Comparison of the performance of the Quantulus GCT 6220 and the Quantulus 1220 for the determination of 55Fe and 63Ni

Comparison of the performance of the Quantulus GCT 6220 and the Quantulus 1220 for the determination of 55 Fe and 63 Ni J.L. García-Le´ on a,* , M. García-Le´ on b , G. Manj´ on a a Universidad de Sevilla, Departamento de Física Aplicada II, Escuela T´ ecnica Superior de Arquitectura, Av. Reina Mercedes 2, 41012, Sevilla, Spain b Universidad de Sevilla, Departamento de Física At´ omica, Molecular y Nuclear, Facultad de Física, Av. Reina Mercedes S/n, 41012, Sevilla, Spain ARTICLE INFO Handling Editor: Chris Chantler ABSTRACT An experimental study of the response of the newer Quantulus GTC 6220 liquid scintillation spectrometer has been conducted with the objective of determining the performance of this counter for the determination of 55 Fe and 63 Ni in the environment. Its response has been compared to that of the well-established Quantulus 1220. For this, Guard Compensation Technology (GCT) was used to reduce the Lower Limit of Detection (LLD in what follows). Two scintillation cocktails, Hidex ProSafe HC+and Revvity OptiPhase HiSafe 3 were compared in this work. The parameters employed for this assessment were background count rate, counting efficiency, and LLD. 1. Introduction The importance of measuring 55 Fe and 63 Ni activities is due to their presence in the construction materials of nuclear power plants that are treated during decommissioning tasks. Additionally, 55 Fe and 63 Ni are present in typical waste produced during regular operation of nuclear reactors such as spent resin, filters, and effluent water (Lee et al., 2023). 55 Fe and 63 Ni are formed by neutron activation of their stable isotopes. Typically, neutrons produced in the reactor core are captured by the stable Fe and Ni present in the concrete of the biological shield of the reactor or in the steel of the reactor components. 55 Fe (T 1/2 =2.747 ± 0.008 y) decays by electron capture (B´ e et al., 2006; Cassette, 2024; Kossert, 2020) and 63 Ni is a weak β emitter with E βmax =66.977 keV and T 1/2 =101.2 ±1.5 y (Audi et al., 2003; Coll´ e et al., 2008; Lee, 2005). The use of liquid scintillation spectrometers is usually considered the better choice for the measurement of the activity of the mentioned radionuclides. However, liquid scintillation spectrometers must be carefully applied to the measurement in the case of low-level activity, which is usually found in the environment. Indeed, 55 Fe and 63 Ni are released during the operation (Mikelic et al., 2007; Navarrete et al., 1995) and decommissioning (Hou et al., 2005; Leskinen et al., 2024) of nuclear facilities. This raises the need to use a sensitive technique capable of measuring both radionuclides to assess the environmental impact of both the operation and decommissioning of such facilities. Our first approach was to use a Quantulus 1220 liquid scintillation spectrometer for this purpose. Nowadays, this system is of limited availability, and other liquid scintillation spectrometers must be alternatively used for 55 Fe and 63 Ni determinations. In this paper, we explore the possibility of employing the newer Quantulus GCT 6220 for the measurement of both radionuclides. The performance of the two LSC systems has been compared along with two different scintillation cocktails, Hidex ProSafe HC+and Revvity OptiPhase ProSafe 3. The effect of quenching on the counting efficiency and LLD has been studied for both scintillation cocktails. 2. Experimental The details regarding the measurement of the activity concentrations of 55 Fe and 63 Ni with a Quantulus 1220 have already been published in (García-Le´ on et al., 2023). In this paper, using a similar experimental procedure, its response for both radionuclides is compared to that of the newer Quantulus GCT 6220 liquid scintillation system. Thus, sets of blank vials and vials spiked with known activities of 55 Fe and 63 Ni and different amounts of a color quenching agent, Na 2 CrO 4 , were systematically prepared and measured in both liquid scintillation spectrometers for the two scintillation cocktails included in our study. This quenching agent was selected due to its wide availability, ease to dye the mixture inside the scintillation vial with a yellow colour using a small and controlled amount of reagent. On the other hand, it has been used previously by other authors for similar studies (Zhai et al., 2022). In real * Corresponding author. E-mail address: [email protected] (J.L. García-Le´ on). Contents lists available at ScienceDirect Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem https://doi.org/10.1016/j.radphyschem.2025.112704 Received 29 October 2024; Received in revised form 23 January 2025; Accepted 14 March 2025 Radiation Physics and Chemistry 233 (2025) 112704 Available online 14 March 2025 0969-806X/© 2025 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/ ). samples, coloring is produced by the stable Fe that is added in the beginning of the radiochemical procedure (Hou et al., 2005) or is naturally present in the matrix, while in our case such effect is replicated by adding Na 2 CrO 4 . 2.1. Vial preparation Polyethylene vials provided by Meridian Biotechnologies Ltd. were used. 17.5 ml of one of the scintillation cocktails were added into the vial. Then it was spiked and filled up to 20 ml using 1 M H3PO4 for 55 Fe vials and 3 M HNO3 for 63 Ni vials. The reason for this choice is that Fe is usually dissolved in 1 M H 3 PO 4 and Ni in 3 M HNO 3 , according to the last step of our radiochemical procedure of choice (Hou et al., 2005). As for blank vials, they were prepared similarly, but spike solution was not added. Acid solutions were prepared employing low-tritium water obtained by distillation of mineral water known to be low in tritium to reduce the tritium signal and further diminish the background count rate. The spiked vials were measured for 15 min in the Quantulus 1220 and 10 min in the Quantulus GCT 6220, while the background measurements lasted 10 h each. 2.2. Scintillation cocktails We have compared the performance of two scintillation cocktails: Revvity OptiPhase HiSafe 3 and Hidex ProSafe HC+. Revvity OptiPhase HiSafe 3 is a multipurpose scintillation cocktail typically used in our laboratory for LSC measurements, while Hidex ProSafe HC+is a newer Nonylphenol Ethoxylates(NPE)-free solution that is also multi-purpose. Our objective is to evaluate the difference between the two in terms of background, efficiency, and LLD. According to the REACH regulation (EU, 2017), NPE-based substances require authorization to be manufactured due to environmental and health concerns, making the search for NPE-free scintillation cocktails a priority in some cases. It is of our interest to determine whether commercially available solutions can be competitive for the determination of 55 Fe and 63 Ni in environmental samples. 2.3. Tracer solutions Tracer solutions were obtained from Laboratoire Etalons d’Activit´ e (LEA) (France). 55 Fe activity concentration was (127.662 ±6.383) Bq/g as of November 04, 2020, being the serial number for the sample 797251/3. 63 Ni activity was (209.945 ±7.348) Bq/g as of November 04, 2020, with a serial number 7767D1/2. 2.4. Quenching agent Color quenching was considered as it is known to affect the response of liquid scintillation spectrometers (García-Le´ on et al., 2023). Amounts of Na 2 CrO 4 ranging from 0 to 100 μ mol were added to spiked and blank vials to examine the background behavior and efficiency depending on the quenching, spectrometer, and scintillation cocktail of choice. 2.5. Liquid scintillation spectrometers The suitability of two liquid scintillation spectrometers for 55 Fe and 63 Ni determinations has been studied and compared in this work: Quantulus 1220 and Quantulus GCT 6220. The first spectrometer was analysed in a previous work (García-Le´ on et al., 2023), and its performance compared to the new Quantulus GCT 6220 spectrometer. For the determination of low radioactivity concentrations, the background of the counter is a key parameter. It is a relevant contributor to the Lower Limit of Detection, which can be defined as follows (Eq. (1)) (Currie, 1968): LLD =4.66  B /t √ ε [Eq. 1] Where B is the background of the counter, t the measurement time and ε is the counting efficiency defined as shown in Eq. (2): ε =N−B A[Eq. 2] Where N is the count rate obtained for the spiked vial and B is the background count rate measured with a blank. N and B are expressed in counts per second (cps). A is the activity added to the spiked vial in Bq. The techniques used to reduce the background by both spectrometers are fundamentally different. The Quantulus 1220 is a widely known liquid scintillation spectrometer model that achieves a low background by employing traditional techniques: A heavy shielding containing a thick lead block, internally lined with copper and cadmium, together with two guard photomultiplier tubes (PMTs) in anticoincidence with the two main photomultiplier tubes are used as passive and active shielding respectively. The approach to background reduction in the Quantulus GCT 6220 is innovative and different from that in the more traditional Quantulus 1220. It does not rely on thick passive shielding or on the use of additional guard PMTs. Instead, it has a thinner 2 cm lead layer, with an additional solid bismuth-germanium oxide (BGO) scintillator that works as active shielding. The signal emitted by this scintillator can be caught by the coincidence PMTs. It is distinguishable from the light signal emitted by the liquid scintillator, allowing better discrimination between true and background events (Perkin Elmer, 2005). This discrimination process is not completely effective, making it necessary to use the Guard Compensation Technology (GCT), which works by calculating a guard efficiency that accounts for missed background events. Three GCT settings are available: Off, low, and high. According to the manufacturer, low is better used to measure radionuclides that emit radiation in the 0–112 keV window, while high is recommended for 0–28 keV (Perkin Elmer, 2005). GCT was set to high for all the measurements presented here due to the energy of the emissions of our radionuclides of interest, particularly for 55 Fe, with all emissions within this energy region due to being low-energy X-rays and Auger Electrons (B´ e et al., 2006). This choice of GCT setting was made as an optimization to reduce the background in the energy region of 55 Fe emissions, which usually has a higher LLD than that of 63 Ni since the counting efficiency is lower (Eq. (1)) due to the low energy of the radiation it emits.” A GCT calibration must be performed before the measurement if GCT is set to low or high. This is scintillator-dependent, so two GCT calibration files were obtained and used, one for ProSafe HC +measurements and the other for OptiPhase HiSafe 3 measurements. 2.6. Quenching determination Each spectrometer has a specific protocol to determine the quenching of each sample. For the Quantulus 1220, quenching is quantified with a parameter called Spectral Quench Parameter of the External standard or SQP(E). This factor is the end channel of the Compton spectrum obtained with the measurement of the vial of interest while exposed to a 152 Eu external source (Cassette and Tart` es, 2014) The Quantulus GCT 6220 uses a different quenching indicator called Transformed Spectral Index of External standard or tSIE. Its value is calculated for each sample parting from a similar Compton spectrum to that explained previously, this time obtained with a 133 Ba external source (Cassette and Tart` es, 2014; Perkin Elmer, 2005) 2.7. Optimal window determination The LLD is evaluated using the counting efficiency and the backJ.L. García-Le´ on et al. Radiation Physics and Chemistry 233 (2025) 112704 2 ground of the spectrometer for each radionuclide of interest. The best measurement conditions are obtained when a higher Figure of Merit (FOM) is achieved. Usually, FOM is defined as shown in Eq. (3): FOM =( ε ⋅V)2 B[Eq. 3] V being the volume of the sample or the amount of the sample in general terms. For that, the selection of a counting window in the spectrum for count rate determination is a relevant task. The experimental procedure followed to find the optimum counting window has already been published (García-Le´ on et al., 2023), where a full account of the method can be found. A summary is shown in what follows. Two vials, one containing a solution spiked with the radioisotope of interest and the other containing a solution representing a blank procedure, both with similar quenching indicators are selected and measured. After the measurement of both vials, the FOM is calculated using combinations of starting and ending channels for the summing window in the spectra. Maximizing the FOM allows for the selection of the measurement window and the LLD may be (Eq. (1)) evaluated. This procedure is applied for different values of the quenching indicator, as this LLD-optimizing window might depend on quenching. 3. Results and discussion 3.1. FOM and optimal window Using the method described above (García-Le´ on et al., 2023), the optimal window has been determined from the analysis of the spiked and blank vials spectra, both with similar quenching factor. The optimal windows for 55 Fe and 63 Ni were selected independently for different amounts of quenching agent. An unquenched 55 Fe spectrum and its corresponding blank spectrum are shown in Fig. 1. Similarly, a 63 Ni unquenched spectrum and its corresponding blank spectrum are shown in Fig. 2. In both cases, the optimal window, determined by the highest FOM is remarked in each figure. 3.2. Background Blank spectra have been compared for both spectrometers: Quantulus 1220 and Quantulus GCT 6220. For the first, two blank spectra corresponding to unquenched vials containing each of the studied scintillation cocktails (ProSafe HC+and OptiPhase HiSafe 3) are depicted in Fig. 3. The corresponding results for the Quantulus GCT 6220 are shown in Fig. 4. Despite the fact that the two spectrometers use different techniques for background reduction, the relative response between the two scintillators is similar. Indeed, the blank spectra obtained with the Prosafe HC +scintillator present higher count rates, resulting in a greater background in the 55 Fe and 63 Ni regions in both spectrometers. Alternatively, smoother background spectra are obtained when using the scintillation cocktail OptiPhase HiSafe 3. Background levels (in counts per minute), corresponding to the four spectra of Figs. 3 and 4, are presented in Table 1. According to Table 1, the use of the Quantulus GCT 6220 in the GCT high setting and the scintillator OptiPhase HiSafe 3 produces consistently lower backgrounds. The higher background presented by the ProSafe HC +scintillator in the Quantulus 1220 in channels 1–150 in Fig. 3 is effectively reduced by the GCT technology when compared to Fig. 1. Background and 55 Fe spectra obtained with the Quantulus GCT 6220 plotted with the optimal window in terms of LLD. The represented window includes channels 2–6. These spectra were obtained from vials with ProSafe HC+. Fig. 2. Background and 63 Ni spectra plotted with the optimal window in terms of LLD. The represented windows include channels 2–23. These spectra were obtained from vials with OptiPhase HiSafe 3 in the Quantulus GCT 6220. Fig. 3. Low energy region of the unquenched background spectra corresponding to both scintillation cocktails in the Quantulus 1220 liquid scintillation spectrometer. J.L. García-Le´ on et al. Radiation Physics and Chemistry 233 (2025) 112704 3 Fig. 4. 3.3. Efficiency and quenching Once the optimal window has been selected for the corresponding quenching factor, the efficiency versus quenching parameter empirical functions may be considered. Indeed, the counting efficiency decreases when quenching is higher, i.e., when the quenching factor is lower. In that case, spectra are moved to the lower energy region, and the count rate within the optimal window decreases. Efficiency calibrations were obtained for both radionuclides, as it is required to perform a measurement in the case of samples of unknown activity and to calculate the LLD. A high efficiency is essential to optimize the LLD along with reducing the counting uncertainty. Fig. 5 shows the efficiency calibrations obtained from the measurement of several spiked vials with different amounts of quenching agent for the measurement of 55 Fe with both scintillation cocktails and spectrometers, while Fig. 6 is similar, but with the results for 63 Ni. A double x-axis is used due to each liquid scintillation spectrometer employing a different quench indicating parameter: SQP(E) for Quantulus 1220 and tSIE for Quantulus GCT 6220. In the case of 55 Fe (Fig. 5), all calibrations have the expected behaviour: The counting efficiency shows a decrease with quenching. Considering the empirical function to adjust the experimental results, these functions can be adjusted as a quadratic polynomial (sometimes linear) shape. The highest efficiency values are obtained for the combination of the Quantulus GCT 6220 and Prosafe HC+, although the efficiency decreases quickly as the quenching factor decreases. Regarding 63 Ni (Fig. 6), the efficiency decreases when quenching is most important as expected. However, for the combination of OptiPhase HiSafe 3 and Quantulus GCT 6220, the counting efficiency does not appear to be quenching-dependent, at least within the quenching factor range used in this work. The cause behind this might be the variable summing window selected for this work. Due to the reduction in background when in GCT mode, the scintillation cocktail Prosafe HC+in conjunction with the Quantulus GCT 6220 presents a high counting efficiency with a competitive background. Considering the counting efficiency and its quenching dependence, the use of Quantulus GCT 6220 with GCT set to high has proven to be a better choice than the Quantulus 1220 for the determination of 55 Fe and 63 Ni, when using any of the two studied scintillators. 3.4. Lower Limit of Detection Once the counting efficiency and its quenching dependence as well as the background have been established, the LLD can be evaluated. This is particularly relevant in the case for 55 Fe, as the emissions of this radionuclide are significantly less energetic than those of 63 Ni, making Fig. 4. Unquenched background spectra of both scintillation cocktails obtained in the Quantulus GCT 6220. Note that the y-scale is identical to that of Fig. 3 to ease comparison. Table 1 Background count rate for every combination of radionuclides, spectrometers, and scintillation cocktails. Cocktail and spectrometer 55 Fe BKG cpm 63 Ni BKG cpm ProSafe – Quantulus 1220 5.61 ±0.10 6.54 ±0.10 OptiPhase HiSafe 3 – Quantulus 1220 0.89 ±0.04 1.82 ±0.06 ProSafe – Quantulus GCT 6220 – GCT high 0.52 ±0.04 1.11 ±0.04 OptiPhase HiSafe 3 – Quantulus GCT 6220 – GCT high 0.08 ±0.03 0.38 ±0.03 Fig. 5. Efficiency calibrations for the measurement of 55Fe in the Quantulus 1220 and Quantulus GCT 6220 with both scintillation cocktails of interest. Fig. 6. Efficiency calibrations for the measurement of 63Ni in the Quantulus 1220 and Quantulus GCT 6220 with both scintillation cocktails of interest. J.L. García-Le´ on et al. Radiation Physics and Chemistry 233 (2025) 112704 4 them less efficient to count. Furthermore, 55 Fe activity is often less than that of 63 Ni due to its shorter half-life, making decay between radionuclide production and analysis relevant. For this reason, our aim is to improve the LLD for the determination of the first radionuclide while still taking the second into account. For that, selecting a combination of spectrometer and scintillator that yield a low LLD is crucial for conducting environmental impact research, as activity concentrations are expected to be low. Figs. 7 and 8 show the LLDs obtained for different levels of quenching for both spectrometers and scintillation cocktails. In both cases, the lower counting efficiency when increasing quenching levels makes the LLD higher as expected according to Eq. (1), especially for measurements made with the Quantulus 1220. This means that quenching should be taken care of during the radiochemical separation of environmental samples by choosing reagents that will allow it to reduce it (Hou et al., 2005). Figs. 7 and 8 support the idea that switching to the Quantulus GCT background reduction technique produces a lower LLD, which is an improvement in the measurement of environmental samples. Additionally, the LLD obtained on this new machine appears to be less affected by quenching, although the counting efficiency is still lowered according to Figs. 5 and 6. When the two scintillation cocktails are compared, the greater background obtained with ProSafe HC +appears to be partially compensated for by the higher efficiency according to Eq. (1), leading to a comparable LLD for both cocktails with the newer Quantulus GCT 6220 in the case of the measurement of 55 Fe (Fig. 7). However, this greater background is not compensated for by the increase in efficiency in the case of the 63 Ni measurement (Fig. 8). 4. Conclusions The interest in measuring 55 Fe and 63 Ni has increased in the last few years due to the decommissioning of nuclear power plants. Liquid scintillation spectrometry has been considered a competent technique for measuring the activity of both radionuclides. The Quantulus 1220 system has been used extensively for this purpose, but recently the newer liquid scintillation spectrometer Quantulus GCT 6220 has been introduced in the market, and our aim was to find whether it is suitable for the problems addressed by the older, now discontinued Quantulus 1220. In this work, both LSC systems have been studied and compared for the measurement of 55 Fe and 63 Ni with the use of two different scintillators. The experimental determination of the background and efficiency was preceded by an exhaustive study on the optimal window for measurements based on the optimization of FOM. It has been proven that the newer Quantulus GCT 6220 spectrometer may be competent in terms of LLD and counting efficiency, even though this new system requires a more demanding maintenance, as well as a daily selfnormalizing calibration and six-month lasting GCT calibration. This study will be followed by the measurement of real samples, which are expected to produce satisfactory results as the radiochemical procedure of our choice has proven to yield radiologically pure (Hou, 2018) and chemically replicable 55 Fe and 63 Ni vials. This means that it is expected to produce vials that do not contain interfering radionuclides that would otherwise need to be considered, while the effect of the combination of all types of quenching in real samples remains to be assessed. Two scintillators have been compared in this work. The effect of using a multipurpose NPE-free scintillation cocktail on the measurement of 55 Fe and 63 Ni has been assessed through background, counting efficiency, and LLD and compared to the use of a traditional NPE-based cocktail. The NPE-free scintillation cocktail of this study, Revvity ProSafe HC+, has shown an overall improvement in counting efficiency, with the drawback of a greater background in the low-energy region leading to higher LLDs in most cases. However, the LLD for the determination of 55 Fe may be similar to that of NPE-based cocktails when the summing window is limited to the lowest channels and the measurement is made setting the GCT to high. CRediT authorship contribution statement J.L. García-Le´ on: Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. M. García-Le´ on: Writing – review & editing, Supervision, Resources, Project administration, Funding acquisition, Formal analysis. G. Manj´ on: Writing – review & editing, Supervision, Resources, Project administration, Methodology, Funding acquisition, Formal analysis, Conceptualization. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:G. Manjon-Collado reports financial support was provided by Nuclear Safety Council. M. Garcia-Leon reports financial support was provided by Government of Spain. 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 Fig. 7. LLDs in Bq/vial obtained for the measurement of 55Fe for both liquid scintillation spectrometers and scintillation cocktails of study. Fig. 8. LLDs in Bq/vial obtained for the measurement of 63Ni for both liquid scintillation spectrometers and scintillation cocktails of study. J.L. García-Le´ on et al. 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