Atmosphere, background

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

Figure 11.13 shows a typical DO AS spectrum measured in air after correcting for atmospheric background light and an electronic offset (Stutz and Platt, 1997). Below the spectrum are shown reference spectra for the gases that contribute to the atmospheric spectrum, scaled by the a, factors determined using Eq. (H). In this case, O, N02, SOz, and HCHO all contribute, leaving a residual spectrum with a peak-to-peak absorbance of 6 X 10 4. 

To describe the atmospheric background levels of POPs over Europe... 

Figure 14-11 shows contoured soil-air CO2 data collected from a depth of 1 m, overlying a breccia pipe which is heavily mineralised at depths of 150-250 m. The anomaly peak is only 0.3% CO2 (only ten times the atmospheric background) but the circular gas halo is centred over the mineralisation (Lovell and Reid, 1989 Reid and Rasmussen, 1990). The fact that these low concentrations accurately reflect mineralisation at these depths is a testament to the sensitivity and precision of the gas-chromatographic method of analysis that was used, and to the low background. 

GalballyIE. 1985. The emission of nitrogen to the remote atmosphere background paper. NATO ASI SerC 159 27-53. 

Atmospheric Background Light from Cosmic Rays... 

Andreae, M. O. (1985) The emission of sulfur to the remote atmosphere Background pap>er. In The biogeochemical cycling of sulfur and nitrogen in the remote atmosphere (Eds. J. N. Galloway, R. J. Charlson, M. O. Andreae and H. Rodhe). Reidel, Dordrecht, pp. 5-25 Andreae, M. O., R. W. Talbot, T. W. Andreae and R. C. Harriss (1988) Formic and acetic acid over the central Amazon region, Brazil 1. Dry season. Journal of Geophysical Research 93, 1616-1624... 

The first two steps are carried out under high vacuum, which allows ions to move freely in space without colliding or interacting with other species. Collisions may lead to fragmentation of the molecular ions and may also produce a different species through ion-molecule reactions. These processes will reduce sensitivity, increase ambiguity in the measurement, and decrease resolution. In addition, the atmospheric background will introduce interference. 

By definition, MCF is equal to C/ /Cq. Atmospheric background irradiance contrast reduction is... 

It should be noted that the atmospheric background MCF, Eq. (22a) or (22b), is not a function of spatial frequency. However, for a sinewave target, such an MCF also is an MTF. It represents a constant damping of the overall... 

The contrast degradation caused by atmospheric background causes the overall system MTF to be damped uniformly across the spatial frequency spectrum. As shown in Fig. 7, this leads to a reduction in or an increase in Ax in Eq. (6), thus impairing resolution. For orientation, recognition, and identification requirements, as opposed to simple detection, should be divided by the required umber of TV lines or spatial fi equencies (discussed in Biberman, 1973, or Kopeika, 1998, concerning the Johnson chart). 

FIGURE 7 Reduction in usable resolution imposed by uniform damping of modulation contract function (MCF) by atmospheric background. MTF, modulation transfer function. 

This type of sensor is capable of locating leaks from a distance of up to 20 m, with typical response times of 0.1-1 s. Over the range of a few metres the sensors exhibit sensitivities close to that required for detecting the atmospheric background (about 1.6 ppm). 

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