SYMPOSIA PAPER Published: 15 November 2019
STP161820180060

Detection Limit for Ra-228 in Drinking Water by Gamma Spectrometry

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We derived a new formula for the detection limit (DL) of 228Ra in gamma spectrometry. We also revised the gamma-peak model and derived new formulas for the continuum background under the peak and its variance. We studied the detectability of 228Ra in drinking water using a modified gamma-spectrometry method. In this method, radium is coprecipitated with the barium sulfate carrier and then is dissolved to a 10-mL counting geometry. The chemical recovery is determined using a 133Ba tracer. We assayed 228Ra using a 911-keV gamma peak from 228Ac after reaching a secular equilibrium. Typically, 2.5 L of water is used in the analysis, and the gamma measurement lasts for 1,000 min. In this study, we measured background spectra separately for 4,000 min. Several germanium (Ge) gamma spectrometers were studied with the relative efficiencies ranging from 83 to 140 %, in shielding configurations with and without cosmic muon rejection. Detectability of 228Ra was studied in both the laboratory control samples (LCS) and method blanks (MB). For the LCS, we obtained the range of DL activities from 0.86 to 0.31 pCi/L (32 to 11 mBq/L), when shielding changed from a standard configuration to the muon-rejection configuration, respectively. For the MB samples, the range of DL activities was from 0.90 to 0.14 pCi/L (33 to 5 mBq/L), respectively. We also calculated the sample volumes sufficient to achieve the detection of 1 pCi/L (37 mBq/L). For all samples, the volume ranged from 2.25 to 0.34 L. Conversely, we calculated the minimum counting times to achieve the activity of 1 pCi/L (37 mBq/L), which ranged from 814 to 59 min. These results show that the low-background gamma spectrometry satisfied the regulatory DL for 228Ra equal to 1 pCi/L (37 mBq/L) and offered considerable improvement in detectability over a conventional gamma spectrometry.

Author Information

Semkow, Thomas, M.
Wadsworth Center, New York State Dept. of Health, Albany, NY, US Dept. of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, One University Place, Rensselaer, NY, US
Khan, Abdul, J.
Wadsworth Center, New York State Dept. of Health, Albany, NY, US
Menia, Traci, A.
Wadsworth Center, New York State Dept. of Health, Albany, NY, US
Li, Xin
Wadsworth Center, New York State Dept. of Health, Albany, NY, US Dept. of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, One University Place, Rensselaer, NY, US
Chu, Liang, T.
Wadsworth Center, New York State Dept. of Health, Albany, NY, US Dept. of Environmental Health Sciences, School of Public Health, University at Albany, State University of New York, One University Place, Rensselaer, NY, US
Torres, Miguel, A.
Wadsworth Center, New York State Dept. of Health, Albany, NY, US
Bari, Abdul
Wadsworth Center, New York State Dept. of Health, Albany, NY, US
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Developed by Committee: D22
Pages: 146–161
DOI: 10.1520/STP161820180060
ISBN-EB: 978-0-8031-7683-6
ISBN-13: 978-0-8031-7682-9