4 LOCKHART AND PATTERSON backscattering with lead-shielded, end-window and RaC. The efficiency of the equipment toward these radon daughters is shown in Table 2 for the various absorber arrangements. The accumulated counts to be expected following the filtration of air samples containing 100 picocuries (pCi) of radon per m? for 20 min at the rate of 1 m? per min are given in Table 3 for the 10-min periods starting at 1 min and at 61 min following the termination offiltration for different values of p and for different absorber thicknesses. The choice of these two time intervals was made on the basis of the sensitivity of the resulting count rate ratio to changes In p, as described in the previous report (1). The indicated count ratios during the two fixed ume periods G-Mtubes of 5.6-mg/cm? windowthickness using conventional B-counting equipment. Counting was timed and recorded byuse of Ametron Count Recorders (Streeter-Amet) which had been modified to operate on a 10-minute cycle. ea pow mee Mee tetae ment. Typical calibration data are shownin Fig. 5; counung was done through a series of aluminum absorbers to determine the most appropriate system to use in subsequent measurements of RaB | COUNTING EFFICIENCY (PERCENT) rs ie a aati ae maggie oe ee eee id ee eee Yak ae The equipment was calibrated as described in the previous report (1) by use of standards of C387, Pb?! (Bi?!), and UX, prepared to simulate as nearly as possible the filter size and arrange- (f1-11 andl f61.71) have been emploved to construct curves for translating measured ratios and total counts into p values and radon concentrauions, as shown tn Figs. 6 and 7, respecuvely. The radon concentration so determined is that which would have resulted from a single, instantaneous enmussion of radon from the soil: the radon value cor- O- responding to the steady emission of radon can be O Oo | J 2 Bmax ENERGY (meV) 3 4 Fig. 5 — Counter cabration curves as a function Of Baur, energy and absorber thickness found by adjusting the above value by a factor obtainable from Fig. 4. The greatest sensitivity in the determination of pts obtained when a 30-mil aluminumabsorberts employed, since this effectively stops all radiation except that associated with the RaC component. On the other hand the counting rates are low with resulting increased stauistical uncertainty in TABLE 2 Efficiency of 8 Counung Equipment Toward RaB and RaG Deposited on a Glass Fiber Filter Counting Efficiency From Fig. 5 and After Estimated Correction for the Effect of Conversion Electrons (%) Powtw) A i is shies Ait ca Dee ee Cait nie Activity RaB (Pb?) 0.65 (100%) (25% low-energy con- No Absorber 5-mi! Al Absorber 10-mil Al Absorber 30-mil A' Absorber From Graph; After Corr. From Graph; After Corr. From Graph After Corr. From Graph After Corr. 10.90 13.63 4.10] 4.51 1.82 1.87 0.0 0.0 13.79 14.48 10.17 10.68 7.75 8.14 3.06 3.2] version electrons) RaC (Bi?!*) 3.17 (23%) 1.65 (77%) (5% high-energy con- version electrons)