A STUDY OF DISEQUILIBRIUM BETWEEN ~(220)Rn AND ~(216)Po FOR ~(220)Rn MEASUREMENTS USING A FLOW-THROUGH LUCAS SCINTILLATION CELL

摘要

Lucas-type scintillation cells (LSCs) are commonly used for rapid measurements of ~(220)Rn concentrations in flow-through mode in field and for calibration experiments in laboratories. However, in those measurements, equilibrium between ~(220)Rn and ~(216)Po is generally assumed and two alpha particles are considered to be emitted per ~(220)Rn decay due to very short half-life of ~(216)Po. In this paper, a small, yet significant disequilibrium existing between ~(220)Rn and ~(216)Po has been examined and shown that less than two alpha particles are actually emitted per ~(220)Rn decay in the cell when flow is maintained. A theoretical formula has been derived for the first time for a correction factor (CF) to be applied to this measured concentration to account for the disequilibrium. The existence of this disequilibrium has been verified experimentally and is found to increase with the increase in the ratio of flow rate to cell volume. The reason for the disequilibrium is attributed to the flushing out of ~(216)Po formed in the cell before its decay due to the flow. Uncertainties in measured concentrations have been estimated and the estimated CF values have been found to be significant for the flow rates considered above 5 dm~3 min~(-1) for a cell of volume 0.125 dm~3. The calculated values of the CF are about 1.055 to 1.178 in the flow rate range of 4 to 15 dm~3 min~(-1) for the cell of volume 0.125 dm~3, while the corresponding experimental values are 1.023 to 1.264. This is a systematic error introduced in ~(220)Rn measurements using a flow-through LSC, which can be removed either by correct formulation or by proper design of a measurement set-up.