Influence of the Dose Rate

Dose rate (kGy/h) Dose (kGy) Change in service life (h) 

Sohematic diagram of the dependence of halfvalue dose on dose rate compared with half-value doses (strain at break) for a polyvinylchloride-based cable material irradiated at 60 °C [ 13] 

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Figure 4.7 Influence of the dose rate on damage accumulation in 4H-SIC for fixed ion mass and ion energy (a) damage at the peak of the Si sublattice versus increasing dose rate and increasing implantation temperature (b) evaluation of the energy activation for the phenomenon responsible of the shift versus temperature shown in part (a). (From [26], 2003 American Institute of Physics. Reprinted with permission.)...

Figure 13 Scheme of the influence of the dose rate on the competition between the inter-metal electron transfer and the coalescence processes during the radiolytic reduction of mixed metal ion solutions. Sudden irradiation at high dose rates favor alloying, whereas low dose rates favor coreshell segregation of the metals because of metal displacement in the clusters. 

Fig. 2 Influence of the dose rate and the polymer concentration on the structures obtained by irradiation (a) intermolecular cross-linking at medium concentrations and low dose rates, and (b) intramolecular cross-linking at low concentrations and high dose pulses. (Reprinted from [41], copyright 2009, with permission of Elsevier)...

Figure is. Scheme of Ae influence of the dose rate on ft,e competition between the inter-metal eiectrofi... 

In Figure 8 the doses per unit radon concentration are plotted as a function of the measured ventilation rate. The NEA conversion factor for low and moderate ventilation (NEA,1983, table 2.10) is multiplied by the appropriate equilibrium factor. In the figure no influence of the ventilation rate on the doses is found. 

Because the dynamics observed by means of pulse radiolysis indicated that the displacement process was not instantaneous, it was suggested that very short, intense irradiation, with a dose sufficient to achieve the complete reduction of all the ions, could efficiently prevent the segregation, due to electron transfer between the metals. Therefore, the method could enable the formation of alloyed clusters, of major interest for various applications, particularly catalysis. The positive influence of high dose rates, which quench the atoms in an alloyed cluster, has been demonstrated a bilayered cluster would be obtained from the same system by irradiation at a lower dose rate. " Moreover, as for monometallic clusters (Section 3.13.4.3), the high dose rate favors nucleation rather than growth, and the final sizes of the alloyed clusters are particularly small. " " ... 

Radiation is carcinogenic. The frequency of death from cancer of the thyroid, breast, lung, esophagus, stomach, and bladder was higher in Japanese survivors of the atomic bomb than in nonexposed individuals, and carcinogenesis seems to be the primary latent effect of ionizing radiation. The minimal latent period of most cancers was <15 years and depended on an individual s age at exposure and site of cancer. The relation of radiation-induced cancers to low doses and the shape of the dose-response curve (linear or nonlinear), the existence of a threshold, and the influence of dose rate and exposure period have to be determined (Hobbs and McClellan 1986). 

The parameters which predominantly influence the acid effect in radiation grafting of styrene monomer to polyethylene film are the structure of solvent, the concentration of monomer and the dose rate. Because these three variables are inter-related, it is difficult to predict, a priori, the conditions required to yield an optimum in grafting. In this respect the type of solvent used is particularly important. 

A number of issues influenced the selection of the dose-response model form and the treatment of the data prior to fitting the model. First, shoot weight and shoot length are continuous response measurements therefore, use of a standardized logistic model form is not appropriate. Second, the natural variation in plant growth often resulted in apparent increased shoot weight and shoot length measurements relative to the control at low herbicide application rates. A dose-response model needs to perform well even when some measurements in treatment levels exceed the controls. 

In the cardiovascular system the effect on the heart rate is prominent. The depressive influence of the nervus vagus on the pacemaking activity in the heart is concentration dependently reduced and thereby the heart rate increases. This can be therapeutically useful in various forms of bardycardia, especially if they are caused by a vagus overstimulation, for example in the carotis-sinus syndrom. There is hardly any effect on the vasculature except a vasodilatation in the thoracic region after very high doses of atropine. 

Figure 2. Influence of preirradiation dose of polyethylene films in air on the rate of grafting observed on heating at 135°C. in acrylonitrile (2)...

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

In contrast to the case of annihilation, for the tunnelling recombination not only the dose (= pt, i.e., number of all created defects) but also the dose rate p plays a substantial role since, in view of the relatively large lifetimes of close-lying defects, the appearance of a third defect influences their recombination. We see from Fig. 7.15 that, the smaller is the dose rate p the closer the similar defects lie, and the better the clustering is marked. We can interpret the curves of Fig. 7.15 (c) and (d) as the creation of periodically arranged similar defects. 

Radiation-induced chlorination of polyisobutene in carbon tetrachloride was studied at various temperatures. The process is a chain reaction with a G value of about 10 to 105, depending on the reaction conditions. At very low dose rates (0.1 to 0.2 rad I sec), the chlorination rate is directly proportional to the dose rate. At higher dose rates, the rate approaches a square-root dependence on the dose rate. The termination reaction and the influence of oxygen are discussed. The reaction is first order with respect to chlorine concentration. An activitation energy of about 4 kcal/mole was obtained. In connection with the chlorination reaction, degradation of the polyisobutene takes place. This degradation was followed by osmometric measurements. The structure of the chlorinated product was briefly investigated by IR spectroscopy. 

The influence of the polymer structure on the irradiation grafting has been examined in the case of styrene to high pressure and low pressure polyethylene films (114). The most important factors which determine the efficiency of grafting are the degree of crystallinity, the thickness of the films and the dose rate. 

The dose rate seems to have no significant influence on G(HDBP) and G(H2MBP) (25), or on the yield of RNO and RN02 compounds (27). 

The styrene anion was extremely short lived when cyclohexane and hexane were used as solvents, but contrary to the findings of Katayama et ah and of Metz et ah, Schneider and Swallow stated that the decay of the anion followed second-order kinetics with a first half-life of about 3—4 fisec. for styrene (10-3M) in cyclohexane and hexane. Evaluation of the decay curves at 390 m/x, led to values for k/e 7 X 106 cm. sec.-1 for styrene in the pure state and fc/c = 3.8 0.6 X 107 cm. sec.-1 for a 10-3A/ solution of styrene in cyclohexane. The decay kinetics were not influenced by varying the dose rate of the pulse by a factor of about 6, thus confirming that the decay is second order. If a G value of 0.2 is assumed for the formation of free ions in hydrocarbons, an extinction coefficient... 

Parameters k and k2 can be easily related to the hydrodynamic conditions (flow rate, stirring rates) and to the current density by empirical equations. The influence of the current density can also be related to the reagent dose for parameter k and to the bubble generation for parameter k2 (the flow rate of cathodically generated hydrogen is proportional to the current density). Thus, this semiempirical model considers easily and simultaneously the gas-liquid mass transfer, the collections of solid particles in electroflotation processes, and the effect of the current density. 

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