TM 5-6635-386-12&PFigure 1-4. BS and AC PositionsNOTEThe tester’s raised (collimated) BS position blocks shallow gamma rays to provide deeper reading for soils. Lower ACposition permits utilization of shallow rays to decrease depth of effective reading for shallower materials. (Surfaceroughness effect increases with AC position, however, and site prep requires care).If the radioactive source is NOT collimated, the sourcebeing positioned at the bottom of the tester permitsgamma rays to stream at an angle backwards towardsthe detector, the tester will pick up 90% of its returnedradiation from approximately 1.5 - 1.75 inches deep.Both positions will read the same overall depth for 99%of its returned radiation.Thus, if the intention of the reading is to read a soilembankment of more than 2-inches thickness, theneither BS or transmission should be used to insure thedeepest possible reading.If the density of a thin lift overlay is desired or any lift ofless than 2-inches thickness, then it would be desirableto use the AC position and to place the additional surfaceseating care required in these shallow positions.Figure 1-5. Effective Depth-Inches1-17. TRANSMISSION ROUGHNESS ERRORTransmission is inherently an accurate mode ofmeasurement in all respects. It is a linear measurementwith depth and is not fooled by varying densities near thesoil surface and is comparatively immune to surfaceroughness problems.1-14
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