Riet Callens

Induced transparency for gamma radiation via nuclear level mixing

A significant reduction of absorption of single gamma photons has been observed using the Mössbauer spectra of 57Fe in a FeCO3 crystal. The absorption deficit can be ascribed to partially destructive interference for absorption because of two indistinguishable absorption paths. The necessary coherence is created by means of level mixing produced by a suitable combination of a magnetic dipole and electric quadrupole interaction.

Time-integrated synchrotron Mössbauer spectroscopy

A new hyperfine interaction technique, time-integrated synchrotron Mössbauer spectroscopy, is presented. This technique uses synchrotron radiation to measure hyperfine splittings of nuclear levels in the energy domain. A comprehensive theoretical description is given, several simulations are presented and some experimental results are discussed. Time-integrated synchrotron Mössbauer spectroscopy can open new perspectives, especially in the study of nuclear resonances that are not accessible with conventional Mössbauer spectroscopy or time-differential synchrotron Mössbauer spectroscopy.

Slowing down of gamma photons

By a transformation to a rotating reference system, a nuclear level mixing scheme is equivalent to a scheme with pure quadrupole splitting and a strong RF-drive between quadrupole split lines. It is shown that this scheme is similar to the one used in optics in which electromagnetically induced transparency (EIT) and low group velocity have been demonstrated. One can therefore consider the slowing down of gamma photons. However, with gamma photons one always works with single photons and the question remains whether the concept of group velocity can be applied.

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.

Stroboscopic detection of nuclear resonance in an arbitrary scattering channel.

The theory of heterodyne/stroboscopic detection of nuclear resonance scattering is developed, starting from the total scattering matrix as a product of the matrix of the reference sample and the sample under study. This general approach holds for all dynamical scattering channels. In the forward channel, which has been discussed in detail in the literature, the electronic scattering manifests itself only in an energy-independent diminution of the scattered intensity. In all other channels, complex resonance line shapes of the heterodyne/stroboscopic spectra are encountered, as a result of the interference of electronic and nuclear scattering. The grazing-incidence case will be evaluated and described in detail. Experimental data of classical X-ray reflectivity and their stroboscopically detected resonant counterpart spectra on the [(nat)Fe/(57)Fe]10 isotope periodic multilayer and antiferromagnetic [(57)Fe/Cr]20 superlattice are fitted simultaneously.

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.

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.