Zero-dose yields

In Table 8 we show the final product distribution observed in the radiolysis with low LET radiation for cyclohexane. Such product distributions have been determined for a great number of liquids With increasing dose the products accumulate in the liquid, and reactions of the intermediates (radicals etc.) with these products become more probable, thus causing the product distribution to change. At sufficiently low doses the so-called zero-dose yields, or initial yields, can be obtained. The radiation chemical yields in principle are also dependent on the dose rate. High dose rates cause higher stationary concentrations of the reactive intermediates (radicals, ions, etc.) and shorter lifetimes due to the increased rate of the reaction of the intermediates with one another. Reaction of radicals with each other may then be favoured, e.g. compared to H-atom abstraction. This effect has been observed in a few cases. In practice, however, the dose rate effects... 

Fig. 10 Secondary properties of various suspensions yielding zero dose variation. (From Ref. 16.)... 

Garrison (12, H, 51) and Maxwell (39, 40) consistently report much higher G values for carbonyl and ammonia production (G = 3 to 5) than those in Table II. Their values are based on extrapolations back to zero dose (39) or referred to the ammonia plus amine G value at zero dose obtained by Maxwell, Peterson, and Sharpless (39) and are intended to reflect initial radical yield values important in theoretical treatments (12, 51). Examining the data in these references shows that G values at higher doses approach those reported in Table II. For example, Maxwell (40) shows an observed G value at 1 megarad of about 0.4 for carbonyl production compared with the zero-dose extrapolation value of about 4.5. 

The EPA uses the linearized multistage model (LMS)—illustrated in Figure 9.34—to conduct its cancer risk assessments. It yields a cancer slope factor, known as the ql (pronounced Ql-star), which can be used to predict cancer risk at a specific dose. The LMS assumes a linear extrapolation with a zero dose threshold from the upper confidence level of the lowest dose that produced cancer in an animal test or in a human epidemiology study. 

The initial yield is obtained by extrapolation to zero dose. 

In many cases, the vields varv with the total absorbed dose. We then gave the initiol yields Gj that is the vields extrapolated to zero dose whenever this extrapolation was impossible, we indicated the yields corresponding to the weakest dose used. When the yi dd indicated by the author refers to a sin e dose or to a limited number of doses without their effects on the vield having been studied, we gave the dose or Hoses used. 

Synchrotron irradiation of 2,2-dimethylbutane yielded C-labelled methane, ethane, ethylene, acetylene, propane, propylene and butadiene. The yields of methane and ethylene -f- ethane relative to acetylene, extrapolated to zero dose, where 0.12 0.03 and 0.49 0.00, respectively. Iodine does not affect the purely hot atom reactions producing... 

FIGURE 11.17 Symmetrical and asymmetrical dose-response curves, (a) Symmetrical Hill equation with n = 1 and EC5o = 1.0. Filled circle indicates the EC50 (where the abscissa yields a half maximal value for the ordinate). Below this curve is the second derivative of the function (slope). The zero ordinate of this curve indicates the point at which the slope is zero (inflection point of the curve). It can be seen that the true EC50 and the inflection match for a symmetrical curve, (b) Asymmetrical curve (Gompertz function with m = 0.55 and EC50= 1.9). The true EC50 is 1.9, while the point of inflection is 0.36. 

MV yield was only 2.5 % (Fig. 16). The MV signal is present immediately after the flash. At low laser intensity the signal decays to practically zero within about 10 ps. The signal decays very little at the highest laser dose (Fig. 17). These observations are explained by the following mechanism The electrons react during the laser flash with adsorbed ions -I- e MV. Some of the positive... 

This model tends to approach a zero probability rapidly at low doses (although it never reaches zero) and thus is compatible with the threshold hypothesis. Mantel and Bryan, in applying the model, recommend setting the slope parameter b equal to 1, since this appears to yield conservative results for most substances. Nevertheless, the slope of the fitted curve is extremely steep compared to other extrapolation methods, and it will generally yield lower risk estimates than any of the polynomial models as the dose approaches zero. 

Since the total amount of drug in the body is the intravenous dose, 350 mg (i.e., 5 mg/kg x 70 kg), and the estimated plasma concentration at zero time is 7 mg/mL, substitution of these numbers in the equation yields the apparent Vd ... 

Due to the accurate knowledge of the dose used, a control of the progressive extent of the radiolytic reduction was achieved. However, quite often, puzzling data were reported when the zero-valent metal was formed, such as an induction time for precipitation, a sensitivity of the radiolytic yields to the initial presence of added particles, and an unusually weak reproducibility [9,10]. Moreover, oxidation of silver atoms by molecular oxygen was observed [7], although the process was thermodynamically improbable for a noble metal such as silver. 

The expression on the left side of Eq. (9) represents the so-called apparent quantum yield

quantum yield of disappearance of A can be approximately determined also by graphic extrapolation of the dependence

[Pg.138]

For dosimetry, the reaction of the reaction of OH with DMSO which yields methanesulfinic acid (92% Veltwisch et al. 1980 Chap. 3.2) is usually used. This allows one to put the conductance signals on a quantitative basis (calculation of G values), and the rates of reactions that are kinetically of first order can be determined for the time dependence of the signal evolution. DMSO dosimetry yields only a relative dose. For the determination of second-order rate constants, however, the exact dose must be known, and this can be determined by the zero conductivity change dosimetry or neutralization kinetics dosimetry (Schuchmann et al. 1991). 

Rzad and Schuler" studied the radiation chemistry of a solution of " C-cyclopropane in hexane over the concentration range 10 " to 10 M. The main radioactive products, which appear to result from ion molecule reactions, are propane formed by H2 transfer (50 %) and by H transfer (20 %) and mixed nonanes (30 %) formed by the addition of CaHg unit to a hexyl ion. At the lower concentrations, very pronounced dose dependence of the yields was observed. This was ascribed to a competitive formation of olefins in the radiolysis. For cyclopropane-cyclohexane solutions the chemical processes seem to be considerably more complicated. The observed yield of total radioactive products extrapolated to zero concentration of cyclopropane are 0.05 and 0.11 G units for hexane and cyclohexane, respectively. These limiting yields are of the order of magnitude of and appear to be related to, the free ion yields in these systems. Since cyclopropane was found to react with hydrocarbon ions" it is used quite often as a scavenger for positive ions, as in the work of Davids and coworkers . 

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