Parent radionuclides

Pb is a ft emitting radionuclide with a half-life of 10.64 h. It is the parent radionuclide of the a-emitting 212Bi. The decay scheme is shown below ... 

Radioactivity The process of spontaneous disintegration by a parent radionuclide, which releases one or more radiations and forms a daughter nuclide. When half the radioactivity remains, that time interval is designated the half-life (Tb 1/2). The Tb 1/2 value gives some insight into the behavior of a radionuclide and into its potential hazards. 

If the decay constant of the parent radionuclide is much lower than that of the daughter radionuclide (i.e., Ai << A2), equation 11.32 can be reasonably reduced to... 

In equation 11.77, 4 is a numerical constant depending on the geometry and decay constant of the parent radionuclide. If the half-life of the parent is long in comparison with the cooling period, A takes a value of 55, 27, or 8.7 for volume diffusion from a sphere, cylinder, or plane sheet, respectively. If decay rates are faster, A progressively diminishes (see table 1 in Dodson, 1973, for numerical values). 

Because the quantity of unstable parent radionuclides P remaining at time t is related to the decay constant (cf eq. 11.22) through... 

Ultra short lived radionuclides, with a half-life of a few seconds to a few minutes are readily available from long-lived parent radionuclides adsorbed to an organic or inorganic ion exchange support matrix (1-3). These radionuclide generator systems are an inexpensive alternative to an on-site cyclotron, especially for positron emitters used for positron emission tomography (PET). 

Automated radionuclide generators capable of providing precise dose delivery of multi-millicurie amounts of short-lived positron emitters on demand from a safe and easily operated system are an attractive alternative to on-site cyclotrons for positron emission tomography. The availability of curie quantities of parent radionuclides from national laboratories and the development of microprocessor automation makes it feasible to utilize these generators in the clinical setting. 

When a radionuclide decays, it does not disappear but is transformed into a new nuclear species of higher binding energy and often differing Z, A, J, tt, and so on. The equations of radioactive decay discussed so far have focused on the decrease of the parent radionuclides but have ignored the formation (and possible decay) of daughter, granddaughter, and so forth, species. It is the formation and decay of these children that is the focus of this section. 

Secular equilibrium when the rate of decay of the parent radionuclide is equal to that... 

To illustrate three common radiochemical separation techniques - precipitation, solvent extraction, and cation exchange - in parts 7A, 7B, and 1C. These methods separate thorium from its uranium parent. Radionuclide recovery is measured by comparing count rates to the original sample, not by carrier yield determination. 

A typical radionuclide generator consists of a column filled with adsorbent material in which the parent radionuclide is fixed. The daughter radionuclide is eluted from the column once it has grown as a result of the decay of the parent radionuclide. The elution process consists of passing through the column a solvent that specifically dissolves the daughter radionuclide leaving the parent radionuclide adsorbed to the column matrix. 

A generator should ideally be simple to build, the parent radionuclide should have a relatively long half-life, and the daughter radionuclide should be obtained by a simple elution process with high yield and chemical and radiochemical purity. The generator must be properly shielded to allow its transport and manipulation. 

The physical methods of geochronology are based on the radioactive decay of unstable (parent) radionuclides into stable (daughter) isotopes such as Rb to Sr to ° Pb Th to 208pb 235pj jQ 207p(j. Rc to Os Lu to Hf and to °Ar. There are... 

There are two isotopes of helium. In addition to the cosmologically produced " He and He, " He is produced as a-particles during radioactive decay of various parent radionuclides, and the... 

The major exposure of the population to natural radiation arises from inhalation of the short-lived radioactive progeny of the radioactive noble gas radon-222, which in turn is a sixth-generation radioactive decay product of natural uranium. The amount of radon-222 present in the air depends on many factors (e.g., gas permeability in soil and rock, relative humidity, and barometric pressure) but is necessarily linked to the geological concentration of the uranium parent radionuclide. There is about an eightfold range of concentrations of uranium in different types of rocks and soils. 

A conceptual analogy of secular equilibrium is two linked water tanks of equal bottom area, each being drained by siphons of different internal diameter (Fig. 5.14). In this analogy, the higher tank contains a height of water, Hp, representing total number of parent radionuclides, Np. The inner diameter of the siphon draining the upper tank represents the decay constant of the parent, Xp. The... 

Patel 1991), where and are the number of atoms of the parent at r = 0 and after time t respectively, and k is the decay constant (see Chap. 2). The decay rate per unit time, Nk, decreases with time (assuming no production of the parent radionuclide). The decay constant is an intrinsic property of a given radioisotope, and is, therefore, unaffected by environmental variables such as time, temperature, pressure, or element abundance. 

In a successive decay, a parent radionuclide p decays to a daughter nuclide d, and d in turn decays to another nuclide c, and we are interested in the decay rate of d over time. Thus,... 

The parent radionuclide decays by several yS-particle transitions (two shown), producing metastable Tc with 87.5% intensity, while 12.5% decay directly to long-lived Tc. Subsequently, metastable Tc decays by isomeric transition to Tc with a half-life of 6.02 h and emission of 140.5 keV gamma radiation. Tc decays with a half-life of 212000 years to stable ruthenium-99 (Boyd 1982). 

When a radionuclide decays to a daughter of half-life much shorter than that of its parent, the daughter builds up to an amount that remains in constant ratio to the amount of the parent, and the amount of the dau ter then decreases at a rate controlled by the half-life of the parent. In this case, the daughter is said to be in equilibrium with the parent, even though the amount of the parent radionuclide may be changing with time. For example, for the batch decay scheme that led to Eq. (2.14), suppose that Xi>Xi, and assume that for times of interest Xjt> 1. Equation (2.14), written in terms of decay rates, then reduces to... 

The valuable tracer Tc used in nuclear medicine can be obtained as the pertechnetate ion using a Mo- Tc generator (see Radioisotope Generator, which takes advantage of the equilibrium between the parent radionuclide Mo and the daughter radionuclide Tc. The separation between the two radionuclides is done on an alumina column by the selective elution of [ Tc04] with a saline solution. 

Positron emission occurs only when the energy difference between the parent radionuclide and the products exceeds 1.02 MeV (the energy equivalent of the sum of the masses of an electron and a positron). The atom s recoil, as for beta-particle emission, is a few electron volts. At lesser energy differences, a proton in the nucleus can be converted to a neutron by electron capture, i.e., the capture by the nucleus of an atomic electron from, most probably, an inner electron shell (see discussion below of CEs). The process of electron capture parallels positron emission and may occur in the same isotope. It is accompanied by emission of a neutrino and characteristic X rays due to the rearrangement of atomic electrons. Electron capture may also be signaled by the subsequent emission of gamma rays. Examples of these decays are given in Sections 9.3.4 and 9.3.6. 

The SF process that results in two nearly equal mass fragments (a process called symmetric fission ) has been observed in Fm (1.5 s). More commonly, SF occurs as asymmetric fission, a split of the parent radionuclide into two unequal large FF. As in neutron-induced fission, many different asymmetric mass (and charge) divisions with varying yields can result, with mass numbers from about 70 to 170, each with many isotopes. Hundreds of different nuclides can be produced. Figure 2.1 displays the predominantly asymmetric mass yields as a function of mass number (dubbed mass-yield curves ) that have been measured for several SF and neutron-induced fission nuclides. 

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