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Crystal tests in AB/ATB/EA (previously SL-EA)

This study focuses on using bent crystals to provide collinear KS and KL beams for the NA48 CP-violation experiment, as well as investigating long-term stability and radiation hardness. Tests were conducted with proton and ion beams, yielding promising results. The mechanical design of the crystal holder also played a crucial role in the success of this application.

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Crystal tests in AB/ATB/EA (previously SL-EA)

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  1. Crystal tests in AB/ATB/EA (previously SL-EA) H.W.Atherton,C.Biino, M.Clément, N.Doble, K.Elsener, L.Gatignon, P.Grafstrom, J.B.Jeanneret, U.Uggerhoj, In close collaboration with many others, in particular friends from Aarhus, CERN, Darmstadt, Dubna, Grenoble, Johannesburg, Strassburg, Stuttgart presented by L.Gatignon / AB-ATB-EA • Studies of proton channeling • Radiation hardness for protons • Studies of ion channeling • NA48 application

  2. Incentive: Application of a bent crystal to provide simultaneous and collinearKS and KL beams for the NA48 CP-violation experiment Approach: • Started cooperation with E.Uggerhoj and S.P.Moller and their NA43 colleagues to gain experience with bent crystals. • Study performance and limitations of bent crystals • Prepare the specific application for NA48 • Investigate long-term stability – radiation hardness • As the opportunity presented itself, some of these studies were • extended to Pb and In beams

  3. Few 1012 ppp > Few 104 ppp

  4. STUDIES WITH PROTON BEAMS • Try to understand bending phenomenology • Measure limits • Work towards design of crystal holders • Use mainly H8 beam Initial attempt: H8 microbeam, 450 GeV/c protons Horizontal divergence 3 microrad Spot size 2Hx 1V mm2 FWHM Intensity at the 104 – 106 level <110> Silicon crystal, 10x50x0.9 mm3 Front and back ends 3x8 mm2 fully depleted solid state detectors Note: outgoing beam: sq=2Yp = 15 mrad

  5. Non-uniform radius of curvature ≈ 10% fully bent through 7 mrad Ball bearings (“fluid”) Too much material(bkgd into detectors) Crystal deformationdue to balls to remove variation in Rcurv over length of crystal

  6. Causes “foot” 1992: Next H8 test <111> Si crystal, 52x9x0.9 mm3 Incident beam badly tuned:divergence 35 mrad More uniform bending radius but curvature not monitored

  7. Bending efficiency:

  8. 1993: Comparison of 450 and 200 GeV/c (110) Si crystal, 50x10x0.9 mm3 4 solid state detectors Classical 3-point bender

  9. 450 GeV/c microbeam:

  10. 450 GeV/c: microbeam (sq= 3 mrad) 200 GeV/c: secondary beam (larger sq) 3 mrad bend

  11. Preliminary results for axial channeling:

  12. In a p- (negative beam): (111) Si crystal,Bending angle is 3.1 mrad (to the left of the plots), hence Rcurv = 6 m (i.e. cannot expect 3.1 mrad bending) Axial channeling: some clear deflection seen but no particles are bent by more than 0.5 mrad Planar channeling: even weaker, but significact planar deflection effect is seen

  13. 1996-1997: Germanium crystal (110) Ge crystal:50x10x1 mm3 bent over 30 mm of its length Beam divergence: 450 GeV/c microbeam: 3 mrad 200 GeV/c sec beam: 7 mrad (approx. 60% p, 40% p+)

  14. 200 GeV/c hadrons 450 GeV/c protons Results for bending efficiency of Ge: up to 60% Good agreement with theoretical expectations

  15. Radiation hardness A crystal has been irradiated in the T6 primary target for a full year with a 450 GeV/c slow-extracted proton beam of 5 1012 ppp over 2.4 s every 14.4 s. The crystal was not aligned, the beam divergence was 0.2 mrad RMS. Total flux: 2.4 1020 p/cm2 over an area of 0.8 x 0.3 mm2 The bending efficiency was measured before and after the irradiation Contact radiography:

  16. The measurement was done with a classical 3-point bender. Deflection efficiency before irradiation: 50% Unfortunately the crystal broke into 2 pieces after the irradiation. Hence the distance between the two outer pins had to be reduced from 30 to 25 mm. This lead to a smaller radius of curvature and a correspondingly reduced channeling efficiency. Nevertheless the channeling and bending efficiency could be measured as a function of efficiency

  17. Deflection efficiency measurement after irradiation 2.4 1020 p/cm2 C.Biino et al., CERN-SL-96-30-EA, published as EPAC 96:2385-2387

  18. FOR NA48

  19. Crystal serves to • bend beam back over 7.2 mrad • attenuate proton beam by large factor • It replaces: • a 5 m long dipole magnet • a very small diameter pinhole • It avoids muons to be bent back • into the experiment

  20. MECHANICAL DESIGN OF CRYSTAL HOLDER:

  21. Reduce transmission by using coupling between H and V planes:

  22. Transmission through the crystal : ≈ 2 10-5

  23. NA48 experience • Never had to replace a crystal • Never had to retune the crystal during a run • Very stable element in the beam • We consider it a very successful application • A crystal was also used as a photon converter to enhance photon conversion efficiency for a given thickness of the KS anti counter (C.Biino)

  24. STUDIES WITH ION BEAMS • Tests were performed with fully stripped Pb82+ beams at 400- GeV/c (33 TeV) • Theoretical expectations: • Yp = 7 mrad, like for protons • Ions steered away from nucleihence reduced nuclear interactiuonsand reduced Weiszaecker-Williams break-up • Fraction channeled driven by beam divergence and dechanneling as for protons • Expect 16.8±2.5% deflection efficiencyfor a 4 mrad bend over 50 mm Si

  25. First attempt (1996): Achieved 14±2%, consistent with theory

  26. 1999 (H4 beam): 105 ions per SPS cycle 60x18x1.5 mm (110) Si crystal, bent over 55 mm with the 3-point bender also used in H8 Beam divergence 43 mrad FWHM, measured by goniometer scan and double scan with two steering magnets (change only angle at crystal)

  27. Hodoscope scans Dechanneling due to 3-point bender

  28. 2001: Study of background and dechanneling (H4) 60x60 mm scintillator, operated at different HV but fixed threshold: sensitive to differentcharge states

  29. Ions interact at rate much lower than in amorphous Silicon Indications that most of the lead comes out as Pb82+ But cannot exclude e.m. dissociation

  30. Very recent results from 2003 Indium run (H2 beam): MUSIC – MUltiple Sampling Ionisation Chamber Si crystal, 60 mm long, bent over 56 mm Scan of S2.S3 coincidence

  31. MUSIC output: Bent beam Direct beam Evidence of strong reduction (by factor ≈20) of nuclear charge changing interactions

  32. Fig. 5. The fragmentation probability suppression η versus charge number in the upper region of Z for the bent beam for angles 7.5 mrad (filled squares: perfect alignment, open triangles: aligned at Yp/2), 11.9 mrad (open circles) and 19.8 mrad (filled triangles).

  33. A list of papers (1) Proton channeling

  34. A list of papers (2) Radiation Hardness Ions NA48 LHC

  35. Final remarks and conclusions • The H8 beam line is an excellent tool for crystal studies and qualificationIts intrinsic divergence can be much smaller than the critical angle Yp • Detailed studies of proton channeling could be done successfully in H8with nice results and record deflection efficiencies for the full beam ! • A crystal has been irradiated in the T6 target to a beam of 5 1012 pppwith a 0.8x0.3 mm2 FWHM spot size over a full SPS year.A 31% loss of deflection efficiency was observed for 2.4 1020 p/cm2 • The H8 tests contributed to the development of a successful applicationof a bent crystal in the simultaneous Ko beams for NA48. The crystal performed very well over many years and has been extremely stable. It never needed to be replaced. • Ion channeling has been studied in H4 (could have been done in H8).The channeling efficiency for Pb is similar as for protons, the outcomingparticles are mostly of the same species as the incoming beam.

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