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We present a novel method for detecting the solid–liquid phase boundary of compressed iron at high temperatures using synchrotron Mossbauer spectroscopy (SMS). Our approach is unique because the dynamics of the iron atoms are monitored. This process is described by the Lamb–Mossbauer factor, which is related to the mean-square displacement of the iron atoms. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated ^(57)Fe sample inside a diamond-anvil cell, and the characteristic SMS time signature vanishes when melting occurs. At our highest compression measurement and considering thermal pressure, we find the melting point of iron to be T_M=3025±115 K at P=82±5 GPa. When compared with previously reported melting points for iron using static compression methods with different criteria for melting, our melting trend defines a steeper positive slope as a function of pressure. The obtained melting temperatures represent a significant step toward a reliable melting curve of iron at Earth's core conditions. For other terrestrial planets possessing cores with liquid portions rich in metallic iron, such as Mercury and Mars, the higher melting temperatures for compressed iron may imply warmer internal temperatures.

In the first known kinetic application of the technique, synchrotron 57Fe-Mossbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic 57Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 · 2H2O], was reduced by Na2S2O4 and then reoxidized by K2Cr2O7 by means of a novel flow-through cell. Synchrotron Mossbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mossbauer spectroscopy yielded comparable values for Mossbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mossbauer data, the synchrotron technique seems well suited for aqueous rate experiments as a result of the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state.

The phonon density of states (DOS) in thin films of polycrystalline α-Fe was measured by inelastic nuclear resonant scattering of synchrotron radiation. The thin-film DOS exhibits significant deviations from the DOS of bulk Fe, which we attribute to phonon lifetime broadening in the confined geometry. The measured DOS can be described with a damped harmonic oscillator model for the phonons with different quality factors Q=25(2) and Q=13(1) for layer thicknesses of 28 and 13 nm, respectively.

Inelastic nuclear resonant absorption method has been applied to study lattice dynamics. The data evaluation procedure for such experiments using synchrotron radiation is presented. Various moments of the measured spectra provide model-independent information on vibrational excitations, such as the recoilless fraction, the average kinetic energy per nucleus, and the average force constant. In addition, the partial phonon density of states is extracted assuming a harmonic lattice model. A measurement performed on α -iron is shown as an example.

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.

Synchrotron Mossbauer Spectroscopy, SMS, is an emerging technique that allows fast and accurate determination of hyperfine field parameters similar to conventional Mossbauer spectroscopy with radioactive sources. This new technique, however, is qualitatively different from Mossbauer spectroscopy in terms of equipment, methodology, and analysis to warrant a new name. In this paper, the authors report on isomer shift and quadrupole splitting measurements of Mohr`s salt, Fe(NH{sub 4}){sub 2}(SO{sub 4}){sub 2}{center_dot}6H{sub 2}O for demonstration purposes. Theoretical calculations were performed and compared to experiments both in energy and time domain to demonstrate the influence of thickness distribution and preferential alignment of powder samples. Such measurements may prove to be useful when the data collection times are reduced to few seconds in the third generation, undulator based synchrotron radiation sources.

The principal and engineering aspects of a dedicated synchrotron radiation beamline under construction at the Advanced Photon Source for nuclear resonant scattering purposes are explained. The expected performance in terms of isotopes to be studied, flux, and timing properties is discussed.

We have constructed the first working model of a novel diffraction geometry for the high-resolution monochromatization of hard (5–30 keV) synchrotron radiation. The monochromator has an energy bandpass of 6.7 meV FWHM and an angular acceptance of 22 μrad (4.5 arcseconds) for 14.4 keV X rays - an energy resolution ΔEE of 4.6 × 10−7. With a second set of crystals, an energy bandpass of 23 meV FWHM and an angular acceptance of 7 μrad was achieved for 23.8 keV photons. The performance of the monochromator has been verified, in a nearly ideal measurement, using nuclear-fluorescent X rays with an energy bandwidth on the order of 10 neV.

A white beam collimator has been built to increase the efficiency of a diamond high-heat-load premonochromator. The collimator is composed of a water-cooled beryllium compound refractive lens. We demonstrate a 30% increase in spectral flux of the diamond premonochromator (at 22.6 keV) by using the collimator.

Discovery of Mossbauer effect [1] in a nuclear transition was a remarkable development. It revealed how long-lived nuclear states with relatively low energies in the kiloelectron volt (keV) region can be excited without recoil. This new effect had a unique feature involving a coupling between nuclear physics and solid-state physics, both in terms of physics and sociology. Physics coupling originates from the fact that recoilless emission and absorption or resonance is only possible if the requirement that nuclei have to be bound in a lattice with quantized vibrational states is fulfilled, and that the finite electron density on the nucleus couples to nuclear degrees of freedom leading to hyperfine interactions. Thus, Mossbauer spectroscopy allows peering into solid-state effects using unique nuclear transitions. Sociological aspects of this coupling had been equally startling and fruitful. The interaction between diverse scientific communities, who learned to use Mossbauer spectroscopy proved to be very valuable. For example, biologists, geologists, chemists, physicists, materials scientists, and archeologists, all sharing a common spectroscopic technique, also learned to appreciate the beauty and intricacies of each other’s fields. As a laboratory-based technique, Mossbauer spectroscopy matured by the end of the 1970s. Further exciting developments took place when accelerator-based techniques were employed, like synchrotron radiation or “in-beam” Mossbauer experiments with implanted radioactive ions. More recently, two Mossbauer spectrometers on the surface of the Mars kept the technique vibrant and viable up until present time.

The time response of resonant nuclei in a rotating sample coherently excited by synchrotron radiation pulses maps to an angular scale. At sufficiently high rotational speeds, the temporal evolution of the nuclear decay can be recorded with a position sensitive detector. This has been demonstrated by nuclear resonant scattering of synchrotron radiation from a 57Fe metal foil rotating at 10 kHz. The method has the potential to yield time resolutions of a few 10 ps, thus overcoming principal limits of existing techniques.

Grazing‐incidence antireflection (GIAR) films are used to suppress electronic reflectivity, while maintaining good nuclear resonant reflectivity to help monochromatize the synchrotron radiation down to the μeV level of energy resolution. Such monochromators will have superior throughput by a factor of 100–500 compared to natural linewidth of individual nuclei. 119SnO2 GIAR films of various thicknesses were designed and fabricated on Pd‐coated quartz substrates. The performance of these films was evaluated based on the measurement of off‐resonance x‐ray reflectivity using MoKα radiation and conversion electron Mossbauer spectroscopy. A Pd film was deposited on a quartz substrate at room temperature and was subsequently annealed at 500 °C in an Ar atmosphere at a substrate temperature of 200 °C, SnO2 was sputtered onto the Pd by reactive sputtering in an Ar and O2 atmosphere. Employing quantitative analysis of surface and interface roughness, a series of experiments was also conducted to determine optimal s...

With an undulator-based synchrotron source and a tunable x-ray monochromator, we produce an x-ray beam with a flux of 4×108 photons/s in an energy bandwidth of 920±110 μeV (ΔE/E≈6.2×10−8) at a photon energy of 14.4 keV. The tunability of the monochromator allows for a measurement of lattice excitations in α-Fe using the technique of inelastic nuclear resonant scattering from 57Fe. The phonon density-of-states are extracted from the measurement and compared to the calculated phonon density-of-states derived from neutron-scattering measurements.

The design and performance results for a cryogenically stabilized high-resolution monochromator for 23.880 keV (lambda = 52 pm) X-rays are presented. The four-crystal-reflection monochromator is suitable for nuclear resonant scattering measurements from 119Sn compounds using synchrotron radiation. The design includes a low-vibration cryostat that maintains two of the four crystal reflections at a temperature where the coefficient for thermal expansion of the crystalline material (silicon) vanishes. Test results demonstrate a 1.3 meV bandwidth with negligible broadening due to vibrations and a spectral efficiency of 37% when used with an undulator source.

A silicon (840) channel‐cut crystal with a large asymmetry is used to linearly polarize synchrotron radiation generated by an undulator source at the 14.413 keV nuclear resonance of 57Fe. The resulting σ‐polarized radiation is then scattered from an 57Fe foil placed in a magnetic field that effects σ→π polarization conversion within the resonant bandwidth. A second crystal of the same type is placed in a crossed position to suppress the nonresonant radiation while transmitting the π‐polarized resonant radiation. A polarization suppression factor of 6×10−7 has been obtained. The suppression of the nonresonant radiation allowed monitoring the decay of the nuclear levels after 1 ns of their excitation.

Nuclear resonant inelastic X-ray scattering (NRIXS) of 14.4125 keV synchrotron radiation was used to measure directly the partial vibrational density of states (VDOS), g(E), of vapor-quenched amorphous Tb1−x 57Fe x thin film alloys (0.20 ⩽ x ≤ 0.82). The structure of the samples was characterized by X-ray diffraction and Mossbauer spectroscopy. The behavior of the Lamb-Mossbauer factor obtained from measured vibrational excitation probability densities indicates vibrational softening with increasing Tb content. A plot of g(E)/E 2 versus E 2 proves the existence of non-Debye-like vibrational excitations below about 10 meV, becoming more pronounced with rising Tb content.

Monochromatization with crystal diffraction has been achieved to a resolution (lambda/delta lambda) beyond 10(8). The monchromator is specifically designed for 23.880 keV synchrotron radiation (lambda = 51.9 pm) for applications involving nuclear resonant scattering from 119Sn. The design uses asymmetrically cut silicon (12 12 12) crystal reflections from two single-crystalline monoliths oriented in a dispersive geometry. A transmitted energy bandwidth of 140 +/- 20 mu eV was measured, corresponding to a resolution of 1.7 x 10(8). Methods of improving efficiency, wavelength stability and resolution are discussed.

Physical review letters 84(5), 1007 - 1010 (2000). doi:10.1103/PhysRevLett.84.1007, Published by APS, College Park, Md.

The phonon density of states was measured by observing the nuclear resonant fluorescence of ${}^{57}$Fe versus the energy of incident x rays from a synchrotron radiation beam. An energy resolution of 6 meV was achieved by use of high-resolution crystal optics for the incident beam. Extremely low background levels were obtained via time discrimination of the nuclear fluorescent radiation.

Physical review letters 74(17), 3475 - 3478 (1995). doi:10.1103/PhysRevLett.74.3475, We report the observation of nuclear resonant diffraction of synchrotron radiation from a 57Fe/Cr multilayer. The multilayer consists of 25 57Fe(17 Å)/Cr(10 Å) bilayers with the Cr layer thickness chosen so as to exhibit antiferromagnetic (AF) coupling of the 57Fe layers. Because of the AF alignment of the layers, the magnetic periodicity of the multilayer is twice the electronic periodicity, resulting in a pure nuclear Bragg reflection that appears during nuclear resonant diffraction. The pure nuclear Bragg reflection presents a means of filtering synchrotron radiation to the level of 10–8 eV at the nuclear resonance energy of 14.4 keV., Published by APS, College Park, Md.

A 4-bounce dispersive crystal monochromator reduces the bandpass of synchrotron radiation to a 10-50 meV range without sacrificing angular acceptance. The monochromator includes the combination of an asymmetrical channel-cut single crystal of lower order reflection and a symmetrical channel-cut single crystal of higher order reflection in a nested geometric configuration. In the disclosed embodiment, a highly asymmetrically cut (α=20) outer silicon crystal (4 2 2) with low order reflection is combined with a symmetrically cut inner silicon crystal (10 6 4) with high order reflection to condition a hard x-ray component (5-30 keV) of synchrotron radiation down to the μeV-neV level. Each of the crystals is coupled to the combination of a positioning inchworm and angle encoder via a respective rotation stage for accurate relative positioning of the crystals and precise energy tuning of the monochromator.

The insertion-device-based, third-generation, synchrotron radiation sources now under construction in Europe, the USA, and Japan bring new opportunities and challenges in the design and manufacture of x-ray optics. These high brightness sources provide new opportunities to overcome some of the outstanding problems associated with nuclear resonant monochromatization of synchrotron radiation. New methods such as polarizing monochromators, and zone plates provide alternative methods for production of (mu) eV-neV resolution in the hard x-ray regime. The design principles, and characterization, and performance of crystal monochromators and of nuclear coherent scattering optics, including Grazing Incidence Anti Reflection (GIAR) films, multilayers, zone plates, as well as single crystals are discussed.

We have designed and constructed a novel linear actuator with 3 A closed-loop control resolution and 50 mm travel range. Two major ultraprecision motion control techniques have been applied to this actuator: (i) a novel laser Doppler encoder system with multiple-reflection optics [D. Shu et al., Proc. SPIE 3429, 284 (1998)]; and (ii) a specially designed high-stiffness weak-link mechanism with stacked thin metal sheets having sub-Angstrom driving sensitivity with excellent stability [D. Shu et al., Nucl. Instrum. Methods Phys. Res. A 467–468, 771 (2001). In this paper, we present recent test results of this actuator with an atomic force microprobe. Applications of this new technique to synchrotron radiation instrumentation are also discussed.

It is demonstrated how inelastic nuclear resonant scattering can be used for the investigation of lattice dynamics. The required instruments are explained. The feasibility of measuring a variety of nuclear resonances is discussed with respect to the synchrotron radiation intensity at the Advanced Photon Source.