Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Helmholtz-Institut Jena, Jena, Germany GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany Günter Weber Hard X-Ray Linear Polarimetry

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, FAIR Atomic Physics:

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Outline of the Talk Motivation and introduction to Compton polarimetry Recent lineaer polarization measurements - Radiative electron capture (time-reversed photoionization) - Bremsstrahlung (spin transfer from electrons to photons) - U 91+ Lyman-  1 (disentanglement of alignment and multipole mixing) Summary and additional activities

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, General Motivation: towards “complete” measurements Observables: (Total) Cross section  Angular distribution W(  ) Spectral distribution E Polarization P Ultimate goal: obtain all observables at once. → most stringent test of theory → maximum information on incident particles E B

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Compton Polarimetry for Hard X-Rays Pair Production Compton Scattering Crystal Optics Liquid Crystals photon energy [ eV ] Bragg Reflection Photoionization Thomson/Rayleigh Scattering E φ Compton Scattering EE e-e- E  Linear Polarization Sensitivity

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E Si(Li) Compton Polarimeter developed within SPARC Angular distribution of scattered x-rays reflects the incident photon linear polarization.  E S. Hess, Phd thesis (2010) Delievered in 2008 Scattering process and absorbing take place inside a double-sided segmented detector crystal. 64 mm 7 mm

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E B dipole E B E B e-e- Z K L free-free Bremsstrahlung Radiative Processes for Highly Charged Ions E B dipole E B E B Radiative Recombination (RR) Radiative Electron Capture (REC) K L free-bound e-e- Z Time-reversal of photoionization. Continuation of RR for continuum states.

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E B dipole E B E B 2p 3/2 1s 1/2 Characteristic Transitions K L bound-bound Radiative Processes for Highly Charged Ions Determined by alignment of the excited state.

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, The Experimental Storage Ring (ESR) at GSI Internal Gas Target Ions: p + … U 92+ Intensity: up to 10 8 stored ions Energy: 4 – 400 MeV/u (  =0.7) Circumfence: 108 m Frequency: up to 2 MHz Gas Target Ion Beam

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Radiative Electron Capture

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Radiative Electron Capture Studies (REC) at the ESR Gas Target E KIN K L Radiative Recombination (inverse photoeffect) K-REC L-REC U 92+ H Projectile Ion Gas Target U Cross section d σ /d Ω Polarization 100% Non-relativistic scenario sin 2 θ angular distribution 100% linear polarization

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, U 92+ → N 2, H MeV/u 400 MeV/u 800 MeV/u K-REC polarization measurements new Si(Li) polarimeter (> 1000 pixels) S. Hess (2010), H. Ding (2014) 16 pixel Ge(i) detector S. Tashenov et al., PRL 97 (2006) Photon emission angle ( deg ) Observation angle ( deg ) preliminary Xe 54+ → H 150 MeV/u mid-Z high-Z preliminary Photoeffect: Electron is emitted into the B-field direction Necessary ion energy range will become experimentally accessible at the HESR at FAIR. (manuscript in preparation)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Bremsstrahlung Studies

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Polarization of electron-nucleus bremsstrahlung Unique advantages at storage rings: no screening effects (bare projectile) electrons from the tip region can be observed preliminary (manuscript in preparation)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E unpolarized ion beam E Unpolarized collision system: Emitted photons are polarized along the reaction plane. Polarized collision system: Orientation of photon polarization depends on the spin polarization of the colliding particles. target polarized ion beam reaction plane Collision symmetry broken: Rotation of scattered photon distribution Probing spin-polarized particle beams by means of Compton polarimetry Proposed for the RR process into spin-polarized ions: A. Surzhykov et al., PRL 92 (2005) → Polarization transfer from particles to photons → Spin diagnostic !

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Photon energy: 92.5 ± 3 keV Unpolarized electrons Transversely polarized electrons Test case: Spin-Transfer in Electron-Atom Bremsstrahlung → unpolarized versus transversely polarized electrons Reconstructed Compton events KαKα KβKβ 100 keV e - → 130 deg See also: S. Tashenov et al., PRL 107 (2011) R. Märtin et al., PRL 108 (2012)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Lyman-  1 Transition in U 91+

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Alignment Studies at the ESR Gas Target: 2p 3/2 Population by Radiative Electron Capture into bare Ions Collision axis µ=±1/2 µ=±3/2 Alignment parameter for the 2p 3/2 state: E KIN K L Radiative Recombination L-REC 2p 3/2  =+3/2  =+1/2  =-1/2  =-3/2 Non-statistical population of magnetic substates.

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Alignment Parameter 2p 3/2 1s 1/2 Lyman-  1 µ=+3/2 µ=+1/2 µ=-1/2 µ=-3/2 Alignment analysis by decay photon spectroscopy: Angular Distribution and Linear Polarization Non-statistical population of the magnetic sublevels (non-zero A 2 ) Anisotropic emission pattern: Non-vanishing linear polarization: Note: Emission characteristics fully determined by A 2 and observation angle . What is it good for? The alignment parameter reveals subtle details of the population process of an excited HCI state. Breit interaction in electron impact excitation Dielectronic recombination in Be-like sytems Coherence in time-reversed photoeffect (REC) C. J. Bostock et al., PRA 80 (2009) A. Gumberizde et al., PRL 110 (2013) S. Fritzsche et al., PRL 103 (2009) Z. Hu et al., PRL 108 (2012) S. Tashenov et al., PRL 113 (2014)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, In high-Z systems higher-order multipole transitions come into play. Z scaling for transitions in H-like ions: So far only E1 - what about higher multipoles at high Z? Nuclear Charge Z Transition rate [ s -1 ] U 91+ Lyman-  1  E1 ~ s -1,  M2 ~ s -1

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E1 transition only E1-M2 interference: Enhanced Lyman-  1 Anisotropy in High-Z ions Lyman-  1 angular distribution: E1 + M2 Th. Stöhlker et al., PRL 79 (1997) A. Surzhykov et al., PRL 88 (2002) Interference between the E1 and the M2 decay branch lead to an increased anisotropy of the Ly-  1 emission pattern. So-called “Multipole Mixing” is well-known for nuclear transitions.

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, E1-M2 interference: Decreased Lyman-  1 Linear Polarization in High-Z Ions Exact relativistic calculations by A. Surzhykov: E1-M2 interference in Ly-  1 leads to Increased Anisotropy Decreased Polarization E1 & M2 E1 only A. Surzhykov, Hyperf. Interact (2003) Degree of linear polarization:

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Standard Ge(i) detectors Particle detectors Dipole magnet Lyman-  1 Polarimetry of H-Like Uranium: Confirmation of reduced Linear Polarization U 92+ → H 96.6 MeV/u G. Weber et al., PRL 105 (2010) Reduction in the degree of linear polarization due to E2-M1 interference is confirmed.

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Combined angular distribution and polarization measurements: A model-independent estimation of the E1-M2 transition ratio EquationObservableExperimentTheory only E1 E1 + M2 with  = / Decoupling of the collision dynamics (alignment parameter A 2 ) and the atomic structure (amplitude ratio  = / ) enables a model independent estimation of both values: → Precise estimation of the E1-M2 transition ratio! Amplitude ratio Theory Ratio of transition rates Γ M2 / Γ E1 = ± 2.8% Motivated a study of QED contributions by Surzhykov et al. (manuscript in preparation)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Storage Rings / EBITSs / HCI Effects of higher multipoles and retardation on the polarization properties of radiative recombination Combined angular distribution / polarization measurement of the 2p 3/2 state: precision life time determination Electron-nucleus bremsstrahlung studies in inverse kinematics Electron Accelerators Study of spin-polarization transfer from electrons to photon (bremsstrahlung) Synchrotrons Polarization transfer in elastic photon scattering (Rayleigh and Delbrück scattering) Ratio of transition rates Γ M2 / Γ E1 = ± 2.8% G. Weber et al., PRL 105 (2010) R. Märtin et al., PRL 108 (2012) Summary and Outlook Bremsstrahlung of polarized electrons Multipole mixing for 2p 3/2 → 1s 1/2 S. Hess / H. Ding, to be published REC for Xe 54+ (150 MeV/u)

Günter Weber FAIRNESS 2014, Vietri sul Mare, Italy, Atomic Physics Division of GSI – Mille Grazie! Thank you for your attention !