1. History of Crystal Extraction at the SPS (RD22)
Walter Scandale
CERN
Geneva, 07 March 2005
CARE-HHH-APD-CC05 workshop
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

2. walter scandale, History of Crystal Extraction at the SPS (RD22)
The ante-fact - Aachen 1990
Giuseppe Fidecaro’s request:
Provide 7 TeV/c extracted beams for B-physics in LHC
Provide an appropriate Insertion Region
Use silicon bent crystals to assist crystal extraction
My answer:
IR for extraction difficult but feasible
Request to evaluate the crystal channeling efficiency
Proposal of an experiment in the SPS to test extraction feasibility and the proton interaction with the superficial crystal layer
W. Scandale, Proc. LHC Workshop, eds G. Jarlskog and D. Rein, Aachen, 1990, vol. III p. 760.
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

3. walter scandale, History of Crystal Extraction at the SPS (RD22)
The RD22 proposal
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

4. walter scandale, History of Crystal Extraction at the SPS (RD22)
The RD22 collaboration
Contactperson
Spokesperson
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

5. The experimental set-up
Goniometer with a stepsize of 2 µrad
Stepping Motor with a stepsize of 50 µm
The RD22 Collaboration, CERN DRDC 91-25
Special tank located the upstream empty half-cell of the long straight
section # 5 in the SPS - total length of the set-up about 20 m
Two Si crystals 3 cm long, bent by about 8.5 mrad, mounted on goniometers
3 scrapers (2 horizontal and 1 vertical) to find the beam position and horizontal angle
Laser monitoring system for the alignment of the crystals
Scintillators to estimate extracted beam intensity
Scintillator hodoscope to estimate horizontal and vertical extracted beam profile
Miscrostrip gas chamber 0.1·0.1 mm2 pixel size
CsI scintillating screens 0.2·0.2 mm2 pixel size
(later) a scintillator horizontal fisc to measure the extracted beam profile; thickness initially 1 mm, later 0,2 mm.
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

6. walter scandale, History of Crystal Extraction at the SPS (RD22)
The basic crystal
A.A. Asseev et al, Nucl.Inst.Meth. A330 (1993) p39
Eff.=54±2 %
Silicon crystal
30 mm long,
Serpukov style holder
Eff.=33±5 %
Eff.=16±3 %
Test in single pass mode
F. Costantini, Nucl.Inst.Meth. A333 (1993) p125
Main drawbacks
Varying radius of curvature
Anticlastic vertical bending
The RD22 Collaboration, CERN DRDC 94-11
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

7. walter scandale, History of Crystal Extraction at the SPS (RD22)
The “perfect” crystal
The RD22 Collaboration, CERN DRDC 94-11
The U-shaped crystal
More constant radius of curvature
Less anticlastic curvature
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

8. The goniometer and the laser
Use of the laser table
Crystal pre-aligned
Continuous monitor of the crystal alignment
Goniometer with a stepsize of 2 µrad
Stepping Motor with a stepsize of 50 µm
The RD22 Collaboration, CERN SL
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

9. The halo formation set-up
The RD22 Collaboration, CERN DRDC 91-25
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

10. walter scandale, History of Crystal Extraction at the SPS (RD22)
The impact parameter
The RD22 Collaboration, Phys. Lett. B 357 (1995)
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

11. The two mode of extraction
Kick mode
Impact parameter 0.1 to 5 mm larger that the amorphous layer
Enhanced single pass extraction
Large extraction rate
Difficult to estimate losses and channeling afficiency
Not a parasitic mode
The RD22 Collaboration, CERN DRDC 92-51
Diffusion mode
Impact parameter < 1 µm smaller that the amorphous layer
Controllable diffusion speed
Controllable extraction rate
Easy to estimate losses and channeling afficiency
Parasitic mode
Difficult to predict b and b’
Diffusion mode
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

12. Extraction efficiency
The basic crystals
Large channeling efficiency measured
Consistent with simulation expectation
Large angular response related to multi-turn effect
The RD22 Collaboration, CERN DRDC 94-11
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

13. Extraction efficiency
The U-shaped crystals
Higher channeling efficiency measured
Consistent with expectations (better surface cutting; no anticlastic curvature)
Large angular response (multi-turn effect)
The RD22 Collaboration, CERN SL
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

14. Extraction efficiency
The crystal with 30 µm thick amorphous layer
The RD22 Collaboration, Phys. Lett. B 357 (1995)
In diffusion mode
Average impact parameter varying from 0.05 to 1 µm
Maximum impact parameter varying from 0.5 to 10 µm
b is much smaller that the size of the amorphous layer
The only possible extraction mechanism should be driven by the multi-pass effect
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

15. walter scandale, History of Crystal Extraction at the SPS (RD22)
Energy dependence
Dechanneling vs beam energy
Critical angle c  p-1/2
Dechanneling due to hits on e- and dechanneling due to bending > LD  p > LB = LD (1-F)2
G. Arduini et al., CERN SL and SL
Channeling probability
Scattering angle
Gaussian distribution
<> = 0
Multiple scattering and dechanneling determine the energy dependence of the extraction efficiency
For a given beam energy and crystal bending angle there is an optimal crystal length
Extrapolations of crystal efficiency to the LHC beam energy are reliable (but they depend to some extent on the assumed size of the amorphous layer, which is not known)
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

16. Ion extraction efficiency
G. Arduini et al., CERN SL and SL
Angular scan smaller than with protons
Electromagnetic breakup cross section large
Multiturn effect smaller than with protons
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

17. walter scandale, History of Crystal Extraction at the SPS (RD22)
Conclusive remarks
Primae of RD 22:
Channelling efficiency consistently of the order of 10 % (more than an order on magnitude larger than in previous experiments)
Experimental evidence of multiturn effect in extraction mode
Robust method to evaluate the extraction efficiency
Experimental validation of simulation code at high energy
Extraction of Led ions
All these results were never exploited for collimation
neither for HEP in collider mode
Why a new experiment ? ---> Round Table discussion
The experience at U70 is very promising
There is a strong incentive to propose collimation upgrade (we need however guidance from RHIC ad FNAL experiences) and to exploit crystal in LHC experiments
Considerable improvement were made in crystal production
More homogeneous bending radius using strip crystals with anticlastic curvature (the Russian way)
“perfect” surface with no amorphous layer using chemical hatching (the Italian way)
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)

18. walter scandale, History of Crystal Extraction at the SPS (RD22)
Some Literature
W. Scandale, Proc. LHC Workshop, eds G. Jarlskog and D. Rein, Aachen, 1990, vol. III p. 760.
The RD22 Collaboration, CERN DRDC 91-25
The RD22 Collaboration, CERN DRDC 92-51
A.A. Asseev et al, Nucl.Inst.Meth. A330 (1993) p39
The RD22 Collaboration, CERN DRDC 94-11
The RD22 Collaboration, CERN SL
The RD22 Collaboration, Phys. Lett. B 357 (1995)
G. Arduini et al., CERN SL and SL
G. Arduini et al., CERN SL and SL
7 March 2005
walter scandale, History of Crystal Extraction at the SPS (RD22)