Radioactivity definitions

Often solvents do not extract 100% of the total radioactive residue. In this case, knowledge about the concentration of the target analyte(s) in the extract and the filter cake is necessary. Even if large amounts of radioactivity remain in the solid residual materials, the extraction efficiency may be sufficient if this unextracted radioactivity is permanently bound to the matrix or if it is associated with compounds which are not included in the residue definition. Finally, in all cases a well performed metabolism study can provide the answers needed, even where residues in the edible parts of treated crops or animals do not occur. If incurred residues do not occur, clearly the determination of extraction efficiency is not required. 

The concept most commonly used when dealing with radioactive nuclides is activity. By definition, the activity of a number of atoms of a nuclide is the number of decay events per unit of time. The law of radioactivity tells us that this activity is equal to the decay constant times the number of atoms. 

Figure 5. A schematic representation of superposed steady-state reservoirs of constant volumes Vi (fractional crystallization is omitted in this schema). At steady-state, Vi/xi=V2/x2=..., where x is the residence time. This is analogous to the law of radioactive equilibrium between nuclides 1 and 2 Ni/Ti=N2/T2=...A further interest of this simple model is to show that residence times by definition depend on the volume of the reservoirs.

Disintegration, Nuclear—A spontaneous nuclear transformation (radioactivity) characterized by the emission of energy and/or mass from the nucleus. When large numbers of nuclei are involved, the process is characterized by a definite half-life (see Transformation, Nuclear). 

Since effects of radioactive decay on the molar mass can be neglected, q>bas Sr can be replaced with an excellent precision by expressed through equation (1.3.2) as a function of mass fractions / and concentrations CSr requires... 

Definition of the background activity of microinjected oocytes is a clear prerequisite for any functional cloning procedure, as a positive signal needs to be distinguished from the endogenous transport activities. Key characteristics of the carrier, such as a dependency on sodium ions or a specific inhibitor are very helpful. In addition, unspecific binding of the radioactive transporter substrate to the oocyte membrane needs to be examined carefully. 

REACH is very wide in its scope covering aU substances (see definition of substances in Section 2.4.1) whether manufactured, imported, used as intermediates or placed on the market, either on their own, in preparations or in articles, unless they are radioactive, subject to customs supervision, or are nonisolated intermediates. Waste is specifically exempted. Food is not subject to REACH, as it is not a substance, preparation, or article. Member States may exempt substances used in the interests of defense. Other substances are exempted from parts of REACH, where other equivalent legislation applies for fiufher details refer EU (2006i). 

A simple way to characterize the rate of a reaction is the time it takes for the concentration to change from the initial value to halfway between the initial and final (equilibrium). This time is called the half-life of the reaction. The half-life is often denoted as ti/z. The longer the half-life, the slower the reaction. The half-life is best applied to a first-order reaction (especially radioactive decay), for which the half-life is independent of the initial concentration. For example, using the decay of " Sm as an example, [ Sm] = [ Sm]o exp( kt) (derived above). Now, by definition,... 

Because a diffusion profile does not end abruptly (except for some special cases), it is necessary to quantify the meaning of diffusion distance. To do so, examine Equation 3-40a. Define the distance at which the concentration is halfway between Co and to be the mid-distance of diffusion, Xmid- The concept of Xmid is similar to that of half-life ti/2 for radioactive decay. From the definition, Xmid can be solved from the following ... 

The decay equation can also be expressed in terms of the radioactive activity (A), i.e., the number of decays per unit time per unit mass of sample. By definition, activity is the same as the decay rate, and can be written as... 

The Poisson distribution describes the results of experiments in which we count events that occur at random but at a definite average rate. Examples of the Poisson distribution include the number of emails we receive in a one-day period, the number of babies bom in a hospital in a two-day period, the number of decays of a radioactive isotope in a one-day period. 

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