A study on the investigation of the minimum detectable activity using the 3-PM LSC method

We investigate the low level detection limit of the liquid scintillation counting method using the three photomultiplier tubes as detectors. For this aim, we have measured the activity, in decaying from 10 to 7 Bq, so as to obtain the decay curve of the pure beta emitting nuclide 89Sr. The detection efficiency K was derived by applying the TDCR(Triple to Double Coincidence Ratio) method. The efficiency function is obtained overall the regions where 40 ≤ K ≤ 94% by applying the multi channel time scaling and computer discrimination analysis method at the same time. To determine the activity, the logic sums of double coincidence rates, derived on the regions of 85≤K, were fitted to the least squared linear function and finally the activity is determined by extrapolating the linear function to K=100 %. According to our data resulted in analyzing the decay curve, we can sure that the 3-PM LSC method is suitable for the measurement of the environmental samples of the activity down to the value of 8 Bq