Introduction to nuclear resonant scattering with synchrotron radiation

The concepts leading to the application of synchrotron radiation to elastic and inelastic nuclear resonant scattering are discussed. The resulting new experimental techniques are compared to conventional Mössbauer spectroscopy. A survey of situations that favor experiments with synchrotron radiation is offered.

     

Monochromatization of synchrotron radiation for nuclear resonant scattering experiments

An introduction to monochromatization of synchrotron radiation in the energy range of 5–30 keV is presented for applications involving nuclear resonant scattering. The relevant relationships of the dynamical theory of Xray diffraction are used to explain basic concepts of monochromatization. These relations are combined with raytracing techniques to design highenergyresolution monochromators. Transmissionoptimized and energyresolutionoptimized designs that achieve high energy resolutions (10⁶)< E/E < 10⁸) are discussed separately. Practical silicon monochromators of both types are presented for a variety of nuclear resonances in this energy range.     

Basic features of coherent nuclear resonant scattering of synchrotron radiation

The time dependences of spatially coherent nuclear resonant scattering of synchrotron radiation show a wealth of characteristic features which originate from the interference of several radiation components and from dynamical nuclear scattering. These effects will be of importance for the future fields of time domain Mössbauer spectroscopy and of nuclear resonant -ray optics. The basic features are discussed and experimental evidence is reviewed.     

Nuclear resonant scattering of synchrotron radiation

Hyperfine Interactions -     

Nuclear resonant scattering of synchrotron radiation

The principal ideas of the theory and the main results of the experimental studies of the coherent resonant scattering of-radiation by nuclear ensembles in matter are briefly over-viewed. An analysis of transmission of the Mössbauer-radiation and of synchrotron radiation through a nuclear resonant medium is suggested using an approach based on the optical theory. The feasibilities of the nuclear resonant scattering of synchrotron radiation as a new technique for studying the hyperfine interactions and some other phenomena of the physics of condensed matter are considered.     

Nuclear resonant scattering using synchrotron radiation

A one-dimensional quantum model for nuclear resonant scattering using synchrotron radiation has been developed. This model gives a clear physical interpretation of the most prominent features of the coherent forward scattering process namely, the “speed-up” and “dynamical beat” effects. The form of the solution, for the time-dependent forward scattered intensity of the resonant radiation from the resonant medium after synchrotron radiation excitation, is a finite series. This unique solution can be interpreted in terms of a summation over all multiple forward scattering paths the radiation takes in reaching the detector. The resonant medium is represented by a linear chain of N effective resonant nuclei. The analysis starts from a coupled set of quantum mechanical equations for the relevant amplitudes in frequency space. Transformation to the time domain gives an analytical expression for the forward scattered intensity. The contribution of every order of the multiple scattering processes from the N effective nuclei appears naturally. The expression gives a clear physical understanding of all relevant aspects of resonant forward nuclear scattering. Furthermore, the present formalism allows the consideration of incoherent processes. This permits the study of processes in which there is gamma emission with recoil or emission of internal-conversion electrons. This revised version was published online in August 2006 with corrections to the Cover Date.     

Temperature dependence of phonon energy spectra with nuclear resonant scattering of synchrotron radiation

Summary The phonon energy spectra of a polycrystalline α-Fe foil were observed at 150 K and 300 K by using the nuclear resonant scattering of synchrotron radiation. In each spectrum, inelastic scattering was observed at both sides of the elastic peak. It was found that the ratio of the elastic-scattering component and the asymmetry of the intensity of the side bands observed at 150 K are larger than those observed at 300 K, respectively. The observed temperature-dependent spectra are in good agreement with the spectra calculated from the phonon energy distribution function. One of the advantageous features of this method is that the excitation of only a specific element is possible. Our results show that this method is applicable to the study of lattice dynamics and opens a new field of the nuclear resonant scattering spectroscopy. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Perturbed nuclear scattering of synchrotron radiation

Mössbauer spectroscopy in the time domain with SR as a source may become a powerful tool in investigations of time-dependent hyperfine interactions caused by external perturbations. This is firstly due to the easy possibility to synchronize the excitation of the nuclei with an external perturbation and secondly due to the insensitivity of Mössbauer time spectra to mechanical vibrations which are generated in samples by these perturbations. As a particular case of perturbed nuclear scattering, the coherent scattering under conditions of 90°- and 180°-switching of the magnetic hyperfine field direction is considered. The results of the first experimental studies are discussed.     

Nuclear resonant scattering of synchrotron radiation from thin films

Hyperfine Interactions - The optical properties of thin films containing Mössbauer isotopes undergo dramatic changes in the vicinity of a nuclear resonance. Remarkable phenomena are observed...     

Nuclear resonant scattering of synchrotron radiation by multilayer structures

Multilayer structures form a particular class of samples employed in nuclear resonant scattering of synchrotron radiation. Their specific properties lead to unusual energy and time characteristics of nuclear resonant scattering, which differ much from those of single crystals. The analysis of these distinctions is presented. Several approaches to achieve pure nuclear reflections with multilayers are discussed. Finally, we review the studies of multilayer structures with nuclear resonant scattering of synchrotron radiation.

     

How to do resonant nuclear experiments with synchrotron radiation

Summary The use of synchrotron radiation to excite low-lying nuclear resonances is a rapidly developing field showing great promise for hyperfine spectroscopy, phonon spectroscopy and kinetic studies, crystallography, and fundamental physics experiments. Recent technical advances in synchrotron sources, optics, and fast detectors have drastically increased signal rates and expanded the range of samples that can be studied. A typical experiment today uses a high-brightness synchrotron source having X-ray pulses well-separated in time, a meV-bandpass monochromator using perfect crystals of silicon or germanium, a sample containing resonant nuclei, and an avalanche photodiode timing detector. Both coherent and incoherent scattering can be observed; the coherent scattering is used for hyperfine spectroscopy and studies of diffraction interference phenomena, and the incoherent-scattering signal promises to be very useful for phonon spectroscopy and other studies of excitations in condensed systems. At this point seven nuclear isotopes have been used to resonantly scatter synchrotron radiation, but the number is rapidly increasing. Paper presented at the ICAME-95, Rimini 10–16 September 1995.     

High-pressure studies with nuclear scattering of synchrotron radiation

Hyperfine Interactions - The nuclear forward scattering (NFS) of synchrotron radiation is especially suited for probing magnetism at high pressure (h.p.), here in the Mbar range, by the nuclear...     

Pure nuclear resonant scattering of synchrotron radiation by a multilayer structure: Theoretical analysis for 169Tm

The possibility of observing pure nuclear resonant scattering of synchrotron radiation by a multilayer structure containing the ¹⁶⁹Tm isotope is analyzed theoretically. The main problem is the need to suppress the enormous background of radiation scattered by electrons. Two methods for the destructive interference of a synchrotron radiation beam in reflection at grazing incidence by a layered system containing Tm nuclei in one of the layers are considered, and their efficiency as applied to the conditions of third-generation synchrotron radiation sources, such as in the European Synchrotron Radiation Facility (ESRF), is calculated. An electron scattering suppression efficiency parameter is formulated as the ratio of the integrated nuclear scattering intensity (with a time delay) to the total prompt electron scattering intensity in assigned ranges of angles and energies. In the first method thin films of a special type on a substrate, viz., GIAR films, are used. In the second method a new effect, which is termed the Bragg antipeak effect and involves the destructive interference of a wave that is Bragg-diffracted in a multilayer superlattice and a wave reflected on the upper boundary of the sample, is employed. The physical properties of the Bragg antipeak effect are considered, and it is found that its efficiency is sufficient for practical use. Zh. éksp. Teor. Fiz. 114, 3–22 (July 1998)     

Inelastic nuclear resonant absorption of synchrotron radiation in thin films and multilayers

Inelastic nuclear resonant absorption of synchrotron radiation is an efficient and unique method for the direct measurement of vibrational density of states (VDOS) of thin films and interfaces that contain Mössbauer isotopes. This is demonstrated for the ⁵⁷Fe nuclear resonance in the case of amorphous and crystalline Tb–Fe alloy thin films and buried Fe/Cr interfaces in epitaxial α-Fe(0 0 1)/Cr(0 0 1) superlattices.

     

Dynamical beats of forward-scattered resonant synchrotron radiation as a nuclear polariton effect

The decay of the nuclear exciton (immobile collective excitation), created by a pulse of synchrotron radiation, is analyzed. It is shown that in the later phases of the decay, the exciton becomes localized at the sample’s frontal surface. Inside the sample, the secondary gamma-quanta, emitted by the contracting exciton, are converted into polaritons (mobile nuclear excitations) characterized by different frequencies and equal group velocities. On the sample’s back surface, the polariton interference causes a beating structure of the transmitted radiation, observed in experiments.     

Artificial neural networks applied to the analysis of synchrotron nuclear resonant scattering data

The capabilities of artificial neural networks (ANNs) have been investigated for the analysis of nuclear resonant scattering (NRS) data obtained at a synchrotron source. The major advantage of ANNs over conventional analysis methods is that, after an initial training phase, the analysis is fully automatic and practically instantaneous, which allows for a direct intervention of the experimentalist on‐site. This is particularly interesting for NRS experiments, where large amounts of data are obtained in very short time intervals and where the conventional analysis method may become quite time‐consuming and complicated. To test the capability of ANNs for the automation of the NRS data analysis, a neural network was trained and applied to the specific case of an Fe/Cr multilayer. It was shown how the hyperfine field parameters of the system could be extracted from the experimental NRS spectra. The reliability and accuracy of the ANN was verified by comparing the output of the network with the results obtained by conventional data analysis.     

Hybrid forms of beat phenomena in nuclear forward scattering of synchrotron radiation

In nuclear forward scattering (NFS) of synchrotron radiation, inter-resonance interference leads to a quantum beat (QB), and intra-resonance interference to a dynamical beat (DB). In general both interference processes determine the time evolution of NFS. Only in the case of far distant resonances the resulting interference pattern can be interpreted as a well distinguishable combination of QB and DB. Multiple scattering by near neighbouring resonances, by contrast, leads to a hybridisation of QB and DB. In particular, asymmetrical continuous distributions of resonances make QB and DB blend into a fast hybrid beat with thickness dependent period and distribution sensitive modulation.

     

Relaxation as seen by nuclei: Comparison between conventional Mössbauer spectra and nuclear resonant scattering of synchrotron radiation

The mechanism of spin‐lattice relaxation has been investigated in the “picket‐fence” porphyrin [Fe(CH₃COO)(TP_pivP)]^-, a high‐spin iron(II) complex with unusual large quadrupole splitting of 4.25 mm s^-1, by conventional Mössbauer spectroscopy as well as by nuclear resonant forward scattering (NFS). Superparamagnetism with a blocking temperature of about 8 K is observable by both methods in the spectra of bacterioferritin from S. olivaceus. From these two examples general conclusions about the merits of both methods can be drawn.