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.