Critical dose rate

In aquatic environments, Spear (1981) spotlights three research needs (1) development of analytical procedures for measurement of individual dissolved zinc species, notably the aquo ion and zinc chloride, and for nondissolved species that occur in natural waters (2) separation of natural from anthropogenic influences of sediment-water interactions on flux rates, with emphasis on anoxic conditions, the role of microorganisms, and the stability of organozinc complexes and (3) establishment of toxicity thresholds for aquatic organisms based on bioaccumulation and survival to determine the critical dose and the critical dose rate, with emphasis on aquatic communities inhabiting locales where zinc is deposited in sediments. These research needs are still valid. 

It gives the critical dose rate (irradiation intensity) pc as a function of the elastic interaction between similar defects and the temperature ... 

The critical dose rate pc necessary for initiating the aggregation process is the smaller, the lower the temperature, the stronger elastic attraction of similar particles and the slower the diffusion (greater the activation energy for hopping). This conclusion is in a complete qualitive agreement with the results obtained recently in terms of a quite different mesoscopic approach [63-65],... 

All these findings are in agreement with both experimental data for NaCl crystals and the recent mesoscopic studies [65, 68] where the critical dose rate was found to be... 

On the other hand, the present microscopic approach leads to the critical dose rate arising from equation (7.2.13) ... 

Both equations (7.2.16) and (7.2.18) have the same dependence on the relative diffusion coefficient, D — D + D, but different dependence on the elastic interaction between defects. However, in both cases the stronger similar defect attraction, the lower is the critical dose rate. In the mesoscopic approach this effect is less pronounced (logarithmic vs. linear dependence) and here pc is considerably higher. It seems that this approach is able to detect only those mesoscopic-size aggregates which are already well-developed - unlike the microscopic formalism able to detect even the marginal aggregation effects. 

In the case of heavy contamination, early warning is essential. Gamma radiation, which in nearly all cases will be associated with radioactive material emitted in an accident, can be easily measured. Many instrument systems can measure dose rates caused by gamma radiation from environmental levels (and therefore well below any critical dose rate) up to extremely high levels. The higher the level, the faster and easier it can be measured. Therefore, on the instrumentation side, the requirements for fast early warning can be met. 

According to US-EPA s Glossary of IRIS Terms, the critical effect is The first adverse effect, or its known precursor, that occurs to the most sensitive species as the dose rate of an agent increases. ... 

The U.S. Environmental Protection Agency has proposed that "The annual alpha radiation dose rate to members of the critical segment of the exposed population as the result of exposure to transuranium elements in the general environment should not exceed either 1 millirad per year to the plumonary lung, or 3 millirad per year to the bone" (54). The USEPA also derived a soil contamination level of 0.2 pCi/m2 Tl cm depth, soil particles less than 2 mm) as a reasonable "screening" level for which the resultant dose rates to the critical segment of the exposed population could be reasonably predicted to be less than the guidance recommendations. 

Lyons, E.T., Drudge, J.H., Tolliver, S.C., Swerczek, T.W. and Collins, S.S. (1989) Determination of the efficacy of pyrantel pamoate at the therapeutic dose rate against the tapeworm Anoplocephala perfoliata in equids using a modification of the critical test method. Veterinary Parasitology 31, 13-18. 

Ambiguity- there should be no statements that might require interpretation by the scientific staff during conduct of the study. This applies particularly to those factors where numbers are critical. Examples are dose rates or application levels, sacrifice or harvest intervals, and replication requirements. The protocol, however, must also retain a degree of flexibility in those areas where exact definition is not needed or cannot be determined prospectively. Specifications should not be so detailed that there is no allowance for equivalent substitution. Usually, it is not necessary to specify brand names however, there may be times when experience dictates that a specific brand or manufacturer are required to perform a given function. In those cases, of course, specificity is not only desirable but mandatory. 

Influence of Dose and Dose Rate. With 0.1-mm. thick PTFE films the grafting did not proceed homogeneously, despite the low dose rate used. All grafted films exhibited crumpled edges, as was observed earlier when PTFE films were grafted near the critical conditions, where... 

Karpov er af. (1977) studied the polymerization in the presence of a number of anionic emulsifiers below the critical micelle concentration. High molecular weight polymers with low concentrations of impurities were obtained at high rates. The overall rate was found to be about 0.5 order with respect to the dose rate with an activation energy of 5 kcal/moI, in reasonable agreement with those reported by Barriac et al. (1976). 

Critical effect A chemical often elicits more than one toxic effect, even in one species, or in tests of the same or different durations. The critical ef-fect(s) is the first adverse effect(s) or its known precursor(s) that occurs as dose rate increases. The critical effect(s) may change among toxicity studies of different durations, may be influenced by toxicity in other organs, and may differ depending on the availability of data on the shape of the dose-response curve. 

The dose required for amorphization is a function of the kinetics of simultaneous dynamic recovery processes. The recovery process is accelerated at elevated temperatures and, in many cases, is greatly increased by radiation-enhanced defect migration. These simultaneous recovery processes may be associated with defect recombination or annihilation, epitaxial recrystallization at crystalline-amorphous interfaces (Carter and Nobes 1991), or nucleation and growth recrystallization in the bulk of the amorphous state. For any crystalline solid, there is a critical temperature, above which the rate of amorphization is less than the rate of recovery, thus amorphization cannot occur. However, Tc also depends on the energy and mass of the incident beam, as well as the dose rate. 

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