- Conference date: 07–12 October 2007
- Location: Buzios, Rio de Janeiro (Brazil)
It is common practice to measure radon gas concentration as a surrogate of its decay products by using a known value of the equilibrium factor. Unfortunately a similar approach has been often used to infer the thoron gas concentration from the measurement of in spite of the fact that the concept of an equilibrium factor is no more meaningful. In order to study the profile of thoron and its decay products in the vicinity of the wall, a passive monitor consisting of a 25cm‐long diffusion tube has been used. The most difficult problem in practice is to keep this monitor‐tube perpendicular to vertical walls. This problem has been solved by using a 25cm‐long nose of a Pinocchio mask. The most important characteristic of the long‐nose‐geometry for thoron measurements is that it makes it possible to estimate the largest indoor concentrations of thoron, which occur at zero distances from the source under stagnant air conditions. When thoron measurements are carried out without the help from Pinocchio, it is important that the monitors lay adjacent to the thoron emanating surface. Different types of compact passive monitors are available which make it possible to measure thoron in the vicinity of the exhaling surfaces. In these cases, a combination of two different passive monitors is used in order to differentiate the thoron from radon. Because of the lack of correlation between thoron gas and its decay product, a correct dose estimate requires the measurement of the concentration of both radionuclides ( and ) separately. While the thoron gas should be measured in the vicinity of the exhaling surface just the opposite is true for the which should be measured tens of cm away from said surface. Passive measurements of are typically carried out by track detectors with suitable degraders of the alpha‐particles energy. Once again Pinocchio can be very helpful, since by positioning the at the tip of his nose, he can ensure the exposure of the detector always at the same distance from said exhalation surface.
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