ADDITIONAL MEASUREMENTS OF BRUSH ATTENUATION AND CALCULATION OF BRUSH CORRECTION FACTOR DOE/ERSP TECH NOTE NO.1.1 DATED: 3 August 1979 AUTHOR: R. Jaffe, EG&G There has recently been renewed interest in the question of the attenuation factor attributable to brush covering an IMP measurement area. Consequently, the original Tech Note1 (undated, about November 1977, by F. Tomnovec) was examined, and two additional experiments were conducted. The purpose of this note is to discuss the original tech note and to present additional data. The first experiment to be discussed is a direct measurement of brush weight per unit area. The second experiment is placing a known 241Am source under brush cover, and calculating brush attenuation, the reciprocal of which is the brush correction factor (BCF). These experiments confirm the original factor proposed for BCF of 1.15 for a high density brush cover. Original Work and Analysis The original work (in October-November 1977) was done on Pearl. IMP access lanes were cut through and 241m readings taken at ten locations. The 84th Army Engineers then carefully cut by hand the brush in a seventy foot circle, removed it, and the IMP remeasured these points, These data were analyzed, and the effect of brush determined. BCF is the ratio of clear-area readings to brush-covered-area readings. BCF was calculated as 1.147 for a 100% brush-covered area. The concept proposed was to determine the total open area fraction and then calculate: BCF = 1.147 /(Open Fraction x 1.147 + (1 - Open Fraction)) = 1.147 /(1+0.147 (Open Fraction)) which is rounded and simplified to: = 1+0.15 (1 - Open Fraction). ‘There was no correlation in the experimental data with brush height, which may be explained as a canopy of brush cover independent of brush height, which is reported to be characteristic of the dominant seaevola brush. The density of brush growth and fraction of brush-covered area are both included in the brush coverage observation recorded at each measurement location by the IMP operator. An objection has been raised to the original tech note concerning the omission from the analysis of one of the ten experimental measurements. As the author is not available for consultation, it is necessary to speculate about the reasons for the omission. These may be: that for the location in question, the open area fraction is about a factor of two higher than for the next highest open-area location; or that in subsequent debris removal, an atypically large decrease in 241Am was noted, implying a localized concentration pattern, which would be undesirable for BCF determination. For whatever reason, data from this location, 5-S-3, were not included. There were four measurements taken before debris removal at that point: DATE 10-08-77 241 Am (pCi/g) READING COMMENT 35.9 "Average 7' brush/two areas 18' dia open grass/dead brush in road/stake under growth." 10-13-77 45.1 Brush cleared. 10-20-77 43.3 300 second data acquisition time. 11-18-77 41.3 B-1-9