Radioactive decay transition energy

Internal transition A mode of radioactive decay, where an excited nucleus transfers energy to an electron and expels the electron from the atom. Internal transition is responsible for transforming certain arsenic isomers from higher to lower energy states (Table 2.1). 

One less-well-known technique, which has many experimental aspects in common with Mossbauer spectroscopy, deserves special attention at this point, since it gives valuable information about the electric-field gradients and the magnetic hyperfine interactions of radioactive nuclei in solids at ambient conditions and under pressure. In this technique, two y-rays with different energies from two different transitions of an individual nucleus in a radioactive-decay cascade are recorded consecutively. The spatial and temporal perturbation of the emission probability by the hyperfine fields is registered in the corresponding perturbed angular correlation (PAC) spectra. 

Radioactive decay involves a transition from a definite quantum state of the original nuclide to a definite quantum state of the product nuclide. The energy difference between the two quantum levels involved in the transition corresponds to the decay energy. This decay energy appears in the form of electromagnetic radiation and as the kinetic energy of the products, see Element and Nuclide Index for decay energies. 

The mode of radioactive decay is dependent upon the particular nuclide involved. We have seen in Ch. 1 that radioactive decay can be characterized by a-, jS-, and y-radiation. Alpha-decay is the emission of helium nuclei. Beta-decay is the creation and emission of either electrons or positrons, or the process of electron capture. Gamma-decay is the emission of electromagnetic radiation where the transition occurs between energy levels of the same nucleus. An additional mode of radioactive decay is that of internal conversion in which a nucleus loses its energy by interaction of the nuclear field with that of the orbital electrons, causing ionization of an electron instead of y-ray emission. A mode of radioactive decay which is observed only in the heaviest nuclei is that of spontaneous fission in which the nucleus dissociates spontaneously into two roughly equal parts. This fission is accompanied by the emission of electromagnetic radiation and of neutrons. In the last decade also some unusual decay modes have been observed for nuclides very far from the stability line, namely neutron emission and proton emission. A few very rare decay modes like C-emission have also been observed. 

Once an electron is ejected from an atomic orbital due to internal conversion, electron capture, or some other process involved in radioactive decay, a vacancy is created in the electron shell which can be filled in several ways. Electrons from higher energy orbitals can occupy the vacancy. The difference in the binding raergy of the two shells involved in the transition will be emitted from the atom as X-rays. This process is called fluorescent radiation. 

As a common consequence of any interaction of nuclear radiation with matter, electron vacancies are created in the K, L, M shells of the atoms. Radioactive decay can also create vacancies in the daughter atoms (electron capture, internal conversion). Electron vacancies can cause X-ray transitions or - as shown by Auger (1925) - the vacancy is filled at the expense of a shell electron emission with the energy... 

The sample matrix plus standards for the elements of interest are irradiated for a select period of time in the neutron flux of a research reactor. After irradiation and appropriate radioactive decay, the y energy spectrum is measured by counting the sample with a high resolution (to separate various y-transitions of close-by energies) y-detection system. The NAA technique provides highly resolved analysis of elemental composition by the identification of characteristic y-ray energies associated with different isotopes. Quantitative analysis is provided by element-to-element comparison of the number of y-rays emitted per unit time by the unknown sample to the number of y-rays emitted per unit time by the calibration standards. 

A typical Mossbauer experiment thus involves an oscillating radioactive source that contains a parent isotope (e.g., "Co for Fe), a stationary absorber that is usually the sample, and a detector. The Mossbauer spectrum consists of a plot of y-ray counts (relative absorption) as a function of the velocity of the source. In the source the radioactive isotope feeds the excited state of the Mossbauer isotope, which decays to the ground state. The energy of the recoil-free emitted radiation is Doppler modulated. Resonant absorption occurs when the energy of the y-ray just matches the nuclear transition energy for a Mossbauer atom in the absorber. This is detected by the decreased... 

The most common type of source for Fe Mossbauer spectroscopy consists of elemental Co incorporated into a host metal lattice such as rhodium or copper. In the case of Sn measurements, " Sn-enriched CaSnOa or BaSnOa is used as a source. Schematic diagrams of the radioactive decay schemes for these two isotopes are shown in Figure 5. In addition to these transitions, internal conversion processes may give rise to emission of radiation of other energies. For example, in the case of Fe, the / = state may decay via the ejection of a X-shell 7.3-keV electron, and the hole created be filled by an L-shell electron, leading to the emission of either a 6.4-keV electron (Auger process) or X-ray in order to conserve energy. 

Isomers—Nuclides having the same number of neutrons and protons but capable of existing, for a measurable time, in different quantum states with different energies and radioactive properties. Commonly the isomer of higher energy decays to one with lower energy by the process of isomeric transition. 

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