BRUSH ATTENUATION FACTOR DOE/ERSP TECH NOTE NO.1.0 DATED: December 1977 AUTHOR: F. Tomnovec, EG&G Both the in situ van and the aerial survey are designed to measure the characteristic 59.5 keV gamma-ray radiation from 24l1Am. On the islands of Enewetak Atoll, the dense brush undergrowth provides significant attenuation for this low-energy radiation. In an effort to determine the degree of attenuation, an experiment was performed on Pearl. Ten sites were carefully chosen to get various average heights of brush and the in situ van (hereafter identified as the IMP) made a measurement at each of these points. The 84th Army Engineers then carefully cut by hand the brush in a seventy-foot circle, removed it, and the IMP remeasured these points. Table B-1-1 is a resume’ of the IMP operator's impressions of eachsite. It should be noted that several sites had some clear areas; Table B~-1-2 indicates the magnitude of the clear area to the total effective area. The effective area is here defined as the actual area times the IMP's detector efficiency. This is an averaging method that allows us to disregard the exact location of each clear spot to the detector. To properly allow for the effect of the clear area seen by the IMP detector we must add all the clear areas together. Let us look at the logic and a sample calculation of one station, 6-S-1. 6~-S-1 IMP measurement before clearing of brush = 14.8 pCi/gm IMP measurement after clearing of brush = 16.2 pCi/gm Figure B-1-l.a Figure B~1-1.b Figure B~1-1.¢ 3 100% a We measured this = 16.2 pCi/gm Ratio = clear area —_— 100% Brush Brush We measured this We can't measure this = 14.8 pCi/gm Road = 17.4% clear area 16.2 = 14.8 - 0.174 (16.2) 0.826 16.2 =o 11.9812 but we can calculate it 16.2 ——_ 14.50508 =_sC 11.116 85 0.826 FIGURE B-1-1. MEASUREMENT OF 24l1A4m IN CLEAR AND BRUSHY AREAS We would have liked to measure Figure B-1-l.a/Figure B-1-1.c directly but our IMP cannot negotiate the heavy brush so a road is cleared by a bulldozer and we can make the measurement in Figure B-1-1.b. We merely make a calculation of the radiation seen by the IMP detector of any clear area, and subtract it from the reading of Figure B-1-1.b. The resultant is an IMP measurement of the remaining radiation attenuated by the brush. In this case 82.6% of the IMP measurement is from the brush covered area and 17.4% is from the clear area. When one divides the remaining radiation from the brush by the area of the brush we get 14.5 pCi/g, which is the measurement when there is 100% brush attenuation, the condition of Figure B-1-l.c. The ratio of Figure B-1-1.a to Figure B-1-1l.¢c gives us our brush attenuation factor. This brush attenuation factor is 14.7% for a 100% brush covered area. Therefore, every IMP egpourement point has a clear area, the road plus any other clear area. An example of its use is as ollows: B-1-1