ts RsitAR“RONNIEmere «= 13 NRL REPORT 6374 of aerosol, little of the radon daughter activity remains airborne. The progressive decrease in observed radon with time is in accord with the loss of aerosol through coagulation and deposition, during which umethe relative surface area of the wall increases. The wall effect is greatest in the large chamber, which contains so much perma- nently mounted equipment that it has a large surface-to-volume ratio. The apparentincrease in radon concentration with introduction of addi- tional smoke, as noted in Table 5, is to be expected if the relative surface areas of aerosol and wall are of significance. The application of correction factors based on the p values does give a rather high degree of internal consistency to the measured radon con- centrations, except for the first few measurements involving extreme degrees of nonequilibrium. Even these fall reasonably well into agreement when corrections are applied for the rapid changes ter as in p occurring during the early collections (see Appendix). In any case the corrected values are morerealistic than those derived from the assump- tion of secular equilibrium between radon and a ee aes erp its daughters. The discrepancy between the ex- perimental and the true radon concentration should not be encountered in measurements made of the radon concentration in the free atmosphere, since in that environment interaction with the ground surface would be much morerestricted. On the other hand, use of this technique to measure radon concentrations in mines, tunnels, or other closed spaces might be expected to show a products may be conveniently determined by 10-min counts starting 1.0 min and 61.0 min after a collection of 20.0: min duration; counting on calibrated equipment with a 10-mil aluminum absorber inserted between the sample and count- ing tube gives a good compromise between sensitivity and the uncertainues introduced by the complex group of conversion electrons emitted and by counting statistics. This method will detect any gross departures of radon in the atmosphere from secular equilibrium with its daughter products and permits a determination of the apparent age of the mixture. In the free atmosphere radonis apparently near secular equilibrium with its daughters at most times; however, during initial stages of inversions and in recendy ventilated spaces equilibrium departures can be expected. Chamber studies using artificial aerosols indicate a loss of radon daughter products to the walls or other surfaces and suggest that underestimation of radon by this methoct may result when the surface-tovolume ratio is high or when mixing promotes intimate contact between the air and a surface. ACKNOWLEDGMENT The authors wish to express their apprectation for help given them during the chamberstudies by Dr. James A. Young and Mr. Wendeli L. Anderson of the Protective Chemistry Branch, NRL. wall effect whose magnitude would depend on the configuration of the restraining walls and the rate of mixing within the volume. CONCLUSIONS Reasonably accurate estimations can be made of the radon concentrations in the free atmosphere by collection on efficient filters of the shortlived descendants of radon and measurement of their $8 activity during two fixed periods fol- lowing collection. The measurement of the extent REFERENCES 1, Lockhart, L.B., Jr, Patterson, R.L., Jr., and Hosler, C.R., “Determination of Radon Concentrations in the Air Through Measurement of Its Solid Decay Products,” NRL Report 6229, March 1965 2. Lockhart, L.B., Jr., Patterson, R.L., Jr., and Anderson, W.L., “Characteristics of Air Filter Media Used for Monituring Aurborne Radioactivity,” NRL Report 6054, March 1964 . Wilkening, M.H.. “Radon Daughter [ons in the Atmosphere,” Chapter 21 in “The Natural Radiation Environ- we *n Was of secular equilibrium of radon andits daughter nent,” J.A.S. Adams and W.M. Lowder, editors, Chicago: The University of Chicago Press (1964)