Surface activity distribution measurements and establishment of a dose rate map inside the destroyed Chernobyl reactor

A Gamma Locator designed for contamination survey inside the reactor hall of the 4th unit of Chernobyl NNP has been developed. The device consists of a detector head and a remote control computer connected by a 150 m long cable. The detector head (dimensions: 500mm by 500mm by 400mm; weight: about 40 kg) is a collimated scintillation gamma detector (the collimation angle is 10°). It is installed on a scanning unit and was placed inside the reactor hall. The Gamma Locator scans all surfaces of the reactor hall with angular steps ( ³1° vertically as well as horizontally) and the particle fluence from the corresponding direction is recorded. The distance between the device head and the measured surface is instantaneously registered by a laser distance gauge. Inside the collimator there is a small CCD camera which makes it possible to obtain a visible image of the measured surface. The effective surface activity levels are presented in colour on the screen of the control computer. The gamma detector essentially consists of a CsI(Tl) scintillator crystal (Æ8 mm in diameter, 2.5 mm in thickness) and a Si photodiode. The detector energy resolution is about 8% for radiation from ¹³⁷Cs. The exposure dose rate distribution in the reactor hall is estimated from the measured effective surface activities (¹³⁷Cs is the main gamma emitting isotope inside the reactor hall). The results of dose rate calculations are presented in colour superposed on a drawing of the reactor hall.