Preparing for a Crystal Experiment Proposal U. Wienands, SLAC Special thanks to S. Mack, U. Ottawa SULI Student 2011.
Channeling in Crystals * Discovered in the 60s: Atoms lined up in strings or planes, give rise to strong fields. beam particles can pass in between, guided by these fields. Nuclear Potential: V (! r ) = Z 1Z 2 e 2 # % 1" r $ (C!, a: screening length) in reality more complicated (thermal vibr., defects etc.) 1 nucleus: 2*10 12 V/cm @ 0.1 Å <110> string: 1.3*10 10 V/cm (Si) (equiv. 4000 T B-field!) r r 2 + C 2 a 2 & ( ' Møller, CAS 2
Bent Crystals (Tsyganov) * Channeled particles may follow a bend in a crystal if! not too small, else transverse energy gets too high pv R c = "Z 1 Z 2 e 2 Nd p for Si (110), 1 TeV/c: R c! 0.4 m In practice, cannot make crystals with a radius that tight Møller, CAS 3
9 mm Si Crystal, 400 GeV p Angular profile (!rad) Volume Reflection dechannel volume capture Scandale et al., CERN Channeling 4 Rotation angle (!rad)
Motivation * Bent crystals have shown potential for beam collimation and extraction CERN SPS (UA9), FNAL Tevatron, Protvino U70 Gain is in peeling off extracted parts rather than dumping on an internal absorber at grazing angle. * Future machines will rely on better, more efficient beam collimation UA9 to mount a crystal in the LHC Linear colliders always had beam collimation as one of their challenges " collider will most likely be challenged as well. * Radiation from crystals in channeling or VR mode may have application as a source. besides being of interest in its own right.
Why at FACET? * Only intense high-energy e beam in the world closest to any linear collider has matching e + beam * FACET has the intensity to allow beam-collimation expt. under somewhat realistic conditions * At FACET we can change the bunch dimensions to change the local density of the bunch study intensity-dependent effects. study how well crystals hold up under intense e /e + beams * At FACET, we can have a low-divergence beam, facilitating better separation of channeled beam from scattered or volume-reflected beam.
Channeling of e + & e @ 23GeV * For Si (110) planar channeling: Critical angle! 43 "rad Tsyganov radius R c! 1 cm; irrelevant as making R < 5 m is difficult if want 1 mr of bending, 5 mm length for 5 m radius or larger. * shorter crystals are better less dechanneling less multiple scattering * e + suffer less dechanneling nuclei attract e & scatter them e + probe of choice for initial measurements
23 GeV e+, 1 mrad bending angle Code by I. Yazynin multiple scattering included
same population transported through 5 m drift
Beam-Collimation Setup Setup for collimation experiment; ß=100 m; "* = 3!10-6 mr
Phase space right after crystal
same population transported from FACET IP to beyond QS1
Detailed tracking through Si (110) continuum model approximation, Lindhard potential e + e + 0-1 -2-3 -4-5 Potential -1 0 1
Detailed tracking through Si (110) e (no scattering)
* 180 GeV e + and e CERN H8
FACET Beamline Map
Beam envelopes, high ß IP 17
Possible initial program * Characterize channeling and volume reflection detect the deflected beam detect the e-m radiation out of the crystal Measure the dechanneling length of interest for e + and e * Characterize difference of volume-reflection angles for e + and e Study crystals with different cutting parameters * Study the spectra of volume-reflection radiation e + and e comparison compare to simulations using detailed tracking. * Measure collimation efficiency for e + and e * Crystals could be obtained from U. of Ferrara V. Guidi et al. are interested in joining a FACET proposal V. Tikhomirov, RINP Minsk. Some UA9 members will join as well, possibly help with goniometer.
Challenges * Need a goniometer in vacuo Learn from CERN experience, may be able to use a CERN or IHEP design Might be able to mount in Kraken chamber * Need detector for channeled e (e + ) close to beam in vacuum envelope preferably robust enough to tolerate direct hit by beam (OTR??) could we do it in an air gap? * Would like a spectrometer for the produced e-m radiation Range Stack? Other ideas?? Separate channeling or VR radiation from bremstrahlung background Alternatively, take the channeled electrons through the spectrometer & detect energy loss.
CERN UA9 Goniometer 20