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

A gamma locator designed to conduct contamination surveys inside the reactor hall of the Chernobyl NNP 4th 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 /sup 137/Cs effective surface activity.