Beyer, Heinrich

Fritzsche, Stephan

Gao, Y.

Grisenti, R. E.

Gumberidze, A.

Hagmann, S.

Hillenbrand, P.-M.

Kozhuharov, C.

Petridis, N.

Spillmann, U.

Stöhlker, T.

Surzhykov, A.

Thorn, D. B.

Trotsenko, S.

Weber, G.

GSI Repository

The Effect of the Breit-Interaction Studied for the Emission Characteristicsof 1s2s22p1/2 Decay in Be-like Uranium ∗S. Trotsenko1,2, A. Gumberidze3,4, Y. Gao2,5, C. Kozhuharov2, S. Fritzsche1,6, A. Surzhykov1,2,6, N.Petridis2,7, D. B. Thorn3,4, H.F. Beyer2, R.E. Grisenti2,7, S. Hagmann2,7, P.-M. Hillenbrand2, U.Spillmann2, G. Weber1, and Th. Sto¨hlker1,2,3,61Helmholtz-Institut Jena, D-07743 Jena, Germany; 2GSI Helmholtzzentrum fu¨r Schwerionenforschung, D-64291Darmstadt, Germany; 3ExtreMe Matter Institute EMMI, D-64291 Darmstadt, Germany; 4FIAS, Frankfurt a. M.,Germany; 5Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China; 6Friedrich-Schiller-Universita¨tJena, D-07743 Jena, Germany; 7Institut fu¨r Kernphysik, Universita¨t Frankfurt, GermanyThe well-known Breit interaction was first worked outby Gregory Breit in order to calculate the fine-structureof atomic helium [1]. Nowadays, the Breit interaction isdescribed as quantum electrodynamics (QED) effect. Itincludes magnetic interactions and retardation in the ex-change of a single virtual photon between the electrons,and affects not only the energy level structure but also thedynamics of atomic processes. The interaction has arousedgreat interest in further exploration and analysis of rela-tivistic contributions to the electron-electron interaction,and its importance has already been confirmed for severalprocesses in the collisions between electrons and highlycharged ions (HCI) [2, 3, 4, 5, 6, 7, 8, 9].More often than not the influence of the Breit interac-tion is small, so it is treated as a minor correction to theCoulomb interaction. However, in certain cases it can evendominate dynamics involving highly charged ions. Naka-mura et al. [10] found that the Breit interaction can en-hance dielectronic recombination (DR) resonant strengthsby almost 100%. Soon afterwards, Fritzsche et al. [11]predicted that the Breit interaction could dominate theCoulomb interaction in the x-ray emission of Li-like heavyions following dielectronic recombination and could evenqualitatively change the angular distribution of x-rays forheavy ions with nuclear charge Z ≥ 73. Three years later,Hu et al. [12] obtained experimental evidence for the pre-diction of Fritzsche et al. by measuring the angular distri-bution of the [1s2s22p1/2]1 −→ [1s22s2]0 transition in di-electronic recombination of Li-like Au with free electronsin an electron beam ion trap (EBIT).In this report, we present the experimental results for an-gular distribution of characteristic x-rays following the res-onant transfer and excitation (RTE) in U89+ collisions withH2 target at the experimental storage ring ESR of GSI. TheRTE is equivalent to the DR processes, but with the dif-ference that the electron is initially in a bound state of tar-get. The experiment performed by colliding Li-like ura-nium (U89+) ions with H2 at the resonance energy (116.15MeV/u) for the U89+[1s22s] H2−−→ U88+[1s2s22p1/2 ]1 pro-cess. The accurate and stable value of the ion beam energywas guaranteed by the electron cooler of the ESR. The lay-out of the experimental arrangement at the gas-jet target is∗Work supported by HI Jena/EMMI/GSI(ESR)Figure 1: The experimental setup at the internal gas-jet tar-get of the ESR.shown in Fig. 1. We measured the angular distribution ofthe [1s2s22p1/2]1 −→ [1s22s2]0 x-ray transition followingthe resonant transfer and excitation.The x-ray emission from the collisions was recordedwith four high purity intrinsic germanium (HPGe(i)) detec-tors placed at 350, 900, 1200 and 1500 angles with regardto the direction of the ion beam. Exploiting time coinci-dences between the x-ray detectors and a particle detectormounted after the ESR dipole magnet, we were able to ob-tain the x-ray spectra corresponding only to the events ofU89+ capturing an electron into singly or doubly excitedstates.Four x-ray spectra have been obtained in the experimentat 350, 900, 1200 and 1500 observation angles correspond-ing to one-electron-capture events. As an example, Fig. 2shows the spectrum recorded at 350 angle. In the spectrum,several radiative electron capture (REC) lines are present.They are denoted according to the shell where the targetelectron is captured into, i.e. L-REC stands for the cap-ture into the L-shell (n=2), M-REC stands for the captureinto the M-shell (n=3), etc. In addition, the RTE inducedpeak ([1s2s22p1/2]1 −→ [1s22s2]0 transition) very close tothe radiative electron capture into the 2sj=1/2 and 2pj=1/2states (L-REC1/2) is found. The REC peaks are signifi-cantly broader than the RTE-induced characteristic transi-tion, due to the Compton profile of the target. This allowsus to fit the RTE and REC lines separately and obtain thecorresponding intensities. This is also possible for x-rayspectrum recorded at 1500. However, at 900 and 1200, thelarge Doppler broadening and a poorer energy resolutionPNI-AP-03 GSI SCIENTIFIC REPORT 2013158 doi:10.15120/GR-2014-1-PNI-AP-03Figure 2: X-ray spectrum obtained at the ESR correspond-ing to one-electron-capture events in 116.15 MeV/u U 89+collisions with the H2 gas target, obtained at 350 observa-tion angle.of the corresponding detectors smear out the difference be-tween the RTE and L-REC1/2 peaks, making it impossi-ble to fit them separately and obtain directly their inten-sities. Therefore, we have to rely on the relativistic RECtheory which has been extensively tested in many differentexperiments [13] and is currently known to provide accu-rate description of the process. Namely, we used the an-gular differential cross sections for L-REC3/2, L-REC1/2and M-REC [14] together with our experimental data forobtaining the intensity of the L-REC1/2. As a cross checkof theory, we also compared the theoretical values with ourexperimental results for L-REC1/2 : L-REC3/2 and L-REC: M-REC ratios at 350 and 1500 where we could obtainthe experimental values independently from the theory. Wefound a fair agreement between the theoretical and exper-imental results, however, in couple of cases a deviation ofabout 10% has been observed. The reason of this deviationis currently unclear. Therefore, we included the uncertaintyof 10% for obtaining the experimental RTE intensity valuesat 900 and 1200 angles. Furthermore, in order to obtain theangular distribution of the RTE induced [1s2s22p1/2]1 −→[1s22s2]0 transition, we normalized its intensity to the oneof the closely spaced L-REC3/2 peak and used the theoret-ical angular differential cross-section for the latter. In thisway, uncertainties related to different solid angles and effi-ciencies of the x-ray detectors are almost completely can-celled out.Our experimental and theoretical angular distributions[11] of the [1s2s22p1/2]1 −→ [1s22s2]0 transition are shownin Fig. 3. From the figure, a good qualitative agreementbetween the experiment and theory agreement can be ob-served. By fitting the equation for angular distribution ofthe electric dipole emission [11] to the experimental an-gular distribution, we received the experimental value foralignment parameter A2 = −0.46 ± 0.07. Our result isdefinitely closer to the prediction from [11] with Breit in-Figure 3: Experimental angular distribution for the[1s2s22p1/2]1 −→ [1s22s2]0 transition following the RTEinto initially Li-like uranium. The dashed line is a theo-retical angular distribution from [11], with the alignmentparameter A2= -0.314. The solid line is from fitting theequation of the electric dipole emission [11] to our experi-mental data, having A2 as a free fit parameter.teraction included (-0.314) than to the one without the Breitinteraction (0.47). This can be considered as a proof for thehigh importance of the Breit interaction for this case. Thereason for the relatively small (∼ 2σ) quantitative devia-tion between our experimental and theoretical results forthe alignment parameter (A2) has still to be clarified.References[1] G. Breit, Phys. Rev. 36, 383 (1930).[2] G. Breit, Phys. Rev. 39, 616 (1932).[3] C. Fontes, D. Sampson, and H. Zhang, Phys. Rev. A 59,1329(1999).[4] R. E. Marrs, S. R. Elliott, and D. A. Knapp, Phys. Rev. 72,4082 (1994).[5] W. R. Johnson, Springer, New York (2007).[6] I. P. Grant, Springer, New York (2007).[7] R. W. Childers, Phys. Lett. B 126, 485-488 (1983), ISSN0370-2693.[8] C.-Y. Wong and R. Becker, Phys. Rev. B 182, 251 (1986),ISSN 0370-2693.[9] I. P. Grant and H. M. Quiney, Int. J. Quantum Chem. 182,283 (2000).[10] Nobuyuki Nakamura et al., Phys. Rev. Lett. 100, 073203(2008).[11] Stephan Fritzsche et al., Phys. Rev. Lett. 103, 113001(2009).[12] Zhimin Hu et al., Phys. Rev. Lett. 108, 073002 (2012).[13] J. Eichler, Th. Sto¨hlker, Physics Reports 439, (2007) 1-99.[14] A. Surzykov, (private communication).GSI SCIENTIFIC REPORT 2013 PNI-AP-03doi:10.15120/GR-2014-1-PNI-AP-03 159

The Effect of the Breit-Interaction Studied for the Emission Characteristics of $1s2s^22p_{1/2}$ Decay in Be-like Uranium

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The Effect of the Breit-Interaction Studied for the Emission Characteristics of $1s2s^22p_{1/2}$ Decay in Be-like Uranium

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The Effect of the Breit-Interaction Studied for the Emission Characteristics of 1s2s22p1/21s2s^22p_{1/2}1s2s22p1/2​ Decay in Be-like Uranium

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The Effect of the Breit-Interaction Studied for the Emission Characteristics of $1s2s^22p_{1/2}$ Decay in Be-like Uranium