Nuclear resonance absorption of y-rays

Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

In Sect. 4.2.1, we have studied that nuclear resonance absorption of y-rays does not occur between nuclei of isolated atoms or molecules (in gaseous or liquid state) because of the large energy loss of the transition energy Eq due to recoil effects. 

Figure 1. Schematic representation of nuclear resonance absorption of y-rays (MOssbauer effect) and nuclear resonance fluorescence...

This y-quantum may be absorbed by a nucleus of the same kind in its ground state, thereby taking up the energy difference q- This phenomenon is called nuclear resonant absorption of y-rays. and is schematically sketched in Figure I. 

Mossbauer spectroscopy, also called recoil-free nuclear resonance absorption, depends upon resonant absorption of y-rays emitted by a radioactive source by atomic nuclei.120 The phenomenon was initially difficult to observe, but the German physicist Mossbauer devised a way in which to record the absorption of a quantum of energy equal to the difference in two energy states of the atomic nucleus. The method depends upon a Doppler effect observed when the sample or source moves. Consequently, Mossbauer spectra, such as that in Fig. 16-18, are plots of absorp-... 

Mossbauer spectroscopy provides measurements of the resonant absorption of y-rays by nuclear transitions from a ground state to an excited state. Like other nuclear techniques, it is based on a phenomenon that is specific to a given isotope and for which no interference from other isotopes is possible. 

Mossbauer spectroscopy is based on the phenomenon of recoil-free resonant absorption of y rays by atomic nuclei, and the spectrum reflects the perturbation induced in the nuclear levels due to the interaction of the Mossbauer atom with its chemical environment. The Mossbauer elements, iron and tin, can be used conveniently as in situ probes in environmental and geochemical applications. Since Mossbauer spectroscopy has now become relatively familiar to chemists, I present here only a brief description of typical experimental techniques used in Mossbauer measurements. There are two types of Mossbauer measurements transmission method and scattering method. 

The basis of the Mossbauer effect is resonance absorption of y-rays (see fig. 2). It means that a y-ray emitted in the de-excitation of a nuclear excited state to its ground... 

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. 

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-... 

Mossbauer effect is the recoilless (also called recoil-free) nuclear resonance emission/ absorption of y rays (see O Pig. 25.1). In the case of a nuclear transition, the de-excited nucleus is normally recoiled by the momentum of the y photon emitted, which makes its resonance absorption impossible by another ground-state nucleus of the same type. In solids, however, recoilless photons can be emitted (and reabsorbed by another ground-state nucleus) with some probability. 

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. 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

The Mossbauer effect is the resonant absorption of low-energy y-rays by nuclei bound in solids in such a way that there is no energy loss due to nuclear recoil. It depends upon the monoenergetic nature of the y-ray emitted from an excited nucleus. When this ray falls on an unexcited nucleus of the same isotope, it will be absorbed if the nuclei are stationary relative to each other. However, if there is relative movement, there will be a Doppler shift in the frequency of the emitted y-ray so... 

The Mossbauer effect is the emission and resonant absorption of nuclear y-rays studied under conditions such that the nuclei have negligible recoil velocities when y-rays are emitted or absorbed. This is only achieved by working with solid samples in which the nuclei are held rigidly in a crystal lattice. The energy, and thus the frequency of the y-radiation involved, corresponds to the transition between the ground state and the short-lived excited state of the nuclide concerned. Table 2.4 lists properties of several nuclei which can be observed using Mdssbauer spectroscopy. 

It will hardly be necessary to recall that it was with the 129 4-keV y-transi-tion in Ir that R. L. Mbssbauer first demonstrated nuclear resonance absorption [64]. The source used was 16-day Os (see Fig. 16.22 for decay scheme) and the absorber was iridium metal. The transmission of the y-rays decreased unexpectedly as the temperature was lowered from 370 K to 90 K. Subsequently he initiated the use of velocity scanning [65, 66], and derived the excited-state half-life as Ij = 0 099 ns. 

Normally, in liquid state, the recoil of the nucleus at the y-ray emission hampers the nuclear resonance absorption. However, Mossbauer spectra of liquids can be observed even at room temperature if the molecules of liquids are trapped in the cavities of a special porous silicate glass of a mean pore diameter of 4 nm. This type of Mhssbauer spectroscopy is called capillary Mossbauer spectroscopy (Burger and Vertes 1983). 

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