IEA Radon_Reporter_April 2025

34 | APRIL 2025 SCIENCE /TECHNICAL Radioactive secular equilibrium occurs when the half- life of the parent isotope is much longer than the half-life of the progeny. As the decay rate of the parent isotope, and hence the production rate of the progeny, occurs at a constant rate, eventually the progeny decay rate equals the production rate, and the activities reach a constant value. In our case, radium-226 is the long-lived parent (half-life 1600 yr), followed by radon and its radon decay products (RDPs). In solids, when radon is created it is either trapped until it decays, or it may be able to migrate through the particle matrix and escape into the open air before it decays back to a solid atom (e.g., Po-218). The “freed” radon disrupts the equilibrium, resulting in a lower concentration of the RDPs in the matrix. The radon that is created on or near the particle surface is much more likely to escape into the air than the radon trapped inside the matrix. The portion of radon that escapes into the air is termed “emanating fraction” and generally ranges from 0.1-0.3 for soils. Alternatively, 70-90% of the radon decays in the solid matrix. In indoor-air environments, radon professionals recognize the term “equilibrium ratio” (ER) as the fraction of RDPs relative to the amount of radon gas and generally assign a value of 0.4-0.6 due to removal mechanisms such as plate-out of the RDPs. Occurrence of true radioactive equilibrium of the decay series (from uranium to lead) occurs in undisturbed rock and soil, but seldom elsewhere, as radon escapes when possible. When equilibrium occurs, every isotope in the decay series has the same amount of activity (pCi), thus it is only necessary to measure any radioisotope in the series to arrive at the value for the other decay products. Once radon is allowed to disperse, the equilibrium is broken, and only sealing against radon loss, and time, will permit equilibrium of the RDPs to re-establish. Upon sealing (no radon loss), equilibrium with its parent (radium-226) takes almost a month as the radon production occurs with a 3.82-day half-life reaching 99% equilibrium with the parent radium in that time. As radon production is predictable, by using the date and time that a sample is sealed airtight, the radon equilibrium (ingrowth) factor can be used to estimate the activity at any time without requiring the full (30-day) period. In contrast, RDP equilibrium with radon is established quickly through a series of short-lived half-lives. The time required to establish equilibrium of the RDPs in a charcoal canister is governed by the Pb-214 and Bi-214 (half-lives of 29 and 20 minutes, respectively) for gamma measurements as well as the alpha-emitting Po-218 and Po-214 isotopes detected by continuous radon monitors. Thus, once the radon is collected as a grab sample or onto charcoal, at least 3 hours (six half-lives) is commonly allowed for RDPs to re-establish equilibrium with the trapped radon so that count rates are maximized and counting error minimized. The degree of radon equilibrium in effluent is important in regard to exposure and dose. Active subslab depressurization (SSD) systems extract soil gas containing radon, RDPs, and moisture. The ER of the emission is likely low due to air movement, but as several alpha-emitting isotopes exist in the effluent, disposal above the roof line is warranted. As a vast majority of the dose is from the RDPs, and not the radon itself, the ER value is important regarding dose. In contrast, emissions from water aeration systems contain only radon gas, without RDPs, and pose less risk upon exit from the stack than SSD systems – since an absorbed dose received from alpha-particle radiation is 20 times more damaging to living tissue than beta or gamma radiation.) A state of radioactive equilibrium from the uranium parent through radium occurs in natural materials, such as granite, before the radon is formed and some may escape. However, there are cases where the uranium-to- radium radioactivity is not in equilibrium. Antique items such as orange Fiestaware and green Vaseline glass have a substantial gamma-ray emission from the uranium oxide coloring ingredient, which is initially devoid of decay products. As the uranium has not decayed anywhere near long enough to re-establish equilibrium of the decay series, there is no radon emission from these household products. An Explanation of Radioactive Equilibrium Michael Kitto