IAEA SAFETY GUIDES - gnssn - International Atomic Energy Agency

This publication is no longer validPlease see http://www-ns.iaea.org/standards/(c)(d)(e)(f)(g)advisable to use plastic cells, especially in dry climates (note: high humiditytends to discharge such spots of charge). The magnitude of this error can beas high as 20% [29]. Non-uniformity is also caused by changes in convectionpatterns inside the cell, resulting from temperature differences between theinside and the outside [30]. It is difficult to determine exact values for theseerrors; however, it is common to find the standard deviation of a Lucas cellcount to be several times greater than the square root o f the count. The magnitudeof this error can be determined by repeated counting of a cell filled witha known amount of radon for extended periods under known measuringconditions.The cells should be purged of sample air promptly after counting, to curtailthe continuing buildup of long lived radon daughters. In single valve cells thisis done by applying a vacuum; in dual valve cells, by drawing through theequivalent of several cell volumes of radon free air or N2. Despite routinepurging, cell background radiation may gradually increase with use of the celland should be checked periodically. It can normally be subtracted from thecount obtained for subsequent samples; if it becomes excessive, it can bereduced by removing the zinc sulphide phosphor from the cell walls andrecoating them. Routine delay of counting by more than about three hourstends to cause higher background counts from the increase in the concentrationof and the deposition of 210Po.Traces of gaseous impurities are reported to alter the cell response slightly[30], but this effect does not constitute a significant problem in practice,certainly not for small cells.The photomultiplier counter system should be periodically standardized (bychecking the operating voltage).It is advisable to have a cell containing a known amount of radium to checkthe reproducibility of the photomultiplier and counting system.A pressure correction should be made for each cell. For example, for a cellof volume 100 cm 3 at an elevation of 1600 m above sea level, the efficiencyincreases by 6%.5.1.2. Thoron measurement with scintillation cellsThoron can be measured with scintillation cells by adapting procedures similarto those for radon, with some exceptions:(a)(b)It is not practicable to wait for daughter products to reach equilibrium levels.Appropriate growth curves corresponding to the time interval betweensampling and counting should therefore be used.If radon is present in sufficient concentration, one cell collecting only radon(using a suitable membrane) and another collecting radon and thoron could beused to make appropriate corrections.26