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Morphological and Structural studies of crystals for
channeling of relativistic particles
CARE HHH-2004
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
University of Ferrara
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.
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
Figure 4
SEM images of the
surface as-cut (left)
and after 30 min
chemical
polishing
(right)
Figure 1
Quantitative analysis of surface roughness is achieved through atomic force microscopy
(AFM). In Fig. 5, AFM micrographs are reported with their standard roughness (Ra).
Figure 5
40 min.
etching
a)
100
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).
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 2
2
cut
10
1
1000
b)
100
10
0
10
20
30
40
etching time (min)
z
50
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.
1000
Ra (nm)
Extraction efficiency for 70-GeV
protons.
Recent
results
[1] (*, strips, 1.8, 2.0, and 4 mm
long), results of 1999-2000; (•, “Oshaped” 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.
30 MIN. ETCHING
Ra (nm)
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 70GeV 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.
y
x
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.
Figure 3
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 a
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 a particles beam (estimated depth analysis ~ 1.5 m) are displayed.
Figure 6
RBS-channeling spectra using 2.0 MeV a 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.
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).
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.
References
[1] A.G. Afonin et al. Physical Review Letters 87 (2001) 094802 -[2] Review of Scientific Instruments 73 (2002) 3170-3173