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Ionisation Chambers
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
The charge (or current) created in the air cavity is conducted to the measuring device (the electrometer) through leads connected to the electrodes. These leads pass through the chamber stem, which is attached to the chamber cavity and designed to hold the whole assembly. Because the charge circulating between the electrodes should be restricted to that created within the air cavity, there must be no other conducting path between the electrodes except that of the ionised air, and therefore, an insulator must be provided between them. Although, in the past, various naturally occurring substances were used as insulators (amber, quartz and ceresin wax), today, synthetic plastics can be used. These are polymethyl methacrylate (PMMA), polystyrene, polyethylene, Nylon, polytetrafluoroethylene (PTFE, Teflon), Mylar film and polytrifluoro-monochloroethylene (Kel-F) (Boag 1966). Surface leakage because of humidity is resisted well by Teflon; however, Teflon is more prone to radiation damage than, for example, polystyrene.
Physics for medical imaging
Published in Ken Holmes, Marcus Elkington, Phil Harris, Clark's Essential Physics in Imaging for Radiographers, 2021
Electrical insulators have all their electrons firmly bound to its molecules. In an insulating material it is much more difficult to disrupt the electrons and they usually break down before the electrons can flow. Good insulators are oil, plastics and rubber.
Ionization Chamber Instrumentation
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
Larry A. DeWerd, Blake R. Smith
The components of an ionization chamber are the shell (also called the wall, thimble, or window), the guard, and the collecting electrode (collector). A schematic of a thimble chamber connected to an electrometer via a triaxial cable (triax) is shown in Figure 2.2. An electric field is established between the ionization chamber components in order to stabilize the ionization collected in the air of the cavity. The electric field is typically generated by a high-voltage power supply or battery within the readout device (electrometer). The schematic of Figure 2.2 is a charge measuring setup; for current measurement, the capacitor is replaced by a resistor. Further details on the electrometer and the triaxial cable are given later. Electrical insulation is provided between each of the ionization chamber components by materials such as polyethylene or PTFE®. The purpose of these insulators is to support the ionization chamber components structurally and to prevent leakage currents. However, even when effective insulators are used, leakage currents may occur when dirt, a hair, or some other type of fiber bridges the insulator. Skin oil is a significant cause of leakage; thus, touching an insulator should be avoided. In general, the signal to noise for a given chamber system always needs to be considered. Ideally, the signal-to-noise ratio should be ≥1000.
The correction of clothing insulation and ergonomic design through 3D CAD reverse engineering
Published in International Journal of Occupational Safety and Ergonomics, 2021
Ivana Špelić, Slavenka Petrak, Maja Mahnić Naglić
Air will become an ineffective insulator when exceeding a certain limit. The forced convection due to body movements creates a bellows effect and becomes more and more pronounced, in addition preventing the linear increase of the total dry heat loss [12,13,24]. The properties of the underlying layers will affect the thermal resistance, including the number and the distribution of the microclimatic air layers. A single large air layer is less thermally resistant than a number of smaller air layers of comparable total thickness, due to the greater convective heat losses. However, the bellows effect is more pronounced in larger air layers. Increased thickness of the inner-layered fabrics also results in a thermal resistance change of as much 200%, especially when the thickness of the inner-layered fabric is increased from 5 to 40 mm [5].
An updated evaluation of potential health hazards associated with exposures to asbestos-containing drywall accessory products
Published in Critical Reviews in Toxicology, 2019
Neva F. B. Jacobs, Kevin M. Towle, Brent L. Finley, Shannon H. Gaffney
In the mid-1960s, it was reported that insulation workers had a high incidence of asbestos-related diseases (Selikoff et al. 1964, 1965). These studies by Selikoff were the first to demonstrate that use of an asbestos-containing insulation (predominately amosite) could pose a significant risk to insulators. Soon thereafter, researchers and industry representatives began to quantify the fiber exposures experienced by other workers, including individuals performing drywall finishing activities (Schmidt 1970; Dotti 1972; Brown 1973; Gypsum Association 1973; National Gypsum Company 1973a, 1973b; Rohl et al. 1975; Rhodes and Spencer 1977; Verma and Middleton 1980). The airborne fiber concentrations reported in these studies generally represented variable short-term, task-based, and peak measurements that were analyzed by phase contrast microscopy (PCM), utilizing a method which measures all fibers (including non-asbestos fibers) greater than five microns in length.
Interaction of obesity and atrial fibrillation: an overview of pathophysiology and clinical management
Published in Expert Review of Cardiovascular Therapy, 2019
Sjaak Pouwels, Besir Topal, Mireille T. Knook, Alper Celik, Magnus Sundbom, Rui Ribeiro, Chetan Parmar, Surendra Ugale
Adipose tissue has complex endocrine functions and is a major source of pro-inflammatory and anti-inflammatory adipokines, which play key roles in the development of obesity-related comorbidities [10]. But also it has structural and functional roles, of which epicardial fat is a good example [11,12]. The adipose tissue pool is composed of at least two functionally distinct types, namely white and brown adipose tissue (WAT, BAT). WAT plays an important role in energy storage, in the secretion of hormones and cytokines that have an impact on appetite regulation, metabolism, insulin resistance, and vascular disease. It is also a thermal insulator and gives protection of organs against mechanical damage [13]. BAT differs in that instead of storing energy, it dissipates chemical energy by non-shivering thermogenesis. This process takes place via mitochondrial uncoupling mediated by expression of tissue-specific, mitochondrial and uncoupling protein 1 (UCP1) [13,14].