Methods using 214 Pb and 214 Bi for early 226 Ra estimation. Determination of Uranium Series Activity Before Secular Equilibrium Is Established Charles A. Wilson IV, 1 Kenneth L. Matthews II, 2 Amin M. Hamideh, 3 and Wei-Hsung Wang 4 Abstract: Timely achievement of uranium series’ secular equilibrium is not always feasi- ble. Our objective is to mathematically justify methods for early uranium series gamma spec- troscopy measurements that can accurately pre- dict naturally occurring radioactive material equilibrium activities long before equilibrium is established. It was believed that, regardless of prior 222 Rn escape, after sealing a sample for a few hours the activities of 222 Rn, 226 Ra, and 238 U could theoretically be determined with a single measurement of both 214 Pb and 214 Bi. However, when accounting for error, this theory did not work as expected (CV = 14.0 in 226 Ra simulation). A similar approach published by Li et al. in 2015 proved to be much more reliable with the error considered, using 214 Pb activities measured at two different times gave signifi- cantly improved results when tested the same way (CV = 0.29 in 226 Ra simulation). Because both 214 Pb and 214 Bi activities are typically available when using gamma spectrometry, we combine these approaches and further in- creased the accuracy of the calculated activities (CV = 0.21 in 226 Ra simulation). Health Phys. 117(4):449–456; 2019 Key words: decay chain; model; naturally oc- curring radionuclides; spectrometry, gamma INTRODUCTION Analysis of naturally occurring radioactive material (NORM) is es- sential. With 222 Rn contributing to 68% of the ubiquitous background activity for the general public (NCRP 2009), accurate measure- ments of the uranium series are vital and analysis can often be time sensitive. While there are several approaches to conduct uranium se- ries measurements, such as alpha spectrometry, liquid scintillation, and mass spectrometry, one of the most common and advantageous practices is the use of gamma spec- troscopy (Li et al. 2015). However, sample preparation can be very time consuming as the most common approach technique is to seal the sample and wait for 28 days for the gaseous progeny to reach secular equilibrium (Tzortzis and Tsertos 2004). Once this step is complete, the radioactive progeny with high energy, high radiation yields, and unique energy peaks can be used as reliable surrogates to determine the activity of other members in the decay chain. There are some approaches that seek to reduce this wait time, but they require assumptions re- garding 222 Rn leakage (Van Cleef 1994), and others warrant further investigation into implications on error and uncertainty (Li et al. 2015). Our objective is to further explore that error and uncertainty. The progeny activities in a se- rial decay chain can be described by the Bateman equation (Bateman 1910). In the uranium decay series 214 Pb and 214 Bi are often used as proxies to estimate the activity of 222 Rn, 226 Ra or 238 U (depending on assumptions that can be estab- lished about the sample). Typically, this assumption requires the sample to be both very old (over 10 6 y) (Chiozzi et al. 2002) and in a condition that does not allow for gas to escape for several days in order to allow 222 Rn and its prog- eny to achieve secular equilibrium (Tzortzis and Tsertos 2004). Once equilibrium is established, 214 Bi energy peaks (609 keV, 1120 keV, and 1764 keV) and 214 Pb energy peaks (242 keV, 295 keV, 352 keV) are at energies that are reasonably distinguishable from other radio- isotopes (Bé and Chechev 2013). Depending upon the type of sam- ple and assumptions regarding 1 Louisiana State University Center for Advanced Mi- crostructures and Devices (CAMD); 2 Louisiana State University Department of Physics and Astronomy; 3 Louisiana State University Radiation Safety Office; 4 Louisiana State University Center for Energy Studies. The authors declare no conflicts of interest. Charles A. Wilson IV is the radiation safety officer at Louisiana State University’s Center for Advanced Microstructures and Devices (CAMD). He recently earned his Ph.D. studying environmental health physics in the Department of Environmental Sciences at LSU. He earned his master’s degree in medical physics and health physics from LSU in 2012. Charles was President of the Deep South Chapter of the HPS and a former Chair of the HPS Student Support Committee and Society Support Committee. He presently serves on the Program Committee and the IRPA task force. His email is cwils35@LSU.edu Operational Topic Operational Radiation Safety 449 Copyright © 2019 Health Physics Society. Unauthorized reproduction of this article is prohibited.