Passive Thoron-Measurements With AndWithout Pinocchios Help

摘要

It is common practice to measure radon gas concentration as a surrogate of its decayproducts by using a known value of the equilibrium factor. Unfortunately a similar approach hasbeen often used to infer the thoron gas concentration from the measurement of ~(212)Pb,in spite of thefact that the concept of an equilibrium factor is no more meaningful. In order to study the profileof 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 aPinocchio mask. The most important characteristic of the long-nose-geometry for thoronmeasurements 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 thoronmeasurements are carried out without the help from Pinocchio, it is important that the monitors layadjacent to the thoron emanating surface. Different types of compact passive monitors areavailable which make it possible to measure thoron in the vicinity of the exhaling surfaces. Inthese cases, a combination of two different passive monitors is used in order to differentiate thethoron from radon. Because of the lack of correlation between thoron gas and its decay product,~(212)Pb,a correct dose estimate requires the measurement of the concentration of both radionuclides(~(220)Rn and ~(212)Pb)separately. While the thoron gas should be measured in the vicinity of theexhaling surface just the opposite is true for the ~(212)Pb,which should be measured tens of cm awayfrom said surface. Passive measurements of ~(212)Pbare typically carried out by track detectors withsuitable degraders of the alpha-particles energy. Once again Pinocchio can be very helpful, sinceby positioning the ~(212)Pbat the tip of his nose, he can ensure the exposure of the ~(212)Pbdetectoralways at the same distance from said exhalation surface.