Morphological and Structural studies of crystals for channeling of relativistic particles

Figure 2

Scheme of the bent crystal plate. Dimensions are expressed in mm, y is the direction tangent to the incident beam on the centre of the crystal, x is oriented along the thickness (crystalline direction (111) of silicon) and z is oriented along the height.The crystals were manufactured at the Semiconductors and Sensors Laboratory of the University of Ferrara as narrow strips, about 2-mm thick in the direction of the beam.

y

x

z

50

0.52

Figure 1

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

A recent design to produce a short crystals is the use of the anisotropic properties of a crystal lattice. From the theory of elasticity it is known that bending a crystal plate in the longitudinal direction causes "anticlastic bending" or twists to appear in the orthogonal direction. For Si (111)-oriented, the crystal plate takes the shape of a saddle as sketched in Fig .2

Figure 3

Image of the beam deflected through mechanically treated (left) and chemically polished crystals (right). The profile of the beam bent by a chemically polished crystal is more uniform and sharp (20 rad vs 100 rad at 70GeV).

S. Baricordi, V. Guidi, C. Malagù, G. Martinelli, E. Milan, M. Stefancich Department of Physics and INFN, Via Paradiso 12, I-44100 Ferrara, Italy

A. Carnera, A. Sambo, C. Scian, A. Vomiero† Department of Physics, Via Marzolo 8, I-35131 Padova, Italy

† also INFN Laboratori Nazionali di Legnaro

G. Della Mea, S. Restello INFN Laboratori Nazionali di Legnaro, Viale Università 2, I-35020 Legnaro (PD), Italy

V.M. Biryukov, Yu.A. Chesnokov, Institute for High Energy Physics, Protvino, Russia

Yu. M. Ivanov, Petersburg Nuclear Physics Institute, Gatchina, 188350, Russia

W. Scandale, CERN, Geneva 23, CH-1211, Switzerland

References[1] A.G. Afonin et al. Physical Review Letters 87 (2001) 094802 -[2] Review of Scientific Instruments 73 (2002) 3170-3173

University of Ferrara

During last decade, the use of bent crystals for beam extraction in circular accelerators has been intensively investigated in several laboratories by virtue of the potential advantages of the method. In the frame of the CERN-INTAS collaboration 2000-132, we recently achieved a substantial progress with crystal-assisted beam deflection at the 70-GeV accelerator at IHEP. Extraction efficiency of the order of 85% was repeatedly obtained for an impinging intensity as high as 1012 protons [1]. Key reason of this successful operation was the use of very short crystals for extraction. Crystal length was selected close to the optimal value foreseen by the physics of proton channeling. Fig. 1 shows theoretical predictions of extraction efficiency as a function of the crystal length, for an impinging proton beam of 70-GeV, as compared to experimentally recorded levels with crystals of different size and design.

Channeling of relativistic particles through a crystal may be useful for many applications in accelerators. In particular, great attention has been paid towards improving extraction efficiency of a proton beam from an accelerator. It has been experimented that significant role is played by the method used to clean the crystal's surfaces that encounter first the proton beam. In particular, it was experimentally observed that chemical cleaning of the surfaces via etching leads to better performance than conventional mechanically treated samples. We investigated the physical reasons for such a behavior through characterization of the surfaces of the crystal. We observed that mechanical dicing of the crystal causes a superficial layer rich in dislocations and lattice imperfections, extending tens of microns into the crystal, which is removed via etching. Such a disordered layer is the first portion of the crystal that would be experienced by the incoming particles and results in a mosaicity degree that exceeds the critical angle of relativistic particles for channeling, i.e., it acts like an amorphous layer.

High values of this efficiency can be obtained through a chemical treatment to the surface of the crystal. It has been demonstrated that chemical etching can be used to remove a superficial layer induced by dicing for manufacturing the sample. Further details on this methodology are reported in Ref. [2].Fig. 3 shows the results of the tests for the chemically polished deflectors vs. those for the unpolished samples. The crystals with chemically polished faces have shown the best efficiency for beam extraction.

CARE HHH-2004

A systematic study of the surface morphology and crystalline perfection of Si crystals just after cut and after chemical etching was performed.Fig. 4 shows two scanning electron microscopy (SEM) images of the surface of a Si crystal. Etching alters the morphology of the cracks by creating larger but less numerous craters.

AS - CUT 30 MIN. ETCHINGFigure 4

SEM images of the surface as-cut (left) and after 30 min chemical polishing (right)

Figure 5

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 (rght down).

Quantitative analysis of surface roughness is achieved through atomic force microscopy (AFM). In Fig. 5, AFM micrographs are reported with their standard roughness (Ra).

Investigation on the crystalline structure within the first microns below the surface relies on ion beam analysis (IBA) by low-energy beams. Low energy protons (<3 MeV) or particles impinging at normal incidence onto the surface of a perfect single crystal would be channeled along crystalline planes and/or axes, with significant shrinkage of backscattered particles due to random interactions with the lattice. An amorphous surface layer, on the other hand, would give rise to an intense backscattering signal. Thus, the fraction of backscattered beam yields information about crystalline perfection near the surface. IBA was carried out at the AN2000 Van der Graaff accelerator of INFN Laboratori Nazionali di Legnaro.In Fig. 6, RBS-channeling spectra for proton beam (estimated depth analysis ~ 12 m) and particles beam (estimated depth analysis ~ 1.5 m) are displayed.

Figure 6

RBS-channeling spectra using 2.0 MeV particles on the sample cut at a speed of 0.5 mm/min (right up). The misaligned (random) spectrum is recorded as a reference. 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.

2.0 MeV RBS-channeling proton spectra (left down) were collected for as-cut and etched samples at 2 dicing speeds (0.5 and 5 mm/min). Up to a penetration depth of 12 m, the etched samples exhibit enhanced channeling for low-energy ions--an indication of removal of the amorphized layer. A lower dicing speed (0.5 mm/min) seems to create a less damaged dead layer.

Considerable progress in crystal channeling research has been obtained over the years. Extraction efficiency of the order of 85% was repeatedly obtained for an impinging intensity as high as 1012 protons. Channeling efficiency proved to be positively affected by superficial etching treatments. Different applications may be envisaged such as beam extraction and collimation in accelerator. Chemically polished samples exhibit a more ordered crystalline structure, indicating the removal of the amorphized layer, which was created during the dicing process. Enhanced crystalline perfection of etched crystals can be related to top-extraction efficiency.