraining simulation results

Challenge

: arrange for enough laser beams to adequately sample the TPC volume so that the binning of the map is sufficient to recover the requisite resolution



how many beams needed??

Applied

(D

x,

D

y)

Measured

(D

x,

D

y)

B.Azmoun

, BNL

14Slide15

Conclusion

The laser system will be helpful to correct track distortions as long as the laser beam position can be known to a precision better than the position resolution of the reconstructed particle tracks and provided that a reliable method which utilizes the laser beams is found to establish a map of correction vectors.

Judging from the experience of ALICE and STAR, we should be able to arrange laser beam tracks within the TPC volume to a precision better than the position resolution of the

sPHENIX TPC (150mm), considering we can likely build a very stable optical system on each TPC big wheel.In addition, we foresee requiring smaller corrections than ALICE and STAR since… Smaller mechanical distortions due to the smaller size of the field cageSmaller drift lengths likely correspond to less track distortion Less space charge Less IBF from readout due to novel gating grid approachNe-based gas also reduces IBF (No constraint on having superb energy resolution since we don’t do pid)Analyze data starting at r=30cm, but most of the space charge is collected at r-20cmLess primary space charge due to smaller luminosity (interaction rate) at RHIC than at LHCPros/cons of magnetic field strength at sPHENIX compared to at ALICE (less diffusion,…,???)B.Azmoun, BNL15Slide16

Backup

B.Azmoun, BNL

16Slide17

Apparatus to measure laser tracks in the lab

Beam

TPC zigzag R/O

2x10mm pads)LaserField cageB.Azmoun, BNL17Slide18

TPC Test Set Up

SS Vacuum Vessel

HV Feed-thru (60kV)

Class IV YAG LaserPhotodiode-Trigger32cm Drift Cell10x0.3mm pad Readout (24ch.)Top Fe55 SourceBottom Fe55 Source24 ch. Readout ElectronicsB.Azmoun, BNL18Slide19

PD -Trigger

Laser

Drfit

Electrons toward Readout(Triple GEM + preamp/shaper)Drift Velocity Measurement of Candidate GasesHV (~32kV)Lens to Focuslaser to a pointPhoto-ionized gasElectron cluster

The PD provides a trigger at the time the

laser produces the cluster of electrons.

The drift time is then measured as the arrival

time of the charge cluster at the readout.

B.Azmoun, BNL

19Slide20

Charge Attachment Measurement for Candidate Gases

Drfit

Electrons toward Readout

(Triple GEM + preamp/shaper)HV (~32kV)Top Fe55 x-ray SourceBottom Fe55 x-ray Source As electron clusters produced by the top Fe55 source drift toward the readout, they lose charge due to charge loss mechanisms within the gas.The amount of lost charge may be measured by taking the ratio of the total charge measured at the readout from the top source (~32cm drift) and the total charge measured from the bottom source (~0cm drift) .

B.Azmoun, BNL

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