Applicability of the closed-circuit accumulation chamber technique to measure radon surface exhalation rate under laboratory conditions

A layer of phosphogypsum was placed at the bottom of two large volume boxes. This system allowed to measure the surface radon exhalation from phosphogypsum without altering the emitting material. The limits and optimal setup of the closed-circuit accumulation chamber technique were thoroughly studied. More precisely, the performance of different radon exhalation fitting methods was analyzed in the reference exhalation boxes, employing different measurement devices and three smaller operational accumulation chambers. As expected, the best approach to obtain the radon exhalation rate depends upon the effective decay constant of the measurement system and the time employed to perform the measurement. The time until the linear approximation can no longer be applied was scrutinized. Although this approximation is usually applied routinely in the literature, the effective time constant of the chamber is often not low enough for the linear fit to be applied safely, providing statistically acceptable measurements that can lead to significative underestimations of the radon exhalation rate.Postprint (author's final draft