Nuclear internal conversion

The decay of radioisotopes iavolves both the decay modes of the nucleus and the associated radiations that are emitted from the nucleus. In addition, the resulting excitation of the atomic electrons, the deexcitation of the atom, and the radiations associated with these processes all play a role. Some of the atomic processes, such as the emission of K x-rays, are inherently independent of the nuclear processes that cause them. There are others, such as internal conversion, where the nuclear and atomic processes are closely related. 

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... 

Resonant y-ray absorption is directly connected with nuclear resonance fluorescence. This is the re-emission of a (second) y-ray from the excited state of the absorber nucleus after resonance absorption. The transition back to the ground state occurs with the same mean lifetime t by the emission of a y-ray in an arbitrary direction, or by energy transfer from the nucleus to the K-shell via internal conversion and the ejection of conversion electrons (see footnote 1). Nuclear resonance fluorescence was the basis for the experiments that finally led to R. L. Mossbauer s discovery of nuclear y-resonance in ir ([1-3] in Chap. 1) and is the basis of Mossbauer experiments with synchrotron radiation which can be used instead of y-radiation from classical sources (see Chap. 9). 

Not all nuclear transitions of this kind produce a detectable y-ray for a certain portion, the energy is dissipated by internal conversion to an electron of the K-shell which is ejected as a so-called conversion electron. For some Mossbauer isotopes, the total internal conversion coefficient ax is rather high, as for the 14.4 keV transition of Fe (ax = 8.17). ax is defined as the ratio of the number of conversion electrons to the number of y-photons. 

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). 

All these conflicts can now be resolved because of what appears to be a deflnitive experiment by Bocquet et al. (4), who directly measured the internal conversion coefficients of the transition from the first nuclear level to the ground state. They directly compared the L, M, N, and O conversion electron intensities in two different states—namely, in stannic oxide and white tin. They found that the 5s electron density is 30% smaller in stannic oxide than in white tin, and since the isomer shift of stannic oxide is negative with respect to white tin, AR is clearly positive. From these data, the Brookhaven group has calculated the value for AR/R for tin-119 as +3.3 X 10". ... 

CONVERSION RATIO. I. The ratio of the number of internal conversion elections to the number of gamma rays emitted in a given time interval hy a single nuclidic species during the de-excitation of one of ns excited energy states. Sometimes known as the imemal-coitcersion inefficient. 2. In a nuclear reactor, the number of fissionable atoms produced per fissionable atom destroyed. See also Nuclear Power Technology. 

Nuclear electromagnetic decay occurs in two ways, y decay and internal conversion (IC). In y-ray decay a nucleus in an excited state decays by the emission of a photon. In internal conversion the same excited nucleus transfers its energy radia-tionlessly to an orbital electron that is ejected from the atom. In both types of decay, only the excitation energy of the nucleus is reduced with no change in the number of any of the nucleons. 

Internal conversion (IC) is a competing process to 7-ray decay and occurs when an excited nucleus interacts electromagnetically with an orbital electron and ejects it. This transfer of the nuclear excitation energy to the electron occurs radiationlessly (without the emission of a photon). The energy of the internal conversion electron, Eic, is given by... 

Figure 9.5 Kinetic energy spectrum of internal conversion electrons for a 412-keV nuclear transition in 198Hg. Superimposed on this spectrum is the accompanying spectrum of (i particles from the [1 decay that feeds the excited state. The peaks labeled K, L, and M represent conversion of electrons with principal quantum numbers of 1, 2, or 3, respectively. (From Marmier and Sheldon, 1969, p. 332.) Copyright Academic Press. Reprinted by permission of Elsevier.

The vibronic coupling operates upon the nuclear and electronic functions of the electronic states involved in the internal conversion (Equation 6.74). Mk is the effective mass associated with the /cth vibrational mode. Because of the different spin labels of the states involved in the intersystem crossing, the spin-orbit coupling gives a finite value to the electronic coupling in Equation 6.74. 

As an example of application of the method we have considered the case of the acrolein molecule in aqueous solution. We have shown how ASEP/MD permits a unified treatment of the absorption, fluorescence, phosphorescence, internal conversion and intersystem crossing processes. Although, in principle, electrostatic, polarization, dispersion and exchange components of the solute-solvent interaction energy are taken into account, only the firsts two terms are included into the molecular Hamiltonian and, hence, affect the solute wavefunction. Dispersion and exchange components are represented through a Lennard-Jones potential that depends only on the nuclear coordinates. The inclusion of the effect of these components on the solute wavefunction is important in order to understand the solvent effect on the red shift of the bands of absorption spectra of non-polar molecules or the disappearance of... 

---