Decreased Global 137Cs Fallout Background on an Inspected Territory as an Evidence for Searching for a Radiation Anomaly of a Clandestine Underground Nuclear Explosion or of an Artifact

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It was found previously that the density of the soil contamination with ¹³⁷Cs in some shafts on the Semipalatinsk Test Site appreciably decreased relative to the ¹³⁷Cs global fallout background. The occurrence of this effect was checked in 1997–1999 around ten shafts prepared for nuclear tests and three shafts after previous nuclear tests. The γ-ray spectra were taken using Ge(Li) and NaI(T1) detectors, and soil samples were taken. The measurements showed that the surface activity of ¹³⁷Cs did decrease in the radius of approximately 100 m around the surveyed shafts. This is caused by the works performed when making the testing shaft, with movement of the soil cover on the site. This effect can serve as an additional evidence for searching for an undeclared nuclear explosion in on-site inspections.