The sediment samples (S1 to S4) were collected at various time intervals between 2018 and 2023 from the shoreline of the Lake Ontario boat launch (Figure 3, marked by a red star), precisely where the water meets the land. Depth was skimmed off the top at approximately 2 cm and sifted in the field to remove large rocks. One topsoil sample (500 g) was collected from a backyard, as shown in Figure 3 (marked by a black cross) at a depth of 6 inches, and sieved for gravel and stones, along with one soil sample from the IAEA-2006 PT exercise that was used. The PT soil sample was milled and sieved to obtain the appropriate fraction at a mesh size of less than 0.1 mm before being homogenized. The soil matrix was characterized, and several samples were pre-screened for radionuclides before spiking. The results indicated that the material was free from radionuclides, except for ¹³⁷Cs, which was detected at 2.6 ± 0.2 Bq·kg⁻¹ based on dry mass (Ref. date: 2006-01-01). Additionally, ²¹⁰Pb was found at 48 ± 1.5 Bq·kg⁻¹ dry mass. The moisture content measured 2.3% ± 0.2% ([24]). The topsoil sample was kept in a cooler and transported to the laboratory, where it was dried overnight at 105˚C to achieve a constant weight. An aliquot of the soil sample (65.6 g) was then weighed and transferred into a 50 ML jar, filled to the top, and sealed with Phenoseal Vinyl Adhesive Caulk (PHENOSEAL, Baltimore, MD 21224) and black electrical tape (S-17841; Uline, Pleasant Prairie, WI 53158) to prevent the escape of ²²²Rn gas from the container. CRM samples were also counted in a 50 ML geometry. Before gamma counting, the sediment sample was allowed to sit for four weeks to ensure ²²⁶Ra was in equilibrium with ²²²Rn progeny (²¹⁴Pb and ²¹⁴Bi). The samples were counted for 60,000 s to 240,000 s on an Ultra-Low Background HPGe detector, depending upon the sample’s size, activity, and geometry. The background was counted for 240,000 s. Radiological testing laboratories must validate their analytical methods using PTs and certified reference materials (CRMs) as quality control tools to provide reliable and valid measurement results for method validation, quality control, and metrological traceability. For this purpose, four CRMs obtained from the IAEA were also analyzed: IAEA-447 (Moss Soil, MS), IAEA-312 (Soil), IAEA-412 (Pacific Ocean Sediment, PO), and IAEA-385 (Irish Sea Sediment, IS).

Figure 3. Location of sample collection (red star: sediment sample, black cross: topsoil sample; Source: Nations Online Project. New York State Map. http://www.nationsonline.org/).

In this study, measurements were performed using a p-type coaxial high-purity germanium (HPGe) detector (Model GX13023, XtRa, Mirion Technologies, CT, USA) with a relative efficiency of approximately 140%. The detector is housed in a copper cryostat with a carbon-composite top-entry window of 0.62 mm thickness, providing sensitivity to photon energies as low as ~10 keV. The spectrometer is installed inside a room constructed of 15-cm-thick pre-World War II steel (Dixie Manufacturing Co., Baltimore, MD, USA), located beneath a 47-story building that provides approximately 33 m of water-equivalent (mwe) overburden for vertical cosmic-ray attenuation. To further suppress ambient radiation, the detector is enclosed in a custom-designed, three-layer ultra-low-background lead shield with a total thickness of 17 cm and is surrounded on five sides by plastic scintillation panels for active muon rejection ([15]). Under this configuration, the system exhibits an integrated background rate of 2.4 counts per minute (cpm) over the gamma-energy range from 50 to 2700 keV, corresponding to approximately 15 counts·s⁻¹·kg⁻¹ of germanium. Energy and efficiency calibrations of the detector were performed as described elsewhere ([16]). Nuclear decay data, including half-lives and gamma-ray emission probabilities, were obtained from the Brookhaven National Laboratory database (https://www.nndc.bnl.gov/nudat3/).

While radiochemical separation techniques are typically required to detect low-level ²⁴¹Am in soils and sediments, where surface activities are often on the order of 20 - 40 Bq·m⁻². Its presence can also be resolved by ultra-low-background gamma spectrometry via the 59.5 keV gamma emission. The sensitivity of the present system enables direct detection of ²⁴¹Am at environmental levels without the need for extensive chemical preconcentration.

²²⁴Ra decays with gamma emission at 240.99 keV (I_γ = 4.10%). However, this gamma line must be deconvoluted from another gamma-ray energy of 241.99 keV (I_γ = 7.26%) emitted by ²¹⁴Pb. ²¹⁴Pb also emits distinct gamma energy lines at 295.22 keV (I_γ = 18.47%) and 351.93 keV (I_γ = 35.72%). These two gamma energy lines were used for interference correction. In soil and sediment, ²²⁴Ra is in secular equilibrium with the radionuclides of the ²³²Th decay series. In this paper, we measured the ²²⁴Ra activity using the gamma energy line of 240.99 keV in soil and sediment samples, correcting for interference from the ²¹⁴Pb. The weighted mean activity of ²¹⁴Pb from the gamma energy lines of 295.22 keV (I_γ = 18.47%) and 351.93 keV (I_γ = 35.72%) was measured through ultra-low background spectrometry and subtracted from the ²²⁴Ra activity measured by the 240.99 keV energy line. The ²²⁴Ra activity is also measured in CRMs, with results compared to those of ²²⁸Ra and ²¹²Pb in the samples. The interference correction was applied using the following equations. The count rate under the 241 keV peak was treated as a sum:

C T ( 224 Ra ; 241   keV ) = C A ( 224 Ra ; 240.99   keV ) + C B ( 214 Pb ; 241.99   keV ) (1).

C_T is the total count rate under the peak area of 241 keV. This includes the contribution from ²²⁴Ra and ²¹⁴Pb.C_A and C_B are the count rates of the peak area of 240.99 keV of ²²⁴Ra and 241.99 keV of ²¹⁴Pb, respectively. C_B is calculated by using the equation below ([12]):

If the 351.93 keV peak of ²¹⁴Pb is also present, then the average of the count rates for both peaks is needed. In the above equation, ℰ γ is the efficiency of the gamma line of the radionuclide. I_γ is the emission probability of the gamma line. I_γ for 241.99 keV is 7.26%, for 295 keV is 18.47%, and for 351.93 keV is 35.72%. The activities of ²²⁴Ra and ²¹⁴Pb are density and coincidence summing corrected. The gamma activities and uncertainties are calculated using the formula given elsewhere ([16]).

²²⁶Ra is determined either directly by measuring the 186.21 keV γ-peak or indirectly by measuring the γ-peaks of ²²²Rn progeny, specifically ²¹⁴Pb and ²¹⁴Bi, after secular equilibrium is established with ²²⁶Ra (approximately 4 weeks). Direct measurement of ²²⁶Ra at 186.21 keV using gamma spectrometry is challenging due to interference from the 185.71 keV γ-peak of ²³⁵U, necessitating correction for the interference from the ²³⁵U (185.71 keV) peak. In this paper, we describe the measurement of ²²⁶Ra activity in soil and sediment using E_γ = 186.21 keV; I_γ = 3.64% gamma energy peak after correcting for interference from ²³⁵U (E_γ = 185.71 keV; I_γ = 57.2%) as detailed below:

The total count rate (C_T) at 186 keV peak is given as:

C_U-235 is the count rate of ²³⁵U under the peak of 185.71 keV, and C_Ra-226 is the count rate of ²²⁶Ra under the peak of 186.21 keV. The count rate of ²³⁵U in the 186 keV peak can be determined using the 143.76 keV peak below.

where ℰ γ is the efficiency of the individual gamma line depending upon the geometry of the container, and I_γ is the emission probabilities (57.2 % for E_γ = 185.71 keV; 10.94 % for E_γ = 143.76 keV). The activity for ²³⁵U is calculated from the weighted mean activities of gamma energy lines of 143.76 keV, 163.33 keV, and 205.31 keV if all appear in the spectrum. The interference-corrected direct ²²⁶Ra activity in soil, sediment, and CRM samples was compared with the activities of ²²²Rn progeny, i.e., ²¹⁴Pb and ²¹⁴Bi.

The ²¹⁴Pb interference-corrected ²²⁴Ra activities measured in soils, sediments, and certified reference materials (CRMs) are summarized in Table 1. A representative gamma spectrum for sediment sample S1, highlighting the principal gamma lines relevant to ²²⁴Ra determination, is shown in Figure 4. Certified values for CRMs are indicated in parentheses in the tables.

In the Lake Ontario sediment samples (S1 - S4), the corrected ²²⁴Ra activities are 23.8 ± 0.7, 23.7 ± 1.5, 23.1 ± 1.6, and 23.4 ± 2.7 Bq·kg⁻¹, respectively, yielding a mean activity of 23.5 ± 0.2 Bq·kg⁻¹. These values are in close agreement with the corresponding ²²⁸Ra activities of 24.3 ± 0.2, 20.6 ± 0.2, 22.3 ± 0.2, and 23.8 ± 0.2 Bq·kg⁻¹ (mean: 22.8 ± 0.8 Bq·kg⁻¹). The agreement between ²²⁴Ra and ²²⁸Ra, as well as with other ²³²Th decay products (²¹²Pb, ²¹²Bi, and ²⁰⁸Tl), is illustrated in Figure 5 and indicates that secular equilibrium is maintained in these sediments.

The near-unity ²²⁴Ra/²²⁸Ra ratios observed across all sediment samples suggest that ²²⁴Ra remains in near-secular equilibrium with its parent ²²⁸Th. This behavior reflects stable geochemical conditions within the Lake Ontario surface sediments, with no evidence for recent disturbance, resuspension, or pore-water exchange processes that would preferentially mobilize short-lived ²²⁴Ra. The consistency of this ratio across samples collected at different times further supports the validity of equilibrium-based gamma-spectrometric assumptions for these sediments.

Table 1. Radionuclide activities in soil and sediment samples in Bq·kg⁻¹.

*Values taken from the reference: [11]; ** Value taken from the reference: [7].

Figure 4. Gamma spectra of the sediment sample (S3) showing major gamma lines.

Figure 5.²²⁴Ra and ²²⁸Ra in soil and sediment samples.

Independent confirmation of the interference correction is provided by CRMs. For IAEA-412 (PO), the certified ²²⁸Ra activity is 36.2 ± 2.3 Bq·kg⁻¹, while the measured value in this study is 36.4 ± 0.5 Bq·kg⁻¹. The corresponding corrected ²²⁴Ra activity is 38.7 ± 3.2 Bq·kg⁻¹, in agreement within uncertainty. Similar consistency between ²²⁴Ra and ²²⁸Ra is observed in IAEA-385, IAEA-447, IAEA-312, and IAEA-2006 PT soil samples, demonstrating that the ²¹⁴Pb interference correction yields accurate ²²⁴Ra activities across diverse matrices.

Direct ²²⁶Ra measurements obtained from the 186.21 keV gamma line were corrected for interference from the closely spaced ²³⁵U gamma line at 185.71 keV. The corrected ²²⁶Ra activities are listed in Table 1 and compared with indirectly derived ²²⁶Ra activities based on the short-lived ²²²Rn progeny, ²¹⁴Pb, and ²¹⁴Bi. Figure 6 illustrates the strong agreement between these independent determinations.

Figure 6.²²⁶Ra, ²¹⁴Pb, ²¹⁴Bi activities in soil and sediment samples.

In the Lake Ontario sediments (S1 - S4), the corrected ²²⁶Ra activities range from 22.6 ± 1.3 to 24.6 ± 1.4 Bq·kg⁻¹, with a mean value of 24.2 ± 0.9 Bq·kg⁻¹. These values are consistent with the corresponding ²¹⁴Pb activities (mean: 22.5 ± 0.8 Bq·kg⁻¹) and ²¹⁴Bi activities (mean: 22.0 ± 0.1 Bq·kg⁻¹), confirming that secular equilibrium between ²²⁶Ra and its progeny is preserved in these sediments.

In soil samples, the BS soil shows a corrected ²²⁶Ra activity of 32.4 ± 7.7 Bq·kg⁻¹, in excellent agreement with the ²¹⁴Pb and ²¹⁴Bi activities of 33.1 ± 0.2 Bq·kg⁻¹. Similarly, in the IAEA-2006 PT soil, the corrected ²²⁶Ra activity of 50.0 ± 15 Bq·kg⁻¹ aligns with the ²¹⁴Pb and ²¹⁴Bi activities of 47 ± 2 and 44 ± 2 Bq·kg⁻¹, respectively.

CRM measurements further validate the correction approach. For IAEA-447 (MS), the measured ²²⁶Ra activity of 26.8 ± 5.6 Bq·kg⁻¹ agrees with the certified value of 25.1 ± 2.0 Bq·kg⁻¹. In IAEA-312 (soil), the measured activity of 296 ± 28 Bq·kg⁻¹ falls within the certified range of 250–287 Bq·kg⁻¹ (95% confidence interval). For sediment CRMs IAEA-385 (IS) and IAEA-412 (PO), the measured ²²⁶Ra activities (22.8 ± 0.6 and 26.9 ± 3.3 Bq·kg⁻¹, respectively) closely match the certified values, confirming that the combined ²³⁵U interference correction and ultra-low-background counting approach yields accurate ²²⁶Ra determinations.

Activities of additional natural and anthropogenic radionuclides are summarized in Table 2. The ⁴⁰K activity in the BS soil sample is 598 ± 15 Bq·kg⁻¹, while sediment samples S1 - S4 range from 531 ± 11 to 632 ± 12 Bq·kg⁻¹, with a mean value of 566 ± 8 Bq·kg⁻¹. These values fall within the global range of 140 - 850 Bq·kg⁻¹ reported by [30], indicating typical lithogenic contributions.

Table 2. Anthropogenic and natural radionuclide activities in soil and sediment in Bq·kg⁻¹.

*Below detection limit as explained in [16].

The measured ¹³⁷Cs activity in the soil sample is 1.4 ± 0.1 Bq·kg⁻¹, while sediment samples show lower activities ranging from 0.79 ± 0.02 to 1.1 ± 0.02 Bq·kg⁻¹ (mean: 0.93 ± 0.03 Bq·kg⁻¹). These values are consistent with reported ranges for surface soils and sediments in the United States, where ¹³⁷Cs activities vary widely depending on fallout history and sedimentation processes ([20]; [9]). The relatively low ¹³⁷Cs activities in the Lake Ontario sediments suggest limited recent anthropogenic input and post-depositional redistribution.

The ²¹⁰Pb activity in the BS soil is 66.5 ± 3.1 Bq·kg⁻¹, while sediment sample S1 shows an activity of 24.4 ± 1.17 Bq·kg⁻¹; samples S2 - S4 are below the detection limit. This pattern is consistent with atmospheric deposition of excess ²¹⁰Pb and variable sediment mixing. The activities of ²³⁴Th and ^234mPa are in close agreement in both soils and sediments, confirming near-secular equilibrium with parent ²³⁸U. The measured ²³⁵U activities in BS soil (1.7 ± 0.7 Bq·kg⁻¹) and sediments (mean: 1.01 ± 0.12 Bq·kg⁻¹) are consistent with global background levels reported by [30].

CRM results further demonstrate the accuracy of the measurements. For example, in IAEA-2006 (PT) soil, the measured ⁶⁰Co activity of 58.3 ± 2.5 Bq·kg⁻¹ agrees with the certified value of 56.1 ± 1.4 Bq·kg⁻¹. Similarly, measured ¹³⁷Cs and ²¹⁰Pb activities in IAEA-447 (MS), IAEA-412 (PO), and IAEA-312 (soil) closely match certified values, confirming the reliability of the ultra-low-background gamma spectrometry system.

The combined results demonstrate that interference-corrected ultra-low-background gamma spectrometry enables accurate determination of ²²⁴Ra and ²²⁶Ra in soils, sediments, and CRMs. The agreement between corrected activities, decay-chain progeny, and certified values supports the robustness of the methodology for routine environmental monitoring, regulatory assessments, and characterization of both Naturally Occurring Radioactive Material (NORM) and Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) materials.

However, the approach relies on assumptions of secular equilibrium and accurate deconvolution of closely spaced gamma peaks. Deviations from equilibrium, matrix heterogeneity, or uranium-series disequilibrium could introduce additional uncertainty and should be assessed on a case-by-case basis. Complementary analytical techniques, such as alpha spectrometry or radon emanation measurements, may further strengthen interpretations in complex or disturbed systems.