Half-life The time

Initiators. The degree of polymerization is controlled by the addition rate of initiator(s). Initiators (qv) are chosen primarily on the basis of half-life, the time required for one-half of the initiator to decay at a specified temperature. In general, initiators of longer half-Hves are chosen as the desired reaction temperature increases they must be well dispersed in the reactor prior to the time any substantial reaction takes place. When choosing an initiator, several factors must be considered. For the autoclave reactor, these factors include the time permitted for completion of reaction in each zone, how well the reactor is stirred, the desired reaction temperature, initiator solubiUty in the carrier, and the cost of initiator in terms of active oxygen content. For the tubular reactors, an additional factor to take into account is the position of the peak temperature along the length of the tube (9). 

Biological half-life The time required for one-half of the material accumulated in a tissue to be removed. 

Half-life The time required for the concentration of a pollutant to decay to half its original value. 

Half life The time required to convert half of the original amount of reactant to product, 294 first-order, 294 second-order, 296... 

Half-life The time it takes for half of any given amount of matter to undergo radioactive decay. 

Half-life The time in which the concentration of a chemical in the environment is reduced by half. 

In the meantime, E. Rutherford (NLC 1908 ) studied the radioactivity discovered by Becquerel and the Curies. He determined that the emanations of radioactive materials include alpha particles (or rays) which are positively charged helium atoms, beta particles (or rays) which are negatively charged electrons, and gamma rays which are similar to x-rays. He also studied the radioactive decay process and deduced the first order rate law for the disappearance of a radioactive atom, characterized by the half-life, the time in which 50% of a given radioactive species disappears, and which is independent of the concentration of that species. 

Half-life The time required by metabolism or elimination to reduce the concentration of a chemical by 50%. 

Elimination half-life ty, plasma half-life) the time necessary for the concentration of drug in a bodily compartment (usually plasma) to decrease by one half as a... 

Half-life The time required for half the atoms in a sample of a radioactive isotope to decay. 

It is further shown in Chapter 15 that the needed rate constant is traditionally provided in terms ofthe half-life (/ ), the time needed for one-half ofthe isotope to disintegrate. The needed relationship between k and is given by... 

Half-life. The time required for the disappearance of one half of a substance. 

The thermal time constant of a reactor characterizes the dynamics of the evolution of the reactor temperature. In fact, since it contains the ratio of the mass proportional to volume with the dimension L3 to the heat exchange area with the dimension L2, it varies non-linearly with the reactor scale, as is explained in Section 2.4. Some values of the time constant obtained with normalized stainless steel reactors [1] are summarized in Table 9.3. The variation by a factor of about 7, over the range considered here, is critical during scale-up. The heating or cooling times are often expressed as the half-life, the time required for the temperature difference to be divided by two ... 

Equation 11.2-25 is the formal definition of half-life, the time the second-order reaction takes to reduce the initial concentration to half the initial cq. Following the methodology of... 

The rate of degradation of an enzyme is reflected in its half-life - the time taken for 50% of the protein to be degraded. Most enzymes that are important in metabolic regulation have short half-lives, and are termed labile enzymes. 

Half-life the time required for half of the initial number of atoms of a radionuclide to decay. 

Elimination half-life The time it takes the body to eliminate or break down half the dose of a xenobiotic. 

Half-life The time taken for the concentration of a xenobiotic in a body fluid to decrease by half. 

Prismane is an example of another interesting strained compound. Because it contains three-membered rings fused with four-membered rings, it should be even more strained than cubane. Prismane was prepared in 1973. It is a liquid that is stable at room temperature but explosive under some conditions. In toluene at 90°C its half-life (the time it takes for one-half of the compound to decompose) is 11 h. Note that prismane is isomeric with benzene. In fact, it was one of the structures proposed for benzene in the early days of organic chemistry. 

Each element is distinguished by the number of protons in the nucleus (the atomic number). The number of neutrons is usually similar to the number of protons, although the number of neutrons may vary. Atoms with the same number of protons but different numbers of neutrons are called isotopes. For example, the most common kind of carbon atom has six protons and six neutrons in its nucleus. Its mass number (the sum of the protons and neutrons) is 12, and we write its symbol as 12C. About 1% of carbon atoms have seven neutrons the mass number is 13, written 13C. A very small fraction of carbon atoms have eight neutrons and a mass number of 14. The 14C isotope is radioactive, with a half-life (the time it takes for half of the nuclei to decay) of 5730 years. The predictable decay of 14C is used to determine the age of organic materials up to about 50,000 years old. 

Radioactive half-life The time during which the decay rate of a radioactive nuclide decreases by a factor of two. 

The energy of these emissions covers a wide range of values but is typically 190 million electron volts (MeV) for fission, 17 MeV for fusion, 5 MeV for alphas, 1 MeV for gammas, and 0.5 MeV for betas. The rate of radioactive decay is expressed through the half-life, the time required for the decay rate of the unstable nuclide to decrease by a factor of two. The half-lives range from less... 

---