Nuclear excitation

AIterna.tives to y-Ray Emission. y-Ray emission results ia the deexcitation of an excited nuclear state to a lower state ia the same nucHde, ie, no change ia Z or. There are two other processes by which this transition can take place without the emission of a y-ray of this energy. These are internal conversion and internal pair formation. The internal-conversion process iavolves the transfer of the energy to an atomic electron. 

Internal Conversion. As an alternative to the emission of a y-ray, the available energy of the excited nuclear state can be transferred to an atomic electron and this electron can then be ejected from the atom. The kinetic energy of this electron is where E is the energy by which the... 

The emission of y rays follows, in the majority of cases, what is known as P decay. In the P-decay process, a radionuclide undergoes transmutation and ejects an electron from inside the nucleus (i.e., not an orbital electron). For the purpose of simplicity, positron and electron capture modes are neglected. The resulting transmutated nucleus ends up in an excited nuclear state, which prompdy relaxes by giving offy rays. This is illustrated in Figure 2. 

The ratio F/Eq of width F and the mean energy of the transition Eo defines the precision necessary in nuclear y-absorption for tuning emission and absorption into resonance. Lifetimes of excited nuclear states suitable for Mossbauer spectroscopy range from 10 s to s. Lifetimes longer than 10 s produce too... 

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). 

In the last column of Table 7.1, the most popular radioactive precursor nuclide is given together with the nuclear decay process (EC = electron capture, = beta decay) feeding the Mossbauer excited nuclear level. 

The effect of a positive quadrupole interaction on the ground and excited nuclear states of Ru is shown schematically in Fig. 7.33 as adapted from a publication by... 

With the two excited nuclear states Ei and 2 given in terms of the quadmpole... 

Nuclide—A species of atom characterized by the constitution of its nucleus. The nuclear constitution is specified by the number of protons (Z), number of neutrons (N), and energy content or, alternatively, by the atomic number (Z), mass number A (N+Z), and atomic mass. To be regarded as a distinct nuclide, the atom must be capable of existing for a measurable time. Thus, nuclear isomers are separate nuclides, whereas promptly decaying excited nuclear states and unstable intermediates in nuclear reactions are not so considered. 

The emission of electromagnetic y-radiation is the natural accompaniment of most nuclear processes and provides the route for an excited nuclear isomer to decay to its ground state. A typical decay sequence is set out in Figure 10.4. 

X. Because there is damping of nuclear motion into e-h pairs, excited e-h pairs at electron temperature Te must also be able to excite nuclear coordinates by fluctuating forces Fx that satisfy the second fluctuation dissipation theorem given as... 

If the spin state interconversion is faster than the excited nuclear state lifetime, that is x 10 7 second, then the observed spectrum is an average of the spectra of the two spin states. Until recently this condition had been observed only for iron(III) complexes with thiocar-bamate or selenocarbamate ligands—ferric dithiocarbamates (119), monothiocarbamates (98), or diselenocarbamates (42). Since 1982, however, there have been a number of reports of other iron(III) complexes which also display an averaged Mossbauer spectrum (56, 57, 108 111, 124, 153, 155). 

The two techniques which have been used effectively to set limits on the rates of spin state interconversions are Mossbauer and EPR spectroscopies. As described in Section III,E, the lifetime of the excited nuclear state involved in the Mossbauer effect is 10"7 second. Thus the observation of the Mossbauer spectrum can immediately classify the spin state lifetime as greater than or less than 10"7 second. Both conditions have been observed, as was described in Section III,E. 

METASTABLE NUCLEI. Nuclei in excited nuclear slates that have measurable lifetimes (exceeding 10 "-III 1 second). 

The Coulomb interactions leading to the isomer shift result in energy level displacements that differ for the ground and the excited nuclear states. Therefore, for a given source, absorbers with different electronic structures (e.g., different valences) will be characterized by different isomer shifts, that is, different shifts of the resonance peaks with respect to zero relative velocity. 

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