Wire screens atoms

Following Barry, James et al (1972), and Thomas and Hinchliffe (1972) investigated the use of wire screens for collecting 218Po atoms or ions. Experiments were done in the absense of aerosol particles, yielding collection efficiency as a function of screen dimensions and face velocity. Information was developed on the fraction of deposited o-activity that could be counted from the front and back sides of the screens. 

A study of published data indicates that the Cheng-Yeh theory reasonably well describes the collection efficiency of single wire screens, for both atoms and particles, and for a wide variety of screen geometries, using the BF constant. 

Figure 2. SEM images of results from a selfpassivation experiment (a.) Kapton H , (b.) Si02 coated Kapton HN (Provided by Astral Industries Inc.), and (c.) 8.75 wt % SigOjj cage MC-POSS-PI. Samples were exposed to 2.3 x 1020 atoms/cm2, scratched, covered by a wire screen and exposed again to 2.3 x 1020 atoms/cm2. The area covered by the wire screen was (a.) the right side, (b.) the bottom portion, and (c.) the left side.

Papastefanou and Bondietti (1991a, 1991b) performed experiments on the diffusive deposition of aerosol particles on wire screens and, in particular, used Pb deposition as a measure of the collection efficiency of the screens for aerosol-associated attached radionuclides in outdoor air, at Oak Ridge National Laboratory, Oak Ridge, Tennessee (35 58 N, 84 17 W) during the summer period. Stainless steel wire screens (60, 200, as well as 40 and 100 mesh/inch) were used in the experiments to collect the unattached species of radon decay products in ambient aerosols. Glass fibre filters were used as back-up to collect the radon decay products which passed the wire screens. The screens were separated from the back-up filter by a spacer screen (4 mesh/inch) to prevent contamination by the filter deposit (e.g., " Pb atoms) via a-recoil. 

Muller (1951, 1956) developed this instrument, which for the first time enabled extensive details of the atomic structure of a solid surface to be seen directly. Figure 1.1 illustrates schematically the basic construction of a FIM. The specimen is prepared in the form of a fine wire or needle, which has been chemically or electrochemically polished to a sharp point with an end radius typically 50-100 nm. It is mounted along the axis of a vacuum chamber, about 50 mm from a phosphor screen (perhaps 75 mm in diameter). The specimen is mounted on an electrical insulator within a cryostat, and it can be raised to a high positive potential (3-30 kV) by means of the leads attached. 

FIGURE 4.15 Proton transfer QM system Proton water wire, proton donor Glu242, and proton acceptor Prop D. The boundary of the QM/MM system is at beta-carbons of Glu242 and Prop D, where the dangling bonds are snbstitnted with H-atoms. This system interacts with the medium via LJ (walls of the QM cavity) and Conlomb forces screened by the external solvent and the membrane. 

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