Network topology and the theory of rubber elasticity

The earliest investigations on rubber elasticity, commencing in the 19th century, were necessarily limited to phenomenological interpretations. The realisation that polymers consist of very long molecular chains. commencing c. 1930, gave impetus to the molecular theory of rubber elasticity (1932-). according to which the high deformability of an elastomer, and the elastic force generated by deformation, stem from the configurations accessible to long molecular chains. Theories of rubber elasticity put forward from 1934-1946 relied on the assumption that the junctions of the rubber network undergo displacements that are affine in macroscopic strain. The theory of James and Guth (1947) dispensed with this premise, and demonstrated instead that the mean positions of the junctions of a ‘phantom’ network consisting of Gaussian chains devoid of material properties are affine in the strain. The vital significance of the distinction between the actual distribution of chain vectors in a network and their distribution if the junctions would be fixed at their mean positions went unnoticed for nearly 30 years. Experimental investigations, commencing with the incisive work of Gee in 1946. revealed large departures from the relationship of stress to strain predicted by the theories cited. This discrepancy prompted extensive studies, theoretical and experimental, during succeeding years. Inquiry into the fundamentals of polymer networks, formed for example by interlinking very long polymer molecules, exposed the need to take account of network imperfections, typically consisting of chains attached at only one end to a network junction. Various means were advocated to make corrections for these imperfections. The cycle rank ζ of the network has been shown (1976) to be the fundamental measure of its connectivity, regardless of the junction functionality and pattern of imperfections. Often overlooked is the copious interpenetration of the chains comprising typical elastomeric networks. Theories that attempt to represent such networks on a lattice are incompatible with this universal feature. Moreover, the dense interpenetration of chains may limit the ability of junctions in real networks to accommodate the fluctuations envisaged in the theory of phantom networks. It was suggested in 1975 that departures from the form predicted for the elastic equation of state are due to constraints on the fluctuations of junctions whose effect diminishes with deformation and with dilation. Formulation of a self-consistent theory based on this suggestion required recognition of the non-affine connection between the chain vector distribution function and the macroscopic strain in a real network, which may partake of characteristics of a phantom network in some degree. Implementation of the idea was achieved through postulation of domains of constraint affecting the equilibrium distribution of fluctuations of network junctions from their mean positions. This led in due course to a theory that accounts for the relationship of stress to strain virtually throughout the ranges of strain accessible to measurement. The theory establishes connections between structure and elastic properties. This is achieved with utmost frugality in arbitrary parameters.     

The effect of pitch and collimation on radiation dose in spiral CT

1 Introduction Spiral computed tomography (CT) was intro- duced in clinical practice in 1980s. Its distinctive characteristic is continuous data acquisition and con- tinuous tube rotation with high speed while simulta- neously translating the patient through the gantry.[1] Owing to these functions the spiral CT possesses the ability to scan an entire organ volume within a single breathhold. Thus, the anatomical section is contiguous and therefore the reliable demonstration of small le- sions…     

Accuracy of the zero crossings of the second directional derivative as an edge detector

In this paper the accuracy of the second directional derivative edge detector is analyzed, based on a number of idealized edge models. The results are compared with those for the Laplacian edge detector. Errors are shown to be small under a number of conditions. These conditions are less severe for the second directional derivative than for the Laplacian edge detector. Spurious or phantom edges can be removed by checking the sign of the third directional derivative, though this is not enough to remove all large errors. Indeed, it is also shown that large errors will be obtained if no threshold is set on the magnitude of a third order derivative.This work was supported by the Belgian National Fund for Scientific Research (NFWO).     

A Conflict Detection Scheme for Concurrency Control of R-tree

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Evaluation of radiation exposure upon visual organs during a head CT scan

In order to analyze and compare the absorbed radiation dose and dose rate of visual organs (eyes, corneas, and lenses) of the head with the surrounding areas of the head during CT scans, a mathematical head phantom was created. In addition, the amount of radiation exposure reduced when using an eye shield was also analyzed. According to the results, it was found that the absorbed dose was highest when using 120 kV (as compared to 80 kV and 100 kV) and the absorbed dose rate of the visual organs compared to the head was about 1.35% for the eyes, 0.9% for the corneas, and 0.095% for the lenses. Finally, it was shown that the dose reduction rate of each organ when using an eye shield was about 24% for the eyes, about 22% for the corneas, and more than 40% for the lenses.     

近距离放射治疗剂量计算固体水模型的设计与实现

讨论了新型近距离放射治疗粒子源125I和103Pd的使用,并以美国医学物理学家协会推荐的近距离放射治疗剂量计算草案为依据,利用蒙特卡罗程序MCNP4C模拟国际近距离放射治疗公司(IBt)生产的新型粒子125I源在固体水模型中的剂量计算几何模型;MCNP4C模拟得到的几何水模型可对AAPM工作组43号报道推荐的剂量测定参量如剂量率常数、径向剂量函数和各向异性函数进行精确计算;近距放射治疗源的剂量学特征就可以通过计算这些剂量测定参量得到.     

BNCT project at Tehran Research Reactor: Current and prospective plans

Boron Neutron Capture Therapy (BNCT) for brain tumor treatment is under development at the Tehran Research Reactor (TRR). This paper presents all current research activities that were performed during recent years as well as the prospective of BNCT research at TRR. The theoretical and experimental investigations show that TRR has a very good potential to consider it as a pilot facility for BNCT research in the Middle East and could be facilitated for clinical applications. In this way, there are some steps and also some challenges which are described in the paper.