Measurements of super(220)Rn in air using a flow-through Lucas cell and multiple time analysis of recorded pulse events

A multiple time analysis technique has successfully been applied for measurements of environmental levels of super(220)Rn using a flow-through Lucas radon detector. The method is based on selective alpha counting of the short-lived nuclide super(216)Po. Owing to its short half-life (0.145 s), the alpha particle emitted at the decay of a super(220)Rn atom will, within a fraction of a second, be followed by an alpha particle from the decay of super(216)Po. By measuring the time interval between successive alpha counts followed by a multiple time analysis, the super(216)Po activity and thereby the super(220)Rn activity can be calculated. The method applied to a commercial instrument with a 160 ml Lucas cell as detector shows that a mean super(220)Rn air concentration of less than 20 Bq multiplied by m super(-3) can easily be determined even if the super(222)Rn air concentration is 10 times higher and varies. At lower super(222)Rn levels a super(220)Rn concentration less than 1 Bq multiplied by m super(-3) can be assessed using this technique. The method will be described together with some examples of its practical use.