The determination of grand surface radioactive contamination. The application of in-situ gammaspectrometry using a flying helicopter.

In-situ gamma spectrometry can be used to determine the qualitative and quantitative deposition of radioactive materials on the ground surface. By applying the in-situ spectrometry method using either a helicopter or an airplane, large areas can be scanned in a short period of time. In this report the results of in-situ gamma spectroscopic measurements taken from a helicopter are described. Measurements were carried out using a single point source, a field of 36 point sources, and using the present ground contamination due to the fall-out from the Chernobyl accident and atom bombs. The results of these measurements were used to determine calibration factors, which were in agreement with a calibration obtained using more simple (and less expensive) laboratory measurements in combination with flux calculations. Detection limits for the measurement of surface contamination were determined. At a height of 50 meters above the surface and using a measurement time of two minutes, the minimally detectable surface contamination was 1.1 kGqm -2 for a Cs-137 contamination and 2.1 kBqm-2 for an I-131 contamination. Fall- out determinations based on measurements taken at a height of 50 meters were in agreement with determinations taken at a height of one meter, and with the results obtained measuring soil samples. The in-situ gamma spectroscopy, using either a helicopter or an airplane, is a fast and powerful method for mapping surface contamination.DGM/