§-S-4 22.3 pCi/g 241Am Open area is 626 ft.2 Clear area ae The effective area seen by the IMP is the area multiplied by the detector efficiency. Table B-1-3 is a computation of the value including the effect of the road. Corr. Factor (626 ft2/3621 ft2) + 17.4% (Road) 0.173 + 0.174 = 0.347 = 1.147 ° (0.347)(1.147) + 0.653 - 1.147 . 0.398 + 0.653 _ = 1 147 1.051 = 1.091 22.3 pCi/g x 1.091 = 24.3 pCi/g The original concept of the experiment was that a common attenuation coefficient would be found and then one would multiply this coefficient by the average height of the brush. It was soon apparent that there is no common attenuation coefficient. Table B-1-4 shows the computation of the brush attenuation factor. Table B-1-5 shows the data and that the attenuation coefficient has a coefficient of variation of 65.6%, which is a broad distribution around the average. It became clear on examining the data for 241Am that regardless of the height of the brush the clear to brush ratio had a tight coefficient of variation. Figure B-1-2 is the average data extracted from tables B-1-4,-6,-7 and -8. These averages are for 24 Am, 155 Eu, 137¢s and 89Co. The 88co data, because of the poor statistics, has the average value presented for both 1173.2 and 1332.5 keV and is given the average energy of 1252.8 keV. After the data had been compiled it was noted that the data was less than 1.0, which is a physical impossibility, but a statistical probability due to the low level of 59Co and the small attenuation. The 50Co data is therefore not used in Figure B-1-2. The data in Figure B-1-2 has a straight line fitted to the data points of the brush attenuation experiment. Wayne Bliss suggested that this indicated the brush attenuation was of the form of an umbrella effect or a canopy of leaves. Visual observation indicates that the canopy is real, for branches of the scaevola are relatively clean of intermediate branches, but branches out at the top exposing all of the leaves. Therefore, the height of the scaevola bush is not important. An attempt was made to verify this idea by assuming the canopy of leaves to have an equivalent thickness of carbon (which it is largely composed of) to reduce the 241 Am by 1.147. The thickness necessary to reduce the 60 keV to what is observed experimentally is 0.343 em. This thickness is then used to construct a curve (from the data in Table B-1-9) that is superimposed on Figure B-1-2 to show what effect a simple canopy of carbon would look like. The reasons that the curves are not superimposed at all energies are numerous: 1. The poor statistics of the experiment at high energies, as is evident from the 60Co. 2. The poor geometry as compared to good geometry from which attenuation coefficients are derived, and which we used for carbon. 3. The resolution of the crystal eliminates even a slightly scattered gamma-ray out of the gamma-peak, measured by the intrinsic germanium crystal. A dose measurement with ion chambers would probably cause the two curves to become congruent. In conelusion we find no difficulty in using a single attenuation coefficient of 1.147 and applying it to the data after allowing for the effect of any clear areas. The aerial survey would use the 1.147 correction to all data measured over brush covered areas. B-1-2