Radioactive decay conversion electron

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)...

This same relationship between shift and coordination number is found with iodine compounds (38, 40), Tetrahedrally coordinated in KIO4 has a higher value of octahedrally coordinated F ion in IOe . If we extend this concept to Sn, the Mossbauer data 7,11) indicate that AR/R for Sn is positive and not negative as Gordanskii has recently proposed 4, 27). This conclusion agrees with that of Bocquet et al. (5), who showed by means of electron conversion in the radioactive decay of that il/ o) was larger in white tin than... 

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

The electron is assigned an atomic number of -1 to account for the conversion during radioactive decay of a neutron to a proton and an emitted electron called a beta particle ... 

Recently a new technique has come into use which avoids radioactivity as well as after-effects conversion electron spectroscopy This method uses the fact that in most Mossbauer transitions not only 7-rays but also conversion electrons are emitted. In the case of Co the electron conversion is the major decay mode. Thus, instead of measuring the 7-rays, absorbed or reflected from an absorber, one measures the emission of electrons from the absorber as a function of the velocity... 

Radioactivity is characterized by the emission of energy (electromagnetic or in the form of a particle) from the nucleus of an atom, usually with associated elemental conversion. There are four basic types of radioactive decay (Table 5.4), of which alpha (a) and beta (p ) decay are most common in nature. Alpha emission is the only type of decay that causes a net mass change in the parent nuclide by loss of two protons plus two neutrons. Because two essentially weightless orbiting electrons are also lost when the equivalent of a helium nucleus is emitted, the parent nuclide transmutes into a daughter element two positions to the left on the periodic table. Thus decays by ot... 

The two other decay processes in Table 5.4 are less common in nature. In K-capture, any orbiting electron (usually in an inner shell) combines with a proton in the nucleus to form a neutron. This relatively rare nuclear transformation process (e + p+ —> n°) is just the opposite of that for P decay, meaning that the formed nucleus also has the same mass but is displaced one element to the left on the periodic table. Conversion of to °Ar by K-capture is an example of the chemical conversion that can attend radioactive decay, in this case leading to transformation of a non-volatile alkali metal into the inert gas Ar, the third most abundant gas in the atmosphere. Although no nuclear particle is emitted by K-capture, the attending cascade of electrons into lower orbitals leads to X-ray emission of characteristic energy that can be measured by the appropriate detectors. The last decay process (also rare) involves emission of a positron (p+), a positively charged electron. The nuclear process (p+ n° + p+) has the same net effect as K-capture and is also characterized by X-ray emission. 

If the NIZ ratio is too low for stability, then radioactive decay occurs in such a manner as to lower Z and increase N by conversion of a proton to neutron. This may be accomplished through positron emission, i.e. creation and emission of a positron (/3 or + je), or by absorption by the nucleus of an orbital electron electron capture, EC). Examples of these reactions are ... 

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. 

Gamma rays can interact with the orbital electrons of other atoms, so that the latter are expelled from that atom with a certain kinetic energy (see Ch. 6). A differ t process, called internal conversion, can occur within the atom undergoing radioactive decay. Because the wave function of an orbital electron may overlap that of the excited nucleus, the excitation energy of the nucleus can be transferred directly to the orbital electron (without the involvement of a 7-ray), which escapes from the atom with a certain kinetic energy E. No 7-ray is emitted in an internal conversion process it is an alternate mode to 7-ray emission of de-excitation of nuclei. 

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. 

The most widely used radioactive tag in RIA is iodine 125. Iodine 125 decays by electron capture. It emits a single gamma ray having an energy of 35.48 keV. Four tellurium K x-rays with energies between 27.5 and 31.8 keV are also emitted. In addition there are L and M x-rays with energies of about 4 and 0.5 keV, respectively, as well as a variety of conversion and Auger electrons. Measurement of these relatively weak photons by... 

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... 

Unlike the forms of radioactive decay that we have discussed so far, electron capture involves a particle being absorbed by instead of emitted from an unstable nucleus. Electron capture occurs when a nucleus assimilates an electron from an inner orbital of its electron cloud. Like positron emission, the net effect of electron capture is the conversion of a proton into a neutron. 

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. 

A beta (/5 ) particle is an electron emitted as a result of the conversion of a neutron to a proton in certain atomic nuclei imdergoing radioactive decay. 

The preceding equations hold for decay schemes that involve the simple transformation of one isotope to another. Three types of decay schemes useful for geochronology are special cases that yield somewhat more complicated age equations. Branched decay leads to the production of two different daughter isotopes from a single radioactive isotope. A familiar example is the spontaneous conversion of to both " °Ar (through electron capture) and " °Ca (through /3 decay). In such cases, the age equations must be modified by factors that correct for the fact that not all decays result in the production of the daughter isotope of interest. For the — " Ar... 

The appropriate excited state of the resonant nucleus can be populated from (a) decay of a radioactive precusor, (b) nuclear reaction or (c) by excitation. Method (a) is most frequently employed because of its convenience. A typical example is shown in Fig. 2 for Co(57), which leads to the 14.41 keV first excited level of Fe(57) by electron capture. The internal conversion coefficient, a, for the 14.41 keV y-ray is 9.0. Therefore, only -10% of the nuclear decays originating from B produce the required 14.41 keV photon. 

Nuclear excitation and nuclear resonant scattering with synchrotron radiation have opened new fields in Mossbauer spectroscopy and have quite different aspects with the spectroscopy using a radioactive source. For example, as shown in Fig. 1.10, when the high brilliant radiation pulse passed through the resonant material and excite collectively the assemblies of the resonance nuclei in time shorter than the lifetime of the nuclear excited state, the nuclear excitons are formed and their coherent radiation decay occurs within much shorter period compared with an usual spontaneous emission with natural lifetime. This is called as speed-up of the nuclear de-excitation. The other de-excitations of the nuclei through the incoherent channels like electron emission by internal conversion process are suppressed. Synchrotron radiation is linearly polarized and the excitation and the de-excitation of the nuclear levels obey to the selection rule of magnetic dipole (Ml) transition for the Fe resonance. As shown in Fig. 1.10, the coherent de-excitation of nuclear levels creates a quantum beat Q given by... 

Radionuclides can be roughly divided into a-emitters and P-emitters. The emission of a-particles is associated with a decrease in mass number by four units and in the atomic number by two units. The loss oftwo protons results in an excess oftwo electrons, so the a-decay is accompanied by the emission of electrons (radiation P). In the radioactive P-type conversion, the nucleon in the atom nucleus transforms, a neutron turns into a proton or vice versa, which is associated with the change of particle charge and emitting an electron or positron. The capture of an electron (usually from the sphere K of the electron cloud) by the atom nucleus is also possible, while changing a proton into a neutron. In the electron cloud, the electron from the higher level then jumps to hll the vacant place, which is accompanied by the emission of a photon. ... 

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