Crystal%20Collimation%20at%20RHIC

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Crystal Collimation at RHIC

• Brief RHIC Overview
• RHIC Crystal Collimation System
• Channeling Results
• Crystal Collimation and Background Reduction
• Conclusion

• Angelika Drees
• CC-2005, CERN, Mar 7, 2005

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Collaborators

• BNL
• Ray Fliller (FNAL)
• Dave Gassner
• Lee Hammons
• Gary McIntyre
• Steve Peggs
• Dejan Trbojevic

• IHEP Protvino
• Valery Biryukov
• Yuriy Chesnokov
• Viktor Terekhov

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Run Species Integrated Luminosity Energy
2000 Au-Au 7.3 mb-1 (PHENIX) 70 GeV/u
2001 Au-Au 92.6 mb-1 (PHENIX) 100 GeV/u
2002 Polarized protons 100 nb-1 (STAR) 100 GeV
2003 d-Au 27 nb-1 (PHENIX) 100 GeV/u
2003 Polarized protons 2500 nb-1 (STAR) 100 GeV
2004 Au-Au 1368 mb-1 (PHENIX) 100 GeV/u
2004 Polarized protons 3200 nb-1 (STAR) 100 GeV
2005 Cu-Cu 14 nb-1 (PHENIX) 100 GeV/u
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Typical RHIC Parameters

• 95 norm. Emittance e15 p mm-mrad (Gold), 25 p
  mm-mrad (Cu, p)
• rms momentum spread sp 0.13
• Bunch length sl 0.2 m
• Energy 100 GeV/u (Gold, Cu) 250 GeV/u (p)
• Avg. Store Length 4 hours
• Beam size at collimator 5.3mm (bPHENIX1m)

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2 Stage Crystal Collimation System
Idea Use a bent crystal to channel halo away
from the beam core, intercept with a scraper
(secondary collimator) downstream.
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Crystal Collimator Geometry
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Considerations for Crystal Collimation

• Crystal alignment to beam halo.
• Angular divergence of beam hitting crystal.
• Positioning of secondary jaws/scrapers

However, in RHIC all warm spaces have large a!

• Crystal should be placed at a location that has
  low a and D and a maximun of b so that
• channeling angle is independent of x0
• angular divergence is reduced
• Channeling efficiency is increased
• Operation of crystal collimator is easier

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RHIC Collimation System
Changed after FY2003
STAR
Scraper can move horizontally, vertically and
rotate in horizontal plane
Downstream PIN Diodes
Upstream PIN Diodes
Hodoscope courtesy of Y. Chesnokov and V.Terekhov
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Vessel Cutaway
Pivot
Inchworm
Crystal
Moveable Stage

• Crystal can rotate approx /- 6 mrad
• Measurement Resolution 20 mrad
• Angular Step Size approx. 30 nrad

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Crystal Vessel
Crystal
Crystal Motion
Beam
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Crystal
Serpukov style holder
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Lattice Functions
bPHENIX 2 m FY2003
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Available data sets
scan refers to a scan of the angular range of
the crystal
Run Species bPHENIX Stores Scans
FY2001 Au 5 m 8 27
FY2001 Au 2 m 4 24
FY2001 Au 1 m 12 109
FY2002 p 3 m 11 119
FY2003 Au 2 m 4 20
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Typical Crystal Scan
sx(x0)
Volume Capture
Crystal Aligned
xp
Crystal Channeling
November 12, 2001 Au beam at store.
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Comparison to Simulation

• Model Optics
• Location wrong
• dip width too narrow
• efficiency too large

Design optics do not agree well with data.
However, measured optics agrees better.
Simulation used CATCH and one turn matrix.
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Comparison to Simulation
Volume capture region strongly affected by number
of turns in simulation.
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Channeling Efficiency
Run bPHENIX Channeling Efficiency Channeling Efficiency Channeling Efficiency Channeling Efficiency Channeling Efficiency
Run bPHENIX Design optics Measured optics Simulation Measured width Channeling data
FY2001 5 59 19 2 24 3
FY2001 2 71 39 2 37 1 9 1 28 3
FY2001 1 74 75 1 56 3 20 2 19 3
FY2002 3 79 21 2 26 3
FY2003 2 71 52 2 50 1 26 2 26 3
Channeling Efficiency does not match predictions
from the theory. This is because the beam
divergence on the crystal does not match theory.
Using the measured beam divergence (from sx(x0)
) the efficiency agrees well for most cases.
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Channeling Results

• We observed proton and heavy ion channeling
• RHIC optics did not match model, so initial
  predictions overestimated crystal performance
• Simple theory overestimates channeling efficiency
  lacking multiple turns, model of halo
  distribution too simple.
• Simulation agrees well with data (for the most
  part)
• Channeling efficiency is understood once optics
  and beam halo distribution are understood.
• Accurate knowledge of lattice functions and halo
  distribution is very important

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STAR Background
4 crystal scans with different scraper positions
- xs
Crystal not moved horizontally
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Other Experiment Backgrounds
Only BRAHMS sees a significant effect
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STAR Background Reduction
Scraper only
Raw Background
Crystal collimation does not do better than
scraper alone!
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Crystal Collimation vs. Raw Background
Scraper moves closer to beam
Crystal Collimation reduces Background to
uncollimated rate
Au beam, d-Au run, crystal collimation not
always effective in reducing background.
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Crystal Collimation Results

• Crystal can cause background in experiments.
• Scraper position very important.
• Because of low channeling efficiency, crystal
  collimation was not successful.
• Scraper alone collimated the best.
• Crystal Collimator removed from RHIC.
  Traditional two stage collimation system
  installed for FY2004 run.

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Summary

• Bent Crystals were used for collimation in RHIC
• Crystal Channeling worked as expected once
  lattice functions and halo distribution were
  understood.
• Collimation was unsuccessful because lattice was
  not optimized in area of collimator.
• Crystal caused background.
• Tevatron has installed the BNL crystal vessel
  ready to be commissioned end of this month ?

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Measuring Crystal Angle
By measuring the deflection of the laser beam,
the crystal angle is measured

• Crystal can rotate approx 6 mrad
• Measurement Resolution 20 mrad
• Angular Step Size 30 nrad

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Beam Divergence
Run bPHENIX sx(x0) mrad sx(x0) mrad sx(x0) mrad sx(x0) mrad
Run bPHENIX Design optics Measured optics Simulation Channeling data
FY2001 5 12.3 39 4
FY2001 2 9.98 19 1 20 1 78 4
FY2001 1 8.91 9 1 11 1 38 3
FY2002 3 10.8 58 3
FY2003 2 9.98 14 1 16 1 28 2
Even using the correct optics, the predicted
angular spread is too small. Multiple turns are
not in the theory! Assumed Gaussian halo
distribution!
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Channeling Angle vs. Position
b1m at PHENIX
Design mrad/mm
Measured Optics mrad/mm
Data mrad/mm
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Placing the Scraper
Scattering from scraper
Scattering from crystal
By using both sets of PIN diodes, we know when
the scraper becomes the primary aperture