Figure 1-4. BS and AC Positions

TM 5-6635-386-12&P Figure 1-4. BS and AC Positions NOTE The tester’s raised (collimated) BS position blocks shallow gamma rays to provide deeper reading for soils. Lower AC position permits utilization of shallow rays to decrease depth of effective reading for shallower materials. (Surface roughness effect increases with AC position, however, and site prep requires care). If the radioactive source is NOT collimated, the source being positioned at the bottom of the tester permits gamma rays to stream at an angle backwards towards the detector, the tester will pick up 90% of its returned radiation 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 soil embankment of more than 2-inches thickness, then either BS or transmission should be used to insure the deepest possible reading. If the density of a thin lift overlay is desired or any lift of less than 2-inches thickness, then it would be desirable to use the AC position and to place the additional surface seating care required in these shallow positions. Figure 1-5. Effective Depth-Inches 1-17. TRANSMISSION ROUGHNESS ERROR Transmission is inherently an accurate mode of measurement in all respects. It is a linear measurement with depth and is not fooled by varying densities near the soil surface and is comparatively immune to surface roughness problems. 1-14