Recoilless emission and absorption

Mossbauer effect spectroscopy, MES, Is based on the ability of certain nuclei to undergo recoilless emission and absorption ofY rays (16). The energy and multiplicity of the ground and excited states of a given nucleus are modified by the chemical environment. It Is thus most often necessary to compensate for the differences In... 

The Mossbauer effect involves the resonance fluorescence of nuclear gamma radiation and can be observed during recoilless emission and absorption of radiation in solids. It can be exploited as a spectroscopic method by observing chemically dependent hyperfine interactions. The recent determination of the nuclear radius term in the isomer shift equation for shows that the isomer shift becomes more positive with increasing s electron density at the nucleus. Detailed studies of the temperature dependence of the recoil-free fraction in and labeled Sn/ show that the characteristic Mossbauer temperatures Om, are different for the two atoms. These results are typical of the kind of chemical information which can be obtained from Mossbauer spectra. 

In 1957, Rudolph Mossbauer, during his graduate studies, discovered an outstanding effect [49] that has generated an entire field in physics, that is, a very high-resolution spectroscopy in the y-ray region of the spectrum named Mossbauer spectroscopy [50-56], The effect consists of the fact that a y photon emitted by an excited nucleus can be resonantly absorbed by another nucleus [50-56], This means that a recoilless emission and absorption has occurred. 

Application of Mdssbauer spectrometry depends on the availability of suitable sources with half-lives of excited states between about 10 and 10 s. The photon energy must not exceed lOOkeV and conversion must not be too high to ensure recoilless emission and absorption. As already mentioned, Fe, the daughter of Co, is the most frequently used Mdssbauer nuclide. Co is used as Mdssbauer source and iron of natural isotopic composition (2.17% Fe) or enriched Fe as absorber. 

The nucleus of an isotope suitable for Mdssbauer spectroscopy must have an excited state of moderately low energy (less than about 200 keV) to permit the occurrence of recoilless emission and absorption, and the excited state must be accessible, preferably by the spontaneous decay of a parent isotope with a... 

However, in contrast, the resonance effect increased by cooling both the source and the absorber. Mdssbauer not only observed this striking experimental effect that was not consistent with the prediction, but also presented an explanation that is based on zero-phonon processes associated with emission and absorption of y-rays in solids. Such events occur with a certain probability/, the recoil-free fraction of the nuclear transition (Sect. 2.4). Thus, the factor/is a measure of the recoilless nuclear absorption of y-radiation - the Mdssbauer effect. 

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

We are interested in the transmission of y-quanta through the absorber as a function of the Doppler velocity. The radiation is attenuated by resonant absorption, in as much as emission and absorption lines are overlapping, but also by mass absorption due to photo effect and Compton scattering. Therefore, the number Tt E2)AE of recoilless y-quanta with energies EXo E + AE traversing the absorber is given by... 

For a harmonic solid the probability of recoilless emission or absorption is given by exp where k is the wave vector of the gamma ray and... 

Since Mdssbauer s first publication (77) on recoilless emission or absorption of y-rays several extensive articles and books (12-24) have... 

Mossbauer spectroscopy therefore appears in principle to be suitable for following changes brought about by solvation. However, the practical application is considerably limited by the fact that the Mossbauer effect (recoilless y-ray resonance absorption) appears only in solid substances, where the Mossbauer nucleus is so firmly bound in the crystal lattice that recoilless y-ray emission and absorption are ensured. 

Recoilless y Emission and Absorption in Condensed Matter, the Mossbauer Effect... 

For free atoms, or atoms in molecules, these recoil effects are typically about five to six orders of magnitude larger than the natural line width, and so there is no possibility of a resonance phenomenon (but having said that, under very special conditions resonances for gases have in fact been achieved). Even in the liquid state, atom or molecule movements are generally too large. However, when both emitter and absorber atoms are bound in solid samples recoilless nuclear resonance absorption becomes observable. But even here the atomic movements due to lattice and molecular vibrations lead to broadening of emission and absorption lines. We must therefore expect temperature-dependent effects in Mossbauer spectroscopy. 

Abstract This chapter describes a general introduction of the Mossbauer spectroscopy. What is the Mossbauer effect and what is the characteristic feature of the Mossbauer spectroscopy These questions are answered briefly in this chapter. Mossbauer spectroscopy is based on recoilless emission and resonant absorption of gamma radiation by atomic nuclei. Since the electric and magnetic hyperfine interactions of Mossbauer probe atom in solids can be described from the Mossbauer spectra, the essence of experiments, the hyperfine interactions and the spectral line shape are discussed. In addition, the experiments and the new resonance technique with synchrotron radiation have been also briefly described. 

In 1956 and 1957, young physicist R. Mossbauer has performed the experiments concerning the scattering of the 129 keV y-ray of Ir by Ir and discovered an increase in scattering at low temperatures. Results obtained and his interpretations were published in 1958 [1-3], which is the beginning of the Mossbauer effects study and its development as the Mossbauer spectroscopy. The Nobel Prize for physics 1961 was awarded to him [4]. Mossbauer spectroscopy is the recoilless emission and the recoilless resonant absorption of the y-ray by the nucleus. After... 

Recoilless Emission and Recoilless Resonant Absorption of y Photons... 

For the absorption process, the same conservation law should be satisfied. For the transition Eq — 14.4 keV in Fe, / is 1.9 X 10 eV which is 10 times large compared to the natural line width of the excited state and no resonance between source and absorber for the free nucleus can be expected. When the nucleus is bounded into the solid, the recoil energy can be dispersed by the excitation of the solid. When the source and absorber are the nuclei embedded into the solid, recoil energy R may be used for the excitation of phonon that is the vibration state of sohd. Phonon is quantized as discrete value in solid and in usual metals the excitation energy of phonon states is the order of 10 -10 eV and there is rather large probability to have a zero phonon excitation in emission and absorption process in other word, the recoilless emission and recoilless resonant absorption of photon. This is the most important characteristic feature of the Mossbauer effect. As a consequence, the y photon emitted by the decay from the first excited state that has a... 

In previous section we treat the Mossbauer spectroscopy using a radioactive y-ray source, which has an extremely narrow line width of the order of 10 eV in the case of Fe resonance. Mossbauer effect is a recoilless emission and a recoilless resonant absorption of low energy y photons in a combination with source and absorber nuclei. 

The theory of this effect was worked out by R. Mbssbauer (Nobel Prize, 1961) and the effect itself is referred to as the Mossbauer effect. It consists of the recoilless emission and resonance absorption of y quanta by nuclei. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

The y-rays from the source (which must be specially prepared to yield a high fraction of recoilless emission) are absorbed by the sample and re-emitted approximately isotropically. Compared with ordinary electronic absorption of y-rays by an equivalent material, detector (A) in the figure measures a decrease in transmitted y-ray intensity when... 

Other fields benefited, too, from the strong neutron fluxes now accessible for irradiations. New phenomena were observed, such as the annealing of hot-atom effects in inorganic solids by postirradiation heating, leading to an increased retention in the form of the original species (Green and Maddock 1949 Rieder et al. 1950). A new and powerful tool was discovered by Rudolph Mossbauer in 1958 recoilless emission ofy rays with natural or nearly natural line width from nuclei embedded in a solid lattice (Mossbauer 1958, 1962). Resonant absorption of such emissions, e.g., the 14.4 keV transition of Fe fed in the decay of Co (T1/2 = 270 d), provides... 

In the simplest (thin-source, thin-absorber) approximation, a simple transmission Mossbauer spectrum (O Fig. 25.6) consists of one single Lorentzian peak the half width of which corresponds to twice of the natural line width F = h ln2/Ti/2 (in energy unit). (The natural line width F is characteristic of both the recoilless emission in the source and the resonant absorption in the absorber. So doubling is actually the result of adding up. ) The peak width Wq = 2cF/Ey (in speed unit) thus obtained is an asymptotic minimum to which (normally) actual experimental peak widths are compared. For the 3/2 —> 1/2 (see Fig. 25.7) transition of Fe, e.g., Wq 0.2 mm/s is the minimum (or natural) peak width. 

Mossbauer measurements are generally performed to analyze the samples containing iron. For this purpose, one uses a specially prepared Mossbauer source of 10-20 mCi (usually Co diffused into Rhodium or Palladium foil) which emits the 14.4 keV photon by excited Fe nuclei produced by the (3-decay of Co. Some of the 14.4 keV photons are emitted by nuclei that share the recoil momentum with macroscopic bits of the matter in which they are imbedded ( recoilless emission ) with the result that their fractional spread in energy, or line width AE/E is extraordinary narrow, of the order of 10 . A sample containing Fe-atoms in an environment that permits recoilless absorption of 14.4 keV photons, is placed between the source and the proportional counter. The sample of iron or compoimd of iron, which should be a thin foil (less than 20 pm), is placed at a distance of 5-10 cm from the Mossbauer source. 

The Mossbauer effect can only be detected in the solid state because the absorption and emission events must occur without energy losses due to recoil effects. The fraction of the absorption and emission events without exchange of recoil energy is called the recoilless fraction, f. It depends on temperature and on the energy of the lattice vibrations /is high for a rigid lattice, but low for surface atoms. 

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