To determine BCF, the effect of this assumed slab shield over the surface must be properly averaged for detector response geometry. The response given in EG&G Report RSSD 78-177 (August 1978) was used. The equation is: BCF = = (8) R (@) =f(@) R (6) exp (-d/ cos 6 ) u il + Wow WoW We st oD Dad ~~ ~— where tan 6 exp (-Fgh see 8) /(@ +H, see 8) flux at angle 6 detector angular response attenuation coefficient = ©»Pt for brush detector view angle linear attenuation coefficient for air height of detector reciprocal of the relaxation length of the source logarithmic distribution in the soil linear attenuation coefficient for soil For the last four factors, the reference value for the IMP calibration factor was used, as discussed in the reference report. The calculation was done numerically considering five degree increments from 0 to 62.5 degrees. The resulting BCF is 1.22. It is worth noting that this is very close to the 1.20 value obtained by calculating BCF at 35 degrees, which is the angle at which 50% of the total detector counts are received, i.e., exp (0.148/eos 35) = 1,20. Response to Source Under Brush At the suggestion of J. J. Giacomini of DRI, an experiment was jointly designed by J. L. Pigg of EG&G and Giacomini. It utilized the on-atoll 241Am source used to calibrate the IMP. Essentially, it involves placing the source under representative brush and determining the count response. Knowing the response obtained for the same geometry with no brush, the BCF can be calculated. The experiment was performed on the island of Kate, and the reference no-brush geometry was tested on Ursula, near the IMP garage. Data for the no-brush test are given in Table B-1-10 and Figure B-1-3. Figure B-1-3 gives the experimental data, normalized to the count response observed with the source directly under the vertical axis of the detector. (The count rate agreed within 8% with that calculated from the inverse square law and the last calibration of that detector.) A calculation of the normalized detector response was made, using the detector angular sensitivity determined for a similar detector (during IMP calibration in July 1977 at EG&G, Las Vegas), and the inverse calculated response is high by about 8%. It is believed that this is due to the non-isotropic nature of the source, which was kept flat on the ground during the experiment, rather than angled toward the detector. (The source dise is recessed slightly inside an annular aluminum ring.) Table B-1-11 gives the brush data and the results of the BCF calculation. The three valid runs taken with this technique give an average BCF of 1.12 for "Medium Dense" brush. In the experienced IMP operator's judgment, this area would be rated as about 60% brush covered. The BCF would thus be calculated as 1 + (0.12 /0.6) = 1.20. Summary and Recommendation The original study gave 1.15 as BCF. Including the tenth point would give 1.17. The direct brush weighing gives 1.22. Placing a source under brush gives 1.20. It is the author's judgment that all available present data show that 1.15 may continue to be used for BCF. The extensive experimental program that would be required to obtain a better value is judged to be not warranted. B-1-11