LSC 검교정시 불확실성 평가에 관한 연구

Abstract

When the calibration on Liquid Scintillation Counter by using a Solid Standard Source of 200,000 DPM(Disintegration Per Minute) is carried out, an uncertainty and error due to an activity and geometry difference with environmental samples can exist. However, an investigation on this was not conducted so far. Therefore, this paper intends to evaluate environmental samples comparatively accurately by decreasing the uncertainty and error which can exist in the process of a calibration on LSC(Liquid Scintillation Counter) instrument through lots of experiments and measurements. For this, after variables that can have an effect on a measurement result were came up with, a sensitivity study was performed. And then, based on a sensitivity study result, an optimal counting condition on an counting instrument was deducted, and the effect by a geometry and activity difference was evaluated quantitatively. Also, in case that the effect by each item described above is represented, the basic cause was examined closely through an additional experiment. In this paper, Radioactivity Error Analysis Method was used for the purpose of verifying calibration results. According to the result of sensitivity study, in case of using the external source exposure time of 75sec and Repeat method, the measuring accuracy and precision of about 1～3% was increased in comparison with the existing method. Also, by measurement and analysis result, the effect due to an activity difference did not appear, and a plastic cell existing into Teflon vial made a role as reflector. The more activity is high, the less the effect of a plastic cell is decreased, and the effect of it can be neglected at the activity of 200,000 DPM. According to the result of sensitivity study by MCNP code, Similarly, in measuring, plastic cells played a role as reflector. From above verification and analysis result, it is judged that there is matter at Solid Standard Source itself, and that the effect by geometry difference does appear largely due to this problem. Also, it was derived that the Liquid Standard Source of more than 1,000 DPM had to be used when the calibration and verification about LSC Instrument were executed(The correlation and correction equation for compensating the effect by geometry difference is presented in the text). The effect by Uncertainty and Error is included at above equation, and measuring accuracy is nearly equal to the accuracy of Liquid Standard Source. Therefore, it is judged that there won't be problem even if equation above represented is used, when the calibration on Liquid Scintillation Counter by using a Solid Standard Source of 200,000 DPM is carried out.