Central Fiber Tracker Status, Physical Alignment, Imperfections

1
Central Fiber TrackerStatus, Physical Alignment,
Imperfections

• Thomas Nunnemann
• Fermilab
• for the CFT Group
• D0-Workshop at NIU
• June 20 - 2000

2
CFT Construction Design

• 8 cylinders
• with axial and stereo double layer of
  scintillating fibers
• stereo angle 3º
• 76,800 channels
• length
• outer 6 (cyl. 3-8) 2.52 m
• inner 2 (cyl. 1-2) 1.66 m
• radius 20 cm 51 cm
• pseudo rapidity coverage
• ? lt 2.0 (reconstruction)
• ? lt 1.6 (trigger)
• 300 wave guides
• length 8 m 11.5 m
• readout with VLPCs
• Visible Light Photon Counter

3
From Scintillation to Reconstructed Tracks

• scintillating fibers on cylindrical layers
• wave guides (clear fibers, connectors)
• Visible Light Photon Counter (VLPCs)
• Analog Front End
• readout, data logging trigger system
• offline reconstruction
• Every step might contribute to imperfection!

4
Tracker Mechanical Construction

• completed within specs
• fiber positioning
• angular ?(rf) 33 µm
• radial ?r 65 µm
• details about geometry, precision etc. later
• only 87 out of 76,800 problematic fibers
• number from ribbon QC
• possible additional damage during mounting (cyl.
  7/8)

5
Wave Guide Production

• clear fibers with double cladding
• 10 built and tested
• production is expected to be completed in August
• wave guide QC
• spread of light yield within wave guide lt8
• ½ problematic channel per wave guide (256 fibers)
• gt 0.25

6
Wave Guide Routing

• optical fibers will be routed between Central and
  End Cap Calo
• tight environment!
• fibers are thermally bent
• Can we install all wave guides without damaging
  any fibers?

7
VLPC Readout

• optical readout with Visual Light Photon Counters
  (VLPC)
• VLPC chips assembled in cassettes
• challenges
• fiber to pixel alignment 10 µm
• operation at 9K (cryogenic system, liquid helium)
• cassette production
• 60-75 of sub-components done
• 20 of cassettes completed
• expected to be finished mid November

8
VLPC Readout II Cassette Test with LEDs

• full characterization of cassettes started in
  Lab3
• gain as function of bias voltage, background rate
• single photo peaks visible, c.f. a)
• good uniformity, c.f. b)
• gain variation within cassette lt5
• 15 cassettes tested, 15,360 ch.
• applying 85 benchmark, number of compromised
  channels
• 24 electrical, 13 optical gt 0.25
• some channels might be recovered, but best parts
  used so far

a)
b)
9
Electronics Status AFE Boards

• Analog Front-End (AFE)
• tests with pre-production prototype
• SVXII chip
• readout similar to SMT
• SIFT chip
• discrimin. for trigger signal
• preliminary results
• ch. spread 1.2 fC, a)
• ch. noise 2 fC, b)
• 1 photo electron (p.e.) corresponds to 7fC
• 1 p.e. trigger threshold might be feasible!

a)
b)
10
Fiber Positioning

• all fiber ribbons were surveyed with CMM
• global precision of fiber positions
• measured desired in D0 coordinate system shown
• angular ?(rf) 33 µm
• radial ?f 65 µm
• incl. corrections for angular offset, pitch,
  residual stereo angle

11
Fiber Alignment (Axial Layers)

• parallel alignment of fibers is demanded by track
  trigger
• sources of misalignment
• ribbon production, ribbon mounting, nesting
  position of cylinders
• nesting position was optimized based on fiber
  survey
• desired position was obtained by 0.5 mil
• offset from optimum increase of av. residual
  stereo angle by 10

stereo slope in rf in mil/100 average abs.
slope 2.2 slope RMS
2.8 specification 4 mils (end-to-end, i.e. 100
12
Fiber Position, Individual Cylinders

• results given for D0 coordinate system
• all axial layers of similar quality
• stereo layers not yet completed
• fiber position RMS slightly better
• discrepancy with desired stereo angle worse
• stereo layer not used in trigger

13
Geometrical Description of Fiber Position

• completed for axial layers
• stereo layers hopefully soon
• generic function including correction with
  parameters for individual ribbons (256 fibers)
  available
• in DO coordinate system
• 3 main fit parameters corrections to
• angular offset
• fiber pitch
• residual stereo slope
• 10 (7) additional parameters describing the
  residuals from the linear fit to the CMM
  measurements
• angular offset for each measurement position
  along fiber
• might be useful for a more accurate description
  with polynomial or spline fit
• some ribbons have kink (glue not compl. cured)
• linear fit does not give acceptable description

14
Geometrical Description II

• not correcting for residual stereo angle (c.f.
  trigger)
• RMS of fiber position increases by only 10
• incl. all corrections
• best limit, if one would know the exact
  longitudinal shape of the ribbon
• RMS reduced by 40

15
Geometry III Description Beyond Linear Fit

• more complex description using residuals of
  linear fit
• CMM measurements within grid of 10 along fibers
• checked that for effective 20 grid no
  improvement
• but improvement for 10 grid likely (indep. check
  not possible)
• correction based on shape of molds
• used for production and mounting (individual mold
  per layer)
• RMS of fiber position can be improved by 10

layer 3 axial p6-fit to mold structure
16
(Im)perfections I

• misalignment
• alignment of fibers within layers and across
  layers within specs
• fulfills needs of track trigger
• due to offline corrections not an issue for
  reconstruction
• but needs to be implemented in reco and MC code
• should be used as start-point for offline
  alignment
• track resolution will profit from most accurate
  as possible description
• Dont forget about relative alignment SMT-CFT!
• problematic due to limited CMM precision
• signal/noise ratio
• light yield of scintillating fibers?
• indication that fibers on tracker have slightly
  higher yield than those in cosmic ray test
• attenuation length of production wave guides?
• noise level on SIFT solved?
• note only problematic for trigger, not for
  reconstruction

17
(Im)perfections II

• problematic channels (significantly reduced gain)
• 0.1 ribbon production
• 0.X several edge fibers on cyl. 7 and 8 might
  have been damaged (connectors had to be shaped)
• 0.25 wave guide production
• 0.X wave guide routing (hopefully no problem in
  the end)
• 0.25 cassettes
• gt should be possible to stay lt1 for the entire
  system
• number of completely dead channels much lower!

18
The End

• We should have a powerful Central Tracker for
  RunII, but it is still a long way to go