Pis'ma v ZhETF, vol. 101, iss. 10, pp. 755-760
© 2015 May 25
Comparative results on the deflection of positively and negatively charged particles by multiple volume reflections in a multi-strip silicon
W. Scandalea'b'e, G.Arduinia, M. Butcher®, F. Ceruttia, M. Garattinia, S. Gilardonia, A.Lechnera, R.Lositoa, A. Masia, A. Mereghettia, E. MetraP, D. Mirarchia'j, S. Montesanoa, S. Redaellia, R. Rossia'e, P. Schoofsa,
G. Smirnova, E.Bagli0, L. Bandiera0, S. Baricordi0, P.Dalpiaz0, G. GermoglP, V.Guidi0, A. Mazzolari0, D. Vincenzic, G. Clapsd, S. Dahagovd'k'1, D. Hampaid, F. Murtasd, G. Gavotoe, F. Iacoangelie, L. Ludovicie, R. Santacesariae, P. Valentee, F. Galluccio^, A. G. Afonin9, Yu. A. Chesnokovgl\ A. A. Durum9, V. A. Maisheev9, Yu. E. Sandomirskiy9, A. A. Yanovich9, A. D. Kovalenkoh, A. M. Taratinh, Yu. A. Gavrikov1, Yu. M. Ivanov1, L. P. Lapina1, W. Ferguson?, J. Fulcher?, G.HalP, M.PesaresP, M. Raymond?, D. Bolognini"1'", S.Hasanm<n,
M.Prest"1'", E. Vallazza0 aCERN, European Organization for Nuclear Research, CH-1211 Geneva 23, Switzerland bLaboratoire de l'Accelerateur Lineaire (LAL), Universite Paris Sud Orsay, 91898 Orsay France CINFN Sezione di Ferrara, Dipartimento di Fisica, Universita' di Ferrara, 44122 Ferrara, Italy dINFN LNF, 40 00044 Frascati (Roma), Italy eINFN Sezione di Roma, 00185 Rome, Italy f INFN Sezione di Napoli, 80126 Napoli, Italy 9Institute of High Energy Physics, RU-142284 Protvino, Russia h Joint Institute for Nuclear Research, 141980, Dubna, Russia lPetersburg Nuclear Physics Institute, 188300 Gatchina, Russia
i Imperial College, London, United Kingdom kLebedev Physical Institute of the RAS, 119991 Moscow, Russia 1 National Research Nuclear University "MEPhI", 119991 Moscow, Russia m Universita' dell'Insubria, 22100 Como, Italy nINFN Sezione di Milano Bicocca, 20126 Milano, Italy °INFN Sezione di Trieste, 34127 Trieste, Italy Submitted 17 April 2015
Bent silicon crystals in channeling mode are already used for beam extraction and collimation in particle accelerators. Volume reflection of beam particles is more efficient than beam channeling; however the mean deflection angle is rather small. An experiment on the deflection of a 400 GeV/c proton beam and a 150 GeV/c 7r~ beam at CERN using a multi-strip silicon deflector in reflection mode is described. The mean deflection angle of beam particles has been considerably increased due to sequential volume reflections realized in the deflector. This gives possibility for a successful usage of the multi-strip deflectors for beam collimation in high energy accelerators.
Bent crystals have found applications in the beam extraction and beam collimation systems of large circular proton accelerators owing to high electric fields produced by the crystals [1-3]. These crystals were used mainly in the planar channeling mode. A new physi-
cal phenomenon, namely the reflection of a high-energy proton beam from the atomic planes of a bent silicon crystal, was recently observed and work started on the use of this phenomenon at accelerators [4-7]. Volume reflection is caused by an interaction between the incident proton and the potential of the bent crystal lattice and
micbMa B >K3TO TOM 101 Bbin. 9-10 2015
occurs in a short length interval in a region tangential to the bent atomic plane.
The probability of a single reflection is high, approaching 98% for 400GeV/c protons . Because of the short characteristic length of the volume reflection process it is very efficient both for positive and for negative particles while channeling is considerably less efficient for negative particles in comparison with positive ones [8, 9]. Efficient channeling in very short crystals of /im length scale has been shown recently for 0.8 and 6 GeV electrons [10,11]. The use of axially-aligned crystals can be considered as a promising option for channeling of negative particles [12,13]. In this work we show that volume reflection in a sequence of crystals can effectively deflect both positive and negative multi-GeV particles through angles of about O.lmrad. This value of the deflection angle is sufficient for important accelerator applications [2,3].
Analytical theory  predicts that, under the same conditions, the deflection angle for negative particles due to volume reflection is ^1.8 times smaller than that for positive particles. However, allowing for the ^ (1 /E)1/2 energy dependence of the reflection angle, in our case for (111) silicon crystals with a bending radius of 7 m, the theory yields similar values for protons and negative pions (Fig. 1). More specifically, the the-
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Fig. 1. The theoretical distributions of deflection angles due to volume reflection in a 2 mm silicon crystal bent along the (111) planes with a radius of 7m for 400 GeV/c protons (1) and 150 GeV/c negative pions (2)
oretical values for mean reflection angle and RMS have been determined to be 11.2 and 5.0/irad, respectively,
for 400GeV/c protons, and 14.3 and 11.7/irad, respectively, for 150 GeV/c negative pions.
Accelerator physics problems like extraction and col-limation require the reflection angle to be several times larger. In order to increase the reflection angle a few variants of multi-crystal devices have been tested in proton beams [14-18]. The design of an eight-strip crystal device used for this study is shown in Fig. 2. Details
Fig. 2. A picture of the multi-strip crystal deflector and scheme of its installation in the goniometer
are described in . The crystals were bent using the anisotropic properties of the crystal lattice. A transverse bend of 0.28 mrad appears when each strip is bent in the longitudinal direction by the metallic holder. Furthermore, since all strips are manufactured from one silicon wafer, good mutual alignment of individual strips holds in both the horizontal and vertical planes. The major faces of the crystal plate were parallel (111) crystalline planes, and the entry face was normal to the (110) crystalline axis. The separate crystal strips are
2 mm in width along the beam, 40 mm in length and 0.9 mm in thickness across the beam.
The experimental setup was as described in . Four microstrip silicon detectors, two upstream and two downstream of the crystal, were used to measure the particle trajectories with an angular resolution of
3 /irad, which is limited by multiple scattering of the particles in the detectors and air. The angular divergence in the horizontal and vertical planes of the incident beam was about 10 /irad. A high precision goniometer allowed orientation of the multi-strip deflector both in the horizontal and vertical plane with an accuracy of 2rad. The scheme of crystal installation in the goniometer is shown in Fig. 2.
Письма в ЖЭТФ том 101 вып. 9-10 2015
In the first stage of the study, a scan of the horizontal orientation angles of the crystal deflector 4>x was performed. A two-dimensional color plot in Fig. 3 shows
-200 0 200 400
Fig. 3. The intensity distribution of a 400GeV/c proton beam (a) and 150GeV/c negative pion beam (b) passed through the eight-strip silicon deflector as a function of the particle deflection angle 9X for different goniometer positions <j>x
the intensity distribution of the beam passed through the crystal as a function of the particle deflection angle 0X and angular position of the goniometer 4>x. Fig. 3a corresponds to 400 GeV/c protons, and Fig. 3b corresponds to 150 GeV/c negative pions.
The mean deflection angle equals zero at the beginning (left) and at the end (right) of the scan due to scattering of particles in the crystal deflector as in an amorphous substance. The particle deflection maxima visible at 0X > 0 near the central region of the scan occurs due to channeling. Two separate maxima of the beam
intensity are explained by misalignment of crystals in the sequence. Further in the angular region, deflections with Ox < 0 occur due to multiple volume reflections of particles traversing the full sequence of strips.
Figs. 4a and b show the distribution of protons and negative pions in the horizontal deflection angle 0X for
Fig. 4. The distributions of horizontal deflection angles 9X in the conditions of sequential volume reflections in the multi-strip deflector marked by the arrows in Fig. 3: (a) -for protons, (b) - for negative pions
a fixed goniometer position indicated in Figs. 3a and b by the vertical arrows where multiple volume reflection (MVR) of particles occurs. The efficiency of one side MVR deflection (0X < 0) is about 94 % for positive particles and about 71 % for negative particles.
The mean reflection angle is 68 /xrad and RMS deviation is 16.5 ytxrad for protons. The mean reflection
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angle is 78 ¿trad and the RMS deviation is 38.4 ¿trad for negative pions. The analytical theory predicts, in the case of a sequence of 8 crystals, a mean angle of 11.2,ttrad x 8 = 89 ¿trad and an RMS = 14.2^trad for protons, and a mean angle 14.3 ¿trad x 8 = 114 ¿trad and an RMS = 33 ¿trad for negative pions. The measured reflection angles are about 80 % of the theoretical values assuming volume reflection on eight strips. This can be explained by the misalignment of two strips for which volume reflection is not realized for the deflector orientation considered.
An additional way of increasing the reflection angle or RMS was achieved at the next stage of the work. In  it was explained theoretically that multiple volume reflections of particles in one bent crystal (MVR OC) can be realized due to the contributions of skew crystalline planes at crystal orientations near the crys-tallographic axis. This possibility was convincingly confirmed experimentally in an extracted beam [13,17,21] and verified in a circulating beam . As noted above, in our case the entrance
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