Calibration Facility To calibrate the above instruments, a calibration facility has been built which consists of a four-quadrant tank and two 47 subsurface systems (see Figure 3). All tanks are 2.4 m deep. The 3.0 m diameter, four-quadrant tank houses four different matrices: control sand, normal backfill sediments, simulated burial grounds, and salt cake. Wells are available (see Figure 3) for sources and detectors. The 41 tanks are 1.2 m in diameter. One contains a mixture of gamma emitters and control sand and the other a mixture of plutonium and sand. The Calibration Facility will be used for attenuation coefficient determinations, calibration of downhole instruments, range of interrogation determinations, effect of moisture, triangulation studies, interference studies, and special studies such as "smear" effects. Other Systems Other systems not included in the above are "passive" activation, void volume detectors, and measurement of nonradionuclide toxic substances. "Passive" activation is the use of metal foils downhole to determine the neutron field, and from this the plutonium plus americium can be calculated. The plutonium can then be calculated by subtracting americium as determined by gamma detectors from the total. Foils such as copper, indium, manganese, aluminum, and zinc are used. Several techniques have been explored for void volume measurements such as radar, transfer impedance, and electromagnetic wave probing. The californium-252 system will be studied for measurement of cadmium, mercury, fluoride, and other nonradionuclide toxic substances. Also, chemoluminescence, X-ray fluo- rescence techniques will be explored for in situ assay of toxic substances. FUTURE All systems discussed above will be updated as improved detectors, electronics, and computer components become available or are developed. New concepts will be explored, such as in sttu measurement of cadmium, mercury, fluoride, and chloride by techniques like chemoluminescence and neutron activation. Measurement of tritium and iodine-129 will be explored using tm sttu laser techniques. Real-time gas analyzers for stack gas and resuspension assays will be studied. New uses of tn sttu instruments will be evaluated, e.g., measurement of subsurface grouting coverage by vanadium activation. 702