Absorption of y radiation

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. 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

The recoilless nuclear resonance absorption of y-radiation (Mossbauer effect) has been verified for more than 40 elements, but only some 15 of them are suitable for practical applications [33, 34]. The limiting factors are the lifetime and the energy of the nuclear excited state involved in the Mossbauer transition. The lifetime determines the spectral line width, which should not exceed the hyperfine interaction energies to be observed. The transition energy of the y-quanta determines the recoil energy and thus the resonance effect [34]. 57Fe is by far the most suited and thus the most widely studied Mossbauer-active nuclide, and 57Fe Mossbauer spectroscopy has become a standard technique for the characterisation of SCO compounds of iron. 

Figure 5.1 Resonant absorption of y-radiation by a nucleus can only take place in the solid state because of recoil effects. The excited nucleus of a free atom emits a y-photon with an energy EirER, whereas the nucleus in the ground slate of a free atom can only absorb a photon if it has an energy equal to Eo+ER. As the linewidth of nuclear transitions is extremely narrow, the emission spectrum does not overlap with the absorption spectrum. In a solid, a considerable fraction of events occurs recoil free (ER=0), and here the emission spectrum overlaps completely with the absorption spectrum (provided source and absorber have the same chemical environment).

The Mbssbauer effect involves resonant absorption of y-radiation by nuclei in solid iron oxides. Transitions between the I = Y2 the I = 72 nuclear energy levels induce resonant absorption (Fig. 7.4). A Mbssbauer spectrum is a plot of the transmission of the rays versus the velocity of their source movement of the source ( Co for iron compounds) ensures that the nuclear environments of the absorber and the source will match at certain velocities (i.e. energies) and hence absorption takes place. In the absence of a magnetite field the Mbssbauer spectrum consists of one (if the absorbing atoms are at a site of cubic symmetry) or two (symmetry distorted from cubic) absorption maxima. When a static magnetic field acts on the resonant nuclei, this splits the nuclear spin of the ground state into two and those of the ex-... 

As mentioned earlier, the Mossbauer effect is observed by recording the absorption of y radiation versus its energy. Energy modulation is produced by the Doppler effect, created by an accurately known relative velocity between the source and absorber, which may be produced purely mechanically or electromechanically. 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

Recoilless nuclear resonance absorption of y-radiation (used by MoBbauer in his Nobel lecture 1961)... 

In the first place, the absorption of y radiation depends on the density of the absorber, similarly to the absorption of a or radiation. In Table 6.3 the mass absorption coefficients for various absorbers and various energies of y radiation are listed. 

The contributions of these partial absorption coefficients to the absorption of y radiation in lead are plotted in Fig. 6.15 as a function of the energy of the y radiation. The strong increase of the contribution of pair formation at higher energies leads to the bending of the curve for the total absorption coefficient in Fig. 6.15 and of the curve in Fig. 6.13. 

Absorption of y radiation for the purpose of radiation protection requires lead walls or thick walls of concrete, as already mentioned in section 6.1. 

The crystal itself is usually a circular cyHiider machined from a single crystal of sodium iodide that was carefully grown to ensure optical clarity for efficient light collection. A small amount of thallium is added to the mother liquor to improve performance. The high atomic number of iodine and the density of sodium iodide (3.7g/cm ) favor the absorption of y-radiation. For this reason, a well counter is often referred to as a y-counter. For a typical well detector, the counting efficiency for expressed as the percentage of decays that produces counts, approximates 70%. 

The situation is very similar in the case of the atomic absorption of Y radiation. While the energy difference between the two nuclear levels is, say, E, the energy of the Y 9uantum must be larger, because part of the photon energy goes to nuclear recoil Eg = E + R > as seen readily in the scheme. 

Very similar information can be obtained with the experimentally less demanding MSssbauer spectroscopy. The resonant absorption of y radiation by certain atomic nuclei ( Fe and Sn several other isotopes of technical interest, e.g., Co, can also be studied in the emission mode) yields information about the close enviromnent of this atom. By comparison with standard samples, the chemical environment of a given atom, the specific type of chemical compound in which it is incorporated, and further morphological information can be derived from the Mosshauer spectra [142]. 

Nuclear y-ray resonance spectroscopy. This technique is based on the resonance absorption of y radiation and is more conventionally known as Mossbauer spectroscopy. The source of the radiation is a nuclide fixed in a solid crystal lattice held below the Debye temperature. In this condition, y radiation of energies less than 150 keV are emitted with no loss of energy. Such quantized y photons can undergo resonance absorption by the appropriate identical stable nuclide in a solid sample matrix. If the chemical environment of the absorbing nuclide is different from the emitter, energy must be added or subtracted from the radiation to establish resonance. This can be achieved by introducing net motion to the source or absorber to establish a Doppler motion energy term. 

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