Dose-rate effect

EB irradiation of polymeric materials leads to superior properties than the 7-ray-induced modification due to the latter having lower achievable dose rate than the former. Because of the lower dose rate, oxygen has an opportunity to diffuse into the polymer and react with the free radicals generated thus causing the greater amount of chain scissions. EB radiation is so rapid that there is insufficient time for any significant amount of oxygen to diffuse into the polymer. Stabilizers (antirads) reduce the dose-rate effect [74]. Their effectiveness depends on the abUity to survive irradiation and then to act as an antioxidant in the absence of radiation. 

A report (37) on the effect of different types of radiation on the elongatlon-at-break of certain commercial cable Insulating materials pointed to several aspects requiring fundamental Investigations on well-characterised materials under defined conditions. It showed the Importance of antl-oxldant stabilisers, particularly In relation to long term ageing. It was concluded that the differences observed could be attributed to dose rate effects rather than to the types of radiation studied. The Irradiated samples were standard dumb hell shaped tensile samples. 

It should be noted that temperature can have a significant effect on the degradation as it controls both the rate of oxygen diffusion into the material and the rates of reaction of the products of the irradiation. The diffusion of oxygen is a limiting factor (as it is with heat ageing) and oxidation is directly connected to the dose rate effect. 

Kale PG, Baum JW. 1981. Sensitivity of Drosophila melanogaster to low concentrations of gaseous mutagens III. Dose-rate effects. Environ Mutagen 3 65-70. 

The dose-averaged specific energy z plays a fundamental role in microdosimetric-based models of radiation action. For instance, it will be shown below that its magnitude relative to D determines the extent to which dose-rate effects are important. If the micro-dosimetric spectrum /i(z) is known, z can be obtained by straightforward integration. There is, however, a second more powerful procedure for obtaining this quantity and this will be now explained. 

The effect of radiation intensity was investigated for three compositions of PVC-styrene. The data for the 77-23 composition are shown in Figure 5. A radiation intensity effect is present even when postirradiation heating is used. When the 84-16 and 91-9 PVC-styrene compositions were similarly studied, no significant dose rate effect was found. 

Since biradical termination of growing chains is associated with dependency of less than the first power with respect to radiation intensity (1), the absence of a dose rate effect may be taken to mean that biradical termination is not important. One may conclude that in the systems containing the lesser amounts of styrene, and thus having sharply decreased capability for internal diffusion, biradical termination is not significant during the actual irradiation. 

An appreciable dose-rate effect on grafting was observed (Table III). At the cobalt-60 dose rate of 0.1 megarad per hour, the extent of grafting was about seven times greater than with the accelerator dose rate of about 100 megarads per hour. 

Soil samples at the epicentre of the nuclear bomb explosion site in Semipalatinsk93 were measured with ESR. Although the dependence on the distance from the epicentre was not presented, ESR will be potentially useful for tooth enamel of residents nearby the accident and the high background contaminated area. It must be noted, however, that the high dose-rate effect will appear in the case of direct A-bomb radiation leading to lower dose estimation than real one. Radiation effect from the contamination, which is the case for residents, may be estimated if artificial dose rate is around 100 Gy/h or less. 

Since oxygen is not consumed in the over-all reaction, it does not affect G Fe3+). This dosimeter shows no dose rate effect under the usual conditions of 7-ray irradiation since all of the above reactions except Reaction 6 are fast, and this reaction has a half-life of 14 seconds in 0.001M ferrous sulfate (1). 

Weller E, Long N, Smith A, Williams P, Ravi S, Gill J, Henessey R, Skornik W, Brain J, Kimmel C, Kimmel G, Holmes L, Ryan L (1999) Dose-rate effects of ethylene oxide exposure on developmental toxicity. Toxicol Sci, 50(2) 259-270. 

Research using gamma-ray irradiation at Seibersdorf has reported pseudo-first-order decomposition of TCE and PCE [17,18]. Flowever, a significant departure from pseudo-first-order kinetics was noted with the e-beam system. This may be due to dose rate effects (1890 Gy s 1 maximum vs. 1.5 Gy s 1 for 60Co), or due to nonuniform dose distribution, or a combination of both [52]. Fig. 6 shows the departure from first-order behavior seen when the e-beam system is used as the source for the irradiation of drinking water containing TCE. 

Before expanding on the role of impurities in defining polymerization rates and yields, it must be acknowledged that there is a dose-rate effect which may be a contributing factor to the discrepancies mentioned. Reports of such an effect have been made, but they are at variance as to the nature of the change in kinetics associated with changes in dose rates (1, 6, 15). In the current work the dose rates have been kept constant. 

Bimetallic Nanoparticles Synthesis and Dose Rate Effect... 

Figure 11. Variation of Tc as a function of dose rate for zircon and monazite, modified after Meldmm et al. (1999a). Reducing the dose rate effectively lowers the critical temper-ature since, for a given dose, a lower dose rate provides more time for thermal recrystallization. The calcula-ted critical temperatnre of natnral monazite is so low that amorphons specimens of pnre monazite are not expected to occnr in natnre.

At the radio-sterilization doses, simulation predicts a greater loss than in the experimental results, possibly because some radiolytic products react with the water radiolysis radicals thus protecting the drug solute [16]. The simulations also predict similar solute concentrations without dose rate effects for E-beam or gamma irradiations whereas the opposite isfound in experimental results [17].The complexity ofthe radiolysis mechanisms at sterilization doses appears with the increase ofthe analytical efficiency. 

Luebeck, E. G., Heidenreich, W. F., Hazelton, W. D., Paretzke, H. G., and Moolgavkar, S. H. (1999). Biologically based analysis of the data for the Colorado manium miners crJiort Age, dose and dose-rate effects. Radiat Res 152, 339-351. 

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