J. Metge

Energy Dependence of Nuclear Recoil Measured with Incoherent Nuclear Scattering of Synchrotron Radiation

The energy dependences of nuclear forward scattering and nuclear 4π scattering of 14.4 keV synchrotron radiation were studied with an energy resolution of 6 meV. Nuclear 4π scattering resulted from internal conversion and therefore represents the energy dependence of resonant nuclear absorption. In the case of the [57Fe(bpp)2][BF4]2 sample the energy dependence of nuclear 4π scattering revealed a 13 meV inelastic broadening of the absorption line. This indicates an excitation of lattice motions in the nuclear absorption process. The technique allows the direct measurement of the energy distribution of nuclear recoil.

Broad-Band Nuclear Resonant Filters for Synchrotron Radiation: a New Source for Nuclear Diffraction Experiments

A grazing incidence antireflection film (GIAR film) containing 57Fe has been used as a broad-band filter in a nuclear-diffraction experiment with synchrotron radiation. A band of 0.5 μeV width has been reflected by the GIAR film consisting of a 57Fe5B4C-layer on a Ta-backing. The electronic reflectivity was reduced to 0.04. The diffracted beam was analysed by the (333) pure nuclear reflection of 57FeBO3. A counting rate of delayed quanta of 8 Hz was obtained. The experiment proves that coherent excitation of hyperfine split nuclear levels took place after a GIAR film reflection.

Nuclear diffraction experiments with grazing incidence antireflection films

The energy dependent reflectivity of grazing incidence antireflection (GIAR) films containing the Mößbauer isotope57Fe has been measured with recoilless 14.4 keV radiation from a radioactive source. The reflectivity is influenced by oxide layers on the surface. Assisted by Mößbauer spectroscopy in transmission geometry and X-ray reflectivity measurements the hyperfine interaction parameters in the layers could be determined. In a nuclear diffraction experiment with synchrotron radiation a GIAR-film has been used as a filter. The diffracted beam was analysed by the (002) reflection of Yttrium Iron Garnet (YIG). The evaluation of the observed quantum beat pattern yielded the energy differential reflectivity of the GIAR-film in agreement with the results from the Mößbauer measurements.

A high resolution spectrometer using nuclear Bragg diffraction

A high resolution spectrometer for synchrotron radiation using nuclear Bragg diffraction has been constructed at a DORIS beamline (DESY, Hamburg). This spectrometer provides a γ-ray beam for hyperfine spectroscopy and for other application which need high resolution in energy and/or time.

A γ-ray detector with good time resolution and high signal-to-noise ratio

Abstract A coincidence detector for an energy range between 8 and 50 keV with a good time resolution and a high signal-to-noise ratio was developed for time-differential experiments using nuclear Bragg diffraction with synchrotron radiation as a source.

Fast Switching of Nuclear Bragg Scattering of Synchrotron Radiation by a Pulsed Magnetic Field

The time dependence of perturbed nuclear Bragg scattering of synchrotron radiation by a 57FeBO3 crystal was studied. The perturbation was an abrupt rotation by 90° of the hyperfine fields in the crystal effectuated by fast switching of an external magnetic field. This perturbation did not destroy the coherence. It transformed the nuclear transitions and caused, therefore, dramatic changes in the time evolution. A new quantum beat pattern of different frequency was immediately established after switching. The intensity and decay rate of the re-emitted radiation sensitively depended on the moment of switching. The frequency of the original beat pattern was restored by switching back.

Nuclear Bragg Diffraction of Synchrotron Radiation in the Presence of Acoustic Vibrations

The time evolution of nuclear Bragg diffraction of 14.4 keV synchrotron radiation in a 57FeBO3 crystal was studied when the crystal was excited to magnetoelastic vibrations by a r.f. field of 2.93 MHz. The vibrations did not modify the time evolution of the diffraction. This result is due to the fact that after the excitation by the SR pulse the nuclear vibrations cause only a phase modulation of the re-emitted radiation. Energy spectra, by contrast, are extremely sensitive to vibrations. Thus the present case is an instructive example of complementarity between energy and time domain Mossbauer spectroscopy. Time domain spectroscopy, in particular, will allow to reveal phenomena, which are blurred in the energy spectra by vibrations.

Relaxation experiments with synchrotron radiation

Relaxation phenomena show up in standard energy domain Mössbauer spectra via line broadening. The evaluation of such spectra is in most cases done by adopting the stochastic theory mainly developed in the 60s and 70s. Due to the time structure and the polarization of the synchrotron radiation nuclear resonance forward scattering in the time domain gives valuable information on relaxation mechanisms. We report here mainly on Nuclear Forward Scattering (NFS) experiments investigating the paramagnetic relaxation of the Fe3+ ion in (NH)4Al0.9557Fe0.05(SO4)2·12H2O and briefly on recent investigations on charge fluctuations in Eu3S4.

Nuclear forward scattering of synchrotron radiation from unmagnetized α-57Fe

Nuclear foward scattering of synchrotron radiation from magnetically polycrystalline and partly magnetized α-57Fe foils has been observed. The forward scattering amplitude in both cases is calculated. The results allow the interpretation of the time spectra from samples which are magnetically polycrystalline or exhibit an unidirectional magnetic texture.

Nuclear Bragg diffraction of synchrotron radiation from the a-sites of YIG

Time differential measurements of nuclear Bragg diffraction of synchrotron radiation were applied to57Fe nuclei at the a-sites of yttrium iron garnet (YIG). The hyperfine parameters were determined by evaluation of the time spectra using the dynamical theory of Mössbauer optics. The observation of nuclear Bragg diffraction allows site selective spectroscopy.