Field radiometry has been developing intensively in the last few years. Field radiometers make it possible to perform efficient examination of the surface soil on radionuclide-contaminated territories. In this paper it is suggested that this class of devices be expanded to study radionuclide contamination of bottom deposits of rivers and reservoirs. The possibility of using a radiometer with a submersible water detector for determining 137 Cs store in bottom deposits is examined. A method for determining the 137 Cs contamination density, using three energy regions of the instrumental spectrum of the submersible detector, is described in detail. The radiometer and possibilities of using it are briefly described for measurement of the 137 Cs distribution profile in the bottom deposits of the Iput’River. This instrument also gives more representative sampling of bottom deposits. Currently, the main method for investigating the contamination of bottom deposits by radionuclides is sampling fol- lowed by laboratory measurements of the activity of the samples. A similar method is also used to study the radioactive con- tamination of soil. However, this method is inefficient and laborious, which is especially so in the study of bottom deposits at large depths, since special samples are required in this case. Moreover, sampling of bottom deposits, in contrast to sam- pling of soil, cannot always guarantee that the samples will be representative. Spectrometric sensors embedded in the bottom of a reservoir or river greatly expand the possibilities of studying radionuclide contamination of bottom deposits. The large number of measurements are required in order to obtain a detailed map of the radionuclide distribution in bottom the deposits. Consequently, the main requirements for the measurement method must be efficiency combined with low cost, i.e., each measurement must be inexpensive and capable of being per- formed simply and rapidly. The methods of soil radiometry satisfy these requirements. The Korad field radiometer with a col- limated spectrometric detector was developed previously for investigating radionuclide distribution in soil [1–4]. Field mea- surements performed using this apparatus showed that it is highly efficient for studying 137 Cs distribution in soil on a con- taminated territory. Description of the Method for Measuring 137 Cs Activity in Bottom Deposits. The basic principles of radio- metric measurement of the characteristics of bottom deposits are close to the above-mentioned determination of the 137 Cs surface activity in soil using a Korad radiometer (Fig. 1). In this method the spectrometric sensor is placed on the bottom of a reservoir (river), and its position is secured with a light plastic housing, which maintains the sensor at a definite height above the bottom. Atomic Energy, Vol. 91, No. 3, 2001 RADIOMETRIC METHOD FOR MEASURING 137 Cs ACTIVITY IN BOTTOM DEPOSITS USING A SUBMERSIBLE WATER DETECTOR V. N. Potapov, 1 S. M. Ignatov, 1 V. M. Chirkin, 1 and V. G. Linnik 2 UDC 539.12.08 Translated from Atomnaya Énergiya, Vol. 91, No. 3, pp. 216–222, September, 2001. Original article submitted June 1, 2001. 1063-4258/01/9103-0749$25.00 © 2001 Plenum Publishing Corporation 749 1 Russian Science Center Kurchatov Institute. 2 V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry.