Four-stage low-pressure cascade impactor

The detection efficiency for unattached fractions is obtained from the product of the geometric efficiency (11.6%), the collection efficiency on the screen (86.1%) with a face velocity 37.1 cms (Thomas and Hinchliffe, 1972 Cheng et al., 1980) and the counting efficiency for unattached fractions on the up-stream side of the screen (80.1 %). The 50% cutoff diameter 

In order to determine radon decay product concentrations, the build-up and decay methods are used (Cliff, 1978 Trembley et al., 1979). The following timetable is used the collection period is 28 min, the first waiting period is 2 min and there are four time-measurement periods of 10 min with a 2-min interval for the readout between them. Using the counting data from the resulting measurements, radon decay product concentrations are estimated by the least square method. 

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The four-stage low-pressure cascade impactor incorporates both the impactor and wire screen methods (Tokonami et al., 1997). This system can measure the activity size distribution of radon decay products in a low level environment within 90 min. Figure 6.9 shows a block diagram of the activity-weighted size distribution instrument. In the first air inlet, unattached radon decay products are collected on a metal wire screen (300 mesh openings 118.2 cm wire diameter 3.75 x 10 cm). A silicon semiconductor detector, SSD, is set opposite the metal wire screens where both collection and detection are concurrent. Output signals from the silicon semiconductor detector are sent through a preamplifier, PA, and the internal amplifier of a multichannel analyser, MCA, and then to the multichannel analyser. 

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