Conversion electrons emission

Transition, Isomeric—The process by which a nuclide decays to an isomeric nuclide (i.e., one of the same mass number and atomic number) of lower quantum energy. Isomeric transitions (often abbreviated I.T.) proceed by gamma ray and/or internal conversion electron emission. 

X and Xy are the partial decay constants (probabilities) of conversion (electron emission) and y-ray emission. The conversion coefficient a is the sum of the partial conversion coefficients of the K shell, L shell,... 

Materials based on boron compounds have been explored for many decades because of their exceptional properties in respect to chemical bonding, crystal structure, and phonon and electron conduction. Especially in the field of energy conversion, electron emission, and neutron absorption, borides occupy many niches of application for which no other material can be employed. Until approximately 1980, the main interest in borides always came, however, from basic research aimed at the understanding of their electronic structure, being either responsible for the unique transport properties or the peculiarities in chemical bonding. It is, therefore, no wonder that the most information about borides was at that time created from the viewpoint of physicists and chemists. 

As an alternative to y-ray emission, an excited nuclear state can also lose its energy by internal conversion electron emission. In this parallel process, the energy of the excited state is transferred directly to an electron of the atomic shell, without previous y-ray emission. The energy of internal conversion electrons depends on the nuclear transition energy and also on the ionization energy of the K, L, M,... electrons. 

If the energy of the transition is greater than 1.022 MeV, the nucleus may lose its excitation energy also by internal electron-positron pair emission. The probability of this mechanism is low it is usually more than 10 times smaller than the probability of y-ray emission. In contrast with the internal conversion electron emission, the internal pair formation coefficient increases with increasing y-ray energy and decreases with increasing atomic number and multipolarity of transition. A review of internal pair formation was given by Wilson (1965). 

It may also occur that, instead of a gamma photon, a shell electron is emitted. This alternative process for nuclear de-excitation is called internal conversion. Electron emission is then followed by the rearrangement of the electronic shell structure, which, in turn, is accompanied by X-ray emission, characteristic of the element concerned, or by the emission of another electron via Auger process. 

The internal conversion coefficient (ICC) of a transition between nuclear levels is defined as the ratio of conversion electron emission intensity to y-ray emission intensity. For the innermost shell, the K shell, this is ... 

The °Co square would be blue for (3 and contains the half-life, the major maximum beta energies in MeV (useful for bremsstrahlung estimation), major gamma energy (in keV) in order of emission probability and the thermal neutron cross-section. The isomer is shown as a white section with decay mode(s) and energies. On this particular chart, electron capture is shown as 8, isomeric transition as I and conversion electron emission as e , along with standard symbols. 

In the heaviest nuclei transitions between excited states are dominated by internal conversion electron emission over gamma emission. It is important to realize that the emission of conversion electrons is a direct process, and does not proceed via an intermediate gamma ray. This is mainly due to the increased probability of finding an atomic electron (its wave function) inside the nucleus where energy can be transferred to it directly. We define the internal conversion coefficient a as the ratio of the number of electrons that get emitted to the number of gamma rays emitted during the decay of a sufficiently large ensemble ... 

When an excited state is converted by ejection of an atomic electron, a high positive charge can be produced through subsequent Auger electron emission. Within the period of molecular vibration this charge is spread throughout the molecule to all atoms, and a Coulomb explosion results. This primary phenomenon occurs, of course, not only as a result of [ decay, but must be taken into account in all cases of nuclear reaction when deexcitation by inner electron conversion occurs... 

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)...

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). 

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). 

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... 

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)]. 

The three unimolecular physical processes that originate from SI are internal conversion to emission of lights and transformation of Sx into Tx. If IC occurs, the net change resulting from electronic excitation is heat transference to the solvent. The other two processes are considerably more interesting. 

Chemisorbed atoms or ions alter some properties of the adsorbents rather seriously. It is especially the work function for electron emission (and, therefore, also the electron affinity) which is either decreased or increased. Conversely this change affects again the adsorption energy. 

Henglein, A. Weller, H. Colloidal semiconductors Size quantization, sandwich structures, photo-electron emission, and related chemical effects, Photochemical Energy Conversion, J. R. Norris, Jr. and D. Meisel, eds., Elsevier New York, 1989. 

---