Decay modes

Isotope Atomic mass Half-hfe, Decay mode... 

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. 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Fig. 4. Decay scheme ofas an example of /5 -decay, showing the spins and parities of the levels populated in the daughter nucleus and the energies in keV of these levels, where (" ) represents the principal decay mode, (—fc.) an alternative mode, and (- - ) is a highly improbable transition.

Exotic Decays. In addition to the common modes of nuclear decay, two exotic modes have been observed. These decay modes are of theoretical interest because theh long half-Hves place strict constraints on the details of any theory used to calculate them. 

PP2- However, there is an alternative theoretical mechanism by which the two Ps could be emitted without any neutriao, denoted PPq- The experimental methods that are used to look for the double P decay mode are often more sensitive to one of these decay modes than the other. The difference ia the expected energy distribution of the electrons is clear from the fact that ia the first case the total decay energy is divided between four particles, including the two antineutfinos that caimot be observed ia the second, it is only divided between the two electrons. As more exotic modes of decay are measured and even larger limits are placed on some of the half-fives, the constraints on theory become even stronger. 

Decay mode Energy, keV Average IB energy, keV Energy bin, keV Average energy in bin, keV Photon intensity, %... 

The j3 -particles that are emitted in the j3 -decay mode are slowed down in the material around the source. When these reach very low velocities they interact with an ordinary electron and the pair is annihilated. The corresponding energy of 2 x E, or 1022 keV, is normally released in the form of two photons of 511 keV each, emitted in opposite directions. 

Information on isochronal annealing of Mo(CO)g has been given recently by Groening and Harbottle The most interesting result in this work was the clearly stepwise nature of the annealing, as is shown in Fig. 6. Curiously, not only the retention values but also the number and positions of the steps show isotropic differences. No clear explanation was offered other than the suggestion that the effect must arise from differences in the decay modes of the two excited nuclides. 

Table 2. Half-lives for the U- and Th- decay series nuclides, with decay modes. 

CAS Decay mode/% Decay mode Energy Intensity Half-life Specific activity0 Gamma... 

Electron capture (EC). In this type of decay, an electron from outside the nucleus is captured by the nucleus. Such a decay mode occurs when there is a greater number of protons than neutrons in the nucleus. 

Electron capture accomplishes the same end result as positron emission, but because the nuclear charge is low, positron emission is the expected decay mode in this case. Generally, electron capture is not a competing process unless Z 30 or so. 

FIGURE 1.14 Predicting decay mode from the relative number of protons and neutrons. 

Predict the decay mode for the following and write the reaction for the predicted decay mode (a) 3516S (b) 179F (c) 4320Ca. 

Isotope PET/SPECT Decay mode (%) Half-life (min) (max/most abundant) (half-value thickness) reactor (R), generator (G)... 

Selected radionuclides symbol, mass number, atomic number, half-life, and decay mode... 

The principal uranium-238 decay series, indicating major decay mode and physical half-time of persistence... 

Table 32.1 Selected Radionuclides Symbol, Mass Number, Atomic Number, Half-Life, and Decay Mode...

Nuclide Symbol Mass Number Atomic Number Half-life3 Major Decay Mode"... 

Figure 32.2 The principal uranium-238 decay series, indicating major decay mode and physical halftime of persistence. (Modified from Cecil, L.D. and T.F. Gesell. 1992. Sampling and analysis for radon-222 dissolved in ground water and surface water. Environ. Monitor. Assess. 20 55-66.)...

G. W. C. Kaye and T. H. Laby, Tables of Physical and Chemical Constants, Longman 1995, gives a table of properties of the nuclides including isotopic abundance or half-life, decay modes, mass excess, neutron capture cross-section and ground-state spin and parity. This publication, with a prospect of regular updates, is available on the website http //www.kayelaby.npl.co.uk/. 

A core-ionized atom has two possibilities to lower its energy, namely Auger decay and X-ray fluorescence (described in more detail in Chapter 7). The Auger yields for processes following core hole creation in the K and L shell are sketched in Fig. 3.25 (right). Obviously, Auger processes are the dominant decay mode in light elements. 

Alpha, beta, and gamma emission are the most common types of natural decay modes, but we do occasionally observe positron emission and electron capture. 

The four decay modes described above all involve emission or giving off a particle, but electron capture is the capturing of an electron from the energy level closest to the nucleus (Is) by a proton in the nucleus. This creates a neutron ... 

Sometimes it is difficult to predict if a particular isotope is stable and, if unstable, what type of decay mode it might undergo. All isotopes that contain 84 or more protons are unstable. These unstable isotopes will undergo nuclear decay. For these large massive isotopes, we observe alpha decay most commonly. Alpha decay gets rid of four units of mass and two units of charge, thus helping to relieve the repulsive stress found in the nucleus of these isotopes. For other isotopes of atomic number less than 83, we can best predict stability by the use of the neutron to proton (n/p) ratio. 

A plot of the neutrons (n) versus the protons (p) for the known stable isotopes gives the nuclear belt of stability. (See your textbook for a figure of the belt of stability.) At the low end of this belt of stability (Z < 20), the n/p ratio is 1. At the high end (Z 80), the n/p ratio is about 1.5. We can then use the n/p ratio of the isotope to predict if it will be stable. If it is unstable, then the isotope will utilize a decay mode that will bring it back onto the belt of stability. 

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