C L'abbe

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.

Step-induced canting of magnetization in Fe/Ag superlattices

We have investigated the reorientation of the easy axis of magnetization in (0 0 1) Fe/Ag superlattices using vibrating sample magnetometry, Mössbauer spectroscopy and nuclear resonance scattering of synchrotron radiation. Clear evidence is found that the Fe-layer magnetization can be oriented considerably out of the plane of the sample at room temperature, even for Fe-layer thicker than 6 ML at which the spin-reorientation transition usually occurs in Fe/Ag. The spin canting is attributed to frustration and a strong contribution of a step-induced anisotropy.

Determination of the magnetic spin direction from the nuclear forward-scattering line intensities

An expression is derived for the line intensities in a nuclear forward-scattering energy spectrum that is obtained via a Fourier transformation of the time dependence of the wavefield. The calculation takes into account the coherent properties of the nuclear forward-scattering process and the experimental limitations on the observable time window. It is shown that, for magnetic samples, the spin direction can be determined from the ratios between the different lines in the energy spectrum. The theory is complemented with experimental results on alpha-iron.

Heterodyne detection of synchrotron radiation

A time integral method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction is presented. The method relies on the interference of radiation scattered by nuclei in two samples, one moving with respect to the other. The method, termed heterodyne detection of synchrotron radiation, gives the same information on hyperfine parameters as the well known differential method. The general formalism is developed for the case where the reference is a single line sample and the investigated sample has magnetic or quadrupole splitting. The first experiments are discussed. A comparison of time differential synchrotron radiation spectroscopy, heterodyne detection and Mössbauer spectroscopy is given.

Example of spatial coherence in nuclear radiation: nuclear emission holography

The importance of coherence in the interaction of (gamma) - radiation with nuclei is demonstrated for nuclear emission holography. Radiation, produced by a radioactive source nucleus, can go directly to a detector or can be resonantly scattered by neighboring nuclei before going to the detector. The interference between these two processes gives rise to the holographic image. The quantum mechanical theory of emission holography with (gamma) -radiation is sketched. An expression for the contrast function in the single scattering approximation is given and discussed. Simulations for a bcc-lattice of 57Fe nuclei show the feasibility of nuclear emission holography. The technique will be very useful to study small clusters of resonant nuclei.

Time-integrated energy domain measurements with synchrotron radiation

A new time integrated method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction or in Bragg directions is introduced. This method gives in principle similar information as the well known time differential method. A brief comparison of both methods is presented. The idea is to excite coherently the nuclei incorporated in two absorbers, one moving with respect to the other. The fields radiated by the nuclei from both absorbers interfere and each time the nuclear energy in one absorber matches, by Doppler modulation, the nuclear energy of the other, an extremum in the time integrated intensity is observed. The results of the first experiments at the Advanced Photon Source at the Argonne National Laboratory will be presented.

Resonant-Detector Mössbauer Spectroscopic Studies of Sn Doped SiO2 Analysed Using Quantum Mechanical Theory

It has already been established that, by using a resonant detector in Mossbauer spec-troscopy, the minimum spectral linewidth is 1.46Γ. Here Γ is the linewidth of the Mossbauer excited-state nuclear level. It is well known that the minimum linewidth obtained in conventional Mossbauer experiments is 2Γ. The quantum mechanical calculation using a nuclear-resonant detector, which predicts this result, is summarized. The fundamental equations describing the system are solved in the frequency domain and applied to the experimental results. The experimental results using the resonant-detector Mossbauer technique and an Sn-doped SiO2 sample are presented. The best fit to the data is obtained using the resonant-detector quantum mechanical theory.

Nuclear emission holography

In this paper the idea of nuclear emission holography is presented. A quantum mechanical theory leading to an expression for the contrast function is sketched and a simulation for α-57Fe is given. It is also shown that the holographic contrast can be increased by collecting data in an appropriate time window.