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Preparation of crystals for channeling

University of Ferrara. Preparation of crystals for channeling. V. Guidi Department of Physics and INFN, Via Paradiso 12, I-44100 Ferrara, Italy. A. Vomiero INFN Laboratori Nazionali di Legnaro, Viale Università 2, I-35020 Legnaro (PD), Italy. Preparation of the samples-1.

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Preparation of crystals for channeling

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  1. University of Ferrara Preparation of crystals for channeling V. GuidiDepartment of Physics and INFN, Via Paradiso 12, I-44100 Ferrara, Italy • A. VomieroINFN Laboratori Nazionali di Legnaro, Viale Università 2, I-35020 Legnaro (PD), Italy

  2. Preparation of the samples-1 • To remove organic and metallic impurities, the wafers are degreased in trichloroethylene, acetone, and isopropanol. • Wafers are cleaned in a solution of water, hydrogen peroxide, and ammonium hydroxide (5:1:1) at 75 °C for 10 min. • A short dipping in diluted hydrofluoric acid (10% in weight). • The wafers are washed in water ,hydrogen peroxide and hydrochloric acid (4:1:1) at 75 °C for 10 min.

  3. Preparation of the samples-2 • Dicing of the samples by a diamond-blade saw avoiding alignment with major crystalline axes. • Defects are induced by the dicing saw (a surface layer estimated to be as thick as 30m is rich in stratches, dislocations, line defects and anomalies). We attempt the removal of such layer by a wet planar etching (HF,HNO3,CH3COOH).

  4. Results-1 70 GeV Example of the bent crystal plate Extraction efficiency for 70-GeV protons. Recent results: (*, strips, 1.8, 2.0, and 4 mm long), results of 1999-2000; (, “O-shaped” crystals 3 and 5 mm), and of 1997 (, strip 7mm). Also shown (o) is the Monte Carlo prediction for a perfect crystal with 0.9 mrad bending.

  5. Results-2 Surface treatment proved to be useful to improve the quality of the extracted beam Images of the 70 GeV protons deflected through mechanically treated and chemically polished crystals Chemically polished Mechanically polished

  6. Study of crystal surface • In order to study the optimal procedure to remove defects, we have prepared several Si-samples. • some samples have been chemically polished by wet planar etching.

  7. AFMmeasurements AFM images of the surface of an as-diced Si crystal (left up) and after 40 min chemical etching (left down). Chemical polishing enhances the standard roughness (Ra) (right up), which tends to decrease for longer etching times (right down).

  8. 2.0 MeV RBS-channeling  particles spectra • As-cut sample • A layer as thick as 30 m is rich in defects 30m

  9. 2.0 MeV RBS-channeling  particles spectra • Chemically polished • HF 49%, HNO3 70%, CH3COOH 100% (2:15:5) • Damage stop

  10. 2.0 MeV RBS-channeling  particles spectra The enhanced yield of the mechanically cut sample (0.5M) with respect to the layer after 30 min of etching (0.5/30C) is attributed to the presence of a more disordered surface structure.

  11. 2.0 MeV RBS-channeling proton spectra • A lower dicing speed (0.5 mm/min) seems to create a smaller dead layer. • The etched samples exhibit enhanced channeling indication of removal of • the amorphized layer.

  12. Conclusions • Extraction efficiency of the order of 85% was repeatedly obtained at IHEP Protvino for an impinging intensity as high as 1012 protons. • Chemical etching improves channeling efficiency and leads to better performance than conventional mechanically treated samples. • We understood the physical reasons for such a behavior through characterization of the crystal surfaces.

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