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