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Scientific Report 2007-2009
Particle physics
P24. Study of proton channeling of bent crystals for beam collimation
in high-energy accelerators
Collimation is an important component of particle accelerator
technique, especially at the Large Hadron Collider
LHC at CERN, where beam of high intensity and
high energy will circulate in the collider ring. At LHC
collimation system serves the purpose of eliminating the
off-orbit particle (beam “halo” particles) to prevent damages
to superconducting magnets. A traditional collimation
system is made of a series of high-density materials
that absorbe particles in the external region of the
beam. A secondary beam halo is therefore generated
with a larger divergence. Subsequent stages of collimation
are needed to eliminate completely the unwanted
particles. A new concept of collimation is such that instead
of instrumenting the first stage of collimation with
an amorphous material acting as absorber, a bent crystal
could be used. Halo particles impinging on it would
be trapped within the crystal planes and deflected to
a well determined direction. In this way particle will
not be scattered in any direction and the efficiency of
collimation will improve. Using crystal would also help
in enlarging the distance of traditional collimating absorber
from the beam core, reducing the impedance of
LHC and allowing higher currents and therefore a higher
luminosity.
Crystal channeling has been observed since several
years. In the 90s it was demonstrated that a proton
beam of 120 GeV/c could be extracted at 8.5 mrad from
the SPS at CERN using a silicon crystal mechanically
bent with extraction efficiency of 10 − 20%. However it
was foreseen that extraction efficiency through channeling
could be brought to higher values by using shorter
crystals, but the realization of the bent crystals of the
requested size and bending was not simple. In the very
last years many progresses were made in producing new
higher quality silicon strip crystals with small length
(down to 1mm) and curvature imparted at the level of
150 microrad.
In 2006 an internazional collaboration with phisicists
from Russia, CERN and Italy has been formed to test
new crystals with protons of 400 GeV/c from the SPS
accelerator at CERN. The aim of the collaboration was
to measure channeling efficiency through different kind
of crystals. The Rome group partecipated to the 2006
PRIN funding to develop a tracking system to measure
the proton deflection angle. The main results of the test
are shown in Fig. 1. A channeling deflection of about
(165±2) µrad is observed when the proton beam hit the
crystal axis with an angle of about 60 µrad with an efficiency
of about 55%. Moreover, the high quality crystals
used in the experiment allowed to discover new effects.
Among those, it was observed the “volume reflection”.
A proton not channeled can pass transversly through the
planes experiencing the periodic atomic potential. If the
crystal has non-null bending, a centrifugal components
adds up. In this way, whenever the transverse energy of
the particle is lower than the potential barrier the transverse
momentum of the particle gets reversed. This particle
is therefore “reflected” (in a direction opposite to
the center of curvature of the crystal) with an angle of
about 13 µrad. The most relevant feature is that this effect
is active for a larger range of relative angle between
the particle direction and the crystal planes and has a
very high efficiency (about 98%). This is therefore much
appealing for beam collimation application. In general
demonstrating the feasibility of using bent crystal to deflect
beam it is an important step for future accelerator
in which big dipole magnet could be substituted by tiny
silicon bent crystal. Those crystal will be in fact the
equivalent of several tens of Tesla magnetic fields!
The Rome group is currently involved in testing such
concept of crystal collimation on a circulating beam at
CERN SPS with preliminary encouraging results.
Figure 1: Beam intensity recorded by the silicon microstrip
detector as a function of the horizontal deflection angle (x
axis) and the crystal orientation (y axis) with respect to the
incoming proton beam. Six regions can be identified: (1) and
(6) non channeling mode; (2) channeling; (3) dechanneling;
(4) volume reflection; (5) volume capture. The wider angular
acceptance of volume reflection compared to channeling is
clearly visible in the figure.
References
1. W. Scandale, et al., Phys. Rev. Lett. 98, 154801 (2007).
2. W. Scandale, et al., Phys. Rev. Lett. 101, 164801 (2008).
3. W. Scandale, et al., Phys. Rev. Lett. 101, 234801 (2008).
Authors
C. Luci, G. Cavoto 1 , F. Iacoangeli, S. Pisano,
R. Santacesaria 1 , P.Valente 1
Sapienza Università di Roma 131 Dipartimento di Fisica