The half-life

Nuclide. Each nuclide is identified by element name and the mass number A, equal to the sum of the numbers of protons Z and neutrons N in the nucleus. The m following the mass number (for example, Zn) indicates a metastable isotope. An asterisk preceding the mass number indicates that the radionuclide occurs in nature. Half-life. The following abbreviations for time units are employed y = years, d = days, h = hours, min = minutes, s = seconds, ms = milliseconds, and ns = nanoseconds. 

Safety. The principal concerns regarding nuclear medical imaging are those associated with the radiopharmaceuticals. Much research has gone into the selection of radiopharmaceuticals exhibiting minimal toxicities, rapid elimination from the body, and short half-life. The radioisotope must be... 

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... 

A second, completely different dose adjustment tule exists. Calvin Kunin proposed to start with the standard dose and to administer one half of the standard dose after one individual half-life. The halfdosage rule of Kunin should only be applied if the individual half-life is longer than the standard administration interval. 

FIGURE 13.13 For first-order reactions, the half-life is the same whatever the concentration at the start of the chosen period. Therefore, it takes one half-life to fall to half the initial concentration, two half-lives to fall to one-tourth the initial concentration, three half-lives to fall to one-eighth, and so on. The boxes portray the composition ot the reaction mixture at the end of each half-life the red squares represent Inc reactant A and the yellow squares represent the product. 

The content of the material in a carbon reservoir is a measure of that reservoir s direct or indirect exchange rate with the atmosphere, although variations in solar also create variations in atmospheric content activity (Stuiver and Quay, 1980, 1981). Geologically important reservoirs (i.e., carbonate rocks and fossil carbon) contain no radiocarbon because the turnover times of these reservoirs are much longer than the isotope s half-life. The distribution of is used in studies of ocean circulation, soil sciences, and studies of the terrestrial biosphere. 

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

It is important to note however that Equation (10) assumes steady state in the Th distribution so that production truly is balanced by decay and export. It is easy to imagine a scenario after a phytoplankton bloom, when the export of POC (and " Th) has decreased or even ceased, such that the water column " Th profile would still show a deficit with respect to caused by prior high flux events. This relief deficit will disappear as " Th grows into equilibrium with on a time scale set by the " Th half-life. The magnitude by which the Th flux is over- or under-estimated depends on whether deficits are increasing or decreasing and at what rate. 

For IV administration, the easiest way to determine the loading dose is in terms of Cp and Cmax. For example, if the desired Cmax is 20 pg/mL and a dose of 100 mg gives a Cp of 10 pg/mL, a loading dose of 200 mg should give a Cp of 20 pg/mL, which is the desired Cmax. If this loading dose is followed by maintenance doses of 100 mg every half-life, the plasma concentrations can be maintained at the... 

Radioactive decay is a stochastic process that occurs at random in a large number of atoms of an isotope (see Textbox 13). The exact time when any particular atom decayed or will decay can be neither established nor predicted. The average rate of decay of any radioactive isotope is, however, constant and predictable. It is usually expressed in terms of a half-life, the amount of time it takes for half of the atoms in a sample of a radioactive isotope to decay to a stable form. 

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). 

PbB concentrations reflect the absorbed dose of lead. However, the interpretation of PbB data depends on a knowledge of the past history of exposure to lead. This is because in the body, bone constitutes the major lead sink and this results in lead having a long body half-life. Thus, in the absence of intense exposure to lead for a considerable period up to its body half-life, the PbB concentrations reflect recent lead exposures. However, if intermittent exposure to lead is occurring in several distinct environments, the PbB concentration reflects both recent and past exposures to lead. Thus, biological effects for populations with the same PbB concentrations may not be the same since different exposure times scales may be involved. This is the reason why free erythrocyte protoporphyrin (FEP) and erythrocyte zinc protoporphyrin (ZPP) have been used as additional biological markers since their elevation is more related to chronic lead exposure than acute lead exposure (see Section 2.7). 

If we start with a particular number of nuclei at a particular time which we can call T=0, then the time at which half of the nuclei originally present are gone is called the half-life. The half-life and the decay probability are related by... 

Half-life The period over which the concentration of a radionucleus falls to half of its initial concentration. 

If the primary loss mechanism of atmospheric reaction is accepted as having a 17h half-life, the D value is 1.6 x 109 mol/Pah. For any other process to compete with this would require a value of at least 108 mol/Pah. This is achieved by advection (4 x 10s), but the other processes range in D value from 19 (advection in bottom sediment) to 1.5 x 10s (reaction in water) and are thus a factor of over 100 or less. The implication is that the water reaction rate constant would have to be increased 100-fold to become significant. The soil rate constant would require an increase by 104 and the sediment by 10s. These are inconceivably large numbers corresponding to very short half-lives, thus the actual values of the rate constants in these media are relatively unimportant in this context. They need not be known accurately. The most sensitive quantity is clearly the atmospheric reaction rate. 

The half-life of a radionuclide represents the amount of time required for half of the sample to decay. Relative stabilities of radionuclides are indicated by their half-life values. The shorter the half-life, the less stable is the radionuclide. 

If a drug is repeatedly administered at dosing intervals that are equal to its elimination half-life, the number of doses required for the plasma concentration of the drug to reach the steady state is... 

L The answer is e. (Kalzang, pp 744, 748J Vitamin is hydroxylated to 2.5-0ITDj (calcifediol). Calcifediol is then hydroxylated in the kidney to the most active form of vitamin D, which is 1,25-dihydroxyvitamin D (cal-citriol). Calcitriol has a rapid onset of action and a short half-life. The administration of calcitriol causes the elevation of serum Ca levels by enhancing the intestinal absorption of Ca. Calcitriol is indicated in vitamin D deficiency,... 

The counting of 222Rn is relatively simple no attempt was made to achieve backgrounds less than 0.1 counts/min. If the activity was higher than 0.2 counts/min, the sample was counted for approximately two weeks to ensure that the activity had the 3.8-day half-life. The problem with 222Rn is that it is difficult to obtain reproducible results, particularly after the sample is removed from and reinserted into the extraction line. This type of reproducibility is necessary if 230Th and 234U are to be reliably determined on aliquots of melt water. 

Half-life The average time in which half the atoms in a sample of a radioactive element decay. 

A At the end of one half-life the pressure of DTBP will have halved, to 400 mmHg. At the end of another half-life, at 160 min, the pressure of DTBP will have halved again, to 200 mmHg. Thus, the pressure of DTBP at 125 min will be intermediate between the pressure at 80.0 min (400 mmHg) and that at 160 min (200 mmHg). To obtain an exact answer, first we determine the value of the rate constant from the half-life. 

We next look for a constant half-life, indicative of a first-order reaction. The initial concentration of 0.88 M decreases to one half of that value, 0.44 M, during the first 100 s, indicating a 100-s half-life. The concentration halves again to 0.22 M in the second 100 s, another 100-s half-life. Finally, we note that the concentration halves also from 0.62 M at 50 s to 0.31 M at 150 s, yet another 100-s half-life. The rate is established as first-order. 

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