erutceL Notes scisyhP in Edited by .J Ehlers, MQnchen, .K Hepp, ZLirich .R Kippenhahn, M5nchen, .H .A WeidenmDIler, Heidelberg and .J Zittartz, nISK Managing Editor: .W BeiglbSck, Heidelberg 83 latnemirepxE Methods ni Heavy noI Physics detidE yb .K Bethge I galreV-regnirpS Berlin Heidelberg New York 1978 Editor Klaus Bethge Institut fur Kernphysik J.W. Goethe-Universit~t Frankfurt August-Euler-StraBe 6 D-6000 Frankfurt a.M. ISBN 3-540-08931-4 Springer-Verlag Berlin Heidelberg New York ISBN 0-387-08931-4 Springer-Verlag New York Heidelberg Berlin This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, -er printing, re-use of illustrations, broadcasting, reproduction yb photocopying machine or similar means, dna storage ni data banks. Under § 54 of the German Copyright waL where copies are made for other than private ,esu a fee is payable to the publisher, the amount of the fee to be determined yb agreement with the publisher. © yb Springer-Verlag Berlin Heidelberg 8791 Printed ni Germany Printing dna binding: Beltz Offsetdruck, Hemsbach/Bergstr. 012345-0413/3512 P R E F A C E The investigation of physical phenomena relating to heavy particles such as atoms, ions and complex nuclei, i.e. the measurement of quan- tities originating in their interactions has grown out of its initial state of trial and error. In review of the present situation of the ex- perimental methods, it can be seen that, in experimenting with heavy particles, it was first tried to extend the techniques and methods ori- ginally developed for light-particle interactions to the new fields. However, information on the limitations of the former methods was soon spred out. Many new ideas had to be developed and tried subsequently be- fore they could be successfully applied. The present selection of topics is due to a widely felt need of new experimental developments. Since the number of heavy-ion facilities all over the world that also devote them- selves to the development of experimental techniques, steadily increases, it was not meant to present a complete review of the whole field but rather to show the present state of knowledge on some of the topics. It was entirely up to the authors to decide how far their articles should cover the material available and where the emphasis was to be put. The authors and the editor should be glad if this summary, as of early 1978, would serve as reference containing useful information for those alrea- dy working in the field or as guide to those who enter the field as new- comers and who want to exert themselves in the development of experimen- tal measuring devices. There are certainly more topics which would be- long in an exhaustive treatment of experimental methods in heavy-ion physics, but the ones treated here are among those which greatly affect the experimental progress. Unfortunately, the articles could not be completed at the same time, so that in one field or the other progress may have occured for the commu- nication of which the editor and the authors would be very thankful. Though a number of pertaining review articles have appeared already in different journals and review series, a collection of some of them in one volume may be helpful to active researchers in the field. Frankfurt am Main, April 1978 The editor Table of Contents Production of Multiply Charged Ions H. Winter, Vienna ................................. Penetration of Heavy Ions Throug~ Matter H. Scb/r~idt-B~cking, Frankfurt a.M ................. 81 Detectors for Heavy Ions B. Martin, Heidelberg, H. Stelzer, Darmstadt ...... 150 Targets for Heavy Ion Beams J. Yntema, Argonne/Ill., F. Nickel, Darmstadt ..... 206 Ma~n@tic Spectrographs for the Investigation of Heavy Ion Reactions T. Walcher, Heidelberg ............................ 236 PRODUCTION OF MULTIPLY CHARGED HEAVY IONS H. Winter Institut fur Allgemeine Physik, Technische Universitit Vienna/Austria Table of contents I Introduction 2 1.1 Production and application of heavy ion beams 2 1.2 Improvements in the development of multiply charged heavy ion sources 3 1.3 General characteristics of MCIS-configurations 4 2 Fundamental MCIS-processes 6 2.1 Ion production 6 2.2 ion losses 10 2.3 Equilibria in MCI-producing configurations; principal MCIS-parameters 13 2.4 Ion beam dynamics 23 2.5 Diagnostics of multiply charged ion source-configurations 26 2.6 MCIS-technology 28 3 Discussion of various MCIS-configurations 30 3.1 Plasma-MCIS 30 3.11 Penning-MCIS 30 3.12 DUOPLASMATRON- and DUOPIGATRON-MCIS 45 3.13 MCI-production with other conventional plasma ion sources 53 3.14 MCIS involving beam-plasma discharges 54 3.15 MCIS involving electron cyclotron resonance plasma heating 56 3.16 MCI-production with high density plasmas 59 3.2 Multiply charged ion sources with enhanced confinement of ions 64 3.21 Electrostatic confinement 64 3.22 The EBIS-multiply charged ion source 65 3.23 Modified EBIS-configurations 70 3.24 Further developments 17 3.3 Further MCIS-concepts 71 3.4 Accelerator-integrated MCIS 72 4 Conclusions 73 Acknowledgements 75 References 76 I Introduction .I I Production and a~plication of heavy ion beams Experi[nental investigation of the interactions between heavy particles like ,sn~cta molecules, ions and bare nuclei leads into one of today's most exciting fields of physi- cal research - heavy ion physics. The experimental methods which must be applied to study such reactions depend pr~narily on the respective particle interaction energy. Beginning from a few 100 eV up to "relativistic" energies of many MeV at least one of the reaction partners must become ionised and accelerated t~at the desired reactions can be started. For the initiation of nuclear processes between light atomic particles interaction energies of about I MeV/amu are necessary; this value increases to more than 6 MeV/amu for heavier reaction partners. Sc~e novel processes which m~y probably lead to a better understanding of the properties of nuclear matter need even considerably higher inter- action energies up to some GeV/amu; moreover, this energy range seems to be very ap- propriate for interesting studies concerning biophysical research and medical therapy. The acceleration of heavy ions up to the above quoted energies cannot be achieved with the usual well established linear or circular accelerators for light ions because of the following reasons (SCHMELZER 1970): - Since accelerators offer only limited voltage per unit of length (for conventional linear accelerators usually about I MeV/m with peak values up to 20 MeV/m) , the path of accelerated ions would beccme very long; therefore, very stringent vacuum require- ments would be set to avoid impractical particle losses. - As well, circular accelerators can not be used with singly charged heavy ions due to the stripping of particles which becomes the more probable the higher the particle energy would increase; since circular accelerators accept particles with a well defined charge to mass number only, the particle losses would beccme unbearable, too. To overccrne these problems, principally three different methods of acceleration can be applied: ).a Acceleration of multiply charged ions: The ions must be produced with enhanced charge to mass numbers by means of a multiply charged ion source (MCIS); after the ion beam formation a first stage of acceleration is provided; at suitable particle energy a stripper leads to drastical enhancement of ion charge states; by a subsequent second acceleration stage sufficient particle ener- gies can be achieved. The initial ion charge states must be chosen by a comprcfnise between bearable expenditures for the MCIS, the first acceleration stage and the post- accelerator as well, whereby many details must be carefully considered. Regarding the MCIS, it must be stressed that at low particle velocities losses due to charge transfer increase drastically with ion charge state. b.) Acceleration of negatively charged ions with a Tandem-accelerator: Alternatively, in a first stage negative ions are accelerated towards a stripper in the terminal of a Tandem-electrostatic accelerator; there the ions are converted into mul- tiply charged ones and accelerated back to ground potential. If for heavier ions suf- ficient high energies are sought, a second stripping process with subsequent accelera- tion is necessary. Although concepts .a ) and ) b. both may use either linear or circular accelerators as second stages, they differ in the following ways: - For concept .a ,) the expenditure for the first stage depends strongly on the capa- bility of the multiply charged ion source. - For concept b. ,) the achievable particle currents are severely limited because of the necessity of negative ion generation and two stripping processes. However, both concepts have found their support and they are applied in the framework of conventional accelerator technology; with the forthcc~aing of superconducting ac- celeration structures, possibly further progress may be achieved in the near future. .c ) Collective particle acceleration: A totally different method consists in the collective acceleration of ions with beams or clouds of relativistic electrons. In the conceptual electron ring accelerator (ERA) stable ring-shaped clouds of fast electrons are produced, compressed, and loaded with ions; by application of diverging magnetic fields, in principle, acceleration up to high energies can be achieved. Another approach involves intense diode discharges where ions will be accelerated by means of relativistic electron beams. In both concepts no separated MCIS is needed because the ions are created by inter- action of relativistic electrons with neutral particles. According to current know- ledge with these techniques it might beccme possible to reach very high ion energies over ccmparatively short distances. However, if such accelerators would beccme feasible, they will offer only very short ion pulses as well as quite poor ion beam quality. 1.2 Improvements in the development of multiply charged ion sources While for heavy ion acceleration aceording to concept .a ) the use of ~ZIS is essential, they can also be applied in many other fields of research and technology; therefore, a comprehensive treatment of the physical and technical problems in connection with MCIS-operation see_ms justified. Not many years ago the meaning prevailed that special research directed to ion sources was superfluous, because in most cases the needed ion currents could be produced by following purely empirical approaches. Nowadays, however, the increased demands of electrc~agnetic mass separation, space propulsion, production of intense neutral par- ticle beams for fusion experiments, and, last but not least, heavy ion acceleration have shown the limits of these methods of trial and error. Therefore, in the last decade increased efforts in connection with production of ion beams have led to great advances for MCIS-development as well as to a better under- standing of their working principles. These achievments are reflected by an increasing number of conferences and symposia concerned with research on heavy ion sources (see references A - F). Early treatments on ion sources have been given by VON ARDENNE (1956) and KAMKE (1956). The report of GUTHRIE and WAKERLING (1949) was of special importance for the further development of plasma ion sources; a cc~prehensive treatment on the same subject was given more recently by GABOVICH (1972). Furthermore, recent reports on ion implantation techniques including heavy ion sources (FREEMAN 1973) and on intense ion beams by GREEN (1974) can be mentioned. In the present article a physical model is presented which serves for a better under- standing of the essential features of the multiply charged ion source-configurations known so far. After a short discussion of important processes in connection with pro- duction and loss ofm ultiply charged ions the model and its principal parameters are explained; furthermore, the interrelations of principal parameters and operational parameters of MCIS are investigated. Following these lines, the most important MCIS-types (both well established ones and new premising conceptual designs) are discussed; hereby, results of own work as well as that of other investigators have been included. Because we take pains to consider also the practical use of multiply charged ion sources, we have used appropriate physi- cal units throughout this treatment. Naturally, the drawn conclusions reflect strongly the personal views of the author; since MCIS-develolm~-nt has entered a state of rapid progress it might just as well be that some working principles not even mentioned here may enable the construction of powerful MCIS in the very near future. .I 3 General characteristics of MCIS-configurations In principle, each MCIS must be treated in connection with the accelerating structure following it. Characterisation of MCIS-properties can be done along the following lines: 1.31 Fundamental characteristics For a complete description of MCIS-properties the following characteristics must be specified: ) a. The species of emitted particles; the species of emitted particles to be accelera- ted; M..particle mass number; M . .n~ainal particle mass number n b.) The charge state (CS) of ions ;z the charge state of ions to be accelerated Zn; the charge to mass numbers z/M resp. Zn/M n- Furthermore, the charge state distribution (CSD) of emitted ions is defined: I . .emitted ion current of particles of a certain species with charge state ; z z I CSD(z)/% = z x 100 I z z Evidently, while still acceptable MC!S operating conditions must be obeyed, the CSD of nc~inal ion charge states should be as high as possible. The CSD of all emitted charge states in their entirety reflect the relative significance of different ion production- and loss processes (cf. ch. 2.3). ) c. The emitted particle current z N resp. Nz/n; this figure sets a limit for the par- ticle currents which can be accelerated; furthernDre, during acceleratoro peration the particle emission should behave as stably as possible. d.) The emittance of injected ions E(z )n ; cf. ch. 2.4 ).e The time structure of ion emission; while d.c.-operation is generally desirable, for most accelerator applications pulsed MCIS-operation will be sufficient. Cc~aonly, the time structure is characterised by duty factor and pulse duration (or by repetition frequency). 1.32 Technical characteristics These show which provisions are to be taken to operate a certain MCIS: ).a Size, weight, power- and cooling requirements of MCIS; these numbers, in turn, specify the capability of the accelerator terminal. ) b. Total emission efficiency/ncrninal particle efficiency; the first figure specifies the ratio between the nc~ninal ion particle current and the sum of the particle currents of all emitted ions and neutrals; therefore, it gives a measure for the capabilityo f the vacuum system. The second figure gives the relation between nc~inal ion particle current and the sumo f all nominal particle currents thus, ; it measures the efficiency of use of the charge material (this figure will be of special ~nportance for rare chaxge materials as e.g. isotopically enriched substances). ).c Source life time; this constitutes the time interval, during which continuous MCIS- operation under acceleratorr equirements will be possible. Usually, MCIS must be con- ditioned after connection to the vacuum systebme fore reliable operation can be started; therefore, the source life time should considerably exceed the period needed for source conditioning. To avoid longer interruption of accelerator operation during replace~e/nt of the MCIS either multiple-source setups or more than one source-terminal can be used. d.) Reliability and service expenditure; these characteristics, although mostly over- looked, influence greatly the total operational behaviour of the accelerator; there- fore, they deserve special attention. Finally, it should be mentioned that still no universally applicable MCIS does exist, and that the qualification of a certain MCIS can only be judged under practical ac- celerator operation; therefore, for all pertinent work this must be regarded as a rule of thumb. 2 Fundamental MCIS-Processes .2 I Ion Production Multiply charged ions can be produced by inelastic collisions between atc~as, molecules or their ions on one side and photons, electrons or heavy particles on the other. As an example, in fig. 2.1 typical courses of the different cross sections are given, if photons, electrons and protons are chosen as projectiles, respectively. ! Ar.p U" Ar÷e T 10 -I~ .~-01 20 50 100 10* ~01 -- EleV Fig. : 2.1 Comparison of ionisation cross sections of Ar for photon, electron and proton impact, respectively. In principle, photoionisation would be the best suited process since photons trans- fer only very little mament~n to heavy particles; however, it must be ruled out as an efficient HI-production process since the cross sections are rather small except near resonance energies, and furthermore, because intense sources of photons in the energy range above 5 eV are still not available. Ionisation by heavy particle bc~bardment may take place with considerable cross sections; but the necessarily high particle velocities will also cause very high mo- m~tt~ transfer and thus will lead to unbearable MCI-energy spread; furthermore, the production of intense ion beams for these purposes would cause too much expenditure. In oontrary to the former processes electron collision leads to high cross sections even at lowe lectron energy; the cross section maximum usually appears at a value of two to five times the respective ionisation potential. Much less momentum transfer to the target particles is achieved than with heavy projectiles; per collision the mean transferred energy amounts to m AE = 2.Ee.~- ' .1.