Recombination of Au49+, Au50+, and Au51+ ions has been studied at the TSR. With Au50+ ions a storage lifetime of only 2 to 4 s was observed with the magnetically expanded electron beam of the cooler at a density of ne = 107 cm-3. This short storage time is a consequence of the highest recombination rate coefficient ever observed with an atomic ion (1.8·10-6 cm3 s-1 at zero relative energy Erel = 0 between electrons and ions). At about 30 meV a huge dielectronic recombination resonance is found with a record small width of only about 15 meV. Such resonances fortuitously occurring near Erel=0 are probably the main reason for the enhanced recombination rates observed with Au50+, with Pb53+ (in a recent experiment at LEAR) as well as with other complex ions. For Au49+ and Au51+ the recombination rates are smaller by an order of magnitude

Bartsch, T

Brandau, C

Doerfert, J

Graham, W G

Lebius, H

Linkemann, J

Muller, A

Savin, Daniel Wolf

Schmitt, M

Schuch, R

Schwalm, D

Uwira, O

Wolf, A

Zong, W

Columbia University Academic Commons

Hyperfine Interactions 108 (1997) 149–154 149Recombination measurements at low energies withAu49+,50+,51+ at the TSRO. Uwira a, A. Müller a, J. Linkemann a,∗, T. Bartsch a, C. Brandau a, M. Schmitt b,A. Wolf b, D. Schwalm b, R. Schuch c, W. Zong c, H. Lebius c, W.G. Graham d,J. Doerfert e and D.W. Savin fa Institut für Kernphysik, Universität Giessen, D-35392 Giessen, Germanyb Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germanyand Physikalisches Institut der Universität Heidelberg, D-69120 Heidelberg, Germanyc Department of Atomic Physics, Stockholm University, S-104 05 Stockholm, Swedend Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfast BT7 1NN,Northern Irelande Institut für Experimentalphysik III, Ruhr-Universität Bochum, D-44780 Bochum, Germanyf Department of Physics, University of California, Berkeley, CA 94720, USARecombination of Au49+, Au50+, and Au51+ ions has been studied at the TSR. WithAu50+ ions a storage lifetime of only 2 to 4 s was observed with the magnetically expandedelectron beam of the cooler at a density of ne = 107 cm−3. This short storage time is aconsequence of the highest recombination rate coefficient ever observed with an atomic ion(1.8 · 10−6 cm3 s−1 at zero relative energy Erel = 0 between electrons and ions). At about30 meV a huge dielectronic recombination resonance is found with a record small width ofonly about 15 meV. Such resonances fortuitously occurring near Erel = 0 are probably themain reason for the enhanced recombination rates observed with Au50+, with Pb53+ (in arecent experiment at LEAR) as well as with other complex ions. For Au49+ and Au51+ therecombination rates are smaller by an order of magnitude.1. IntroductionBefore the construction of storage rings for heavy ions there was a major concernabout recombination losses during the cooling by electrons. Radiative recombinationwith its cross section diverging at zero energy was considered the beam lifetimelimitation particularly for highly charged ions. Also three-body recombination andcollisional radiative recombination were considered [1] but the low electron beamtemperatures presently achieved at storage rings had not been anticipated at that time,∗ Present address: Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany. J.C. Baltzer AG, Science Publishers150 O. Uwira et al. / Recombination of Au49+,50+,51+ ions at the TSRso that the higher-order recombination mechanisms were supposed to be less important.Regarding recombination losses due to dielectronic recombination (DR) near zerorelative energy Erel = 0 in the electron-ion collision system, a case study for Ar15+ions was also carried out [2], but the results did not appear very threatening to storageand cooling of these ions.The scene changed when unexpectedly high recombination rates at Erel = 0 wereobserved in an experiment with 6.3 MeV/u U28+ ions employing a cold dense electrontarget at the linear accelerator UNILAC of the GSI in Darmstadt [3,4]. Within anenergy range between 0 and roughly 1 meV the observed recombination rate droppedfrom about 2 · 10−7 cm3 s−1 to half this maximum value. The electron density wasne = 4 · 108 cm−3. It was immediately recognized that the lifetime of a stored cooledbeam of U28+ ions would only be seconds considering the normal density of electroncoolers, and yet, this had not been widely noticed until after a series of experiments atthe Low Energy Accumulator Ring LEAR at CERN in which Pb53+ ions were storedand cooled for preparation of further acceleration [5,6]. From the observed lifetimesof the beam during cooling at different electron densities a total recombination ratecoefficient of 6.0 · 10−7 cm3 s−1 was inferred. For ne = 4.4 · 107 cm−3 the beamlifetime was reduced to only 2 s, which is intolerable for an efficient handling andacceleration of the ion beam. Fortunately, the neighbouring charge states Pb52+,54+,55+behave quite differently and provide sufficient time for cooling without extensive beamlosses [7].While the recombination rate for Pb53+ and its neighbouring charge states wasdetermined only indirectly at LEAR, recently a special effort was made at the TSRto investigate by direct detection of the recombination products the similarly chargedgold ions Au49+,50+,51+ which are isoelectronic with Pb52+,53+,54+, respectively. Inthis paper we present preliminary results of the measurements. The analysis of thedata material available from the beam time is still in progress, however, the resultspresented here will not change much any more.2. Experiments and resultsFor the measurement of recombination spectra at low relative energies Erel betweenelectrons and ions in storage ring coolers, the effects of the cooling forces [8] have tobe considered. When the energy of the electron beam is switched away from coolingconditions the friction between the electron and the ion beams causes the ion velocityto shift towards the new electron velocity. The cooling forces strongly depend onthe charge and the mass of the ions and hence the drag effect is the most disturbingwith highly charged heavy ions. However, it can be circumvented by fast switchingof the electron beam energy between cooling and data-taking energies. For relativeenergies Erel  kT⊥, where T⊥ is the transverse electron beam temperature and k isBoltzmann’s constant, the cooling forces rapidly decrease. Thus, for relative energiesbeyond about 0.5 eV a switching scheme with dwell times on data-taking energy ofO. Uwira et al. / Recombination of Au49+,50+,51+ ions at the TSR 151the order of tens of milliseconds is sufficiently fast to avoid problems with the dragforces and hence with the definition of Erel. Efficient schemes of that type are appliedto recombination and ionization studies using fast high-voltage amplifiers. However,at lower relative energies defeating the drag effect requires additional effort. For thisreason, a new data taking scheme has been developed [9] for the TSR in which theswitching between cooling and data-taking is carried out with a low-voltage high-power amplifier that allows switching times much smaller than 1 ms. Thus, the dwelltime on data-taking can be reduced to about 1.5 ms without too much reduction ofthe duty cycle. The counting rates of recombined ions together with all other relevantparameters are recorded as a function of time with bins of 100 µs duration. Withthe resulting time resolution it is possible to monitor the development of the electronenergy and the drag effect of the electron beam on the circulating ion beam. Ifnecessary, only small time spans after each switching of the electron energy can beused to build the recombination spectrum. The duty cycle of this scheme may godown when only a small time span can be used; on the other hand, the data taken inthis way will yield more accurate results.The analysis of the detailed time spectra has revealed a time dependence in res-onance peak energies. This observation can be used to recover data with improvedenergy resolution. In particular, widths of resonance peaks occurring at energies greaterthan 0.2 eV will become smaller. The analysis of the time spectra with this aspect isstill in progress. We anticipate that the absolute rate data presented in this paper (whichwere taken with the 1.5 ms dwell time mode) will need little correction after finalizingthe evaluation. Figure 1 shows the recombination spectrum taken with Au50+ ions inthe TSR. The ion energy was 3.6 MeV/u, close to the maximum possible at the accel-erator facilities in Heidelberg for these heavy ions. The data are plotted on a doublelogarithmic scale in order to visualize the extremely sharp peak at Erel = 0 (whichlooks rather massive on the double-log scale). Within approximately 4 · 10−4 eV therecombination rate drops from its maximum, which is as high as 1.8 · 10−6 cm3 s−1,to half this value. This is the highest recombination rate coefficient observed so farin merged beams experiments with atomic ions. The solid line in fig. 1 represents acalculation of the rate for radiative recombination (RR) based on the Bethe–Salpeterformula [10] which was modified by the corrections previously described by Andersenet al. [11]. The enhancement factor beyond the expectation of RR theory is about 60.At the electron density ne = 107 cm−3 of the adiabatically expanded electron beamof the cooling device the lifetime of the circulating ion beam during electron coolingwas only about 2 seconds with the settings of the present measurement.Apart from the recombination maximum at Erel = 0 there are huge dielectronicrecombination peaks. The one with the lowest energy is already at Erel = 30 meV.Detailed analysis of the shape of this resonance yields electron beam temperaturescorresponding to the thermal energies kT⊥ = 16 meV and kT‖ = 1 ·10−4 eV. The fullwidth at half maximum of this resonance is only about 15 meV, which is the smallestwidth ever observed in electron collision experiments with multiply charged ions. TheDR resonances are so densely spaced in energy that one can easily imagine strong152 O. Uwira et al. / Recombination of Au49+,50+,51+ ions at the TSRFig. 1. Recombination rates of electrons with 3.6 MeV/u Au50+ ions measured at the TSR as a functionof the relative energy in the electron–ion center-of-mass frame. The curve denoted by RR is a calculationof the rate for radiative recombination. It agrees with the experimental rate observed at energies between4 and 5 eV where no resonances appear to be present. For further details see the text.Fig. 2. Recombination rates of electrons with 3.6 MeV/u Au49+ ions measured at the TSR as a functionof the relative energy in the electron–ion center-of-mass frame. The solid line is the rate calculated forradiative recombination. It agrees with the experimental rate observed at energies around 1.2 eV whereno resonances appear to be present. For further details see the text. The inset shows the data in thelowest energy range on a double logarithmic scale.O. Uwira et al. / Recombination of Au49+,50+,51+ ions at the TSR 153resonance contributions also to the peak at zero energy. The DR resonance energyof an ion depends on the individual structure of this ion; hence, DR resonances ofAu50+ may be fortuitously placed near Erel = 0. One can then expect completelydifferent recombination rates in isonuclear ions with different charge states and differ-ent electronic structure. Therefore, we also measured recombination rates of Au49+and Au51+ ions. The result of the Au49+ experiment is shown in fig. 2. Indeed, themaximum recombination rate at Erel = 0 is roughly a factor of 10 lower than that ofAu50+. DR resonances are not nearly as densely spaced as in Au50+ and probably areabsent at Erel = 0. Nevertheless, there is still a considerable enhancement (by abouta factor of 4) of the measured rate beyond the expectations according to the theoryfor RR. The solid line in fig. 2 represents a calculation of the RR rate with the sameapproach as that used for Au50+.A similar result is obtained with Au51+ ions. There, however, no obvious resonancesare found at all at energies from 0 to 1 eV.3. Summary and outlookRecombination of ions in charge states near 50 has been investigated at the TSR.The recombination rates at zero relative energy in the electron–ion collision systemmeasured with Au49+, Au50+ and Au51+ are very similar to those inferred fromstorage lifetimes of isoelectronic lead ions studied at the LEAR. Apparently, the hugerecombination rates observed for Au50+ and for Pb53+ ions are caused by the individualelectronic structure of these ions which happens to support dielectronic recombinationresonances at very low energies. The ions Au49+ and Au51+ as well as Pb52+ andPb54+ have a different electronic structure and do not provide as many DR resonancesat low energies. We hypothesize that this is the reason why the recombination ratesat Erel = 0 are much smaller than those observed with Au50+ and Pb53+ ions.AcknowledgementsThe authors thank the staff and the technicians of the TSR for their assistance inthese experiments. This work was supported by the Human Capital and Mobility Pro-gramme of the European Community, by the Gesellschaft für Schwerionenforschung(GSI), Darmstadt, by the Max-Planck-Institut für Kernphysik, Heidelberg, and by theGerman Ministry of Education, Science, Research and Technology (BMBF).References[1] H.F. Beyer, D. Liesen and O. Guzman, Part. Accel. 24 (1989) 163.[2] C. Carlsund, N. Elander, R. Mowat, D.C. Griffin and M.S. Pindzola, Physica Scripta T 22 (1987)243.154 O. Uwira et al. / Recombination of Au49+,50+,51+ ions at the TSR[3] A. Müller, S. Schennach, M. Wagner, J. Haselbauer, O. Uwira, W. Spies, E. Jennewein, R. Becker,M. Kleinod, U. Pröbstel, N. Angert, J. Klabunde, P.H. Mokler, P. Spädtke and B. Wolf, PhysicaScripta, T 37 (1991) 62.[4] S. Schennach, A. Müller, M. Wagner, J. Haselbauer, O. Uwira, W. Spies, E. Jennewein, R. Becker,M. Kleinod, U. Pröbstel, N. Angert, J. Klabunde, P.H. Mokler, P. Spädtke and B. Wolf, in: Physicsof Highly-Charged Ions, eds E. Salzborn, P.H. Mokler and A. Müller, Supplement to Z. Phys. D:Atoms, Molecules and Clusters 21 (1991) 205.[5] S. Baird, J. Bosser, M. Chanel, J. Duran, R. Giannini, P. Lefèvre, R. Ley, R. Maccaferri, S. Maury,I. Meshkov, D. Möhl, G. Molinari, F. Motsch, H. Mulder, U. Oeftiger, J.C. Perrier, E. Roux,G. Tranquille and F. Varenne, CERN, Geneva, Internal Report PS/AR/Note 95-06 (MD), March1995.[6] S. Baird, J. Bosser, C. Carli, M. Chanel, P. Lefèvre, R. Ley, R. Maccaferri, S. Maury, I. Meshkov,D. Möhl, G. Molinari, F. Motsch, H. Mulder, G. Tranquille and F. Varenne, Phys. Lett. B 361 (1995)184.[7] S. Baird, J. Bosser, C. Carli, M. Chanel, R. Ley, R. Maccaferri, S. Maury, I. Meshkov, D. Möhl,G. Molinari, E. Syresin, G. Tranquille and F. Varenne, CERN, Geneva, Internal Report PS/AR/Note96-08 (MD), May 1996.[8] A. Wolf, this volume.[9] O. Uwira, PhD Thesis, Giessen 1997, in preparation.[10] H. Bethe and E. Salpeter, Quantum Mechanics of One- and Two-Electron Systems (Springer, Berlin,1957).[11] L.H. Andersen and J. Bolko, Phys. Rev. A 42 (1990) 1184.

Recombination Measurements at Low Energies with Au49+,50+,51+ at the TSR

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Recombination Measurements at Low Energies with Au49+,50+,51+ at the TSR

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