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Crystal collimation for LHC
Valery BiryukovIHEP Protvino
Vincenzo GuidiFerrara University and INFN
Walter ScandaleCERN
CERN, Geneva, 24 April 2003
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Borrowed from Ray Fliller’s talk at Paris EPAC 2002
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Crystal Channeling
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Beam line (70 m long) made of 3 crystals, IHEP
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• Beam focusing by crystal
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Crystal design as used at IHEP Protvino and RHIC Crystal is 3 to 5 mm along the beam
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Crystal
5mm
Beam Direction
Crystal Courtesy of IHEP, Protvino
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New crystal design (“strip”) gave 85% efficiency at IHEP
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Typical beam phase space at crystal location, IHEP
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1- circulating beam, 2- extracted beam, IHEP
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Crystal extraction efficiency as measured since Dec 1997.85% is measured even when all stored beam is dumped onto crystal
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Deflected (left) and incident (right) beams as seen downstream of the crystal
• Prior to the test, the crystal was exposed in the ring to 50-ms pulses of very intense beam (about 1014 proton hits per pulse).
• No damage of crystal was seen in the test, after this extreme exposure.
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Beam profile at collimator face with NO crystal, 70 GeV
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Misaligned x
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Crystal collimation
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Effy vs Energy
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45 GeV
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12 GeV
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Crystal lifetime is order of 5*1020 proton/cm2
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RHIC Crystal Collimator Setup
8 Upstream PIN diodes
4 Downstream PIN diodes
Data fill focus on upstream PIN diodes
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Layout of RHIC experiment on crystal collimation
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RHIC measurements, EPAC 2002
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Simulations of LHC crystal collimation
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Simulations with smaller bending, 0.1 mrad
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Two bending options compared: 0.2 and 0.1 mrad
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Efficiency vs bending angle
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Background suppression factor vs crystal bending
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FNAL simulations for Tevatron crystal scraping, PAC 1999
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Conclusion
• Simulations and experiments promise
10-fold improvement in backgrounds
at TeV accelerators if bent crystal is used as primary scraper.
• No problems with high intensity or lifetime.
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Extraction parameters
• Protons • Energy at 1.3-70 GeV• Intensity 1012 protons
in spills of 2 s duration• Efficiency greater than
85%• Equivalent to 1000 T
dipole magnetic fieldExtraction efficiency vs. crystal length at 70 GeV
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Structure of the bending crystal
y
x
z50
0.52
• Dimensions
0.5250mm3
• 1/R is the curvature
experienced by channelled
protons
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Bending device
• Bending exploits anticlastic effects due to anysotropy of crystalline Si
• For the (111) direction the sample takes the shape of a saddle
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Preparation of the Si samples I
• Starting material is prime-grade, (111) oriented 525-m-thick silicon wafer
• In previous runs there came out that a surface layer as thick as 30 m was rich in scratches, dislocations, line defects and anomalies that would reduce channelling efficiency
• Such a layer originated in the mechanical cutting for manufacturing the samples
• Thus we attempted removal of the layer
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Preparation of the Si samples II
• Preliminary cleaning to organic and metallic impurities from the surface of the wafers by H2O2, NH4OH, HF, HCl,...
• Coverage of the largest surfaces by Apiezon wax• Cutting of the samples by a diamond-blade saw
avoiding alignment with major crystalline axes.
• Planar etching (HF, HNO3 and CH3COOH, 2:15:5) with a timing set for 30 m thinning.
More info in Rev. Sci. Instrum. 73 (2002) 3170-3173
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7 meter
VACUUM PIPE
CRYSTAL
S1
S4S3
S2
EM
Images of the beam deflected through mechanically treated (left) and chemically polished crystals (right)