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Environmental Technologies Part 9

Tham khảo tài liệu 'environmental technologies part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Air Radioactivity Monitoring in Serbia 153 Under normal conditions the rate of natural radionuclides originated from radon and thoron is dominant in the measurements after 78 minutes showing pronounced daily variations maxima in early mornings and minimums in the afternoons. This daily pattern is closely connected with meteorological conditions in the ground level air and the best early indication for increased level of anthropogenic radionuclides in air. Measurements of total beta activity in air in 24- hour s aerosols samples were introduced in the late 1960ties and they completely replaced the automatic devices in 1976. The activity were determined by 2n proportional alpha beta counters in air voluminous samples 600 m3 taken on filters with effective diameter of 13 cm. The samples were measured 5 hours and 5 days after the sampling. Under no anthropogenic pollution first measurement provided data on the level of radionuclide thorium-B and its daughters and the other one on the activity of long lived radionuclides in the aerosols originated from the nuclear tests or the reactor emission. As alpha activity data are very significant fission products are in general beta gamma emitters so when increased beta activity after 5 hours is followed by an adequate increasement of alpha activity it could be attributed to natural radionuclides. On the contrary non proportionality of alpha and beta activity in air would undoubtifully pointed to anthropogenic origin of radioactive pollution. Monitoring of gamma emitting radionuclides in aerosols by gamma spectrometry started in the mid 1960ties first with an Na Tl detector well-type 3x3 crystal with 100 channel amplitude analyzer . Today three high computerized gamma spectrometric systems with HP Ge detectors are in use see 5.1 . Under normal conditions composite monthly filter samples previously ashed up to 370oC are analyzed while when radioactivity is increased filters could be analyzed without previous preparation. To .