Counting Efficiencies for Alpha Particles Emitted from Wire Screens

Abstract

Counting efficiencies for alpha particles emitted from the front and the back of 30-, 105-, 200-, and 400-mesh wire screens were measured for ultrafine radon daughter aerosols deposited at face velocities in the range 5.1 to 30.8 cm s⁻¹. Mean activity median diameters for the ultrafine ²¹⁸Po, ²¹⁴Pb, and ²¹⁴Bi particles were 0.70 ± 0.16, 1.1 ± 0.3, and 1.0 ± 0.2 nm (0.062, 0.033, and 0.038 cm² s⁻¹), respectively, as determined from graded wire screen array analysis of the test atmosphere. For wire screen collection efficiencies < 0.8, the “front-to-total” (FT) ratio, denned as the ratio of measured alpha activity from the front of the screen to the total alpha activity (front and back), was found to be insensitive to the screen and sampling parameters, with a mean value of 0.67 ± 0.02. With increasing collection efficiency, the FT ratio was found to increase, up to a maximum value of 0.86 ± 0.03 for collection efficiencies > 0.999. Alpha-particle losses within the screens (screen loss factors) were determined by comparison with counting efficiencies for radon daughters deposited onto membrane filters. For the four screen types studied, the mean screen loss factor at a face velocity of 21.2 cm s⁻¹ was 1.04 ± 0.01. A Monte Carlo simulation of alpha-particle losses within a simple woven wire screen showed that the FT ratios were sensitive to the functional form of the deposition of the radioactive aerosol around the wire cylinders of each screen. Screen loss factors derived from the Monte Carlo analysis were found to be insensitive to the deposition on the wire, but dependent upon the counting geometry, in particular the distance between the wire screen and the detector.