Double Electron Capture in Relativistic U92З Collisions

Physica Scripta. T92, 429^431, 2001 Double Electron Capture in Relativistic U92 Collisions Observed at the ESR Gas-Jet Target

G. Bednarz1, A. Warczak1,P.S¨ wiat1, Th. StÎhlker2,3,H.Beyer2,F.Bosch2,R.W.Dunford4,S.Hagmann2,7,E.P.Kanter4, C. Kozhuharov2,A.KrÌmer2, D. Liesen2, T. Ludziejewski2,5,X.Ma2,P.H.Mokler2 and Z. Stachura6

1Institute of Physics, Jagiellonian University, Reymonta 4, 30-059 Krako¨ w, Pol an d 2 Gesellschaft fur Schwerionenforschung (GSI), 64220 Darmstadt, Germany 3 Institut fÏr Kernphysik University of Frankfurt 60486 Frankfurt, Germany 4 Argonne National Laboratory, Argonne, IL, USA 5 Institute for Nuclear Studies, Sè wierk, Poland 6 Institute of Nuclear Physics, Krako¨ w, Pol an d 7 Kansas State University, Kansas, USA

Received July 31, 2000; accepted August 15, 2000 pacs ref: 34.50.Fa, 34.70.e

Abstract . radiative double electron capture (RDEC) ^ a one-step Processes associated with double electron capture into bare U92-ionshave process, where the energy and momentum gained by been observed under single collision conditions. Regions of very low cross capture of two correlated electronsisconverted into sections, close to mbarn, have been explored successfully. In particular, an one photon with approximately twice the energy of a attempt to register photons with twice the energy of single K-REC photons single REC photon. In analogy to REC, the RDEC hasbeen performed. Moreover, K-REC spectra associated with double charge can be treated astime inversionof double exchange have been analysed in terms of their angular distribution. As a result, photoionization. Using the principle of detailed balance, evidence for correlated electron capture hasbeen found. one obtains, in the high energy limit, an approximation

forthecrosssectionofRDEC(sRDEC)[4]: 1. Introduction 0:0932 s 2To s A Á Z À 1Á Á ph Á s Z 2 Radiative electron capture (REC) of a single electron, RDEC t 2 REC t Z sph To observed in fast collisions of fully stripped high-Z ionswith light target atoms, is a dominant charge-exchange process where A 1 stands for the phase-space fraction accessible [1]. Here, the fundamental electron^photon interaction to RDEC [4], sph isthe crosssectionfor singlephotoioniz- mechanisms can be studied complementary to photoionization caused by a photon with energy of ho 2ho). Very ation experiments when considering REC as time reversal recent theoretical consideration of RDEC [5], within a of photoe¡ect. Recently, considerable e¡orts, directed onto non-relativistic approximation, gives for sRDEC avery ^6 electron-photon interaction, went towardsdetailsof double small fraction of sREC which variesbetween 10 ^9 photoionization of two-electron systems. This phenomenon, (Z 18) and 10 (Z 92). Here again, there isno direct in particular, dealswith very challenging problemsof atomic experimental evidence for the process. The only exper- physics where the electron-electron interaction should be iment aiming at observing of RDEC photons [4] provided taken into account thusentering the area of correlation uswith an upper limit estimateof sRDEC (for Z 18) e¡ects[2]. which wasvery closeto the nonrelativisticpredictions In order to follow thisguideline, the main intention of the given in [5]. However, it was suggested in [5], that in present experiment was to observe processes associated with the high-Z region, due to relativistic e¡ects, the corre- capture of two electrons into bare and fast heavy ions. sponding RDEC cross section should be strongly Measurementsofprojectile X-raysassociatedwith double enhanced with respect to the nonrelativistic prediction. charge exchange give access to the investigation of the Therefore, at high-Z, a scattering of theoretical predic- following radiative processes: tionsfor sRDEC, covering six orders of magnitude, requiresurgently an experimental clari¢cation. . double radiative electron capture (DREC) a two-step process in which two uncorrelated electrons are captured in one collision and two photons are emitted, both with 2. Experiment the energy of single REC photons. The cross section The experiment wasperformed at the heavy ion storagering, for thisprocess( sDREC) can be calculated within the inde- ESR, at GSI in Darmstadt. Bare U92+ ionsat an energy of pendent electron approximation and presented in the form 286 MeV/u have been used in collisions with gaseous N2- [3]: and Ar-targets, with densities ranging from 4.7Á1011/cm3 p up to 5.9Á1012/cm3 [6]. After passing through the target, s 0:13 Á Z Á aÀ2 Á s2 Z 1 DREC t 0 REC t the ions were charge state analysed in the next ESR bending

where Zt isthe target atomic number, a0 Bohr radius, and magnet and collected in a movable position-sensitive multi- sREC stands for the cross section for single REC. So far, wire proportional counter (MWPC). In Fig. 1, the charge- thisprocesshasnotbeen observedand veri¢ed experi- state distribution for U92+! Ar collisions is shown. The sep- mentally; aration between the two neighbouring charge states (91 and

# Physica Scripta 2001 Physica Scripta T92 430 G.Bednarz et al.

90) amountsto about 80 mm. Due to thislarge separation tion zone at a multitude of di¡erent observation angles with (the diameter of the ESR beam tube amountsto 250 mm) respect to the beam axis. For our current investigation an it wasnecessaryto tune the trajectory of the primary beam array of germanium detectors, covering observation angles out of the centre in order to detect both charge states of in the range from almost 0 up to 150 hasbeen installed. interest simultaneously on the detector. As observed in The X-ray detectorswere triggered by signalsfromthe par- Fig.1,therateofsingledown-chargedU91 isover four ticle detector. ordersof magnitude larger than for double down-charged U90 ions. In order to observe e¤ciently processes related to double capture, the particle detector wasplaced at a position where no single down-charged ions could hit the 3. Data analysis 92+ detector. Further on, single collision conditions for double InthecaseofU ! N2 collisions, single electron capture is electron capture were tested by measuring the yield U90+ predominantly determined by REC, with a measured cross ions as function of the target density (see Fig. 2). The linear section of 880 Æ 100 b which isin accordance with our pre- dependence observed in the ¢gure points clearly to single viousexperimental and theoretical results[1].In thiscol- collision conditions, a crucial requirement of the lision system double capture should be mediated mainly measurement. In this context, Fig. 1 presents ¢rst experi- by two uncorrelated REC processes. The cross section value mental evidence for double electron capture occurring in for the process of (8 Æ 3 mb), measured for the ¢rst time in singlecollisionsofbare uranium ionswith Ar atoms. thisexperiment, isin good agreement with Eq. (1) (10.4 mb). To register X-ray emission related to double capture Fig. 3 clearly shows that in this collision system the cross events, the atomic physics photon detection chamber at section associated with double charge exchange is about ¢ve the internal jet target of ring hasbeen used[7,8]. This orders of magnitude smaller than that for single capture environment allowsusto view the beam/jet target interac- channel.

Fig.1. Charge state spectrum of U-ions after passage of U92-ionsthrough a thin Ar-target. The primary ion beam could not be registered simultaneously.

Fig.3. Cross sections measured in the experiment: triangles ^ single capture; squares ^ double capture; circle ^ RDEC estimate. Lines show theoretical predictions: solid line ^ REC; dashed line ^ DREC (formula (1)); dotted Fig.2. Ratio of double charge-exchange yield over the number of ionspassing line ^ RDEC (nonrelativistic approximation [5]); dash-dotted line ^ RDEC through the N2-target (in arbitrary units). (relativistic corrections included [5]). Physica Scripta T92 # Physica Scripta 2001 Double Electron Capture in Relativistic U92 Collisions Observed at the ESR Gas-Jet Target 431

For U92+ ! Ar collisions about 75% of the cross section normalisation procedure as above and taking into account for single electron capture is due to REC [1]. The other part the corresponding X-ray e¤ciencies of the detectors, of the cross section is due to non-radiative electron capture di¡erential cross sections of 1.25 mbarn/sr (at 90)and (NRC). Therefore, the measured cross section value for 17.5 mbarn/sr (at 120) with uncertaintiescloseto 100% double charge exchange (360 Æ 70 mb) ismostprobably were determined. Averaging these two values and assuming composed of the cross sections for all the possible com- an isotropic distribution for RDEC, an estimate for sRDEC binationsof the uncorrelated REC and NRC transitions. waspossible( sRDEC 100 mb). Thisdata point ispresented According to Eq. (1) the contribution consisting of two in Fig. 3, aswell. It issituatedabout four ordersof magnitude uncorrelated radiative transitions(DREC) amountsto above the prediction of an non-relativistic approach [5] and 54.9 mb. Signi¢cant deviation of the measured cross section about two ordersof magnitude below the predictions from thisvalue (comp. Fig. 3) isprobably related to a strong involving relativistic corrections [5]. Our experimental contribution of NRC to double capture. ¢nding for sRDEC suggests that for the case of high-Z ions In addition, the angular distribution of K-REC photons thisprocesscontributestoa considerable amount to the inte- associated with double electron capture was registered. gral double electron capture probability and pointsto an However, only in the case of U92 ! Ar collisions the stat- increasing role of electron^electron correlation. Signi¢cant istical signi¢cance was su¤cient for an analysis (Fig. 4). uncertaintiesdue to poor statisticsof the presentexperiment The corresponding di¡erential cross sections were deter- require, however, continuation of these dedicated measure- mined by normalising the photon yields to the number of mentswhich shouldreveal the role of thisvery rare atomic K-REC photons measured in coincidence with single process in heavy ion-atom collisions. capture where the angular distribution is experimentally known [7]. It turnsout from the data plotted in Fig. 4 that 4. Conclusions the di¡erential cross sections at 60 and 120 deviate from the emission pattern known for K-REC associated with For the ¢rst time X-ray processes associated with double single electron capture (dashed line). The observed angular charge exchange under single collisions conditions have been distribution (see solid guide line in Fig. 4) can be a signature observed in heavy U92-ions at relativistic energies. In par- of quadrupole emission which suggests an in£uence of the ticular, crosssectionsforradiative electron capture have second electron involved into the capture process. However, been studied for both, correlated and uncorrelated two- due to the relatively large experimental uncertainties, this electron transitions. In the case of the uncorrelated process suggestion has to be veri¢ed in future experiments. (DREC) the experimental cross-section value is described During the whole experiment only a few photonswith rather well by theory. twice the energy of single K-REC photons associated with For the correlated RDEC transition the measured double electron capture have been observed. These events cross-section value seems to be very close to that for we attribute to the expected RDEC process. Using the same uncorrelated two electron capture, which pointsto an increasing role of electron-electron correlation in the studied collision system. A very similar conclusion can be drawn based on the angular distribution of K-REC photons associated with double charge exchange. In thiscasetheory isnot able to give a reasonable estimation of the observed e¡ects. However, all these results should be considered with caution because of relatively poor statistics, a price one has to pay when entering the area of rare processes.

Acknowledgements We would like to express many thanks to the ESR sta¡ for excellent tuning of the machine anticipating extreme requirementsof the experiment. Thiswork was supported by KBN (Poland) and the German Ministry for Education and Research (grant WTZ POL-226-96). R.W.D., E.K. and S.H. were supported by the U.S. D.O.E. O¤ce of Basic Energy Sciences, Division of Chemical Sciences.

References 1. StÎhlker, Th. et al., Phys. Rev. A 51, 2098 (1995). 2. McGuire, J. H., ``Electron Correlation Dynamicsin Atomic Collisions'', (Cambridge University Press, 1997). 3. Meyerhof, W. E. et al., Phys. Rev. A 23, 3291 (1985). 4. Warczak, A. et al., Nucl. Instr. Meth, B 98, 303 (1995). 5. Yakhontov, V. L. and Amusia, M.Ya, Phys. Rev. A 55,1952(1997). 6. Reich, H. et al.,Nucl.Phys.A626, 417c (1997). 7. StÎhlker, Th. et al., Phys. Rev. Lett. 82, 3232 (1999). Fig.4. Angular distribution of single K-REC photons associated with double 8. StÎhlker, Th. et al., in: ``X-Ray and Inner Shell Processes'', AIP Con- electron capture. Dashed line describes the shape of the distribution of ference Proc. 506 (Eds. D. S. Gemmel, E. P. Kanter and L. Young), K-REC photons associated with single electron capture [7]. Solid line serves (Chicago, Illinois, 1999) p. 389. only to guide the eye.

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