IAEA-SM-354/93P XA9951340 DEVELOPMENT OF AN UNDERWATER CHERENKOV DETECTOR TO REVEAL SOURCES OF TECHNOGENIC RADIONUCLIDES A.M.CHERNYAEV, I.A GAPONOV. Russian Research Center "Kurchatov Institute", Moscow, Russia L.V.LAPUSHKINA National Research Institute "Electron", St. Petersburg, Russia For monitoring marine radioactive contamination sources the underwater scintillation detectors are extensively used, permitting in situ detection of radionuclides both from a surface ship and from underwater vehicles [1], Detection of gamma-emitting radionuclides by the underwater scintillation detectors does not involve any difficulties. The problem consists in detection of the nuclides, when their beta-decay is not accompanied by gamma radiation. First of all, it is true for strontium-90, which is the most significant nuclide from ecological viewpoint. To enhance the potentialities of in situ radioactivity monitoring in marine environment, complex research aimed at the development of an underwater Cherenkov detector for measuring of beta-decay nuclides, featuring a high sensitivity not only in deep-sea but in near the surface sea water and characterized by a high level of the natural light background, has been undertaken at RRC KI . To suppress the light background this project makes use of an original method of optical filtration of light fluxes by the wavelength, making allowance for differences in spectral characteristics of the Cherenkov radiation and atmospheric light, as well as the optimal combination of optical and geometrical parameters of basic elements in the photodetector [2], A schematic diagram of the Cherenkov detector being developed is presented in Fig. 1 and it consists of a high-strength capsule (1) with a transparent inlet window (2), enclosing a solar-blind filter (3), a photomultiplier (4) and an electronic module (5). Ns^^^^^<ss^^ FIG. 1 Structural schematic diagram of the underwater Cherenkov detector. 406 The spectral sensitivity range of the Cherenkov detector is concentrated at the band of 220-300 nm. When passing to the spectrum visible range, the sensitivity decreases abruptly. Moreover, the sensitivity drop occurs in such a way that atmospheric light integral values at the output from the detector can not be distinguished, at the background of statistical fluctuations of the Cherenkov radiation integral values, from natural potassium-40 radionuclide present in sea water. The result mentioned is achieved by using the solar-blind filter incorporating an air-isolated alkali metal layer deposited on a transparent substrate in combination with the photomultiplier using either TeCs or TeRb photocathode. Besides, the solar-blind filter contains an additional interference filter made from materials, which are transparent in the working range of the spectrum. Our estimates show that the threshold sensitivity of the detector module for strontium-90 in the sea water surface layer under the most unfavorable conditions will make up approximately 400 Bq/m3 if the measurement duration is 1000 s. REFERENCES [1] Proceedings of the Workshop on Monitoring of Nuclear Contamination in Arctic Seas, 18- 19 January 1995. NRL/MR/6610-95-7674. [2] Patent RU 2092871. Data of patent 03.04.96. The bulletin of discoveries, inventions and trade marks No 28, 10.10.97. Detector of Cherenkov radiation. A.M.Chernyaev, A.Yu.Rumyantsev, I.A.Gaponov , L.V.Lapushkina et al. 407.