Radioactive decay. See

Elements slightly heavier than uranium, produced by radioactive decay (see later), are found in tiny amounts in natural uranium ores. Plutonium (element 94) has also been found in nature, a product of the element-forming processes that happen in dying stars. So it is a tricky matter to put a precise number on the natural elements. 

It designates a type of radioactive decay. See beta particle. 

For the well-mixed reservoirs, a conservation equation is written in which gain of 14C by inflow to the box (atmosphere or surface ocean) is balanced by the outflow to other boxes plus radioactive decay (see Section 2.8) of the tracer during its time in the reservoir. For the deep ocean, conservation is described by a partial differential advection-diffusion equation. The... 

Because strontium is an element, its atoms do not degrade by environmental processes such as hydrolysis or biodegradation. However, radioactive strontium will be subject to radioactive decay and transformation to other elements. Eventually, all of the radioactive strontium will be transformed into stable zirconium by the process of radioactive decay (see Section 4.2) ... 

The lanthanoids resemble each other much more closely than do the members of a row of t/-block metals. The chemistry of the actinoids is more complicated, and in addition, only Th and U have naturally occurring isotopes. Studies of the transuranium elements (those with Z > 92) require specialized techniques. The occurrence of artihcial isotopes among the /-block elements can be seen from Appendix 5 all the actinoids are unstable with respect to radioactive decay (see Section 24.9), although the half-lives of the most abundant isotopes of thorium and uranium ( Th and t = 1.4 x lO and 4.5 x 10 yr respectively) are... 

Another characteristic time parameter of radioactive decay is the mean life r, which is the expected value of the y (1, A) exponential distribution associated with radioactive decay (see Chap. 9 in this Volume) ... 

PARENT A radionuclide that transforms to another by radioactive decay. See daughter. 

The time taken for the concentration of reactant to he halved during a chemical reaction is called the half-life. Identical kinetic behaviour is exhibited by substances undergoing radioactive decay (see Chapter 2), with the exception that this physical process is unaffected by changes in temperature. For a reaction that has an overall order of 1, the half-life is constant and is independent of the initial concentration of the reactants (see Figure 16.7). 

The parameter is obtained from the compartment-specific half-life or residence times that apply to bulk removal by processes such as photolysis, hydrolysis, oxi-dation/reduction, microbial transformations, and radioactive decay (see Section 8.5). This parameter is estimated as 0.693/ri/2, where ri/2 is the half-time for a given removal process. 

Cold cathode A non-thermoelectron-emitting cathode that emits electrons, usually by secondary electron emission, under ion bombardment or by radioactive decay. See also Field emission. 

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