Calibration Experiment on the Detection of Fissioning Material in Soil by Means of Neutron Detection

Abstract

The mass of a fissioning material and its distribution in soil can be estimated by analyzing the results of the detection of neutron and/or γ radiation in wells drilled in a clean zone near the assumed location of the material [1]. The PRIZMA.D program [2] with the BAS library [3] and the MCNP program [4] with the ENDF-BV library, which were verified on calibration experiments, are used in the mathematical simulation of an experiment, choosing the forms of the material and the power of the radiation to obtain the best agreement between the computational and experimental dependences obtained when working under field conditions.Calibration experiments when working out the procedure were performed in 2001 and 2003 under laboratory conditions with a point source -an IBN-8 constant Pu-Be source with neutron yield ∼1.6·10 6 sec -1 or a small object made of the fissioning material. The main purposes of the experiment were to check the utility of the method for detecting fissioning materials in soil, on the basis of neutron detection, and obtain experimental data suitable for verifying the computational procedures. Sand with a known chemical composition was used as the soil.Experimental Arrangement. The experiments were performed on a 175 × 80 × 114 cm measurement volume with a well simulator in the form of an aluminum tube with outer diameter 95 mm and wall thickness 7.5 mm, inside which a SNM-18 neutron detector was positioned (Fig. 1). The tube was placed perpendicular to the plane of the bottom along the entire height of the container. Two tubes, made of aluminum alloy with one tube placed inside the other, were laid in the central plane of the container. The outer stationary tube functioned as a casing; its outer diameter was 38 mm and its wall thickness was 2 mm. A second tube, with outer diameter 30 mm and wall thickness 1.5 mm, was placed inside the casing tube. A capsule with the source or an object made of the fissioning material was secured at the center of the inner tube. When the inner tube is moved only the distance between the capsule and the detector changes; the rest of the configuration remains unchanged.To increase the water content, sand was poured from the measurement container into two boxes, water was added, and the mixture was manually mixed, after which it was reloaded, taking samples to determine the water content. Three series of measurements were performed with water content in the sand 0.3, 3.8, and 8.1 mass% (dry, moist, and wet sand) and density 1.39, 1.45, and 1.47 g/cm 3 , respectively. A sample of hard rock (stone) with known chemical composition and water content and a cavity -air and water -were used as nonuniformities; they were buried in sand near the detector. The cavity was made from a 1.5 liter household plastic container. Water was poured into and out of the cavity along two hoses, so that its position during the experiments remained unchanged for one load of sand.The experiments were performed in the building where the fissioning material is stored. Despite the thickness, the co...