Standardized low-level measurement methods for environmental studies

Since 1958 this centre (BARC) has been carrying out an extensive programme of country-wide monitoring of radioactive fallout and natural radioactivity. This required the development and setting up of a large number of low-level alpha, beta and gamma counting systems and standardization of sampling, measurement and analysis procedures for a variety of sample matrices. Interference in gamma spectrometry due to 41A produced by nuclear research reactors at this centre required the setting up of a low-level counting laboratory away from the centre. This laboratory was constructed in 1973 at the BARC Hospital complex, about 6 km from the reactors and was fully equipped and commissioned in 1976. The entire nuclear instrumentation has been done in this Air Monitoring Section. Experience gained in the early years of the development of low-level counting systems and their standardization procedures have been discussed in earlier publications. Low-level systems are required primarily for (a) beta counting and (b) gamma spectrometry. Optimization of the overall capabilities in this regard requires: (1) Increasing the counting efficiency by correct choice of detector systems; (2) Decreasing the background by graded massive shielding and electronic systems, e.g. anticoincidence shielding; (3) Increasing selectivity and sensitivity either by employing selective chemical separation methods, or by the high-resolution systems, sample preconcentration, in the use of optimum sample geometry for the best counting efficiency. Developments in all the above areas carried out at this laboratory are briefly reviewed. The approach followed here is best suited for a developing country with limited financial resources. The lowest detection limits for various sample matrices for the systems currently in use are also given. (author)