1. References [1] Röhlsberger, Toellner, Sturhahn, Quast, Alp, Bernhard, Burkel, Leupold, Gerdau; Phys. Rev. Lett. 84 (2000); p. 1007 [2] Röhlsberger, Quast, Toellner, Lee, Sturhahn, Alp, Burkel; Appl. Phys. Lett. 78 (2001); p. 2970 [3] Nuclear Resonant Scattering of Syn- chrotron Radiation; ed. By Gerdau, de Waard; Hyperf. Interact. 123/124 (1999) 125 (2000) Summary At high energies it seems impossible to build efficient high-resolution monochromators for Nuclear Forward Scattering. Presently, the Nuclear Lighthouse Effect is the only method to access Mössbauer isotopes with high transition energies and short lifetimes, together with Synchrotron Radiation Perturbed Angular Correla- tion, which investigates the incoherent channel of de-exitation. Nevertheless, although Small Angle Scattering is much reduced at high x-ray energies, time descrimination between prompt SAX and interesting delayed quanta is often necessary. Further restriction arise from the limited sample environment. Very recently, first Nuclear Forward Scattering spectra of 61 Ni were obtained at 180 K and 115 K, which al- ready from a technical aspect concerning the cooling, is a success. Other interesting Mössbauer isotopes at high transition energies are displayed to the right. The Nuclear Lighthouse Effect Th. Roth 1,2, O. Leupold 1,3, R. Röhlsberger 3, K. Quast 3, H.C. Wille 1, R. Rüffer 1 1 European Synchrotron Radiation Facility, Grenoble, France email: troth@esrf.fr 2 Technische Universität München, Germany 3 University of Rostock, Germany ID 18 Nuclear Forward Scattering of Synchrotron Radiation Nuclear Forward Scattering is the time based analogue to the energy re- solved Mössbauer spectroscopy. Synchrotron radiation probes the sam- ple containing the Mössbauer isotope. Even with best monochromatisation, the energy width of the incident beam is broader then the nu- clear level or its hyperfine splitting. Thus all nuclear levels are excited simul- taneously. The col- lective and cohe- rent part of the radiative decay is peaked in forward direction. It will show interference of the different energy levels in case of a hyperfine splitting (quantum beats) and dy- namical beats due to multiple scattering.  high intensity, polarized light, highly collimated  complex data analysis, separation of prompt and delayed quanta needs time structured SR Mössbauer Spectroscopy Nuclei showing the Mössbauer effect absorb and emit  -rays of a certain energy without recoil (in a crystal). The spectral width of these  -rays is extremely narrow. The energy of the emitted line can be tuned via the Doppler effect by mo- ving the source. This enables one to investi- gate samples containing that Mössbauer isotope, often via transmission measurements with resonance absorption. This technique is very helpful in, e.g., detecting hyperfine interactions, the electron density at the nuclei and structure of a chemical bound.  Extremely high energy resolution Relatively direct data-analysis  Poor collimation and intensity Nuclear Lighthouse Effect Like in the case of Nuclear Forward Scat- tering, the synchrotron radiation creates a collectively excited state (nuclear ex- citon) in the Mössbauer isotope. In a rotating sample, these excited states aquire a phase shift while evolving in time. The radiative decay pro- ceeds therefore in a deflected direction: The time spectrum is mapped onto an angular scale. This allows one in principle to record time spectra with a position sensitive detector without timing electro- nics, independently of the time structure of the exciting radiation. The detector load is drastically reduced, as the delayed signal de- flected and is thus not in direction of the direct beam  no timing mode and no fast electronics in certain applications, mono- chromatisation less critical, high time resolution possible  small angle scattering background, tiny sample environment intensity sample High resolution monochro- mator Premonochro- mator Synchrotron with well separated bunches Detector and timing electronics Prompt peak Delayed quanta Position-sensitive detector Synchrotron radiation Mössbauer isotope X-ray detector Radioactive Mössbauer source 57 Fe example e-e- Plot of all known Mössbauer isotopes as a function of resonance energy and lifetime. In green: Isotopes that have been applied in nuclear scattering experiments with synchro- tron radiation. In red: Isotopes that become accessible with the technique of the nuclear lighthouse effect. Perspectives on new samples Scattering angle [mrad] Distance [mm] 20 Nuclear Lighthouse Effect measurement with 57 Fe The Nuclear Lighthouse Effect at the 14,4 keV resonance of 57 Fe with 15 kHz rota- tion frequency of the sample. Image plate recording. Small Angle Scattering background Small angle scattering reduces drastically at high x-ray energies. Approximated ID18 spectral flux e.g. for the 61 Ni isotope: with three 32mm undulators in 16 bunch mode: approx. 10 5 -10 4 photons / (sec  eV) at 67.4 keV in 17th harmonic via Si 333 reflection Nuclear Lighthouse Effect at high energies  Small Angle Scattering is much reduced and leads to low background. Early decay times become accessible.  Absorption in the rotor material becomes less critical  Undulator flux is lower  Low Lamb-Mössbauer factor of isotopes in that range, thus sample cooling is a must Monochromatisation The angular acceptance of crystal Bragg reflections decreases with increasing x-ray energy. At high energies, High Resolution Monochromators using high indexed re- flections are therefore inappropriate. With Compound Refractive Lenses, the incoming beam can be collimated, before it passes a simple silicon mono- chromator, using the Si(333) or Si(444) reflections. In-vacuum slits Slit blades of tungsten single crystals to reduce Small Angle Scattering Stator rotor assembly Modified from standard NMR equipment. Bearing and driving by gas flow. Rotation frequencies up to 12 kHz available at ID18. Cooling by cryogenic cooling gas flow operation from 100 to 300 K Beamstop for direct beam 61 Ni Detector Position sensitive detector or scanning detector Nuclear Lighthouse Effect with a 6  m thick 57 Fe foil rotating at 5 kHz. No timing was used to discriminate the SAX background. Instead, measurements on the resonance energy, 14.4 keV, and away from the resoance are substracted to obtain the delayed signal. The filling mode of the synchrotron was 2 1 / 3 First Nuclear Forward Scattering time spectra of 61 Ni obtained with Nuclear Lighthouse Effect As the ratio SAX to delayed signal turns out to be quite important for the resonance of 61 Ni, fast detectors and timing are necessary to obtain these time spectra. A stack of 24 avalanche photo diodes is used during last 16 bunch mode. Fitting of the spectra is under way. SAX background at 14 keV SAX background at 80 keV Small angle scattering, SAX, background estimated from