, > . etn . . 0 Poa wae nee gt ae ce ee ee Dc el Me a eT ee ee a . The Extent of Radioactive Equilibrium Between Radon and Its Short-Lived Daughter Products in the Atmosphere L. B. Lockwart, Jr. ann R. L. PATrersON, JR. Physical Chemistry Branch Chemistry Division A set of experimental conditions has been derived which optimizes the accuracy with which the radon content of the atmosphere and its apparent residence time drere can be determined from measurement of the filterable gross @ activity during two separate 10-minute periods following the end of a 20-minute collection period. The 20-minute collection is made on a filter of essentially [00% retentivity (glass fiber filter) and counted for 8 activity through a 10-mil (70. mg/cm?) alurmnum absurber on equipment of known efficiency for the 8 particles emitted by RaB and Ra. The rane of measured counts during the 61through-71-minute decay period to that during the t-through-l]-minute period is employed to obtain the atom ratio of RaC to RaB (p) in the air sampled. the apparent age (7) of the conglomerate of radon and its daughters, and the total counts to be expected from anv given radon concentration in the air. Extensive examples of atmospheric measurements are given which suggest that under most conditions in the free atmosphere radonis essentially at secular equilibrium) with its daughter products. Other collections made following addition of radon to a restiicted voliime show the expected ine rease in p with time, which confirms the basic validity of the procedure. INTRODUCTION In a previous report (1) it was shown that the radon concentration of the atmosphere could be deduced from measurementof its short-lived decay products by their collection on a filter for a fixed period of time followed by f-activity measurements during two subsequent periods. In addition to the usual errors associated with air flow measurements and counter calibrations, two sources of error either could not be controlled or were unresolved: the large statistical error inherent in any counting process when a limited number of random events occur and the uncertainty in evaluating the counting efficiency for the conversion electrons emitted by RaB (Pb*"*} and RaC (Bi#!4), The radon present in the air sample, moreover, was assumed to have resulted from an instantaneous emission of radon gas from the soil; this was, of course, an obvious oversimplification. This report considers means of optimizing the accuracy and sensitivity of this method by determining those counting conditions which reduce the effect of the conversion electrons and their NRL. Problem A02-13; Projects RR 004-02-42-5151 and AEC AT 49-7(2435). This is an interim report; work on this problem 3s con- tinuing, Manuscript submitted November 9, 1965. uncertain calibration and which also permit the greatest sensitivity in the determination of concentration and age from -activity measurements made during two fixed decay periods. THEORETICAL CONSIDERATIONS The growth of radon daughter products in the atmosphere from a fresh radon source, their growth and decay ~n a filter during a subsequent 20-minute period of air filtradon, and the radioactivity decay relauonships between the solid daughter products collected on the filter were presented in the previous report (1). These data have been revised and extended to permit interpretation of the apparent radon age in terms of its conunuous emission from the soil rather than its instantaneous emission. A summary ofthis extension of information is given in Table 1. Plots of the RaC/RaB atom ratios (p) as functions of elapsed time are shown in Fig. 1 for the conditions of instantaneous and continuous addition of radon to the atmosphere. It is evident that the higher p values will represent a much longer ume ofgrowth of the radon daughters wheninterpreted in terms of the constant introduction of fresh radon. Similarly, -the radon/RaB activity ~ ‘ oa