Characterization of a strip crystal previously in use at FNAL Vincenzo Guidi, Andrea Mazzolari CERN, March 24, 2009 University of Ferrara and INFN - Italy 4th Crystal Channeling Workshop 2009 Alberto Carnera, Davide De Salvador University of Padova and INFM - Italy

Outlook Morphological characterization: optical interferometry and AFM Structural characterization: RBS-channeling and XRD Characterization of a strip crystal previously in use at FNAL Conclusions

Crystal bending Crystal bending is accomplished through anticlastic deformation Angle θ (degrees) Principal to anticlastic radii

Optical characterization Optical profilometry with 2 nm resolution Determination of the primary and anticlastic curvatures Correction of torsion

Planarity and roughness of lateral faces An application of crystals would be the collimation of beam halo in next generation of hadron machines (e.g. the LHC) Particles in the halo drift outwards at the rate of ~2 nm per turn. Since the tune is not integer, the particles will hit the crystal every ~10-20 turns and thereby the first impact parameter of the particles onto the crystal will be in the range of ~100 nm

Planarity and roughness of lateral faces It is demands a crystal with a roughness lower than 100 nm on the lateral faces of the crystal Sub-nm roughness was achieved with ACE  Roughness can be measured by AFM or optical interferometry  Planarity can be measured by optical interferometry

Miscut angle by XRD Nominal lying (110) Real lying (110) The angle between the nominal and the real lying of planes is the miscut angle. It should be of the order or less than 1 mrad

The entry face of the crystal Sub-nm roughness was achieved High-quality surfaces is achieved via anisotropic chemical etching: ST9 The entry face is crucial because it captures the particles for channeling. It can be analyzed by -- HRTEM (locally) -- RBS channeling (globally)

RBS - channeling mode Rutherford backscattering under channeling mode is a powerful method to investigate the structure of a surface Channeled particles penetrate deeper into the crystal and are more difficult to emerge out of the surface, resulting in a decrease of the current

RBS spectra Examples of records by anisotropic chemical etching Low signal of backscattered particles means good crystalline quality 4 He + p APL 91 (2007)

Structure of surfaces The surface  min is defined as the ratio of the RBS yield under channelling alignment and random condition extrapolated at the surface channel. The higher the degree of crystalline order in the lattice the lower the surface  min due to the reduction of dechanneling from the defects in the crystal. Experimental  min for a Si (110) crystal with probe particles of 2 MeV. An ideally perfect surface yields  min = 2% probeas cut ACE 4 He + 16%2.2% p18%2%

Characterization of a sample previously in use at FNAL We retrieved and applied the procedure we used in our lab at that time, namely a wet isotropic planar etching exposed for two minutes Subject: Progress made with INTAS Date: Fri, 1 Jul :02: Dear Walter, We are glad to inform you that we have managed with INTAS support to perform a nice crystal deflector which is ready for FNAL experiments, with bending of 150 mrad, and which doesn't require further testing before installation into accelerator. The wafer used for this deflector was perfectly polished in Ferrara. [omissis] Best regards, Valery

Morphology of the lateral face No chemical etching was applied at side surfaces as opposed to our methodology Bending radius on the center = 15.7 m Torsion = 20  rad/mm FNAL FE

Morphology of the entry face FE sample shows knurling due to the blade of the dicer, which are not present in the other Presence of large “chipping” at the edges, an indication of rough cut of the sample FEFNAL

XRD characterization Miscut angle of lateral faces was 80 μrad Mosaicity was determined at the entry face

RBS channeling RBS-channeling characterization AxisBeamFace Measured  (%)Reference  (%) [111]2 MeV He+Lateral4.3 ± [1-10]2 MeV He+Entry4.6 ± The quality of the surface is modest, probably not adequate for the energy at FNAL RBS measurements highlighted that the [1-10] axis is 7° out of axis, i.e., rather an unusual value [1-10] [111]

Conclusions Example of characterization of a strip-like crystal Characterization of a crystal previously in use at FNAL The sample shows features that are significantly different than for our methodology Proposal for experimentation with such a crystal at H8