Mossbauer

The Mossbauer effect, a physical phenomenon discovered by Rudolf Mossbauer in 1957, refers to the resonant and recoil-free emission and absorption of y-rays by atoms bound in a solid. In general, y-rays are a kind of electromagnetic wave with very short wavelengths less than 10-8 cm, which are produced by nuclear transitions from an unstable high-energy state to a low-energy state. The energy of the emitted y-ray corresponds to the energy of the nuclear transition minus an amount of energy that is lost as recoil to the emitting atom. If the lost recoil energy is small compared with the energy linewidth of the nuclear transition, then the y-ray energy still corresponds to the energy of the nuclear transition and the y-ray can be absorbed by another atom of the same type as the first. Such emission and subsequent absorption is called resonance. Additional recoil energy is also lost during absorption, so in order for resonance to occur the recoil energy must actually be less than half the linewidth for the corrseponding nuclear transition. The amount of lost energy is described by the equation Er = Egamma /2Mc2, where ER is the energy lost as recoil, Egamma is the energy of the y-ray, M is the mass of the emitting or absorbing body, and c is the velocity of light. In the case of a gas the emitting and absorbing bodies are atoms, so the mass is quite small, resulting in a large recoil energy, which prevents resonance.

Due to the fundamental quantum nature of solids, atoms bound in solids are restricted to a specific set of vibra-tional energies called phonon energies. If the recoil energy is smaller than the phonon energy, there is insufficient energy to excite the lattice to the next vibrational state, and a fraction of the nuclear events, called the recoil-free fraction, occurs such that the entire crystal, rather than just the atom, acts as the recoiling body. Since the mass of the crystal is very large compared to that of a single atom, these events are essentially recoil-free. In these cases, since the recoil energy is negligible, the emitted y-rays have the appropriate energy and resonance can occur. In general, the y-rays have very narrow linewidths. This means they are very sensitive to small changes in the energies of nuclear transitions. In fact, the y-rays can be used as a probe to observe the effects of interactions between a nucleus and its electrons and those of its neighbors. This is the basis for Mossbauer spectroscopy, which combines the Mossbauer effect with the Doppler effect to monitor such interactions.

Mossbauer spectroscopy is a spectroscopic technique based on the Mossbauer effect. In its most common form, Mossbauer absorption spectroscopy, a solid sample is exposed to a beam of y-radiation, and a detector measures the intensity of the beam that is transmitted through the sample. The y-ray energy is varied by accelerating the y-ray source through a range of velocities with a linear motor. The relative motion between the source and sample results in an energy shift due to the Doppler effect. In the resulting spectra, y-ray intensity is plotted as a function of the source velocity. At velocities corresponding to the resonant energy levels of the sample, some of the y-rays are absorbed, resulting in a drop in the measured intensity and a corresponding dip in the spectrum. The number, positions, and intensities of the dips (also called peaks) provide information about the chemical environment of the absorbing nuclei and can be used to characterize the sample. In order for Mossbauer absorption of y-rays to occur, the y-ray must be of the appropriate energy for the nuclear transitions of the atoms being probed. Also, the y-ray energy should be relatively low; otherwise the system will have a low recoil-free fraction, resulting in a poor signal-to-noise ratio. Only a handful of elemental isotopes exist for which these criteria are met, so Mossbauer spec-troscopy can only be applied to a relatively small group of atoms. The sources commonly used for the Mossbauer spectra are 57Co, 119mSn,125Sb,133Ba,195Au, and 182Ta. The most important source is 57Co and the Mossbauer leap of 14.4 keV of 57Fe is generated from the disintegration of 57 Co. Thus 57Fe is by far the most common element studied using the technique.

The Mossbauer spectra provide the information of the inner magnetic fields in the nuclei and the hyperfine fields. The Mossbauer effects can be used to study the properties of nanoparticles/nanocapsules. Figure 17 gives the 57Fe Mossbauer spectrum at room temperature of the Fe(B) nanocapsules [122]. The relative amounts of different phases and the hyperfine fields in the phases can be obtained for the nanocapsules. The hyperfine fields of the phases in the nano-capsules may slightly differ with those in their bulk counterparts, due to the size effect. The disadvantage of the Moss-bauer study is the limitation that it can detect information on the materials with elements the same as after disintegration of the source.

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