Background: The on-line low-level liquid radioactivity concentration measurement device is designed for monitoring the radioactivity level of nuclear power plant waste water.The relation between the radioactivity concentration of 137Cs in the liquid of pipeline and the counting rate of measurement device

i.e.the response coefficient is must be known.It is generally given by experimental calibration.But the calibration is difficult for on-line measurement.Purpose: This paper attempts to study the response coefficient of on-line measurement device by calculation.Methods: The response coefficient is calculated by Monte Carlo method and in which γ-ray transport processes in detection device are simulated.The response counts in NaI detector derived from 137Cs in the liquid are calculated.Results: The response coefficient of off-line measuring device is calculated firstly.The result is in good agreement with the experimental calibration and the difference is small than 3%.It is indicated that the method is correct and the calculation model is reasonable.Then

the response coefficient of on-line measuring devices is calculated according to the on-line measurement device design and the structure of NaI detector.The minimum detectable concentration is given according to the response coefficient and the background count rate.Conclusions: The response coefficient of on-line measurement device is 1.249×10 4 s 1 Bq 1 m3 and the minimum detectable concentration can reach(4–5)×103Bq m 3when the thickness of lead shield is 80 mm.