The decay rate of 57 Fe nuclei in an 57 FeBO 3 crystal excited by 14.4 keV synchrotron radiation pulses was controlled by switching the direction of the crystal magnetization. Abrupt switching some nanoseconds after excitation suppresses the coherent nuclear decay. Switching back at later times restores it, starting with an intense radiation spike. The enhanced delayed reemission is due to the release of the energy stored during the period of suppression. Suppression and restoration originate from drastic changes of the nuclear states and of the interference within the nuclear transitions.

Monochromatizationof synchrotron radiation down to about 10 eV at an energy of 14.4 keV has been achieved by double nuclear Bragg diffraction from "Fe-yttrium iron garnet single-crystal films set for the electronically forbidden (200) reflection. The monochromatized y quanta have been analyzed with respect to time delay and capability of resonance absorption. By setting of appropriate energy and time windows a pure beam of resonant y quanta at a rate of about 1 Hz is available.PACS numbers: 76.80.+y, 07.85.+ n, 61.10.Fr y quanta with an energy width of 10 6 to 10 s eV, which is typical for low-lying nuclear states, have a coherence length of 0.2 to 20 m. Thus a highly brilliant beam of such quanta would open new perspectives to experimental y optics. Unfortunately, the usual radioactive sources are monochromatic but not brilliant whereas synchrotron radiation is very brilliant but white.Resonant nuclear diffraction of synchrotron radiation as has been first pointed out by Ruby' is a suitable method to achieve the desired beam, which in addition would have a well-defined time structure and polarization pattern. A first attempt to observe resonant nuclear excitation in Fe has been published by Cohen, Miller, and West.In order to maintain the outstanding brilliance of the synchrotron radiation only coherent reflections come into consideration, i.e. , either grazing-incidence reflec-tions3 or nuclear Bragg and Laue diffraction. 4 5 Obviously the 14.4-keV resonance of Fe is a good choice because the excitation energy is easily available from any synchrotron radiation source which covers the xray range and because the mean life v = 140 ns of the excited state allows the application of delayedcoincidence techniques. Furthermore, the chemistry and crystallography of iron compounds is well known, so that the needed large-size single crystals can be made.While present experience with grazing-incidence antireflection films indicates that four sequential reflections are required in order to obtain the desired suppression of the nonresonant prompt radiation, only two nuclear Bragg reflections will be sufficient if pure nuclear reflections are used. The greatest progress with this technique has been published by Chechin et al. , who reported an enhancement of delayed counts behind an o. -Fe203 single crystal positioned for the I777) pure nuclear reflection.

Curious "doublet" resonances in the KL double photoexcitation spectrum of argon are explained as a strong admixture of 3d' excited states to the principal [1s2p]4p excitation. A simulated "ab initio" EL photoabsorption spectrum including shake-up and shake-off contributions agrees well with the experimental data.

On the base of the stochastic model of M. Blunae for time-dependent hyperfine interactions the perturbation factors of the angular correlation function are evaluated. The results of numerical calculations for some special configurations of stochastic fluctuating field gradients as well as magnetic fields are presented. In the medium range where the fluctuation rates are comparable with the nuclear precession frequencies, interesting features are worked out for the spinrotation spectra. For the limiting case of very fast fluctuations it is proved that the well-known exponential decay of anisotropy as predicted by the theory of Abragam and Pound appears.

Asymmetric nuclear-resonance broadening, as originating, e.g., from magnetic-hyperfine-field distributions in magnetic alloys, has strong effects on the time evolution of nuclear forward scattering of synchrotron radiation. In thin samples of an Invar alloy, resonance broadening and the resulting dephasing in time cause a fast decay of the coherent scattering signal. In thick samples, the asymmetry of the broadening strongly affects dynamical scattering. Quantum beat and dynamical beat blend into a fast hybrid beat with thickness dependent period and field distribution sensitive modulation.

The lattice parameters of -Al 2 O 3 have been measured in a temperature range from 4.5 to 250 K with a relative accuracy of better than 6 Â 10 À6 . The experimental method uses Bragg backscattering and the recently proposed Mo È ssbauer wavelength standard, i.e. the wavelength ! M = 86.025474 (16) pm of the nuclear resonance radiation of 57 Fe (Shvyd'ko et al., 2000), which has previously been applied successfully to measure the lattice parameters of -Al 2 O 3 at temperatures between 286 and 374 K . The experimental data in the range from 4.5 to 374 K are consistent with the Debye model of thermal expansion. At 4.5 K, the thermal expansion coef®cient is as low as 1.2 (9) Â 10 À10 K À1 in the a direction.

We demonstrate an interferometer for hard x rays with two back-reflecting sapphire crystal mirrors--a prototype x-ray Fabry-Pérot interferometer. A finesse of 15 and 0.76 mu eV broad Fabry-Pérot transmission resonances are measured by the time response of the interferometer. Interference patterns are observed directly in spectral dependences of reflectivity.

A coherently excited nuclear state in a rotating sample acquires a phase shift during its time evolution that is proportional to its angular momentum and the rotation angle. As a consequence, the radiative decay of the excited state proceeds into the rotated direction, and the time spectrum of the nuclear decay is mapped onto an angular scale. This effect has been observed in nuclear resonant scattering of synchrotron radiation from a 57Fe metal foil rotating at 18 kHz.