黄土包气带中核素迁移的直接测定

DIRECT MEASUREMENT OF RADIONUCLIDE MIGRATION IN LOESS AERATED ZONE

野外试验场核素迁移试验通常采用的方法是在示踪核素投放后不同时间在试验坑取土样 ,测定不同深度处的核素比活度分布。本文介绍了为中国辐射防护研究院野外试验场包气带土壤中核素迁移而开发的直接测定方法 ,主要介绍测量原理、装置、刻度及部分测定结果。试验所用的示踪核素源层为布放在地表下 1m深处体积为 55mm× 6 mm的85Sr、13 4Cs、60 Co与黄土的混合物。直接测量装置包括测井用探头 [ 50 mm× 30 mm的 Na I(Tl)探测器及其铅屏蔽体 ]、探头升降及深度测读部分、数据获取系统和测井四部分。文中给出了在天然条件 (C坑 )和人工喷淋条件 (F坑 )下示踪核素投放后不同时间的85Sr的垂向活度分布曲线。试验结果表明 ,F坑中示踪核素投放后第 2 6 4天 85Sr峰位向下迁移了 5cm左右 ,第 4 30天已向下迁移了约 8cm;而在 C坑 ,在约 2年试验期内85Sr的峰位几乎仍在原处。本方法所得结果基本上与采集土芯样测量的结果相符。

To take soil samples from test pit at different time after tracers were put in and to measure the distribution of specific activity at different depth is the usual method in studing nuclide migration in aerated zone at CIRP's Field Test Site. In this paper the method of direct measurement of radionuclide migration in aerated zone is reported, including the principle of measurement, equipment and its performance,the calibration method and its some results. Mixture layer of 85 Sr, 134 Cs, 60 Co with soil which dimension is φ55 mm×6 mm was buried at the centre of test pit bottom 1 m undeground,the direct measuring apparatus includes φ55 mm×30 mm NaI (Tl) detector for well logging and its lead shield,an elevator and depth shower,eletronics system for acquiring and analysing data. Curves of activity distribution for 85 Sr in test pit C (under natural condition) and F (under artificial sprinkling condition) on different date after radioactive tracers were put in are given. Results show that in test pit F the peak of 85 Sr was migrated about 5 cm and 8 cm downward 264 and 430 days after the tracer was put in respectively,but in test pit C the peak was almost not moved during testing periods of 2 years. The results agree with the ones obtained by a method of sampling soil core. Finally some aspects of measurement are analysed. (