Decay of radioisotopes

Properties of Particles. From the research of the early part of the twentieth century, the existence of several types of particles was firmly estabhshed, and the properties were deterrnined. The particles that are involved in the decay of radioisotopes are given in Table 4. An additional type of conservation is that in all atomic and nuclear decays, the number of nucleons, ie, protons and neutrons, is conserved and the number of leptons, ie, electrons and neutrinos, is also conserved. 

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. 

Radioisotopes are unstable and decay by particle emission, electron capture, or y-ray emission. The decay is a random process, i.e., one cannot predict which atom from a group of atoms will decay at a specific time. The decay of radioisotopes, therefore, is described in terms of the average number of radioisotopes disintegrating during a period of time. The disintegration rate (or the number of disintegrations per unit time), -dN/dt, of a radioisotope at any time is proportional to the total number of undecomposed radioisotopes present at that time. This may be expressed as follows ... 

There are no major commercial uses for curium because of the extremely small amount produced. In the future, the most important use of curium may be to provide the power for small, compact thermoelectric sources of electricity, by generating heat through the nuclear decay of radioisotope curium-241. These small, efficient power sources can be used in individual homes or remote regions to provide electricity to areas that cannot secure it from other sources. It could also be used as a source of electricity in spacecraft. However, today curium s main use is for basic scientific laboratory research. 

The decay of radioisotope 238U to stable 206Pb in a closed system is described by the following equation ... 

Alpha decay leads to a decrease of the atomic number by two units, Z — Z — 2, and causes an expansion of the electron shell, as illustrated in Fig. 9.5 for the a decay of radioisotopes of Bi. EHffercnccs in the binding energies arc marked for electrons in the inner shells. Furthermore, there arc two surplus electrons after a decay. However, in the case of a decay the excitation effects due to the change of the atomic number are relatively small compared with the recoil effects that have been discussed in the previous section, with the result that the recoil effects dominate. 

Primary - internal Natural decay of radioisotope recoil energy of radioactive decay... 

To date objects that are more than 60 000 years old, techniques involving other isotopes must be used. Rocks and minerals that are up to billions of years old can be dated on the basis of the decay of radioisotopes with long half-lives, such as potassium-40 tm = 1.25 billion years), uranium-238 (ti/2 = 4.5 billion years), and rubidium-87 = 48 billion years). Such techniques are illustrated in Figure 21.9. 

The decay of radioisotopes found in nature often results in the formation of products called daughter nuclei, which may or may not be radioactive. If the resulting products are nonradioactive (stable), the decay stops. However, the products may be radioactive, and decay will continue to... 

FIGURE 5.3.8.1 First-Order Decay of Radioisotope P Through 10 Half-Lives. After 10 half-lives, only 0.1% of the starting amount of isotope remains. 

For irradiation, the ionizing radiation used are gamma rays, generated from the decay of radioisotopes cobalt 60 or cesium 137, X rays, and electrons, the latter two generated by machines for such purposes. The operators of equipment involving ionizing radiation need to be protected from its effects. The ability subsequently to pasteurize or irradiate food should not compensate for best practices to minimize contamination of food before treatment. Moreover, treated food also needs to be protected from subsequent contamination. 

We win learn that radioisotope decays are first-order kinetic processes that exhibit characteristic half-lives. The decays of radioisotopes can be used to determine the ages of andent artifacts and geological formations. 

This equation is for the first order reaction in chemical kinetics. It is also the equation for the decay of radioisotopes in physics. In viscoelasticity... 

Radioactivity is equal to the rate of decay of a given radioisotope. This quantity is proportional to the number of radioactive atoms present, so that for a single isotope,... 

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

The biological half-life of a radioisotope is the time required for the body to excrete half of the radioisotope. The effective half-life is the time required for the amount of a radioisotope in the body to be reduced to half its original amount, as a result of both the decay of the radioisotope and its excretion. Sulfur-35 (tu2 = 87.4 d) is used in cancer research. The biological half-life of sulfur-35 in the human body is 90. d. What is the effective half-life of sulfur-35 ... 

Radioisotopes may occur in the earth naturally as primordial radioisotopes, formed when the planet was created, or be produced by natural or artificial processes. Most fast decaying primordial radioisotopes have long disappeared from the planet since the earth originated about 4.5 billion years ago, such isotopes have decayed and reached a final, stable form. The relatively few primordial radioisotopes still extant in the earth today, therefore, decay very slowly. Among these are potassium-40 and some isotopes of uranium, such as uranium-235 and uranium-238, which are of use for dating archaeologically related minerals and rocks (see Textboxes 15 and 16). 

FIGURE 10 The half-life. It is impossible to predict when a radioisotope or an unstable substance (molecule) will decay or be decomposed. On an average, however, only half of any type of radioisotope or unstable substance (molecule) remains after one half-life (A/2) one-quarter will remain after two half-lives (A/A), one-eighth after three half-lives (A/8), and so on. The half-life is characteristic of every radioisotope and unstable molecule that of radioisotopes is not affected in any way by the physical or chemical conditions to which the radioisotope may be subjected. Not so the half-life of chemically unstable molecules, which is altered by changes in temperature and by other physical and chemical conditions. 

The half-life (t1 ) of a radioisotope is the amount of time it takes for that isotope to undergo radioactive decay and be converted into another. It is also a measure of the stability of the isotope the shorter its half-life, the less stable the isotope. The half-life of radioisotopes ranges from fractions of a second for the most unstable to billions of years for isotopes that are only weakly radioactive. In the case of radiocarbon (carbon-14), for example, the half-life is 5730 years (see Fig. 61). 

The fluorescence decay time is one of the most important characteristics of a fluorescent molecule because it defines the time window of observation of dynamic phenomena. As illustrated in Figure 3.2, no accurate information on the rate of phenomena occurring at time-scales shorter than about t/100 ( private life of the molecule) or longer than about 10t ( death of the molecule) can be obtained, whereas at intermediate times ( public life of the molecule) the time evolution of phenomena can be followed. It is interesting to note that a similar situation is found in the use of radioisotopes for dating the period (i.e. the time constant of the exponential radioactive decay) must be of the same order of magnitude as the age of the object to be dated (Figure 3.2). 

---