Target theory

The design of most process plants relies on redundant safety features or layers of protection, such that multiple layers must fail before a serious incident occurs. Barrier analysis ) (also called Hazard-Barrier-Target Analysis, HBTA) can assist the identification of causal factors by identifying which safety feature(s) failed to function as desired and allowed the sequence of events to occur. These safety features or barriers are anything that is used to protect a system or person from a hazard including both physical and administrative layers of protection. The concepts of the hazard-barrier-target theory of incident causation are encompassed in this tool. (See Chapter 3.)... 

Hitness Models. Models for radiation-induced carcinogenesis have been proposed on the basis of a target theory that assumes that the site of action has some number of particles (N > 1) that are hit by k or... 

Target Theory, 124-126, 132 Template chemistry, 170 Thymine dimers, 133-135 Time-lapse cinematography, 249... 

The radiation chemistry of aqueous solutions may be considered from two points of view. The first, called the Target Theory, considers the direct effect of ionizing radiations on the solute molecules. The second approach regards transformations in the solute molecules to be attributed to interactions with the reactive intermediates formed by the radiolysis of water. Because most aqueous systems are relatively dilute, the latter approach seems statistically more reasonable. Kinetic studies of dilute aqueous systems have indeed borne out this supposition. The radiation chemistry of aqueous solutions then becomes the free radical and redox chemistry of H-, OH-, and e q. 

Radiation target theory predicts an exponential loss of molecules as a function of radiation exposure the rate of this loss is directly proportional to the mass of the molecule. The supercoiled form was observed to decay in this manner. The double-stranded GFP plasmid has a mass of 3300 kDa. In three independent experiments, the amount of supercoiled DNA decreased exponentially with radiation dose (Fig. 2), yielding a target size of 3059 kDa (Table 1), indicating that a single radiation interaction anywhere in the plasmid... 

A simple exponential decrease of measured properties as a function of radiation dose indicates that die effect is due to a single radiation interaction. Radiation target theory predicts that the rate of this decrease is directly proportional to the mass of the structure responsible for that property. Thus the disappearance of supercoiled plasmids is caused by a single radiation interaction anywhere in a mass of 3059 kDa - close to the known mass of the entire molecule. This result does not indicate the fate(s) of the hit supercoiled DNA molecules. 

Other review articles have been published on the radiation inactivation method (Pollard et al., 1955 Kempner and Schlegel, 1979), two of them recently (Jung, 1984 Harmon et al., 1985). Since we do not want to be repetitious, the theory and experimental procedure will only be described briefly, although enough details will be given to allow the reader to follow the discussion. We will focus our attention primarily on the basic concepts of target theory and protein inactivation. 

Some of the basic assumptions of the target theory have been challenged. Lea (1955), Pollard et al. (1955), and Usatyi and Lazurkin (1962) postulated that not all ionizations effectively result in permanent damage to structure and function of proteins. In some proteins two hits could be needed to inactivate a protein. Jung and Schiissler (1968) found altered amino acid contents of both active and radiation-denatured RNAse separated after irradiation. Their data suggest that even active enzyme mole-... 

The temperature effect has not been accounted for in classical target theory (Lea, 1955) because p (equation (1)) was thought to be proportional to a single parameter, target size, which is insensitive to tempera-... 

In radiology, the biological effects of radiation are usually discussed along two lines the matrix effect and target theory. The matrix effect considers the particle-water interaction in which ions, radicals, and excited atoms are produced. This is the dominating effect at large... 

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