Nuclear forward scattering of synchrotron radiation has become an important technique to study magnetic and electronic properties of solids. Observation of the nuclear forward scattering relies on the finite lifetime of an excited nuclear state and a pulse structure of synchrotron radiation. Nuclear resonant scattering, delayed in time, is differentiated from the prompt pulse of incident radiation by a fast detector. The huge intensity of the prompt pulse leads, however, to an overload of the detector which has to be avoided by decreasing the energy bandwidth of synchrotron radiation using appropriate monochromators. For low-energy nuclear resonances, this is routinely achieved using a special monochromator with high-order reflections of perfect silicon crystals, which provide a narrow band pass (~meV) and high angular acceptance matching the angular divergence of synchrotron radiation. However, for nuclear resonances beyond 30 keV this approach is no longer applicable, because the angular widths of high-order reflections become very small.

In this work we show that nuclear forward scattering applied to high-energies can be performed with X-ray optics based on silicon crystals. This was done using low-order reflections with a moderate energy resolution but sufficiently high angular acceptance. With this, in combination with a multi-element detector, the overload problem is circumvented. The method was applied to study the 67.41 keV Mössbauer transition in 61Ni, which is an element of particular industrial and biological importance.

The experiment was performed at ID18, the Nuclear Resonance beamline. The nickel foil enriched in 61Ni to 85% was irradiated by the beam from the high-heat-load Si(111) double-crystal monochromator with a band pass of 15 eV. The radiation after the sample was monochromatised further by two crystals. The first Si crystal with the highly asymmetrical (444) reflection worked as a dispersive element. The second Si(844) crystal was an energy analyser. The observed energy band pass of this monochromator was 120 meV. The time evolution of scattered radiation was measured using a novel 16-element array of Si avalanche photodiodes. The elements of the array were adjusted to the incident radiation at small glancing angle and displaced laterally relative to each other. Each element covered only a part of the beam in order to distribute the load over the photodiodes.

Time evolution of the nuclear forward scattering from the metal nickel foil is shown in Figure 8 for various temperatures and with or without applied external magnetic field. The thickness of the sample, chosen to optimise the count rate, was extremely large. This brought us into a so far unknown regime of nuclear forward scattering, characterised by large effective thicknesses. In this regime the beats in the measured time spectra are determined mainly by multiple scattering. On the other hand, the role of hyperfine magnetic splitting is manifested as a scaling of the time axis which is seen by comparison of the two top curves in the Figure 8. The application of the magnetic field of 4T, which decreases the hyperfine magnetic splitting by about a factor of 2, leads to the stretching of the beat structure. The fit to the data allows us to obtain values of the magnetic field at the nuclear site, which is 7.4(2) T without magnetic field and 3.7(2) T with applied magnetic field. These results are consistent with that obtained by NMR. The pronounced beat structure of multiple scattering allows the precise determination of the effective thickness and, therefore, the Lamb-Mössbauer factor, which equals 0.167(3) for 3.2 K.

Fig. 8: Time evolution of the nuclear forward scattering for metal Ni foil. All measurements except for the upper curve were performed with external magnetic field B=4T. The solid lines show the fit. The arrows emphasise stretching of the dynamical beat structure by the applied magnetic field.


In this work we have studied nuclear forward scattering of the 67.41 keV radiation by 61Ni. The method can be applied to a series of other isotopes. Among them, 73Ge(68.75 keV) is a classical elementary semiconductor, 157Gd(64.0 keV), 145Nd(67.25 keV), 175Yb(76.46 keV) and 170Er (79.31 keV) are important compounds for the magnetism in strongly-correlated systems and 99Ru (89.36 keV) is interesting in catalysis and material science. Thus, routine hyperfine spectroscopy of high-energy Mössbauer isotopes will become available, especially under extreme conditions.


Principal publication and authors

I. Sergueev (a), A.I. Chumakov (a), T.H. Deschaux Beaume-Dang (a), R. Rüffer (a), C. Strohm (a,b) and U. van Bürck (b), Phys. Rev. Lett, 99, 097601 (2007).
(a) ESRF
(b) Physik-Department E13, Technische Universität München, Garching (Germany)