Surface activity and dose rate distribution inside 4-th reactor hall of Chernobyl NPP

A Gamma Locator designed to conduct contamination surveys inside the reactor hall of the Chernobyl NNP 4-th unit is developed. The device consists of detector head and remote control computer connected with a cable (> 150 m). The detector head is a collimated scintillator-based gamma-ray detector. It is installed on a scanning unit and is placed inside the reactor hall. The Gamma Locator scans all hall surfaces with a certain angle step and measures quantum flux from these directions. These data are used to determine the effective surface activity of contaminated surfaces. A distance between the detector head and surveyed surface is measured by a laser distance device. There is a small CCD camera inside the collimator. It obtains a visible image of the measured surface. An exposed dose rate distribution within the investigated volume is reconstructed by using the measured 137Cs effective surface activity