A survey meter is an indispensable radiation detector for monitoring radioactive contamination in case of radiation accidents, and it is used as typical equipment in daily radiation measurement and control at radiation facilities. The survey meter measures count rate (min-1, cpm) . In general, about 30 seconds per each place is necessary for the measurement by using the survey meter with the time constant of 10 seconds, and the time of about 60 seconds per each place is necessary for a more accurate measurement. Therefore measurement at many places takes a lot of time and compels heavy labor to radiation workers. However a quick response is especially important for emergency. In order to solve these problems, the method, which predicts the final value at the initial stage without waiting for the final response, has been proposed. The method treats the condition that the time constant of the survey meter is given, and also the condition that the survey meter is scanned for finding surface contamination. Experiments were carried out using a90Sr/90Y source, 133Ba source and a60Co source. The results show that the principle of the method has been confirmed and the final response can be predicted with reasonable accuracy.
A pulse time interval analysis technique for the determination of short-life nuclide is based on the selective extraction of successiveβ-α (U-series: 214Bi→214Po (T1/2164 μs) →) or α-α (Th-series: 220Rn→216Po (T1/2145 ms) →) decaying pulse pairs (defined as correlated events) with lifetimes of the microsecond or sub-millisecond orders from the mixture activities of natural decay series. In particular, a multiple pulse time interval analysis (MTA) technique is superior to a single time interval analysis (STA) because the correlated events are certainly distinguishable from the random events or background ones consisting of horizontal line. Theoretical approach of MTA distribution was performed from two different ways. One is deduced from four typical cases which occur by a“start event”and a“stop event”. Another is based on probability of correlated events within short time of microsecond or sub-millisecond orders. Consequently, it was confirmed that same theoretical equation for MTA distribution was deduced from different ways. Radioactive nuclides in soil and quartz samples were determined using liquid scintillation counting (LSC) method combined with the MTA technique (LSC/MTA method) . By comparing γ-ray spectrometry, determination of214Po, 216Po, 238U and232Th with 10-3-104 Bq/kg order in quartz samples were recognized to be quantitatively performed using LSC/MTA method on an assumption of radioactive equilibrium. Based on the theoretical equation, it was found that the separate α-and β-pulse outputs from detector decreases the background events to improve further more the counting efficiency for the correlated events.
40K 226Ra, 232Th and137Cs concentration in dip forest soil was analyzed with respect to soil horizon and topography. In addition, a relationship between soil erosion and amount of radioactive nuclide in moving soil was analyzed. 1) Vertical distribution of40K 226Ra and232Th concentration was found non-uniform; which was high in B-horizon while low in A-horizon. 2) 92% of137Cs was kept in A-horizon, and its downward movement was restricted small. 3) 137Cs concentration was found high in crest of which soil erosion was weak, and was found low in valley of which erosion was strong. 4) A clear relationship was found between the moving soil amount, a good indicator of soil erosion, and the137Cs content in it.