Reaching environmental levels of 244Pu by accelerator mass spectrometry at the Centro Nacional de Aceleradores

Journal of Environmental Radioactivity 278 (2024) 107485 Available online 29 June 2024 0265-931X/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Reaching environmental levels of 244 Pu by accelerator mass spectrometry at the Centro Nacional de Aceleradores E. Chamizo a , * , M. L´ opez-Lora a , b , I. Levy c , V. L´ erida-Toro a a Centro Nacional de Aceleradores (CNA), Universidad de Sevilla, Junta de Andalucía, Consejo Superior de Investigaciones Científicas, Sevilla, Spain b Dpto. de Física At´ omica Molecular y Nuclear, Universidad de Sevilla, Sevilla, Spain c International Atomic Energy Agency, Marine Environment Laboratories, Monaco ARTICLE INFO Handling Editor: Sheldon Landsberger Keywords: 244 Pu AMS Pu 236 U IAEA-412 IAEA-465 IAEA-385 CNA ABSTRACT 244 Pu (T 1/2 =81 My) is the longest-lived, most minor, and the most understudied Pu isotope. The anthropogenic production of 244 Pu is linked to nuclear detonations. Reported 244 Pu/ 239 Pu atom ratios in environmental samples range from below 10 −6 to above 10 −3 . This work discusses the performance of the 1 MV Accelerator Mass Spectrometry system at the Centro Nacional de Aceleradores (CNA, Seville, Spain) to analyse 244 Pu at environmental levels. The presence of 232 Th traces in the Pu sample limits the sensitivity of the technique through the formation of the diatomic trication ( 232 Th 12 C) 3+ , of mass 244 u, which must be suppressed by adjusting the stripper gas pressure. A 244 Pu background of 0.0075 fg (2 ×10 4 at) is demonstrated for samples that have undergone a chemical treatment. The reliability of the technique is proved through the analysis of three reference sediments provided by the International Atomic Energy Agency (IAEA-412, IAEA-465, IAEA-385). 244 Pu results are complemented with 239 Pu, 240 Pu, 241 Pu and 236 U and their relative isotopic abundances are discussed. 1. Introduction Anthropogenic actinides are widely distributed in the general environment from a multiplicity of sources (Sources and Effects of Ionizing Radiation, 2000). Baseline levels were imposed by the atmospheric explosions of nuclear weapons in the 1945–1980 period. Thermonuclear tests peaking in the 1960s had a worldwide impact, constituting the Global Fallout (GF) signal. The routine operation of Nuclear Reactors (NRs) and Nuclear Reprocessing Plants (NRPs), and nuclear accidents, have had a regional and/or local impact. In this context, it is key to have analytical tools allowing the identification of anthropogenic actinides contamination sources at a specific site. The abundances of the fissile 239 Pu (T 1/2 =24.110 y), its neutron activation product 240 Pu (T 1/2 =6.561 y), and 238 Pu (T 1/2 =87.7 y), all of them α -emitters, have been the most used tools to unravel Pu contamination sources (Bisinger et al., 2009; Chamizo et al., 2006; Kelley et al., 1999). Over the last decade, several publications (Corcho Alvarado et al., 2022; Lachner et al., 2010; L´ opez-Lora et al., 2023; Steier et al., 2013) have constated the advantages of expanding the Pu isotopic vector to the heaviest and most minor Pu isotopes: 241 Pu (T 1/2 =14.3 y, β - -emitter), 242 Pu (T 1/2 =0.373 My, α -emitter) and 244 Pu (T 1/2 =81 My, α -emitter). In NRs, subsequent single radiative neutron captures on 235 U, 238 U and/or 239 Pu and on their timely β decay products, give light to a cocktail of actinides radionuclides in close connection with the fuel composition, the irradiation time, and the type of reactor (Oversby, 1987; Pigford and Ang, 1975). In contrast, the detonation of atomic weapons, due to the extremely short neutron irradiation times (i.e. short compared to the shortest possible β decay half-life) and the comparatively higher neutron fluxes, produces an anthropogenic fingerprint also device dependent but, generally, significantly different from that of NRs (Diamond et al., 1960; Helfrich and Schneider, 1965). 241 Pu, highly abundant in NRs, can offer key information to unravel contamination sources, and can be used to determine the time elapsed since irradiation in combination with 242 Pu (Steier et al., 2013; Thakur and Ward, 2018). By contrast, 244 Pu can be used as a fingerprint of GF Pu. The formation of 244 Pu from the neutron activation of the short-lived 243 Pu (T 1/2 =4.95 h), demands high, time-constrained, and intense neutron fluxes that are only fulfilled in the detonation of specific nuclear weapons, mostly thermonuclear devices. Thus, its production is marginal in the case of NRs. Consequently, GF Pu is linked to much higher 244 Pu/ 239 Pu atom ratios compared to NRs related sources, opening the gate to a reliable deconvolution of GF and NRs sources, as demonstrated in (L´ opez-Lora et al., 2023). * Corresponding author. E-mail address: [email protected] (E. Chamizo). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad https://doi.org/10.1016/j.jenvrad.2024.107485 Received 4 March 2024; Received in revised form 14 June 2024; Accepted 17 June 2024 Journal of Environmental Radioactivity 278 (2024) 107485 2 The analysis of 244 Pu at environmental levels is only accessible by ultra-sensitive Mass Spectrometry (MS) techniques. The reason is its impracticable activity concentrations in general environmental samples. The assumed GF 244 Pu/ 239 Pu atom ratio based on a soil sample from Salzburg (Austria) is (5.720 ±0.097)x10 −5 (Steier et al., 2013). Surface soil samples from the general environment can be estimated to contain GF 239 Pu activity concentrations below 0.4 mBq/g (i.e. 4.4 ×10 8 at/g), according to the 239+240 Pu activities per km 2 reported in (Krey et al., 1976). Thus, an upper limit for the 244 Pu concentration in soils would be about 7 pBq/g (i.e. 2.5 ×10 4 at/g). The ability of the MS instrument to suppress the major 238 U ionic species is normally not an issue due to the mass difference. The main limitation usually comes from the formation of molecular isobars with similar kinematic properties as the ion of interest and/or the presence of machine background in the involved mass region. Attaining good counting yields is also key for a quantitative analysis. Overall, for a reliable analysis of 244 Pu in environmental samples, chemical procedures featuring good Pu recoveries and MS techniques offering good counting yields and low and well-known background sources become mandatory. In this work, we study for the first time the performance of the 1 MV Accelerator Mass Spectrometry (AMS) system at the Centro Nacional de Aceleradores (CNA, Seville, Spain) to analyse 244 Pu at environmental levels. The limitations of the technique are discussed, and its applicability to the study of sediment samples is assessed through the analysis of three International Atomic Energy Agency Reference Materials (IAEARM): IAEA-412 (Pacific Ocean sediment), IAEA-465 (Baltic Sea sediment) and IAEA-385 (Irish Sea sediment). The obtained 244 Pu/ 239 Pu atom ratios are examined in the context of Pu contamination sources. 241 Pu results are also provided and discussed for these samples. That way, the solidity of the CNA AMS technique for its analysis is demonstrated for the first time. 241 Pu and 244 Pu results are complemented with 236 U and 239,240 Pu, whose analysis methodologies at the CNA have been described in (Chamizo and L´ opez-Lora, 2019; L´ opez-Lora and Chamizo, 2019), respectively. Thus, only the final results are discussed here. 2. Materials and methods 2.1. Reagents and materials In-house distilled analytical grade HNO 3 and HCl acids, and concentrated H 2 O 2 and NH 3 solutions of the highest purity, were used for the radiochemical processing of the above-mentioned IAEA-RM samples. Solutions were prepared using ultrapure water (H 2 O-MQ). Ion chromatography separations were performed using TEVA®, UTEVA® and Pre-filter® resins (2 mL columns of 100–150 μ m particle size) supplied by Triskem International, Inc. (Bruz, France). The 242 Pu, 244 Pu and 233 U spike solutions were provided by National Physical Laboratory (NPL, England), Joint Research Centre Institute for Reference Materials and Measurements (IRMM, Belgium), and IAEA Environment Laboratories in Monaco (unknown supplier), respectively. The 242 Pu and 233 U spike solutions had been previously tested by AMS at the CNA for the absence of 239,240,241,244 Pu and 236 U traces (i.e. below the 10 −5 atomic abundance in each solution). The ColPuS Pu standard and the recently prepared in-house Multi-Isotopic (MI) U-Pu-Am standard (i.e. from the ColPuS standard solution spiked with well-known amounts of 233 U and 243 Am), were used for normalization purposes during the Pu and U AMS determinations. The Fe carrier used for the AMS cathode preparation was obtained from an Fe solution provided by High Purity Standards (HPS, England). The Nb binder was provided by Alfa Aesar Puratronic® (−325 mesh, 99.997% purity). Aluminium cathodes were provided by High Voltage Engineering Europa (HVEE, Amersfoort, The Netherlands). The machine test samples containing 232 Th were prepared from an 232 Th solution provided by HPS that had been previously chemically treated at the CNA to remove U and Pu traces. The machine test samples containing 238 U were prepared from our in-house MM uranium solution (Chamizo and L´ opez-Lora, 2019). New laboratory material was used whenever possible to prevent laboratory contamination. 2.2. Radiochemical procedure To test the performance of the overall 244 Pu measurement procedure at the CNA (radiochemical separation and AMS technique), 3 ×5 g aliquots of the IAEA-412, IAEA-465 and IAEA-385 samples were processed. The adopted chemical procedure is an adaptation to solid matrixes of the developed method at the CNA to separate sequentially Pu, Np, and U fractions from seawater samples (L´ opez-Lora et al., 2019). Thus, the extraction of Np and U is also considered. The discussion of the performance of the method for 237 Np analysis will be part of a different publication (i.e. but references to it have been included where necessary as “( 237 Np)”). The flow-diagram of the procedure is presented in Fig. 1. Sediment aliquots were first calcined in a muffle furnace at 550 ◦C for 5 h. About 2–3 pg of 242 Pu (i.e. isotopic tracer for Pu and non-isotopic tracer for 237 Np) and of 233 U (i.e. isotopic tracer for 236 U) were then added to control the radiochemical procedure (i.e. primary spikes). The samples were then transferred to 250 mL glass beakers with concentrated HNO 3, and anthropogenic radionuclides were put in solution by a leaching procedure with aqua regia during 5 h in a hot plate at about 150 ◦C. The resulting residues were treated with concentrated HNO 3 and H 2 O 2 several times and finally dissolved with a few mL of 8M HNO 3 . The supernatants were separated by centrifugation, taken to dryness, dissolved with a few mL of concentrated HCl, transferred to 1 L glass beakers, and reconstituted to a volume of about 800 mL with H 2 O-MQ. 2 g of K 2 S 2 O 5 and 0.25 g of FeSO 4 were then added, and actinides were coprecipitated with Fe(OH) 2 by raising the pH to 9 with NH 3 . The Fe (OH) 2 precipitates were recovered and treated with HNO 3 several times to eliminate HCl traces, and subsequently reconstituted in 18 mL of 1M HNO 3 to proceed with the ion chromatography separation of Pu +Np on one side (i.e. from TEVA®+Pre-filter® resins), and U on the other (i.e. from UTEVA® resins), as explained in (L´ opez-Lora et al., 2019). The resulting solutions were processed to produce the AMS cathodes to analyse the Pu +Np or U isotopic ratios, and to get information on the chemical yields for Pu or U, as explained in section 4.1. In every case, 1 mg of Fe was added as a carrier for the actinides. The final sample was a mixture of Fe 2 O 3 and Nb, with Fe:Nb in 1:3 mass ratio, pressed into an Al cathode. 2.3. Analysis of the Pu isotopic composition by AMS at the CNA The Pu isotopes were analysed on the 1 MV AMS system at the CNA. Briefly, they were extracted as monoxide anions ( x Pu 16 O) - from a Cs sputter ion source; mass analysed on a first 90◦sector magnet; injected into the accelerator where they undergo the stripping process in He gas at about 650 kV; analysed as x Pu 3+ on a second 90◦sector magnet and on a 120◦electrostatic deflector; and, finally, counted from the total energy signal provided by a gas ionisation chamber. The masses of interested are counted sequentially in pulses of several seconds using the multistep configuration of the ARGUS software machine. The sediment samples in this work were analysed in two different experiments. Firstly, ( 237 Np), 240 Pu, 239 Pu and 242 Pu were studied. The machine settings were established according to the observations reported in (Chamizo and L´ opez-Lora, 2019): the slits settings at the image position of the first 90◦sector magnet (S1 in Fig. 1-c) were set asymmetrical, left side opened to 1 mm and right side to 1.5 mm (i.e. (1,1.5) mm), to minimize the 238 U 16 O − tailing effects on neighbouring masses; the stripper gas pressure was set to 1 ×10 −2 mbar as a compromise between transmission through the accelerator (42%) and background effects of scattered 238 U 3+ molecular fragments on 239 Pu (and on 237 Np). Pulses of 5s ( 242 Pu, 239 Pu) and 10 s (( 237 Np), 240 Pu) were adopted. Additionally, scattered 238 U 3+ molecular fragments passing the second 90◦sector magnet during the analysis of 239 Pu 3+ (and 237 Np 3+ ) (i.e. with the same momentum (p) to charge (q) ratio, p/q) were also monitored in E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 3 5 s pulses. The sequence was repeated 5 times per run. Every sample was measured 4 runs. Secondly, 242 Pu, 241 Pu and 244 Pu were analysed. The slits settings at the entrance of the accelerator (S1 in Fig. 1-c) were set symmetrical to ±3 mm to maximize transmission, and the stripper gas pressure was set to 3 ×10 −2 mbar (42% transmission) to suppress the diatomic trication ( 232 Th 12 C) 3+ surviving the stripping process during the injection of 244 Pu 3+ (see discussion in Section 3.1). Pulses of 5 s ( 242 Pu), 15 s ( 241 Pu) and 25 s ( 244 Pu) were adopted in this case. The presence of the 244 Pu 3+ molecular isobar ( 232 Th 12 C) 3+ was also controlled by monitoring the corresponding 232 Th 3+ molecular fragment during an additional 5 s pulse (see Section 3.1). The sequence was repeated 5 times per run. Every sample was measured 10 runs. The corresponding machine settings and the performance of the system for the target Pu isotopes are summarized in Table 1. In the analysis sequence for the Pu isotopic analysis, every group of 5 samples (i.e. including one reagents blank processed together with the Fig. 1. Flow-diagram of the applied chemical procedure in this work to the studied IAEA-RMs. The method is an adaptation to solid matrixes of the procedure for seawater samples presented in (L´ opez-Lora et al., 2019). Where necessary, references to 237 Np have been made, although the results will be part of a different publication. Table 1 Summary of the adopted measurement conditions for the target Pu isotopes (and 237 Np) in this work: i) measurement times, including the pulsing time for every mass, the number of runs (i.e. every run comprises 5 pulsing sequences), and the overall measurement time; ii) the machine settings, with the adopted slits S1 settings (Fig. 1- c) and the stripper gas pressure; iii) transmission through the low-energy (LE) side, accelerator, and high-energy (HE) side; and iv) performance of the measurement during the experiments in this work, including the output currents from cathodes containing well-known amounts of 242 Pu, the correction factor for the x Pu/ 242 Pu atom ratios (i.e. ratio between the nominal and the measured values) for the ColPuS standard, and the background from instrumental blanks, or machine backgrounds. In the case of the correction factors, the obtained standard deviation of the mean for the different runs is included in parenthesis. (*) In the case of 241 Pu/ 242 Pu, it is indicated the assumed correction factor and standard deviation of the mean, since the ColPuS was not certified for it. 237 Np will be discussed elsewhere. Measurement times Settings Transmission Measurement performance Day Radionuclide Pulsing time (s) Number of runs Overall measurement time (s) Slits S1 (mm) Stripper gas pressure (mbar) LE side Accelerator HE side cps/pg ( 242 Pu) Correction factor ( x Pu/ 242 Pu) Instrumental background (fg) 1 ( 237 Np) 10 4 200 (1,1.5) 0.01 60% 42% 95% 340 – – 239 Pu 5 100 1.09 (1.1%) 0.4 240 Pu 10 200 1.08 (1.5%) 0.2 242 Pu 5 100 – – ( 238 U (p/ q 237 Np)) 5 100 – – 238 U (p/ q 239 Pu) 5 100 – – 2 241 Pu 15 10 750 (3,3) 0.03 95% 42% 95% 520 1 (2.6%)* 0.001 242 Pu 5 250 – – 244 Pu 25 1250 0.96 (2.6%) 0.004 232 Th from ( 232 Th 12 C) 3+ 5 250 – – E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 4 samples of interest) was preceded by: i) a ColPuS Pu standard sample, to control the machine stability and estimate the necessary correction factors (i.e. quotient between the nominal and the AMS atom ratios) for the target Pu ratios in every case (i.e. referenced here to 242 Pu which was added as a spike); ii) a Nb sample for ion-source cleaning purposes after the standard analysis; iii) a sample spiked with a few μ g of 238 U and 232 Th from our in-house solutions, to control 238 U background effects on neighbouring masses and the presence of 232 Th 12 C molecular isobar during the analysis of 244 Pu; and (iv) an instrumental blank sample (i.e. 1 mg of Fe as Fe 2 O 3 mixed with 3 mg of Nb) to control the machine background counts on the masses of interest. Since the ColPuS standard is not characterized for 241 Pu (Dittmann et al., 2019), any standard correction was applied for the 241 Pu/ 242 Pu AMS ratio (Table 1). 3. Performance of the 1 MV CNA AMS system to analyse 244 Pu 3.1. Background from 232 Th 244 Pu is 6 units of mass (u) above the heaviest naturally occurring radionuclide, 238 U (i.e. 99.27% isotopic abundance). The low-energy spectrometer of the 1 MV CNA AMS system allows the discrimination of ( 244 Pu 16 O) - (260 u) from ( 238 U 16 O) - (254 u) at the image position of the first 90◦sector magnet, where the beam size is about 5 mm and those masses are separated by 15.7 mm (Chamizo, 2009). An additional selection of the mass of interest occurs on the second 90◦sector magnet at the exit of the accelerator. Overall, tailing effects on mass 244 u from the main 238 U peak can be neglected even for a configuration allowing the maximum transmission through the spectrometer (i.e. with the slits at the image positions of the two sector magnets and of the electrostatic deflector wide open). This allows the analysis of 244 Pu under an optimal configuration avoiding transmission losses. A constated potential source of background for actinides AMS analysis if 3+charge state is selected is the presence of molecular isobars (Lachner et al., 2013). In the case of mass 244, the existence of the diatomic trication ( 232 Th 12 C) 3+ was observed for the first time during the setup of 244 Cm measurements on the ETH Zürich Tandy facility (Christl et al., 2014). Therefore, for 244 Pu analysis, such molecular isobar must be taken into consideration, due to the abundance of the naturally occurring 232 Th (i.e. 100% isotopic abundance), especially in soil and sediment samples. Surviving ( 232 Th 12 C) 3+ diatomic ions would mimic the trajectory of 244 Pu on the high-energy spectrometer, being finally injected into the gas ionisation chamber, where its total energy signal would be indistinguishable from the 244 Pu. By extrapolation, other more exotic diatomic ions involving 238 U like ( 238 U 6 Li) 3+ if existed could also play a role when the technique is pushed to its limits. Preliminary studies on a set of sediment samples containing Th in the Pu fraction (i.e. chemically processed skipping step 6 in Fig. 1-b) pointed out to the presence of a 244 mass background under the routine 240 Pu AMS settings at the CNA (i.e. stripper gas pressure set to 1.3 ×10 −2 mbar (L´ opez-Lora and Chamizo, 2019)). To study in detail the survival of the diatomic trication ( 232 Th 12 C) 3+ during the analysis of 244 Pu 3+ at our facility, a set of 5 samples containing 1 μ g of 232 Th as oxide in the same matrix as the samples of interest were studied for different stripper gas settings. The ratio between the gas detector counting rates under 244 u mass settings (i.e. ( 232 Th 12 C) 3+ ions), and the ones corresponding to 232 Th 3+ molecular fragments from the dissociation of ( 232 Th 12 C) 3+ (i.e. but reduced by two orders of magnitude to avoid the saturation of the detector electronics by changing the OY-Steerer on the high-energy side) was the adopted control parameter (i.e. ( 232 Th 12 C) 3+ / 232 Th 3+ ). The obtained results are shown in Fig. 2. An exponential decrease with the stripper gas pressure is observed below 2.5 ×10 −2 mbar, showing the destruction of the 244 Pu 3+ molecular isobar ( 232 Th 12 C) 3+ . For higher stripper gas settings, 244 mass counting rates approach the observed instrumental blank levels. Since a content of 1 μ g of 232 Th per AMS sample can be considered a very conservative approach for a real-case scenario involving chemically processed sediment or soil samples, 3 ×10 −2 mbar (i.e. 2.5 times the stripper gas pressure corresponding to the maximum transmission through the accelerator) is adopted as the working stripper gas pressure for the analysis of 244 Pu (Table 1). The anticipated transmission through the accelerator for U 3+ is 42%, 91% of the maximum value (Fig. 2). In a side experiment, the survival of ( 238 U 6 Li) 3+ diatomic ions were also investigated for a set of AMS cathodes spiked with 1 μ g of 238 U following a similar approach. The recorded counting rates for the resulting 238 U 3+ molecular fragments were negligible for stripper gas pressures above 1 ×10 −2 mbar. This could mean that ( 238 U 6 Li 16 O) - is not formed in the ion source. In any case, our results indicate that ( 238 U 6 Li) 3+ can be neglected as a 244 Pu 3+ background source in AMS. 3.2. Sensitivity of the technique for samples without an environmental matrix To study the sensitivity of the 1 MV CNA AMS system to analyse 244 Pu in samples without an environmental matrix, a set of 5 instrumental blank samples were measured using the corresponding settings in Table 1. Another 5 Fe samples were spiked with a well-known amount of 242 Pu to assess the Pu counting yield (i.e. detector counting rate of 242 Pu 3+ per mass of 242 Pu in the cathode). In addition to 244 Pu and 242 Pu, counting rates corresponding to 232 Th 3+ molecular fragments from the dissociation of ( 232 Th 12 C) 3+ were also monitored to control the 232 Th background (i.e. but as explained before, reduced by changing the OY-Stereer setting on the high-energy side). According to Fig. 2, the estimated ( 232 Th 12 C) 3+ / 232 Th 3+ ratio is at the 10 −7 level for the working stripper gas pressure of 3 ×10 −2 mbar. The 5 instrumental blanks accumulated overall 15 counts of 244 Pu in 105 min (i.e. 5 ×21 min) of counting time, which corresponds to 0.0024 cps with 26% statistical uncertainty. 232 Th 3+ count rates averaged 15 cps, meaning that the 232 Th background contribution on 244 Pu was below 10 −5 cps, thus negligible. Considering that the 242 Pu counting yield for the spiked samples was on average 520 cps per pg of 242 Pu in the cathode during the analysis, this translates into a 244 Pu background of about 0.0046 fg (i.e. 4.6 ag, or 1.14 ×10 4 at), for instrumental blank samples. Fig. 2. Average counting rates of ( 232 Th 12 C) 3+ normalized to those of ( 232 Th) 3+ molecular fragments from its dissociation (i.e. but reduced by two orders of magnitude by adjusting the OY-Steerer at the exit of the accelerator) versus the stripper gas pressure (P st ), for a set of 5 Fe samples spiked with 1 μ g of 232 Th. The line is the exponential fit to the data points. The calculated transmission through the accelerator for U 3+ in (Chamizo and L´ opez-Lora, 2019) is also shown. The arrows below the horizontal axis point out to the adopted stripper gas settings in this work for the analysis of ( 237 Np), 239 Pu and 240 Pu (grey arrow) and 241 Pu and 244 Pu (black arrow) (Table 1). E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 5 4. AMS results for IAEA reference materials 4.1. Chemical yield AMS detector counting rates for a specific radionuclide, cannot be directly translated into number of atoms of that radionuclide in the cathode. Counting yield depends strongly on the sample matrix and is time dependent. Besides, the use of the cathode material is variable. Thus, the interpretation of the output counting rates of the primary spike (i.e. added at the sample pretreatment stage, 242 Pu or 233 U in our case, Fig. 1-a) in terms of a quantitative mass of such spike in the AMS cathode, is not straightforward. To overcome this problem in this work, a secondary spike was incorporated into the samples during the cathode preparation stage (Fig. 1-c). That way, the AMS measured ratio between the primary and secondary spikes, could be used to calculate the actual amount of the former in the cathode and, thus, estimate the chemical yield (i.e. fraction of the primary spike that survives the chemical process). Due to availability reasons, 244 Pu was used as a secondary spike. In the case of Pu +Np, two cathodes were produced: i) 10% of the final Pu +Np solution was spiked with 0.1 pg of 244 Pu to determine the 242 Pu chemical yield (Cathode 1 in Fig. 1-c); ii) the remaining 90% solution was processed to measure the Pu isotopic composition (and 237 Np) (Cathode 2 in Fig. 1-c). In the case of U, the 244 Pu secondary spike was added to the whole final U solution. The U cathode was produced by direct evaporation to avoid elemental fractionation (Cathode 3 in Fig. 1- c). During the AMS analysis, the obtained 244 Pu/ 233 U ratios were corrected by our in-house MI standard to consider the different U and Pu ionisation yields in the ion source. The determination of the chemical yield was done once the analysis of the IAEA-RMs had been concluded, to minimize the possible crosscontamination effects on mass 244 in the ion source. Consistently high Pu chemical recoveries, of 81% with a standard deviation of 7%, were obtained for the processed samples (Table S1). As for U, an average 60% recovery with a standard deviation of 20% was obtained (Table S2). These results demonstrate the suitability of the applied chemical procedure to measure the most minor Pu isotopes in sediment matrixes. 4.1.1. 244 Pu results The raw 244 Pu AMS data for the studied IAEA-RMs are presented in Table 2. The detected 244 Pu counts were corrected by the instrumental and the 232 Th background levels. The instrumental background was estimated considering the average 244 Pu counts produced by 3 Fe samples and the processed blank in the same measurement sequence, since they were indistinguishable. Counting times of 21 min were dedicated to 244 Pu for every AMS cathode (Table 1). Lower 232 Th 3+ counting rates and 242 Pu counting yields were observed in the case of the IAEA-RMs compared to the Fe samples (i.e. about 60% lower in both cases), most likely due to matrix effects appearing in the case of processed samples. The observed 232 Th 3+ count rates for the IAEA-RMs pointed out to a good overall suppression of Th in the Pu fraction for the applied chemical procedure (Fig. 1). Considering the 242 Pu yield difference and the instrumental 244 Pu counting rates obtained during these experiments (i.e. 0.0024 cps), a more realistic approach to the 244 Pu AMS sensitivity of about 0.0075 fg (i.e. 7.5 ag, or 1.9 ×10 4 at) can be obtained for samples that have undergone a chemical treatment. The achieved measurement efficiency for 21 min overall counting time is about 1.5 ×10 −4 . The obtained results for the 244 Pu/ 239 Pu isotopic ratios and for the 244 Pu atom concentrations are presented in Fig. 3 (numerical results in Table S1). There is a good agreement between the analysed sample aliquots in each case, demonstrating the reliability and solidity of the applied technique. The lowest 244 Pu concentration was obtained in the case of the sample IAEA-385, with an average value of (1.23 ±0.50) x10 4 at/g, approaching the sensitivity of the technique for samples with an environmental matrix (i.e. of about 3.8 ×10 3 at/g, considering a sample of 5 g). The highest 244 Pu concentration corresponds to sample IAEA-465, with an average value of (1.36 ±0.16)x10 5 at/g. IAEA-412 situates in-between, with an average 244 Pu concentration of (3.30 ± 0.48)x10 4 at/g 244 Pu/ 239 Pu isotopic ratios follow a similar trend, featuring the IAEA-385 sample the lowest ratio, with an average of (8.2 ±3.4)x10 −6 , and sample IAEA-465 the highest, with an average of (1.01 ±0.12)x10 −4 . The obtained average value for sample IAEA-412 is (8.34 ±0.12)x10 −5 . This value is lower than the four reported values for IAEA-412 in (P˘ aces¸il˘ a et al., 2022), ranging from 1.22 ×10 −4 to 2.65 × 10 −4 . Table 2 Raw AMS results during the analysis of 244 Pu of the 3 studied aliquots (tagged as 1, 2, and 3) of the IAEA-412, IAEA-386 and IAEA-465 RMs. The samples were spiked with about 2 pg of 242 Pu. The 244 Pu instrumental background counts (fifth column) were extrapolated from the performance of 3 Fe samples and the processed blank prepared together with the IAEA-RMs aliquots. The final 244 Pu/ 242 Pu ratios are corrected by the ColPuS standard (Dittmann et al., 2019). 244 Pu 244 Pu/ 242 Pu Sample 242 Pu (cps/pg) 232 Th 3+ (cps) Detected counts Instrumental background counts 232 Th background counts Net counts Time (s) at/at Uncert. Instrumental Background 14 3 – 0 – 1250 – – Fe sample (x3), Processed blank (x1) IAEA-385-1 260 2.1 9 3 0 6 1250 3.1E-06 64% IAEA-385-2 150 2.0 5 3 0 2 1250 <1E-6 IAEA-385-3 328 2.7 10 3 0 7 1250 3.5E-06 56% IAEA-412-1 284 3.2 25 3 0 22 1250 1.1E-05 25% IAEA-412-2 459 4.4 28 3 0 25 1250 6.9E-06 23% IAEA-412-3 39 11.8 25 3 0 22 1250 6.3E-06 25% IAEA-465-1 378 0.6 34 3 0 31 1250 3.6E-05 20% IAEA-465-2 362 11.6 59 3 0 56 1250 2.8E-05 14% IAEA-465-3 266 1.2 22 3 0 19 1250 3.6E-05 27% Average (IAEA-RMs) 320 4.4 E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 6 4.1.2. 241 Pu results Following a similar approach as in the case of 244 Pu, the sensitivity of the technique was evaluated for 241 Pu analysis in chemically processed samples. A value of about 0.0010 fg/g (i.e. 1.0 at/g or 2.5 ×10 3 at/g) was obtained from the analysis of the processed blank sample, that clearly exceeded the 0 241 Pu background counts recorded by the Fe samples. This background, linked in this case with the sample processing, did not constrain the analysis of the target samples with 241 Pu atomic concentrations at least one order of magnitude above. The obtained 241 Pu results are plotted in Fig. 4 (numerical results in Table S1). They show consistency for every sample matrix, validating the applied analysis technique. The minimum 241 Pu concentration was obtained for IAEA-412 sample, with an average concentration of (2.41 ±0.16)x10 5 at/g; the maximum was for sample IAEA-385, with an average of (4.31 ±0.14)x10 6 at/g. As for IAEA-465, the 241 Pu average concentration was (8.90 ±0.54)x10 5 at/g. The obtained average 241 Pu/ 239 Pu isotopic ratios were (2.868 ±0.097)x10 −3 for IAEA-385, (6.10 ±0.40)x10 −4 for IAEA-412, and (6.65 ±0.41)x10 −4 for IAEA- 465. They agree with reference values or reported results in the bibliography (Table S1). 4.2. 239 Pu, 240 Pu and 236 U results The obtained 239+240 Pu concentrations agree with certified values in Fig. 3. Obtained 244 Pu/ 239 Pu atom ratios and 244 Pu atom concentrations for the three twin aliquots processed in this work of the three studied IAEA-RMs. Numerical values are displayed in Table S1 and average values in Table 3. LOD stands for Limit of Detection. Fig. 4. Obtained 241 Pu/ 239 Pu atom ratios and 241 Pu atom concentrations for the three twin aliquots processed in this work of the three studied IAEA-RMs. Numerical values are presented in Table S1 and average values in Table 3. LOD stands for Limit of Detection. E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 7 the case of samples IAEA-465 and IAEA-412 but is 25% lower in the case of the three studied aliquots of IAEA-385 (Table S1). The obtained 236 U results imply a similar behaviour, with a concentration in agreement with reported CNA values in the past in the case of sample IAEA-412, but 25% lower for sample IAEA-385 (Chamizo et al., 2015) (i.e. previous 236 U data have not been reported for IAEA-465). This might point out to a heterogenous nature of sample IAEA-385. The measured 240 Pu/ 239 Pu atom ratios agree with either the reference or reported levels in the bibliography in every case (Table S1). The obtained 236 U/ 239 Pu atom ratio for IAEA-412, with an average value of 0.013 ±0.0014, agrees with the variability observed in (Chamizo et al., 2015) for the same sample material. 4.3. Isotopic ratios for IAEA-RMs: comparison with bibliography To the best of our knowledge, only two very recent works have reported results on 244 Pu in IAEA-RMs. In (Corcho Alvarado et al., 2022), information on 240 Pu, 241 Pu, 242 Pu and 244 Pu was provided for IAEA-384 (Fangataufa sediment), IAEA-367 (Enewetak atoll sediment) and IAEA-368 (Mururoa atoll sediment). In (P˘ aces¸il˘ a et al., 2022), 244 Pu results for IAEA-410 (Bikini atoll sediment) and IAEA-412 (Pacific Ocean sediment) were also reported. These results, together with the obtained ones in this work for IAEA-385 (sediment from Sellafield), IAEA-412 and IAEA-465 (Baltic Sea sediment), are summarized in Table 3. 244 Pu/ 239 Pu isotopic ratios are also shown in Fig. 5. The reported GF 244 Pu/ 239 Pu atom ratio based on a soil sample from Austria is (5.7 ±1.0)x10 −5 (Steier et al., 2013). The measured value in this work for sample IAEA-412, (8.34 ±0.12)x10 −5 , is slightly higher. As for the 241 Pu/ 239 Pu atom ratio, a survey on soil samples from all around the globe situates the GF signal at (7 ±1)x10 −4 (referenced to October 2023) (Krey et al., 1976). The obtained 241 Pu/ 239 Pu isotopic ratio for sample IAEA-412, (7.66 ±0.12)x10 −4 , agrees with that value. The measured 240 Pu/ 239 Pu atom ratio, of 0.1720 ±0.0032, does not deviate from the expected number for the worldwide GF average, 0.176 ±0.014 (Krey et al., 1976). Sample IAEA-465 shows a 244 Pu/ 239 Pu atom ratio higher than the above mentioned value for the Austrian soil in (Steier et al., 2013) and for the IAEA-412 in this work, (1.01 ±0.11)x10 −4 . This ratio is, however, similar to the measured one for the 1952–1964 period in a sediment core from a bay in the Swedish coast of the Baltic Sea, (1.10 ± 0.39)x10 −4 (L´ opez-Lora et al., 2023). The same agreement is observed in the case of the obtained 241 Pu/ 239 Pu and 240 Pu/ 239 Pu atom ratios in this work, (6.65 ±0.41)x10 −4 and 0.1716 ±0.0036, respectively, matching the reported ratios for the Baltic Sea sediment core in (L´ opez-Lora et al., 2023), (6.34 ±0.85)x10 −4 and 0.1775 ±0.0079, respectively, and also in agreement with the assumed GF estimations. The lowest 244 Pu/ 239 Pu isotopic ratio corresponds to sample IAEA- 385 in this work, (8.2 ±3.4)x10 −6 . This value is in agreement with the reported results for a sediment core collected at the Irish Sea in the vicinity of the Sellafield NRP in (Steier et al., 2013), for which an upper limit of 3.5 ×10 −6 for the 244 Pu/ 239 Pu isotopic ratio was given. By Table 3 Reported Pu information for IAEA-RMS in the bibliography and in this work. Only samples with 244 Pu results have been considered. (*) 244 Pu/ 239 Pu atom ratios results in (P˘ aces ¸il˘ a et al., 2022) are expressed as ranges for the different studied aliquots. 239+240 Pu 240 Pu/ 239 Pu 241 Pu/ 239 Pu (referenced to 10–2023) 244 Pu/ 239 Pu Reference Sample description mBq/ g Uncert. at/at Uncert. at/at Uncert. at/at Uncert. Bikini Atoll sediment (AEA-410) (1.97E-4-4.50E-4) * (P˘ aces ¸il˘ a et al., 2022) Pacific Ocean sediment (IAEA-412) (1.22E-4-2.65E-4)* 0.591 0.005 0.1718 0.0032 7.6E-04 1.2E-04 8.34E-05 1.21E- 05 This work Baltic Sea sediment (IAEA-465) 1.996 0.020 0.1716 0.0036 6.6E-04 4.1E-05 1.01E-04 1.17E- 05 Sediment from Sellafield (IAEA-385) 2.248 0.017 0.1729 0.0027 2.9E-03 9.6E-05 8.18E-06 3.36E- 06 Enewetak atoll sediment (IAEA-367) 41.1 1.3 0.297 0.0060 1.1E-03 5.9E-05 4.00E-04 6.00E- 05 (Corcho Alvarado et al., 2022) Fangataufa sediment (IAEA-384) 113 6 0.04970 0.00080 1.04E- 04 1.09E- 05 3.50E-04 4.00E- 05 Mururoa atoll sediment (IAEA-368) 33.7 1.1 0.032 0.0010 6.5E-05 1.2E-05 <1.00E-05 Fig. 5. Reported 244 Pu/ 239 Pu atom ratio results in the bibliography for IAEA- RMs. From left to right: 1) values for the IAEA-410 and IAEA-412 RMs expressed as ranges for different aliquots in (P˘ aces¸il˘ a et al., 2022); 2) obtained average results in this work for IAEA-412, IAEA-465 and IAEA-385 RMs (numerical values in Table 3); and 3) values obtained in (Corcho Alvarado et al., 2022) for IAEA-367, IAEA-384 and IAEA-368 RMs. Horizontal lines indicate the assumed values for different Pu sources: Sellafield NRP in green (Steier et al., 2013), GF Pu in orange (Steier et al., 2013), and debris from the Ivy Mike thermonuclear test in 1952 in black (Diamond et al., 1960). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) E. Chamizo et al. Journal of Environmental Radioactivity 278 (2024) 107485 8 contrast, IAEA-385 features the highest 241 Pu/ 239 Pu isotopic ratio in Table 3, of (2.870 ±0.096)x10 −3 . The documented abundances of 239 Pu and 241 Pu in the liquid effluents released by the Sellafield NRPs (Cumbria, United Kingdom) would produce a decrease in the 244 Pu/ 239 Pu and an increase in the 241 Pu/ 239 Pu atom ratios in samples impacted by this regional source (Ray et al., 2020). The obtained result for IAEA-385 agrees with the anticipated trends in the 244 Pu/ 239 Pu and 241 Pu/ 239 Pu isotopic ratios. The measured 240 Pu/ 239 Pu isotopic ratio, of 0.1729 ± 0.0027, do not deviate from the expected one for GF in the Northern Hemisphere. 5. Conclusions This work discusses the performance of the 1 MV CNA system at the CNA to analyse 244 Pu at environmental levels as a continuation of previous studies (Chamizo et al., 2022; Chamizo and L´ opez-Lora, 2019; L´ opez-Lora and Chamizo, 2019). Through the analysis of 232 Th samples we demonstrate the survival of the 244 u diatomic trications ( 232 Th 12 C) 3+ for the stripper gas pressure corresponding to the maximum transmission through the accelerator, which must be increased for their fully dissociation. A non-zero background of 4.6 ag, or 1.14 ×10 4 at, is observed for blank samples without an environmental matrix, which slightly increases to 7.5 ag, or 1.9 ×10 4 at, in the case of processed blank samples. We assume that the observed 244 Pu background counts come from cross contamination in the ion source. Note that the ColPuS standard that contains 244 Pu has been routinely used over the last few years for routine Pu isotopic analysis at the CNA (Dittmann et al., 2019). The technique is applied to the study three IAEA-RMs from distinct regions: IAEA-412, Pacific Ocean sediment; IAEA-385, sediment from Sellafield, and IAEA-465, Baltic Sea sediment. The good chemical yields as determined by AMS (i.e. of about 80% and 60% for Pu and U, respectively), and the similar 232 Th 3+ counts from the dissociation of ( 232 Th 12 C) 3+ ) to instrumental blank samples, constates the good overall performance of the procedure for the analysis of 244 Pu in sediments. Reproducible results are obtained for the three independent aliquots studied in each case, proving the reliability of the technique. The obtained 244 Pu/ 239 Pu isotopic ratios for samples IAEA-412, (8.34 ±0.12)x10 −5 , and IAEA-465, (1.01 ±0.11)x10 −4 , seem to be representative of GF Pu considering the measured values for 240 Pu/ 239 Pu and 241 Pu/ 239 Pu. However, they are higher than the adopted value as representative for GF Pu in the Northern Hemisphere, of (5.7 ±1.0) x10 −5 (Steier et al., 2013). This might indicate a regional dependency of that ratio for GF sources. More results are necessary to shed light on such possible regional pattern. Our study demonstrates the importance of taking into consideration the presence of molecular isobars of the naturally occurring radionuclides for the analysis of the most minor actinides by low-energy AMS at environmental levels. The obtained results in this work can be directly extrapolated to 244 Cm studies. CRediT authorship contribution statement E. Chamizo: Writing – original draft, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. M. L´ opez-Lora: Writing – review & editing, Visualization, Validation, Methodology, Investigation, Funding acquisition, Conceptualization. I. Levy: Validation, Resources, Methodology, Investigation. V. L´ erida-Toro: Validation, Investigation. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Elena Chamizo reports financial support was provided by Spain Ministry of Science and Innovation. Mercedes Lopez Lora reports financial support was provided by European Commission. Isabelle Levy reports administrative support, equipment, drugs, or supplies, and travel were provided by International Atomic Energy Agency. None If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability I have included the results in the paper in a Sipplementary Information file. Further information can be provided upon request. Acknowledgements This work has been financed through the Grant PID2022-140680NB- I00 funded by the Spanish Government (Ministerio de Ciencia, Innovaci´ on y Universidades, MCIN/AEI/10.13039/501100011033) and by the European Union. M. L´ opez-Lora received funding from the European Commission through the HORIZON-MSCA-2022-PF-01-01 MSCA Postdoctoral Fellowships 2022 (Grant agreement ID: 101106068). The IAEA is grateful to the Government of the Principality of Monaco for the support provided to its Marine Environment Laboratories. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvrad.2024.107485. References Bisinger, T., Hippler, S., Michel, R., Wacker, L., Synal, H.-A., 2009. Determination of plutonium from different sources in environmental samples using alphaspectrometry and AMS. Nucl. Instrum. Methods Phys. Res. B 268, 1269–1272. https://doi.org/10.1016/j.nimb.2009.10.150. Chamizo, E., 2009. 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