Modes of Radioactive Decay

Radioactive decay is irreversible and spontaneous, releasing energy. The amount and type of energy depends on the parent element and mode of decay. All radioactive decay results in the release of heat. In fact radioactive decay of long-lived Th, and K is the primary source of heat 

All units used to quantify radioactive decay are defined in terms of number of decays per unit of time. The most fundamental expression of radioactivity is the iqmber of decays per second (1 decay 

Actinides and Their Daughter and Fissjon Products Chap. 13 

For a parent element decaying to produce a single daughter element, the abundance of the parent remaining can be described using first-order kinetics  

From the expression for radioactive decay, one can derive the half-life (r,/2) of a given radionuclide. The half-life is the time it takes for half of the atoms to decay, in other words, the time at which N/Ng = 0.5. Thus, the half-life equals 

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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. 

The pattern of nuclear stability can be used to predict the likely mode of radioactive decay neutron-rich nuclei tend to reduce their neutron count proton-rich nuclei tend to reduce their proton count. In general, only heavy nuclides emit a particles. 

Capture, Electron—A mode of radioactive decay involving the capture of an orbital electron by its nucleus. Capture from a particular electron shell, e.g., K or L shells, is designated as "K-electron capture" or "L-electron capture."... 

The first four modes of radioactive decay can be plotted on a single diagram (Fig. 10.3), which allows for a prediction of the nature of the daughter nucleus from a parent subject to any one of the above processes. 

Figure 10.3 Schematic diagram of the four common modes of radioactive decay.

Effects of different modes of radioactive decay on the position of an isotope on the Chart of the Nuclides. Beta-decay, which changes a neutron to a proton, moves the nuclide up and to the left. Positron decay or electron capture, which changes a proton into a neutron, moves the nuclide down and to the right. And -decay, which is the emission of a 4He nucleus, moves the nuclide down and to the left. 

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). 

Spontaneous fission (symbol sf) was found in 1940 by Flerov and Petrzhak at Dubna, after fission by neutrons had been discovered in 1938 by Hahn and Strassmann in Berlin. Spontaneous fission is another mode of radioactive decay, which is observed only for high mass numbers A. For the ratio of the probability of spontaneous fission to that of a decay is about 1 10 . It increases with the atomic number Z and the number of neutrons in the nucleus. For Fm the probability of spontaneous fission relative to the total probability of decay is already 92%. 

Where the number of both protons and neutrons in an atom is known we are able to identify a specific isotope of a specific element and this is termed a nuclide. Some naturally occurring elements are radioactive and specific isotopes of these elements are called radionuclides. This term implies that their nuclei are unstable and spontaneously decay, transforming the nucleus into that of a different element. Radioactive decay is written in equations that look a little like those for chemical reactions, but they need to express the atomic mass of the elements involved and the type of rotation emitted. A number of modes of radioactive decay are possible, and here we outline some of the common ones. The decay of potassium (40K) can be written ... 

K capture. A mode of radioactive decay in which an electron from the K shell is captured by the nucleus. 

Many of the nuclides in the actinide family—U, Np, Pu, etc.—fission spontaneously as one of the modes of radioactive decay. Usually, for a nuclide with multiple modes of radioactive decay, the half-life of the nuclide is determined from the total decay rate, representing all the decay processes for that nuclide. However, in the case of spontaneous fission, a separate half-life for that process alone is used. Examples of nuclides that undergo spontaneous fission are given in Table 2.5. 

If a graph is made (Fig. 3.1) of the relation of the number of neutrons to the number of protons in the known stable nuclei, we find that in the light elements stability is achieved when the number of neutrons and protons are approximately equal (N = Z). However, with increasing atomic number of the element (i.e. along the Z-line), the ratio of neutrons to protons, the NIZ ratio, for nuclear stability increases from unity to iqiproximately l.S at bismuth. Thus pairing of the nucleons is not a sufficient criterion for stability a certain ratio NIZ must also exist. However, even this does not suffice for stability, because at high Z-values, a new mode of radioactive decay, a-emission, appears. Above bismuth the nuclides are all unstable to radioactive decay by a-particle emission, while some are unstable also to / -decay. 

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. 

Knowledge Required (1) The general composition of atomic nuclei (numbers of neutrons and protons). (2) The modes of radioactive decay available to nuclei. (3) The meanings of the terms alpha particle and beta particle. 

In nuclear equations, reactant and product nuclei are represented by giving their mass numbers and atomic numbers, as well as their chemical symboL The totals of the mass numbers on both sides of the equation ate equah the totals of the atomic numbers on both sides are also equal There ate four common modes of radioactive decay alpha decay, which reduces the atomic number by 2 and the mass number by 4, beta emission, which increases the atomic number by 1 and leaves the mass number unchanged, positron emission and electron capture, both of which reduce the atomic number by 1 and leave the mass number unchanged. 

SECTION 21.2 The neutron-to-proton ratio is an important factor determining nuclear stability. By comparing a nuclide s neutron-to-proton ratio with those in the band of stability, we can predict the mode of radioactive decay. In general, neutron-rich nuclei tend to emit beta particles proton-rich nuclei tend to either emit positrons or im-dergo electron capture and heavy nuclei tend to emit alpha particles. The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus. A nuclide may undergo a series of decay steps before a stable nuclide forms. This series of steps is called a radioactive series or a nuciear disintegration series. 

There is considerable variety in the modes of decay of heavy elements if spontaneous fission is considered a mode of radioactive decay, then there are many chains that meet this criterion of long-to-short. However, this chapter does not include fission except as a mechanism of terminating a chain. 

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