Kirchner, R

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Particle Accelerators, 1994, Vol. 47, pp. 127-132Reprints available directly from the publisherPhotocopying permitted by license only@1994 Gordon and Breach Science Publishers, S.A.Printed in MalaysiaION SOURCES FOR RADIOACTIVE BEAMSR.KIRCHNERGSI Darmstadt, Postfach 110552, D-64220 Darmstadt, Germany(Received 29 October 1993)The ion sources reviewed here, most of them developed for isotope separation on-line (ISOL), are classifiedaccording to their ionizing mechanism, utilizing electrons, heat, light, and penetration of matter. Emphasis is puton the beam-optical characteristics ion current density, energy spread, and emittance and on the ISOL-essentials"efficient, fast, and selective", both for the ion source and the complete targetJionsource-system.KEY WORDS: ISL, radioactive beams, ion sources, production targets, ISOL1 INTRODUCTIONIon sources for radioactive beam facilities have essentially the same task as the ones1,2developed for isotope separation on-line (ISOL). O~ a meeting of accelerator' specialistsit is close at hand to emphazise the demands to an ISOL-source by comparison withthe demands to the more familiar accelerator ion sources. Common is the demand forreliabibility, universality, stability, long lifetime, low emittance and low energy spread.The difference lies in the fact that accelerator sources have to convert (usually unlimitedand well-defined) charge materials into intense ion currents, whilst ISOL-sources haveto convert the limited number of particles produced in nuclear reactions as efficiently aspossible into ions, the corresponding ion current hardly ever reaching the /lA-range. Sincethe "charge material" of ISOL-sources is radioactive, i.e. often of very limited lifetime,and composed of many elements, the conversion should proceed in afast and selective, i.e.element-specific way - requirements, generally without meaning for an accelerator source.A further difference may be that these three main requirements do not only apply to the ionsource proper, but to the system of ion source and its production target.Of the possible mechanisms leading to ionization, Le. electron-bombardment, heat,interaction with light, penetration of matter, and high electric fields, all except the lastone have been utilized for ISOL-sources and serve for the subsequent classification.2 ELECTRON-IMPACT ION SOURCESSince electron-impact ionization cross-sections do not differ too strongly among neighour-ing elements, the sources of this chapter are in principle universal and unselective. Limits of127128 R. KIRCHNERuniversality and thus a certain degree of selectivity may, however, arise from the fact someelements are not sufficiently volatile at the operation temperature of the ion source and arelost due to surface adsorption or diffusion into the bulk of the source enclosure.2.1 Low pressure arc discharge ion sourcesThese sources typically are composed of a cylindrical anode, sealed by two end-plates (thecathode plate with the filament and the anti-cathode) to a hot and gastight enclosure for thegaseous discharge. Ions are created by fast (30-100 eV) glow electrons from the filament.At and above a threshold pressure of 10-3 to 10-2mbar, the primary electron current risesstrongly to its saturation value due to formation of the cathode double-layer. This is the dropof practically the full discharge voltage within short distance of the filament, while the restof the enclosure fills with the practically neutral low pressure arc plasma. The resulting highflux of ionizing particles, often amplified by an external solenoidal field which additionallyincreases the ion-confining forces of the plasma, makes this class of sources quite efficient(typically 30% for Ar, Kr, or Xe). Depending on whether ion emission is performed axiallythrough a hole in the cathode- or anti-cathode-plate, or radially through a slit in the anode,the design is a Hollow-cathode,3 Nielsen4 or Bemas-Nie,s-7 ion source. They all have bothlow emittance (emittance values given in his review refer to 90-95% of the beam intensityand 30 kV acceleration voltage) and energy spread (;S 201l' mm mrad,6 few eV). Due to thepressure threshold (close to which the discharge tends strongly to instabilities) the outputcurrent density is quite high (ji > 10mA/cm2). The high ion currents, especially for theBernas-Nier source, are advantageous, if the ion source is coupled to targets emanatinggreat amounts of material,7 exclude, however, the simple and economic low-intensity massseparator concept due to space charge problems.2.2 Space-charge compensated electron-bombardment sourcesSimilar in design to the low pressure arc sources and like those suited for all elementsof sufficient volatility at the temperature of the enclosure (up to 2500 K), this class ofsources avoids the arc threshold pressure and the associated instabilities by extracting theprimary electrons by a grid. The main types, especially developed for ISOL applications, arethe FEB/AD-source8 with an axial cathode-grid geometry and the radial-geometry EBGP-source.9 The advantage of the former is its rugged design, the advantage of the latter thenon-necessity of a source magnet, its easy conversion into a high-temperature-cavity ionizer(chapter 3.2) and its high ultimate temperature by avoiding electrical insulators withinthe hot enclosure. Typical for these sources are high and widely pressure-independentionization efficiencies (20-70% for the elements above neon). Similar in emittance andenergy spread to the low pressure arc sources, they have substantially lower output currentdensities (ji < ImA/cm 2 ) thus being easily compatible with any mass separator concept.2.3 Electron-cyclotron resonance ion sourcesPrimarily intended as high-charge-state accelerator source, the ECR-source has now alsofound considerable appreciation as 1+-ionsource in ISOL. The reason is that the very highION SOURCES FOR RADIOACTIVE BEAMS 129efficiency for gases,10-12 and especially its capability ofdissociating and ionizing efficientlymolecular gases ofC, N, 0, chemical reactivity preventing their handling in high temperatureenclosures. Its principle is microwave-driven electron-bombardment ionization, the low-density discharge plasma being confined by a magnetic field configuration of axial mirrorplus radial hexapole field. Typical are ion current densities of around 1 mA/cm2 out oforifices from 3 to 10 mm diameter, however, with quite high emittances of70 to 1501l' mmmrad.13,14 A great advantage is the long lifetime and stable running conditions for gasesdue to the absence of wearing parts; disadvantageous is the strong decrease of efficiencywith increasing pressure. Due to the cold enclosure ECR's can hardly compete in efficiencyfor less-volatile elements with high-temperature sources. If, however, high ionic chargestates from the source are required by certain post-accelerator concepts, even efficienciesof around 1% as reached e.g. for high charge states of Ca are of unrivalled importance,although considerably lower than the 25% reached for Ar8+ .143 THERMIONIC ION SOURCESThe ionization efficiency ofthermo-ionizers is strongly dependent on the ionization potential(electron affinity) ofthe element considered. Differences in these quantities for neighbouringelements may make the ionization process intrincally selective.3.1 Positive and negative surface ionization sourcesSurface ionization occurs when a valence electron ofan adsorbed atom has a sufficiently highchance to tunnel into the Fermi-sea represented by the adsorbing material. The ionizationprobability is given by the Saba-Langmuir equation, i.e. it is high and almost temperature-independent for Wi < cP (Wi ionization potential of adsorbed atom, cP work function ofsurface), and strongly dependent on temperature and (Wi - cp) for Wi > cpo Efficientionization is thus restricted to elements with Wi < 5.5 eV, e.g. alkalines and alkalineearths, desorbing from high-workfunction surfaces such as W, Re, and Pt.Analogously efficient negative surface ionization is possible for elements with electronaffinities EA > cp, e.g. the halogenes. It requires low-workfunction surfaces such as LaB6or BaO. Since these materials are limited in operation temperature, an ISOL-design shouldminimize the number of surface collisions,15 to avoid excessively long hold-up times.3.2 High-temperature cavity ion sourcesThese sources are a special class of positive surface ionizers,16 where the ionization takesplace inside a closed (except for the outlet hole) volume17 with the effect that atoms have amultifold chance of being surface-ionized. At high temperture and low plasma densities theionization efficiency may exceed the Saba-Langmuir values by orders of magnitude. Forhigh neutral densities the amplification factor may be as high as the mean number of wallcollisions.18 This makes the coupling of relatively volatile targets unproblematic, as longas the vapour is not surface-ionized or only to low degree. In the opposite case, i.e. for highplasma densities, the efficiency may drop drastically, even below the values given by the130 R. KIRCHNERSaha-Langmuir-equation. Its high temperature (up to >3000 K) gives access to refractoryelements; efficient ionization is, however, restricted to elements with Wi < 7 eV, e.g. thelanthanides. Ion currents may be as low as 1 /LAlcm2, energy spread and emittance are verylow « 1 eV, < 2rr mm mrad).4 LASER ION SOURCESMultistep photo-ionization using intense laser light of 2 or 3 (eventually even of only 119)wavelength is of increasing importance both in trace analysis and ISOL.20- 23 Resonantexcitation at least in the first step with subsequent transition(s) into the continuum or to auto-ionizing- or Rydberg-states, guarantees intrinsic selectivity of the ionization process whichmay, however, be limited due to surface ionization of neighbouring elements. Ionizationefficiencies of the order of 10% are achieved for favourable excitation schemes, even usinglow duty-cycle pulsed lasers. Only these have usually sufficient power (either directly orfor driving tunable dye-lasers) to saturate the transitions. To overcome the low duty-cycle,the photo-jons have to be effectively prevented from recombination, e.g. by confinementin the potential trough of a hot cavity21 or in a gas cell,19 eventually combined with fastejection geometries. The consequently bunched structure of the ion beam (typically 10 kHz,10-30 /Ls) allows for enhanced selectivity by gated detection and may be useful for postacceleration. Emittance and current density are low (comparable to hot cavity ionizers);the energy spread, however, may be some 10 eV, i.e. corresponding to the electrostaticpotentials applied for fast ejection of the ion pulses.5 ION GUIDESThe ion guide technique, developed in Jyvaskyla24 and adopted succesfully also by manyother ISOL-groups,25 utilizes the fact that radioactive nuclei, when recoiling from theirproduction target are highly charged and may be efficiently discharged to charge statue 1+in a stopping gas as e.g. pure helium. It is thus a universal (but consequently unselective)and very fast "ion source", since there are no delays due to solid state diffusion and surfaceadsorption. Restrictions of the technique to light-ion induced reactions and to very thintargets, as well as ion losses due to charge exchange wih impurities, wall collisions, orthree-body-recombination, however, limit the rates of separated isoptopes to values typicallybelow 105/s and thus the applicability for RIB-projects.6 THE TARGET/ION-SOURCE SYSTEMSo far only the contribution of the ion source to the essentials efficient, fast, and selective ofthe target/ionsource-system was considered. The system adds to the losses by incompleteionization the halflife-dependent losses due to the finite processing time between creation ofa reaction product and its release from the system as an ion beam. On the other hand it mayenable selectivity in cases, where this is not possible by the ionizing mechanism. EstablishedION SOURCES FOR RADIOACTIVE BEAMS 131techniques are "chemical evaporation", i.e. the on-line-formation of compounds,26 enablingoften both improved separation of wanted elements and suppression of unwanted. Aparticularly favourable case is the formation of fluorides of Sr and Ba by adding CF4permitting practically 100%-efficient and completely exclusive separation as SrF+ (BaF+)ions.27,28Other techniques utilize the delays caused by solid-state diffusion in the target materialor by surface adsorption at the walls of the system, most effectively by the latter.26,29 Theassociated problems and perspectives are best exemplified by looking at the propagationof reaction products through two somewhat idealized ISOLDE-systems.26,30 Both aresupposed to be operated at 2500K and to use thick Ta targets consisting of 100 j.Lm Ta-foils, and of 10 j.Lm Ta-powder, respectively, thus minimizing the surface/volume-ratio ofthe target material and vice versa. Considered are reaction products of Xe, Yb, and Dy,which have practically the same solid-state diffusion speed in tantalum (D ~ 5 x 10-9cm2/s) but very different mean sticking times on tantalum surfaces (Ta ~ 0, 50 j.LS, and 500j.LS, respectively). Xenon, though not a typical reaction product of Ta targets, is includedto illustrate, how volatile elements, not suffering from sizeable wall sticking times, arestill delayed in the gaseous phase by molecular flow. Calculations based on References 29to 31 show in qualitative agreement with experiment32 that for the foil target the mainlosses arise from solid state diffusion, and that the halflife-dependence of the releaseefficiency differs relatively little for the three elements (no selectivity), allowing even forthe separation of shortlived Dy-isotope. The "much faster" powder target, on the other hand,gives considerably better yields for the volatile Xe, while Dy (and most other lathanides)are strongly discriminated, thus opening for the most volatile lanthanide element Yb a widehalflife-window for reasonably efficient and selective separation.REFERENCES1. H.L. Ravn, Nucl. Instr. Meth., B70 (1992) 1072. P. Van Duppen et aI, Rev. Sci. Instr., 63 (1992) 23813. G, Sidenius, Radiation Effects., 44 (1979), 1454. O. Almen and K.O. Nielsen, Nucl. Instr. Meth., 1 (1957) 3025. V.T. Koslowsky et al. Nucl. Instr. Meth., B70 (1992) 2456. J.C. Puteaux et ai, Nucl. Instr. Meth., B26 (1987) 2137. J.C. Puteaux et aI, Nucl. Instr. Meth., 186 (1981) 3218. R. Kirchner et aI, Nucl. Instr. Meth., 133 (1976) 187 and B70 (1992) 569. J.M. Nitschke, Nucl. Instr. Meth., A239 (1985) 110. V. Bechtold et al. Proc Workshop ECRIS, Jiilich 1986, 24811. P. Decrock et ai, Nucl. Instr. Meth., B58 (1992) 25212. L. Buchmann et al. Nucl. Instr. Meth., B63 (1992) 52113. M. Huyse (Leuven), H. Schulte (GSI), private communication14. P. Sortais (GANIL), priv. comm.15. B. Vosicki et ai, Nucl. Instr. Meth., 186 (1981) 30716. G.J. Beyer et aI, Nucl. Instr. Meth., 96 (1971) 34717. A. Latuszynski et ai, Nucl. Instr. Meth., 125 (1975) 6118. R. Kirchner, Nucl. Instr. Meth., A292 (1990) 203132 R. KIRCHNER19. Z.N. Quamhieh, Nucl. Instr. Meth., B70 (1992) 131 and M. Huyse, private communication20. H.J. Kluge et aI, Proc. Accelerated RIB Workshop, TRIUMF Rep. 85-1,11921. G.D. Alkhazov et aI, Nucl. Instr. Meth., A280 (1989) 14122. V.I. Mishin et aI, Nuci. Instr. Meth., B73 (1993) 55023. V.N. Fedoseyev et aI, Annual Rep. 1992, Inst. Nuc!. Chern., University Mainz (1993)24. J. Arje et aI, Phys. Rev. Lett., 54 (1985) 9925. Section IT and IV in Nuc!. Instr. Meth. B70 (1992) 50 and 213ff26. E. Hagebo et aI, Nucl. Instr. Meth., B70 (1992) 16527. R. Eder et aI, Nuci. Instr. Meth., B62 (1992) 53528. Y. Kawase et aI, Nucl. Instr. Meth., B70 (1992) 14629. R. Kirchner et aI, Nuci. Instr. Meth., B70 (1992) 56 and 18630. L.C. Carraz et aI, Nucl. Instr. Meth., 148 (1978) 21731. G. Rudstarn, CERN-Report 70-3 (1970) 12532. T. Bjornstad et aI, Phys. Scripta, 34 (1986) 578

Ion sources for radioactive beams

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