Crystal Channeling and Its Application at High-Energy Accelerators

233 Pages·1997·8.17 MB·English

by  Dr. Valery M. Biryukov

Accelerator Physics
Editors: F. Bonaudi
c.w.
Fabjan
Springer-Verlag Berlin Heidelberg GmbH
Valery M. Biryukov Yuri A. Chesnokov
Vladilen I. Kotov
Crystal Channeling
and Its Application
at High-Energy
Accelerators
Translated by V. M. Biryukov
With 122 Figures
i
Springer
Dr. Valery M. Biryukov
Dr. Yuri A. Chesnokov
Professor Vladilen 1. Kotov
Institute for High Energy Physics
142284 Protvino, Moscow Region, Russia
Editors:
Professor F. Bonaudi
Professor C. W. Fabjan
CERN, Div. PPE
CH -1211 Geneve 23, Switzerland
Library of Congress Cataloging-in-Publication Data
Blryukov, Valery M., 1961-
Crystal channeling and its application at high-energy accelerators
I Valery M. Biryukov, Yuri A. Chesnokov, Vladilen 1. Koto'v.
p. cm.
Includes bibliographical references.
1. Channel ing (Physics) 2. Crystals--Effect of radiation on,
3. Particle accelerators. I. Chesnokov, Yuri A., 1955-
Ir. Kotov, V. 1. (Vladilen Ivanovich) 111. Title,
QC176.8.C45B57 1996
530.4' 16--dc20 96-28616
CIP
ISBN 978-3-642-08238-2 ISBN 978-3-662-03407-1 (eBook)
DOI 10.1007/978-3-662-03407-1
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Origina1ly published by Springer-Verlag Berlin Heide1berg New York in 1997.
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Preface
"Nature performs not hing vainly,
and makes nothing unnecessary"
Aristotle
Interest in the passage of charged particles through crystals first appeared
at the beginning of this century following experiments on x-ray diffraction in
crystallattices, which provided the proof of an ordered distribution of atoms
in a crystal. Stark [1] put forward the hypothesis that certain directions in a
crystal should be relatively transparent to charged particles.
These first ideas on the channeling of charged particles in crystals were
forgotten but became topical again in the early 1960s when the channeling
effect was rediscovered by computer simulation [2] and in experiments [3]
that revealed anomalously long ion ranges in crystals. The orientational ef
fects during the passage of charged particles through crystals have been found
for a whole range of processes characterized by small impact parameters for
collisions between particles and atoms: nuclear reactions, large-angle scatter
ing, energy losses. Lindhard explained the channeling of charged particles in
crystals [4]. The results of the numerous investigations into the channeling
of low-energy (amounting to several MeV) charged particles in crystals have
been summarized in several monographs and reviews [5~8l.
A new stage in the investigation into the channeling of charged particles is
its extension to high energies. This stage began at the European Laboratory
for Nuclear Research (CERN) in Geneva in 1974. In 1976, Tsyganov pro
posed the possibility of bending the high-energy charged particles by means
of bent crystals. This idea was confirmed in pioneering experiments carried
out in 1979 in a collaboration between the Joint Institute of N uclear Research
(JINR) in Dubna and the Fermi National Accelerator Laboratory (FNAL)
in Batavia, IL. In the first experiments on bent crystals the efficiency of the
particle beam deflection (i.e., the ratio of the intensity of the deflected beam
to that incident on a crystal) was very low (a fraction of 1%). But in the sub
sequent experiments it has been improved to 10%, and has recently reached a
record value of 50% in experiments on the deflection of a 450-GeV proton
rv
beam at CERN.
Experiments carried out in 1990 on the accelerator at the Institute of
High Energy Physics (IHEP) in Protvino, in collaboration with the Sankt-
VI Preface
Peterburg Institute of Nuclear Physics (PINP) in Gatchina, demonstrated
that it is possible not only to deftect charged particle beams in a bent crystal
but also to focus them in the bending plane. This can be achieved if the
exit face of the crystal is shaped to form a cylindrical surface, so that all the
planes tangential to the atomic planes in the exit face of a crystal intersect
at so me distance from the crystal. Bent crystals, with their extremely high
interplanar electric fields (109 V cm -1 or highe r) , can thus be used to control
beams of charged particles with high and superhigh energies. This possibility
has been realized by many research centers working in high-energy physics.
Studies are being carried out into the use of bent crystals for the extraction
of accelerated proton beams, in beam lines, and in certain experiments. In
all three cases so me interesting and promising results have been obtained. In
particular, bent crystals have been used to extract accelerated protons with
energies up to 8 GeV at JINR (1984), up to 70 GeV at IHEP (1989), and 120
GeVat CERN (1993). In the JINR experiment the extraction efficiency was
10-4, but it has been improved to rv 10-2 at IHEP and to 10-1 at CERN.
The experiments proceeding at CERN and started at FNAL are designed
to achieve a highly efficient extraction of a proton beam, so that the results
can then be used to develop an extraction system for supercolliders such as
the large hadron collider (LHC), because the use of crystals for this purpose
seems to be the only method that can ensure simultaneously both the beam
extraction and the experiments carried out in the collider mode.
Considerable interest is generated by the studies into channeling radiation,
in crystals, of electrons and positrons with energies of hundreds of Ge V, which
are being carried out at CERN under the leaders hip of Uggerh0j. These
studies show that at high energies, using the aligned crystals as radiators,
one can obtain photon beams with an intensity two orders higher than the
corresponding value in amorphous matter. Therewith, in a crystal as thin as
< 1 mm the particles lose more than 50% of their energy by radiation. U nder
certain conditions narrOW (rv 10%) peaks appear in the photon spectra at
energies 0.7-0.8 times the primary energy of the electron. The results open
up the possibility for experimental study of, e.g., the photoproduction of
particles (also rare) in a new energy range of photon beams.
This book presents from a unified point of view the results of numerous
studies in high-energy charged-particle channeling in crystals, which imply a
possibility for their application at the modern accelerators in the extraction
systems of accelerated particle beams, in beam lines, and in experiments.
The book is addressed to not only the specialists but also the broad
audience of physicists and engineers working at accelerators and carrying out
the experiments in high-energy physics.
The first two chapters consider the physical aspects ofthe charged-particle
channeling in the straight and bent crystals. Different mechanisms of the par
ticle capture into the channeling mode are analyzed, the diffusion theory of
Preface VII
the particle dechanneling is presented, and the effects due to various imper
fections of the crystal lattice are considered.
The third chapter is devoted to the experimental studies directed at inves
tigating the basic foundations of the channeling theory. The requirements for
machining the crystals are formulated, the bending methods are discussed,
and the experimental techniques are considered in detail. The experimental
results are analyzed and compared to the theoretical predictions.
In the second half of the book (Chaps. 4-6) the crystal applications at
high-energy accelerators are considered.
Chapter 4 is devoted to the problems of particle extraction by means of
a bent crystal. The methods of calculation are presented. The systems for
proton beam extraction from the 70-GeV accelerator at IHEP and 120-GeV
SPS at CERN are considered. The possibilities of beam extraction from the
accelerators at FNAL and LHC are discussed.
Chapter 5 considers the questions of crystal application to particle beam
lines. The results of the studies into the resistance of crystals to radiation
are discussed. The experience of the use of bent crystals for splitting the
beams extracted from an accelerator, and for the creation of the test areas
is generalized. The method of beam focusing by a bent crystal is presented,
and its experimental results are analyzed. Based on the experimental data,
the possibility of using a bent crystal for the diagnostics of particle beams is
shown.
The final chapter is dedicated to the problems of crystal applications in
an experiment. The ideas and proposals for how one can use bent crystals to
measure the characteristics of short-lived particles (decay modes, lifetimes,
magnetic moments) are discussed. The scheme of the experiment at FNAL
for measuring the 17+ -hyperon spin precession in a bent crystal and deter
mining its magnetic moment is considered. The process of the radiation of
high-energy electron and positron beams in crystals is qualitatively consid
ered. The results of the studies performed at CERN of the crystal channeling
radiation for the hundreds-GeV electrons and positrons, which open up possi
bilities for the experimental research in a new energy range of photon beams,
are reported. The scheme of formation of a polarized tagged photon beam is
described and the characteristics of such a beam created at CERN for the
experiments at the [l spectrometer are given.
In conclusion we express our gratitude to Profs. H. Lengeier, C. Fabjan
and F. Bonaudi for their active support of the idea to write this book. We are
much indebted to Profs. E. Uggerh0j, N.F. Shulga and S.S. Gershtein, and
to Drs. V. Maisheev and S. Bityukov for a number of useful comments, and
in particular to Prof. Fabjan whose advice and suggestions have essentially
helped us to improve the book. We are also grateful to L.M. Komarova, T.K.
Lesnikova, and O.P. Laskovaya for a lot of technical help in the work on this
book.
Protvino, January 1997 The authors
Table of Contents
1. Channeling Phenomenon ................................. 1
1.1 Structure of Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Electric Fields in Crystals .... . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Particle Motion in the Potential of Atomic Planes .......... 12
1.4 Dechanneling.......................................... 17
1.4.1 Diffusion Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 17
1.4.2 Single Electronic Scattering. . . . . . . . . . . . . . . . . . . . . . .. 21
1.4.3 Comments on the Diffusion Approach. . . . . . . . . . . . . .. 23
1.5 Energy Loss ........................................... 25
1.6 Axial Channeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 27
2. Beam Deflection by Bent Crystals . . . . . . . . . . . . . . . . . . . . . . .. 31
2.1 Particle Motion in a Bent Channel. . . . . . . . . . . . . . . . . . . . . . .. 31
2.2 Defiection Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 34
2.2.1 Acceptance of a Bent Crystal . . . . . . . . . . . . . . . . . . . . .. 34
2.2.2 Dechanneling in a Bent Crystal .................... 38
2.2.3 Bending Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 40
2.2.4 The Infiuence of Temperature on the Efficiency ...... 42
2.2.5 Channeling in Crystal with Variable Curvature. . . . . .. 46
2.3 Feed-in Mechanisms .................................... 50
2.3.1 Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 50
2.3.2 Bending . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 55
2.3.3 Conclusion...................................... 60
2.4 Computer Simulation .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 60
2.4.1 Introduction..................................... 60
2.4.2 Simulation Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 62
2.4.3 Scattering....................................... 62
2.4.4 Simulation of the Dechanneling Process .. . . . . .. . . . .. 64
2.5 Channeling in an Imperfect Lattice . . . . . . . . . . . . . . . . . . . . . .. 67
2.5.1 Classification of Lattice Defects . . . . . . . . . . . . . . . . . . .. 67
2.5.2 Dislocations..................................... 69
2.5.3 Computer Simulation of Dislocation Dechanneling . . .. 77
2.5.4 Radiation Damage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 80
2.5.5 Conclusion...................................... 81
X Table of Contents
3. Experimental Studies of High-Energy Channeling
and Bending Phenomena in Crystals. . . . . . . . . . . . . . . . . . . . .. 83
3.1 Introduction........................................... 83
3.2 Crystals and Bending Devices . . . . . . . . . . . . . . . . . . . . . . . . . . .. 83
3.3 General Experimental Methods. . . . . . . . . . . . . . . . . . . . . . . . . .. 87
3.4 Deflection Efficiency Measurements . . . . . . . . . . . . . . . . . . . . . .. 90
3.4.1 Simulation of Bending Experiments. . . . . . . . . . . . . . . .. 92
3.4.2 Conclusion...................................... 94
3.5 Energy Loss in Bent Crystals ............................ 94
3.6 Dechanneling Investigation .............................. 98
3.6.1 Fermilab Experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 98
3.6.2 Protvino Experiment ............................. 101
3.6.3 Dechanneling Simulations ......................... 104
3.7 Dynamic Equilibrium ................................... 105
3.8 Volume Capture ........................................ 109
3.9 Influence of Crystal Lattice Imperfeetions
on Beam Deflection ..................................... 115
3.9.1 Spatial Lattice Imperfections ...................... 115
3.9.2 Surface Imperfections and Edge Effects .............. 117
3.9.3 Conclusion ...................................... 122
3.10 Topics Under Development .............................. 122
3.10.1 Bending Particles with Axial Channeling ............ 122
3.10.2 Bending Negative Particles ........................ 124
3.10.3 Deflection of High-Energy Heavy Ions ............... 124
4. Crystal Extraction ........................................ 125
4.1 Introduction ........................................... 125
4.2 Single-Pass and Multiple-Pass Modes of Extraction ......... 126
4.2.1 Scattering in a Crystal ............................ 127
4.2.2 Motion in the Accelerator ......................... 129
4.2.3 Estimate of the Multi-pass Efficiency ............... 131
4.2.4 Angular Acceptance of a Crystal
in the Multi-pass Mode ........................... 133
4.2.5 Extraction with High-Z Crystals ................... 134
4.2.6 Dependence of Efficiency on Machine Parameters ..... 134
4.2.7 Some Ideas on Multi-pass Channeling ............... 137
4.3 Extraction from the CERN SPS .......................... 140
4.3.1 Crystal Transmission of a Single Pass ............... 140
4.3.2 The SPS Extraction Set-Up ........................ 142
4.3.3 A 'Twisted' Crystal ............................... 143
4.3.4 Computer Simulation of Extraction ................. 144
4.3.5 A 'U-shaped' Crystal ............................. 148
4.3.6 A Crystal with an Amorphous Layer ................ 149
4.3.7 A More Realistic Simulation ....................... 149
Table of Contents XI
4.3.8 Crystal Optimization ............................. 150
4.3.9 Conclusions ...................................... 152
4.4 Crystal Extraction from the Tevatron ..................... 152
4.4.1 Qualitative Discussion of the Extraction ............. 152
4.4.2 Simulation ....................................... 156
4.4.3 Conclusions ...................................... 161
4.5 Proton Extraction from the Large Hadron Collider ......... 162
4.5.1 Introduction ..................................... 162
4.5.2 Simulation of a Single Pass ........................ 163
4.5.3 Simulation of the Extraction ....................... 165
4.5.4 Conclusions ...................................... 168
5. The Use of Crystal Deflectors in Beam Lines ............. 169
5.1 Crystals in Intense Particle Beams ........................ 169
5.2 Beam Attenuator ....................................... 170
5.3 Beam Splitting ......................................... 172
5.4 Creation of New Experimental Areas ...................... 174
5.5 Beam Diagnostics ...................................... 175
5.5.1 Method of Measurement .......................... 176
5.5.2 Determination of the Spatial
and Angular Characteristics of a Beam . . . . . . . . . . . . . . 176
5.5.3 Determination of the Distribution
of the Particle Momentum ......................... 178
5.6 Beam Focusing with Crystals ............................ 178
5.6.1 The Focusing Method ............................. 178
5.6.2 Focusing of a Parallel Beam to Form a Point
in the Particle Deflection Plane .................... 179
5.6.3 Focusing a Beam Diverging from a Point-Like Source
into a Parallel Beam .............................. 181
6. Application of Crystal Channeling
to Particle Physics Experiments .......................... 185
6.1 Studying Short-Lived Particles: Ideas and Proposals ........ 185
6.1.1 The Study of Decay Modes of Short-Lived Particles ... 185
6.1.2 Measurement of Magnetic Moments ................. 186
6.1.3 Measurement of Short-Lived-Particle Lifetimes ....... 187
6.2 Measurement of the Magnetic Moment of the 17+ Hyperon
Using a Bent Crystal ................................... 188
6.3 Radiation of High-Energy Electrons and Positrons
in Aligned Single Crystals and Its Application
in Physical Research .................................... 191
6.3.1 Coherent Bremsstrahlung ......................... 191
6.3.2 Channeling Radiation ............................. 196
6.3.3 Application of the Results to High-Energy Physics .... 201