A novel methodology for online measurement of thoron using Lucas scintillation cell

The use of Lucas scintillation cell (LSC) technique for thoron estimation requires a modified methodology as opposed to radon estimation. While in the latter, the α counting is performed after a delay period varying between few hours to few days, in the case of thoron estimation the α counting has to be carried out immediately after sampling owing to the short half-life of thoron (55 s). This can be achieved best by having an on-line LSC sampling and counting system. However, half-life of the thoron decay product 212Pb being 10.6 h, the background accumulates in LSC during online measurements and hence subsequent use of LSC is erroneous unless normal background level is achieved in the cell. This problem can be circumvented by correcting for the average background counts accumulated during the counting period which may be theoretically estimated. In this study, a methodology has been developed to estimate the true counts due to thoron. A linear regression between the counts obtained experimentally and the fractional decay in regular intervals of time is used to obtain the actual thoron concentration. The novelty of this approach is that the background of the cell is automatically estimated as the intercept of the regression graph. The results obtained by this technique compare well with the two filter method and the thoron concentration produced from a standard thoron source. However, the LSC as such cannot be used for environmental samples because the minimum detection level is comparable with that of thoron concentrations prevailing in normal atmosphere.