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Measurements of 22Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and Dry Deposition Samples

Measurements of 22Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and... environments Article Measurements of Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and Dry Deposition Samples Mauro Magnoni *, Luca Bellina, Stefano Bertino, Brunella Bellotto, Maura Ghione and Maria Clivia Losana Physical and Technological Risks Department, ARPA Piemonte, 10015 Ivrea, Italy; l.bellina@arpa.piemonte.it (L.B.); s.bertino@arpa.piemonte.it (S.B.); b.bellotto@arpa.piemonte.it (B.B.); m.ghione@arpa.piemonte.it (M.G.); m.losana@arpa.piemonte.it (M.C.L.) * Correspondence: mauro.magnoni@arpa.piemonte.it Received: 29 December 2019; Accepted: 8 February 2020; Published: 11 February 2020 Abstract: Sodium-22 ( Na, half-life 2.603 years) is a cosmogenic radionuclide mainly produced in the stratosphere by nuclear spallation reactions of cosmic rays on Ar. Due to the very low concentration levels normally reached in the environment, Na poses no significant radioprotection threats: actually, the e ective doses delivered to humans can hardly exceed a few nSv per year, a very negligible value. However, the measurements of this radionuclides can be very interesting for atmospheric circulation and climatic studies. Unfortunately, the diculty of Na detection, due to its very low concentration levels, has prevented the gathering of large and widespread time series of this radionuclide. In this paper, a method for the retrospective measurements of Na in the atmosphere, starting from the gamma spectra (hyperpure germanium detectors (HPGe) detectors) of wet and dry deposition samples stored in our databases is proposed and validated. The method was applied to spectra samples gathered in the context of the Italian National Radioactivity Monitoring Network (RESORAD) and allowed the detection of the very low atmospheric activity concentration values of Na present at ground level. The results obtained with the new method are discussed and compared for validation with the available experimental values. Finally, some possible applications to environmental studies are also highlighted and suggested. Keywords: Na; atmospheric activity concentration; HPGe gamma spectra; retrospective analysis 1. Introduction 22 + Sodium-22 ( Na) is a cosmogenic radionuclide with a relatively long half-file (2.603 years), continuously produced by nuclear spallation reactions of cosmic rays on argon-40 ( Ar) nuclei [1]. It decays into the stable isotope Ne by + emission (90.35%) and electron capture (EC, 9.65%). Its production mainly occurs in the stratosphere and is essentially due to the high energy particles (E > 100 MeV/n) belonging to the galactic cosmic rays component (GCR, galactic cosmic rays). Once produced, Na is quickly attached to the sub-micron particulate suspended in the atmosphere and slowly settles to the ground [2]. It is eciently scavenged by precipitation, and therefore can also be found in meteoric waters, thereby easily entering into the ecosystems. Its concentrations in atmosphere increased substantially during the sixties of the 20th century, in the early phase of Cold War (1945–1963), due to nuclear weapons testing. At that time, traces of Na were also measured in lichens, mosses and wild game [3,4]. Nowadays, since the last atmospheric nuclear weapon detonation occurred in 1980 (Lop Nor, China), the Na levels returned to the typical pre-Cold War values: atmospheric concentrations usually well below 1 Bq/m at ground level [5]. However, the main interest in studying Environments 2020, 7, 12; doi:10.3390/environments7020012 www.mdpi.com/journal/environments Environments 2020, 7, 12 2 of 12 this radionuclide is its use as a tracer of the atmospheric circulation, often investigated also by means of other cosmogenic radionuclides with very di erent half-lives (for instance: Be, t = 53.22 days and 1/2 10 6 Be, t = 1.3610 years) [1]. A sudden increase of the ground level concentrations of the cosmogenic 1/2 radionuclides, for example, can be used as indicator of the intrusion of air masses of stratospheric 22 7 origin [6–9]. In this respect, the study of the ratio Na/ Be could give very interesting information, as was recently pointed out in a recent study (Ho mann, 2018, [10]). Moreover, because the cosmogenic radionuclides’ production rate is a ected by the 11 year sun cycles, the activity concentration values of all cosmogenic radionuclides are also of great interest for monitoring solar activity [11–14]: Na is particularly interesting in this respect, because of its physical characteristics, as its quite long half-life, make it less sensitive to variations of the meteorological conditions. Therefore, the availability of reliable time series of this radionuclide is very important and of great scientific relevance, allowing the gathering of some very interesting information that cannot be obtained using only easier-to-measure radionuclides, for example, Be, usually present in larger concentrations. 2. Materials and Methods In principle, Na can be easily measured by spectrometry with hyperpure germanium detectors (HPGe): actually it emits a strong line at 1274.5 keV with a yield close to unity (99.94%) in a region of the spectrum only slightly influenced by the Compton background of the K high energy emission (1460 keV). There is also another emission at 511 keV with an even stronger yield (180%) but not useful for quantitative determination due to the interference of the 511 keV annihilation peak always present in the background because of the pair production (electron e and positron e ) interactions of radiation with matter, mainly due to the lead shielding. Unfortunately, in spite of its strong emissions, the very low concentration levels typically found in the atmosphere (<1 Bq/m ) make the detection of Na in normal atmospheric particulate samples often very dicult. For that reason, a huge amount of air (tens of thousands of cubic meters, at least) needs to be filtered in order to achieve the necessary sensitivity. Alternatively, an indirect measurement of the atmospheric activity concentrations can be done using deposition data. In fact, wet and dry deposition can be collected for a convenient sampling time  (in our case, 1 month) by means of stainless steel tanks or similar containers. The relationship between the deposition values D (Bq/m ) and their corresponding atmospheric activity concentrations C (Bq/m ) can be deduced from a simple model describing the deposition D of radionuclides in a collection tank by the following di erential Equation: dD +   D = F (1) dt where  is the decay constant of the radionuclide and  is the corresponding downward flux, usually expressed as Bq/(m s). If the flux is assumed to be constant in time, the analytical solution of the above equation is straightforward and the amount of radioactivity collected in the tank during a generic sampling time  is thus given by the following expression: D =  1 e (2) The main limitation of this description is considering the Na flux as a constant: an apparently crude approximation, very far from real conditions, being the deposition mainly governed by precipitations—a typical example of a discrete and non-regular phenomenon. Bulk deposition should be more precisely described as a two components process, as follows: F = F + F , where wet bulk dry for the dry component a very simple relationship holds: F = C  v , in which C is the activity dry d concentration, while v is the average value of the settling velocity of the atmospheric particulate. A much more complicated expression should be used for the wet deposition component instead, involving many experimental parameters, such as the amount and the intensity of the precipitation Environments 2020, 7, 12 3 of 12 event, the height of the atmospheric column scavenged by the rain, the scavenging coecients, etc. However, in real cases, the simultaneous knowledge of all these parameters is seldom available, thereby preventing the possibility of using a “true” theoretical wet deposition mathematical model. Fothat reason, a very simplified description is often proposed, with bulk deposition modelled using the same relationship that holds for dry deposition: F = C  v (3) bulk m in which C is still the activity concentration, while v is a mean deposition velocity experimentally Environments 2019, 6, x FOR PEER REVIEW 3 of 12 evaluated after measuring simultaneously the deposition data (see Equation (2) and the corresponding activity concentration C in atmosphere [15,16]. In doing so we must bear in mind that the physical bulk=C∙vm (3) meaning of v is quite di erent respect to that of v : while v is a mean velocity obtained averaging m d d in which C is still the activity concentration, while vm is a mean deposition velocity experimentally over the distribution of all the velocities of the settling particulate suspended in atmosphere, v is not evaluated after measuring simultaneously the deposition data (see Equation (2)) and the a real velocity, just an empirical parameter encompassing the e ect of dry deposition and precipitation, corresponding activity concentration C in atmosphere [15,16]. In doing so we must bear in mind that the physical meaning of vm is quite different respect to that of vd: while vd is a mean velocity obtained and whose dimensions are those of a velocity. For that reason the numerical values of v are much averaging over the distribution of all the velocities of the settling particulate suspended in greater that those of v , the latter being related only to the much slower dry deposition processes: atmosphere, vm is not a real velocity, just an empirical parameter encompassing the effect of dry experimental measurements performed on Cs gave for v a value around 0.04 m/s [16], while typical deposition and precipitation, and whose dimensions are those of a velocity. For that reason the values of v , strongly dependent on the particulate diameter and other factors as well [17], are typically numerical values of vm are much greater that those of vd, the latter being related only to the much slower dry deposition processes: experimental measurements performed on Cs gave for vm a value in the range 0.1–0.001 cm/s. around 0.04 m/s [16], while typical values of vd , strongly dependent on the particulate diameter and The experimental set up for the collection of the wet and dry deposition samples is a stainless steel other factors as well [17], are typically in the range 0.1–0.001 cm/s. tank placed on the roof of the laboratory building (see Figure 1). The bottom of the tank is always kept The experimental set up for the collection of the wet and dry deposition samples is a stainless wet in order to prevent resuspension during dry periods. The collection of the samples is done at the steel tank placed on the roof of the laboratory building (see Figure 1). The bottom of the tank is always kept wet in order to prevent resuspension during dry periods. The collection of the samples is done end of each month: the tank is emptied and carefully washed with distilled water. The resulting water at the end of each month: the tank is emptied and carefully washed with distilled water. The resulting is then reduced by evaporation (90 C) and brought to dryness. The residue is finally weighted, put in water is then reduced by evaporation (90 °C) and brought to dryness. The residue is finally weighted, a little cylindrical jar (see Figure 2) and counted with hyperpure germanium detectors (HPGe) for 16 h. put in a little cylindrical jar (see Figure 2) and counted with hyperpure germanium detectors (HPGe) for 16 h. Figure 1. The stainless steel tank for sampling wet and dry deposition on the roof of the ARPA Figure 1. The stainless steel tank for sampling wet and dry deposition on the roof of the ARPA Piemonte Piemonte building (Via Jervis, 30, Ivrea, 10015, Italy): the collection area is about 4 m wide. The tank building (Via Jervis, 30, Ivrea, 10015, Italy): the collection area is about 4 m wide. The tank is emptied is emptied on a monthly basis through a tube. on a monthly basis through a tube. These are the standard procedures followed in the context of the Italian Environmental Radioactivity National Monitoring Network (RESORAD): they allow reaching a quite-good sensitivity for most radionuclides. For instance, the MDA (minimum detectable activity), referred to Environments 2019, 6, x FOR PEER REVIEW 4 of 12 137 2 22 2 as Cs, was about 0.015 Bq/m , while for Na a slightly larger value applies, 0.025 Bq/m , due to a lower spectrometric efficiency at the high energy  emission of sodium-22 (1274.5 keV). It can be demonstrated that these deposition MDA values correspond to about 0.2–0.3 µ Bq/m for the activity concentration: very low values are perfectly adequate for monitoring purposes, but still not enough Environments 2020, 7, 12 22 22 4 of 12 for a continuous monitoring of Na in atmosphere, as at ground level the Na activity concentrations are sometimes even lower [5]. Figure 2. Plastic jar for the wet and dry deposition measurements placed on the top of the cap of a Figure 2. Plastic jar for the wet and dry deposition measurements placed on the top of the cap of a hyperpure germanium detectors (HPGe) detector. The dry residue (about 4 g) is uniformly distributed hyperpure germanium detectors (HPGe) detector. The dry residue (about 4 g) is uniformly distributed in a thin cylindrical shaped geometry. in a thin cylindrical shaped geometry. These are the standard procedures followed in the context of the Italian Environmental Therefore, in order to improve the sensitivity of the measurements, single annual samples were Radioactivity National Monitoring Network (RESORAD): they allow reaching a quite-good sensitivity assembled, simply mixing the 12 monthly samples: each monthly sample of a given year (4 g of dry for most radionuclides. For instance, the MDA (minimum detectable activity), referred to as Cs, residue) was transferred into a larger jar (Figure 3) and counted as a new composite annual sample. 2 22 2 was about 0.015 Bq/m , while for Na a slightly larger value applies, 0.025 Bq/m , due to a lower Operating in this way, a significant decrease of the MDA values for deposition is expected, as spectrometric eciency at the high energy emission of sodium-22 (1274.5 keV). It can be demonstrated can be easily calculated using the simple, classic MDA formula given by Currie in 1968 [18]: that these deposition MDA values correspond to about 0.2–0.3 Bq/m for the activity concentration: 4.66 ∙ 𝜌 very low values are perfectly adequate for monitoring purposes, but still not enough for a continuous 𝑀𝐷𝐴 = 𝐷 (4) 22 22 𝜀 ∙ 𝑟 ∙ 𝑆 ∙ √𝑡 monitoring of Na in atmosphere, as at ground level the Na activity concentrations are sometimes 𝛾 𝛾 even lower [5]. where t is the counting time, back is the standard deviation of the background,  is the photopeak Therefore, in order to improve the sensitivity of the measurements, single annual samples were efficiency of the HPGe detector at the specific radionuclide emission energy, r is the  yield of the assembled, simply mixing the 12 monthly samples: each monthly sample of a given year (4 g of dry emission and S is the surface area. From this expression, taking into account Equations (2) and (3), residue) was transferred into a larger jar (Figure 3) and counted as a new composite annual sample. the expression for the MDA activity concentrations can be written as follows: Operating in this way, a significant decrease of the MDA values for deposition is expected, as can 4.66 ∙ 𝜌 ∙ 𝜆 be easily calculated using the simple, classic MDA formula given by Currie in 1968 [18]: 𝑀𝐷𝐴 = (5) ( ) 𝜀 ∙ 𝑟 ∙ 𝑆 ∙ √𝑡 ∙ 1 − 𝑒 ∙ 𝑣 𝛾 𝛾 𝑚 4.66 back MDA = p (4) in which the sampling time  is 1 month for the standard samples and 1 year for the composite "  r  S t sample. The improvement of the MDA values for annual measurements is due to the increased value  of the quantity (1−e ), a factor that largely dominates two negative effects: (1) the decrease of the where t is the counting time,  is the standard deviation of the background, " is the photopeak back photopeak efficiency  caused by increasing of the solid angle of the counting geometry; (2) the slight eciency of the HPGe detector at the specific radionuclide emission energy, r is the yield of the emission and S is the surface area. From this expression, taking into account Equations (2) and (3), the expression for the MDA activity concentrations can be written as follows: 4.66 back MDA = p (5) "  r  S t (1 e )  v 𝜆𝜏 𝑏𝑎𝑐𝑘 𝑏𝑎𝑐𝑘 Environments 2019, 6, x FOR PEER REVIEW 5 of 12 increase of the Compton background standard deviation related to the greater size of the annual sample. In order to boost further the sensitivity performances of the  spectrometry, the counting times were also increased from the standard value (57,600 seconds) up to 200,000 seconds. In Table 1 all the factors contributing to the variation of the MDA values are summarized, with monthly measurements taken as reference. Environments 2020, 7, 12 5 of 12 Table 1. Multiplying factors affecting MDA values: monthly measurements taken as reference. in which the sampling time  is 1 month for the standard samples and 1 year for the composite sample.  Sampling Time (1−e ) @ 1274.5 keV back Counting Time Overall Factor The improvement of the MDA values for annual measurements is due to the increased value of the Monthly me  asurement 1 1 1 1 1 quantity (1e ), a factor that largely dominates two negative e ects: (1) the decrease of the photopeak Annual measurement 0.0947 1.586 1.889 0.537 0.152 eciency " caused by increasing of the solid angle of the counting geometry; (2) the slight increase of the Compton background standard deviation related to the greater sizof the annual sample. In order to Operating in this way we were able to considerably lower the MDA values down to MDAC ≈ boost further the sensitivity performances of the spectrometry, the counting times were also increased 0.05 µ Bq/m , at least a factor of 5–6 better than the previous typical values: MDAC levels of this order from the standard value (57,600 s) up to 200,000 s. In Table 1 all the factors contributing to the variation of magnitude are supposed to be adequate for the detection of the very low ground level Na activity of the MDA values are summarized, with monthly measurements taken as reference. concentrations. Figure 3. Jar containing an annual sample, made up putting together and mixing 12 monthly samples, Figure 3. Jar containing an annual sample, made up putting together and mixing 12 monthly samples, ready to be counted on the top of the cap of a HPGe detector. ready to be counted on the top of the cap of a HPGe  detector. Thus, the 12 monthly samples collected from 2014 to 2018 (five years) were put together, mixed Table 1. Multiplying factors a ecting MDA values: monthly measurements taken as reference. and counted as five annual composite samples. Unfortunately, this approach could not be extended Sampling Time (1e ) " @ 1274.5 keV  Counting Time Overall Factor back to samples older than 6–7 years, because the residual radioactivity present in the samples reaches Monthly 22 undetectable levels due to the relatively short half-life of Na (2.603 years). Therefore, to work around 1 1 1 1 1 measurement the problem, we followed a method recently proposed in [19]. The method, called the spectral Annual 0.0947 1.586 1.889 0.537 0.152 summation technique, is based on the very simple idea to build a virtual, annual spectrum simply by measurement summing channel by channel, the  rays counts recorded in each monthly spectrum: of course, the new virtual spectrum will not be affected by the Na decay issue, as each monthly spectrum was Operating in this way we were able to considerably lower the MDA values down to previously acquired shortly after the collection of the samples. The virtual reconstructed annual MDA  0.05 Bq/m , at least a factor of 5–6 better than the previous typical values: MDA levels of C C spectrum will be equivalent to an annual spectrum of the same size acquired with a much longer this order of magnitude are supposed to be adequate for the detection of the very low ground level Na activity concentrations. Thus, the 12 monthly samples collected from 2014 to 2018 (five years) were put together, mixed and counted as five annual composite samples. Unfortunately, this approach could not be extended to samples older than 6–7 years, because the residual radioactivity present in the samples reaches undetectable levels due to the relatively short half-life of Na (2.603 years). Therefore, to work Environments 2020, 7, 12 6 of 12 around the problem, we followed a method recently proposed in [19]. The method, called the spectral summation technique, is based on the very simple idea to build a virtual, annual spectrum simply by summing channel by channel, the rays counts recorded in each monthly spectrum: of course, the new virtual spectrum will not be a ected by the Na decay issue, as each monthly spectrum was previously acquired shortly after the collection of the samples. The virtual reconstructed annual spectrum will be Environments 2019, 6, x FOR PEER REVIEW 6 of 12 equivalent to an annual spectrum of the same size acquired with a much longer counting time (twelve times the typical counting time of a monthly spectrum), thereby resulting in a substantial reduction of counting time (twelve times the typical counting time of a monthly spectrum), thereby resulting in a the MDA value. substantial reduction of the MDA value. In order to avoid distortion phenomena, the summation operations should be performed with In order to avoid distortion phenomena, the summation operations should be performed with great care: only the spectra acquired with the same spectrometer and with the same channel-energy great care: only the spectra acquired with the same spectrometer and with the same channel-energy calibration data can be summed: even small calibration di erences and slight shifts can shatter the calibration data can be summed: even small calibration differences and slight shifts can shatter the final result, reducing the detection capabilities. final result, reducing the detection capabilities. The typical sensitivity performance of the virtual spectrum can be estimated by means of The typical sensitivity performance of the virtual spectrum can be estimated by means of Equation (5), inserting the proper values of all the relevant parameters. We have found thusly that Equation (5), inserting the proper values of all the relevant parameters. We have found thusly that MDA  0.07 Bq/m , a value very close to the experimental one, obtained with the composite sample C 3 MDAC ≈ 0.07 Bq/m , a value very close to the experimental one, obtained with the composite sample (Figure 3). (Figure 3). 3. Results and Discussion 3. Results and Discussion In Figures 4 and 5 the measured spectra of the annual samples 2018 and 2014 are shown. Both In Figures 4 and 5 the measured  spectra of the annual samples 2018 and 2014 are shown. Both spectra look very similar: the 1274.5 keV 22 Na emission, marked in red, is always present, while in spectra look very similar: the 1274.5 keV Na emission, marked in red, is always present, while in the 2014 spectrum the peak is considerably smaller. The only relevant di erence is the lack of the the 2014 spectrum the peak is considerably smaller. The only relevant difference is the lack of the 477.6 keV Be peak in the 2014 spectrum, because it completely disappeared due to decay. 477.6 keV Be peak in the 2014 spectrum, because it completely disappeared due to decay. Figure 4. The annual spectrum of 2018: The Na peak marked in red is evident in the higher energy part Figure 4. The annual spectrum of 2018: The Na peak marked in red is evident in the higher energy of the spectrum. The other marked peak belongs to Be, the most abundant cosmogenic radionuclide part of the spectrum. The other marked peak belongs to Be, the most abundant cosmogenic in radio atmospher nuclide. e in atmosphere. Environments 2020, 7, 12 7 of 12 Environments 2019, 6, x FOR PEER REVIEW 7 of 12 Environments 2019, 6, x FOR PEER REVIEW 7 of 12 Figure 5. The annual spectrum of 2014: The Na peak is still evident, but much smaller with respect Figure 5. The annual spectrum of 2014: The Na peak is still evident, but much smaller with respect toFigure that in5. Figur The e ann 4. ual Mor spec eover tru ,m the of 2014 Be peak : The completely Na peak is disappear still evident ed , due but to much decay small . er with respect to that in Figure 4. Moreover, the Be peak completely disappeared due to decay. to that in Figure 4. Moreover, the Be peak completely disappeared due to decay. In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the recorded monthly spectra by means of the spectral summation, are shown. recorded monthly  spectra by means of the spectral summation, are shown. recorded monthly  spectra by means of the spectral summation, are shown. 22 7 22 Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak 22 7 22 22 7 22 Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak in the composite spectrum is significantly smaller than that in the corresponding measured spectrum in the composite spectrum is significantly smaller than that in the corresponding measured spectrum in the composite spectrum is significantly smaller than that in the corresponding measured spectrum (Figure 4). (Figure 4). (Figure 4). Environments 2020, 7, 12 8 of 12 Environments 2019, 6, x FOR PEER REVIEW 8 of 12 22 7 22 7 Figure 7. Annual summation spectrum for 2014: Both Na and Be peaks are present. It is worth Figure 7. Annual summation spectrum for 2014: Both Na and Be peaks are present. It is worth noting that in the 2014 measured spectrum (see Figure 5) the Be peak is missing. noting that in the 2014 measured spectrum (see Figure 5) the Be peak is missing. The detection of the Na peaks in the reconstructed spectra obtained with the spectrum summation The detection of the Na peaks in the reconstructed spectra obtained with the spectrum technique is certainly a good achievement. However, in order to quantitively compare the results summation technique is certainly a good achievement. However, in order to quantitively compare obtained the resul with ts ob these tained two with di th ere ent se tw appr o di oaches, fferent an appro additional aches, astep n adis diti needed. onal step Actually is needed. , the Act two udeposition ally, the data two sets depo , Dsitio and n dD ata, sets show , Dvalues a and Dm that , sho di w va erlues from theach at differ other from by eabout ach other one by or der about of on magnitude, e order of as a m magnitude, as shown in Table 2. shown in Table 2. Table 2. Na annual deposition values from 2014 to 2018. Table 2. Na annual deposition values from 2014 to 2018. 2 2 22 Year Measured Da (Bq/m ) Reconstructed Dm (Bq/m ) Rain (mm) Year Measured D (Bq/m ) Reconstructed D (Bq/m ) Rain (mm) a m 2018 0.7604 ± 5.8% 0.0529 ± 10.1% 1520.9 2018 0.7604 5.8% 0.0529 10.1% 1520.9 2017 0.3977 ± 13.1% 0.1022 ± 12.0% 736.8 2017 0.3977 13.1% 0.1022 12.0% 736.8 2016 0.5429 ± 9.6% 0.0207 ± 25.3% 1122.4 2016 0.5429 9.6% 0.0207 25.3% 1122.4 2015 0.5027 9.2% 0.0268 17.9% 866 2015 0.5027 ± 9.2% 0.0268 ± 17.9% 866 2014 0.4800 15.8% 0.0253 18.1% 1655.4 2014 0.4800 ± 15.8% 0.0253 ± 18.1% 1655.4 In Table 2 the annual precipitation values (mm of rain) recorded at the sampling site (Ivrea) are In Table 2 the annual precipitation values (mm of rain) recorded at the sampling site (Ivrea) are reported as well, because it is a relevant parameter for deposition mechanisms. reported as well, because it is a relevant parameter for deposition mechanisms. The reason for these discrepancies can be explained taking into account that, for measured The reason for these discrepancies can be explained taking into account that, for measured values, values, the measured deposition quantity, i.e., the annual deposition Da, is given by the expression: the measured deposition quantity, i.e., the annual deposition D , is given by the expression: −𝜆 ⋅ (6) 𝐷 = ⋅ (1 − 𝑒 ) D =  1 e (6) where  is the downward flux of the radionuclide (Bq/(m ∙s) and a = 1 year is the sampling time, while for the reconstructed deposition Dm the following hold 2 s: where  is the downward flux of the radionuclide (Bq/(m s) and  = 1 year is the sampling time, while for the reconstructed deposition D the following holds: −𝜆 ⋅ (7) 𝐷 = ⋅ (1 − 𝑒 ) in which  = 1 month. being monthly collected deposition. The downward flux  can thus be m D =  1 e (7) evaluated independently using the two different Equations (6) and (7), using as a normalization factor  the proper (1−e ) quantity. One can argue that the comparison between the two methods should be in which  = 1 month. being monthly collected deposition. The downward flux  can thus be made, more significantly, in terms of the activity concentrations C instead of the fluxes . This is very evaluated independently using the two di erent Equations (6) and (7), using as a normalization factor easy to do: Taking into account Equation (3) the following equation (Equation (8)) can be obtained, the proper (1e ) quantity. One can argue that the comparison between the two methods should be made, more significantly, in terms of the activity concentrations C instead of the fluxes . This is very 𝜏𝑚 𝜏𝑎 Environments 2020, 7, 12 9 of 12 easy to do: Taking into account Equation (3) the following equation (Equation (8)) can be obtained, allowing a direct calculation of the atmospheric activity concentration values fromhe deposition data: C = (8) (1 e )  v in which for all data the average value v = 0.04 m/s were used [16]. The activity concentration values calculated with Equation (8) are then plotted for comparison in Figure 8. The overall uncertainty of C is largely dominated by the v component and can be estimated of the order of 20%. (a) (b) 22 22 Figure 8. Comparison of the Na activity concentration values measured and obtained with the Figure 8. Comparison of the Na activity concentration values measured and obtained with the spectral spectral summation summation technique: technique: in in the the first first panel panel ( (a a) ) ar are e displayed displayed the the raw raw data data while while in in ((b b)) the the rain rain normalized normalized ones. ones. In the panel a, the raw data are displayed: at first sight no apparent linear trend appears, while the removal of the data with the highest summation spectral value (year 2017; C 1 Bq/m ) would lead to a quite good linear correlation (R = 0.871). The exclusion of this data could be supported by the consideration that particularly low rainfall values occurred in 2017 (see Table 2): actually, it has already pointed out that the overall amount of precipitation can substantially a ect the “real” v value, a parameter that plays an essential role in the calculation of the activity concentration C. Therefore, a di erent approach can be tried, normalizing all the activity concentration data to the average precipitation rate in the period 2014–2018, as follows: C C  , where Cn are the n = i i i P normalized values, P is the average precipitation value in that period and the subscript i refers to each individual activity concentration value. The normalized results are shown in the panel b: while some discrepancies still remain, the correlation seems improved. The agreement of the two data sets could probably be improved further with a more precise and sophisticated normalization procedure, taking into account not only the overall precipitation values but also the number of precipitation events and their distribution along the year. Nevertheless, in spite of these still open issues, the obtained results show that the spectral summation technique applied to the deposition data are able to detect the low level Na traces in the atmosphere, giving values that in most cases are in fairly good agreement (within 25%–30%) with the measured ones. The reconstruction of long time series of this (and maybe others) radionuclide from old stored Figure 9. In the last few years the Na activity concentration values show an apparently spectra is therefore a realistic perspective, very promising for several environmental studies. increasingly trend. However, as an example, some preliminary interesting results can be shown. Taking as the most reliable estimation for the Na annual activity concentrations, the average of the measured and reconstructed values, and plotting them versus time, an apparent increasing trend appears (Figure 9). Environments 2019, 6, x; doi: FOR PEER REVIEW www.mdpi.com/journal/environments (a) (b) Figure 8. Comparison of the Na activity concentration values measured and obtained with the spectral summation technique: in the first panel (a) are displayed the raw data while in (b) the rain Environments 2020, 7, 12 10 of 12 normalized ones. Figure 9. In the last few years the 22 Na activity concentration values show an apparently Figure 9. In the last few years the Na activity concentration values show an apparently increasingly trend. increasingly trend. This trend is consistent with the corresponding decrease of solar activity that we experienced in those years due to the well-known 11 year solar cycle (in particular, the end of the 24th solar cycle), leading to an increase of the GCR component, and thus to greater production rates of all cosmogenic radionuclides. In Figure 10 the Na activity concentrations data available at the moment are plotted together with the periodically varying, yearly averaged sunspot numbers, taken as a proxy for solar activity [20–22]: as expected, the data clearly seem to be inversely correlated with the average sunspot numbers. The calculation by means of the summation spectra technique of the Na activity concentration in the past few years until the beginning of this century is currently on the way and will allow us to verify Environments 2019, 6, x FOR PEER REVIEW 2 of 2 these conclusions in a broader time range. Environments 2019, 6, x; doi: FOR PEER REVIEW www.mdpi.com/journal/environments Sunspot Numbers vs Na-22 250 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 Year Figure 10. Sunspot numbers (magenta) and Na (blue) ground level atmospheric activity concentrations Figure 10. Sunspot numbers (magenta) and Na (blue) ground level atmospheric activity displayed with their uncertainty bars. concentrations displayed with their uncertainty bars. Sunspot Numbers Annual Average Na-22 [microBq/m ] Environments 2020, 7, 12 11 of 12 Author Contributions: Conceptualization, M.M.; data curation, M.M., L.B., S.B., B.B., M.G. and M.C.L.; formal analysis, M.M., L.B., S.B., B.B., M.G. and M.C.L.; methodology, M.M.; software, L.B. and S.B.; supervision, M.M., L.B. and M.C.L.; writing—original draft, M.M.; writing—review and editing, M.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. LNE-LNHB/CEA Table de Radionucléides. Available online: www.nucleide.org/DDEP_WG/Nuclides (accessed on 30 September 2019). 2. Lal, D.; Peters, B. Cosmic ray produced radioactivity on the Earth. In Handbuch der Physik; Springer: Berlin, Germany, 1967; Volume 46/2, pp. 552–616. 3. Magnoni, M. Chapter 12—Environmental Radioactivity and Radioecology. In Physical Agents in the Environment and Workplaces-Noise and Vibrations, Electromagnetic Fields and Ionizing Radiation; Licitra, G., d’Amore, G., Magnoni, M., Eds.; Taylor and Francis Group; LLC: Boca Raton, USA, 2018. 4. NCRP. Report n 94, Exposure of the Population in the United States and Canada from Natural Background Radiation; NCRP: Bethesda, MD, USA, 1987. 5. Blazej, S.; Mietelski, J.W. Cosmogenic Na-22, Be-7 and terrestrial Cs-137, K-40 radionuclides in ground level air samples collected in Krakow (Poland) over years 2003–2006. J. Radioanal. Nucl. Chem. 2014, 300, 747–756. [CrossRef] [PubMed] 6. Dutkiewicz, V.A.; Husain, L. Stratospheric and tropospheric components of 7Be surface air. J. Geophys. Res. 1985, 90, 5783–5788. [CrossRef] 7. Dutkiewicz, V.A.; Husain, L. Determination of stratospheric ozone at ground level using Be/ozone ratios. Geophys. Res. Lett. 1979, 6, 171–174. [CrossRef] 8. Vieeze, W.; Singh, H.B. The distribution of beryllium-7 in the troposphere: Implication on stratospheric-tropospheric exchange. Geophys. Res. Lett. 1980, 7, 805–808. [CrossRef] 9. Yoshimori, M. Production and behaviour of beryllium-7 isotope in the upper atmosphere. Adv. Space Res. 2005, 36, 922–926. [CrossRef] 10. Ho man, I.; Lewis, B.; Chan, P. Circulation of cosmogenic Na using the global monitoring network o the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO). J. Environ. Radioact. 2018, 187, 8–15. [CrossRef] [PubMed] 11. European Radiation Dosimetry Group. Radiation Protection 85, Exposure of Air Crew to Cosmic Radiation; EURADOS report 1996-01; European Radiation Dosimetry Group: EC, Brussels, Belgium and Luxemburg, Luxemburg, 1996. 12. O’Brien, K.H. Secular variation in the production of cosmogenic isotopes in earth’s atmosphere. J. Geophys. Res. 1979, 84, 423–431. [CrossRef] 13. Damatto, S.R. Be-7 measured at ground air level and rainfall in the city of Sao Paulo. In Proceedings of the INAC 2013—International Nuclear Atlantic Conference, Recife, Brazil, 24–29 November 2013; ISBN 978-85-99141-05-2. 14. Field, C.V.; Schmidt, G.A.; Koch, D.; Salyk, C. Modeling production and climate related impacts of Be concentrations in ice cores. J. Geophys. Res. 2006, 111, D15107. [CrossRef] 15. Magnoni, M. La deposizione umida e secca: Aspetti sperimentali e teorici. In Proceedings of the Congress ‘Twenty-Five Years After Chernobyl Accident: Studies, Remarks and Recent Findings’, Udine, Italy, 10–12 March 2011. 16. Facchinelli, A.; Magnoni, M.; Gallini, L.; Bonifacio, E. 137Cs contamination from Chernobyl of soils in Piemonte (North-West Italy): Spatial distribution and deposition model. Water Air Soil Pollut. 2002, 134, 341–352. [CrossRef] 17. Giardina, M.; Bu a, P. A new approach for modeling dry deposition velocity of particles. Atmos. Environ. 2018, 180, 11–22. [CrossRef] 18. Currie, L.A. Limit of Qualitative Detection and Quantitative Determination. Anal. Chem. 1968, 40, 586–593. [CrossRef] Environments 2020, 7, 12 12 of 12 19. Ho man, I.; Lewis, B.; Chan, P.; Ungar, K. Ho man Analysis of Na using a spectral summation technique on high-volume aerosol samples. J. Environ. Radioact. 2017, 169, 151–158. [CrossRef] [PubMed] 20. SILSO, Royal Observatory of Belgium. Available online: www.sidc.be/SILSO (accessed on 14 November 2019). 21. Hoyt, D.V.; Schatten, K.H. Group sunspot numbers: A new solar activity reconstruction. Part 1. Sol. Phys. 1998, 179, 189–219. [CrossRef] 22. Hoyt, D.V.; Schatten, K.H. Group sunspot numbers: A new solar activity reconstruction. Part 2. Sol. Phys. 1998, 181, 491–512. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environments Multidisciplinary Digital Publishing Institute

Measurements of 22Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and Dry Deposition Samples

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environments Article Measurements of Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and Dry Deposition Samples Mauro Magnoni *, Luca Bellina, Stefano Bertino, Brunella Bellotto, Maura Ghione and Maria Clivia Losana Physical and Technological Risks Department, ARPA Piemonte, 10015 Ivrea, Italy; l.bellina@arpa.piemonte.it (L.B.); s.bertino@arpa.piemonte.it (S.B.); b.bellotto@arpa.piemonte.it (B.B.); m.ghione@arpa.piemonte.it (M.G.); m.losana@arpa.piemonte.it (M.C.L.) * Correspondence: mauro.magnoni@arpa.piemonte.it Received: 29 December 2019; Accepted: 8 February 2020; Published: 11 February 2020 Abstract: Sodium-22 ( Na, half-life 2.603 years) is a cosmogenic radionuclide mainly produced in the stratosphere by nuclear spallation reactions of cosmic rays on Ar. Due to the very low concentration levels normally reached in the environment, Na poses no significant radioprotection threats: actually, the e ective doses delivered to humans can hardly exceed a few nSv per year, a very negligible value. However, the measurements of this radionuclides can be very interesting for atmospheric circulation and climatic studies. Unfortunately, the diculty of Na detection, due to its very low concentration levels, has prevented the gathering of large and widespread time series of this radionuclide. In this paper, a method for the retrospective measurements of Na in the atmosphere, starting from the gamma spectra (hyperpure germanium detectors (HPGe) detectors) of wet and dry deposition samples stored in our databases is proposed and validated. The method was applied to spectra samples gathered in the context of the Italian National Radioactivity Monitoring Network (RESORAD) and allowed the detection of the very low atmospheric activity concentration values of Na present at ground level. The results obtained with the new method are discussed and compared for validation with the available experimental values. Finally, some possible applications to environmental studies are also highlighted and suggested. Keywords: Na; atmospheric activity concentration; HPGe gamma spectra; retrospective analysis 1. Introduction 22 + Sodium-22 ( Na) is a cosmogenic radionuclide with a relatively long half-file (2.603 years), continuously produced by nuclear spallation reactions of cosmic rays on argon-40 ( Ar) nuclei [1]. It decays into the stable isotope Ne by + emission (90.35%) and electron capture (EC, 9.65%). Its production mainly occurs in the stratosphere and is essentially due to the high energy particles (E > 100 MeV/n) belonging to the galactic cosmic rays component (GCR, galactic cosmic rays). Once produced, Na is quickly attached to the sub-micron particulate suspended in the atmosphere and slowly settles to the ground [2]. It is eciently scavenged by precipitation, and therefore can also be found in meteoric waters, thereby easily entering into the ecosystems. Its concentrations in atmosphere increased substantially during the sixties of the 20th century, in the early phase of Cold War (1945–1963), due to nuclear weapons testing. At that time, traces of Na were also measured in lichens, mosses and wild game [3,4]. Nowadays, since the last atmospheric nuclear weapon detonation occurred in 1980 (Lop Nor, China), the Na levels returned to the typical pre-Cold War values: atmospheric concentrations usually well below 1 Bq/m at ground level [5]. However, the main interest in studying Environments 2020, 7, 12; doi:10.3390/environments7020012 www.mdpi.com/journal/environments Environments 2020, 7, 12 2 of 12 this radionuclide is its use as a tracer of the atmospheric circulation, often investigated also by means of other cosmogenic radionuclides with very di erent half-lives (for instance: Be, t = 53.22 days and 1/2 10 6 Be, t = 1.3610 years) [1]. A sudden increase of the ground level concentrations of the cosmogenic 1/2 radionuclides, for example, can be used as indicator of the intrusion of air masses of stratospheric 22 7 origin [6–9]. In this respect, the study of the ratio Na/ Be could give very interesting information, as was recently pointed out in a recent study (Ho mann, 2018, [10]). Moreover, because the cosmogenic radionuclides’ production rate is a ected by the 11 year sun cycles, the activity concentration values of all cosmogenic radionuclides are also of great interest for monitoring solar activity [11–14]: Na is particularly interesting in this respect, because of its physical characteristics, as its quite long half-life, make it less sensitive to variations of the meteorological conditions. Therefore, the availability of reliable time series of this radionuclide is very important and of great scientific relevance, allowing the gathering of some very interesting information that cannot be obtained using only easier-to-measure radionuclides, for example, Be, usually present in larger concentrations. 2. Materials and Methods In principle, Na can be easily measured by spectrometry with hyperpure germanium detectors (HPGe): actually it emits a strong line at 1274.5 keV with a yield close to unity (99.94%) in a region of the spectrum only slightly influenced by the Compton background of the K high energy emission (1460 keV). There is also another emission at 511 keV with an even stronger yield (180%) but not useful for quantitative determination due to the interference of the 511 keV annihilation peak always present in the background because of the pair production (electron e and positron e ) interactions of radiation with matter, mainly due to the lead shielding. Unfortunately, in spite of its strong emissions, the very low concentration levels typically found in the atmosphere (<1 Bq/m ) make the detection of Na in normal atmospheric particulate samples often very dicult. For that reason, a huge amount of air (tens of thousands of cubic meters, at least) needs to be filtered in order to achieve the necessary sensitivity. Alternatively, an indirect measurement of the atmospheric activity concentrations can be done using deposition data. In fact, wet and dry deposition can be collected for a convenient sampling time  (in our case, 1 month) by means of stainless steel tanks or similar containers. The relationship between the deposition values D (Bq/m ) and their corresponding atmospheric activity concentrations C (Bq/m ) can be deduced from a simple model describing the deposition D of radionuclides in a collection tank by the following di erential Equation: dD +   D = F (1) dt where  is the decay constant of the radionuclide and  is the corresponding downward flux, usually expressed as Bq/(m s). If the flux is assumed to be constant in time, the analytical solution of the above equation is straightforward and the amount of radioactivity collected in the tank during a generic sampling time  is thus given by the following expression: D =  1 e (2) The main limitation of this description is considering the Na flux as a constant: an apparently crude approximation, very far from real conditions, being the deposition mainly governed by precipitations—a typical example of a discrete and non-regular phenomenon. Bulk deposition should be more precisely described as a two components process, as follows: F = F + F , where wet bulk dry for the dry component a very simple relationship holds: F = C  v , in which C is the activity dry d concentration, while v is the average value of the settling velocity of the atmospheric particulate. A much more complicated expression should be used for the wet deposition component instead, involving many experimental parameters, such as the amount and the intensity of the precipitation Environments 2020, 7, 12 3 of 12 event, the height of the atmospheric column scavenged by the rain, the scavenging coecients, etc. However, in real cases, the simultaneous knowledge of all these parameters is seldom available, thereby preventing the possibility of using a “true” theoretical wet deposition mathematical model. Fothat reason, a very simplified description is often proposed, with bulk deposition modelled using the same relationship that holds for dry deposition: F = C  v (3) bulk m in which C is still the activity concentration, while v is a mean deposition velocity experimentally Environments 2019, 6, x FOR PEER REVIEW 3 of 12 evaluated after measuring simultaneously the deposition data (see Equation (2) and the corresponding activity concentration C in atmosphere [15,16]. In doing so we must bear in mind that the physical bulk=C∙vm (3) meaning of v is quite di erent respect to that of v : while v is a mean velocity obtained averaging m d d in which C is still the activity concentration, while vm is a mean deposition velocity experimentally over the distribution of all the velocities of the settling particulate suspended in atmosphere, v is not evaluated after measuring simultaneously the deposition data (see Equation (2)) and the a real velocity, just an empirical parameter encompassing the e ect of dry deposition and precipitation, corresponding activity concentration C in atmosphere [15,16]. In doing so we must bear in mind that the physical meaning of vm is quite different respect to that of vd: while vd is a mean velocity obtained and whose dimensions are those of a velocity. For that reason the numerical values of v are much averaging over the distribution of all the velocities of the settling particulate suspended in greater that those of v , the latter being related only to the much slower dry deposition processes: atmosphere, vm is not a real velocity, just an empirical parameter encompassing the effect of dry experimental measurements performed on Cs gave for v a value around 0.04 m/s [16], while typical deposition and precipitation, and whose dimensions are those of a velocity. For that reason the values of v , strongly dependent on the particulate diameter and other factors as well [17], are typically numerical values of vm are much greater that those of vd, the latter being related only to the much slower dry deposition processes: experimental measurements performed on Cs gave for vm a value in the range 0.1–0.001 cm/s. around 0.04 m/s [16], while typical values of vd , strongly dependent on the particulate diameter and The experimental set up for the collection of the wet and dry deposition samples is a stainless steel other factors as well [17], are typically in the range 0.1–0.001 cm/s. tank placed on the roof of the laboratory building (see Figure 1). The bottom of the tank is always kept The experimental set up for the collection of the wet and dry deposition samples is a stainless wet in order to prevent resuspension during dry periods. The collection of the samples is done at the steel tank placed on the roof of the laboratory building (see Figure 1). The bottom of the tank is always kept wet in order to prevent resuspension during dry periods. The collection of the samples is done end of each month: the tank is emptied and carefully washed with distilled water. The resulting water at the end of each month: the tank is emptied and carefully washed with distilled water. The resulting is then reduced by evaporation (90 C) and brought to dryness. The residue is finally weighted, put in water is then reduced by evaporation (90 °C) and brought to dryness. The residue is finally weighted, a little cylindrical jar (see Figure 2) and counted with hyperpure germanium detectors (HPGe) for 16 h. put in a little cylindrical jar (see Figure 2) and counted with hyperpure germanium detectors (HPGe) for 16 h. Figure 1. The stainless steel tank for sampling wet and dry deposition on the roof of the ARPA Figure 1. The stainless steel tank for sampling wet and dry deposition on the roof of the ARPA Piemonte Piemonte building (Via Jervis, 30, Ivrea, 10015, Italy): the collection area is about 4 m wide. The tank building (Via Jervis, 30, Ivrea, 10015, Italy): the collection area is about 4 m wide. The tank is emptied is emptied on a monthly basis through a tube. on a monthly basis through a tube. These are the standard procedures followed in the context of the Italian Environmental Radioactivity National Monitoring Network (RESORAD): they allow reaching a quite-good sensitivity for most radionuclides. For instance, the MDA (minimum detectable activity), referred to Environments 2019, 6, x FOR PEER REVIEW 4 of 12 137 2 22 2 as Cs, was about 0.015 Bq/m , while for Na a slightly larger value applies, 0.025 Bq/m , due to a lower spectrometric efficiency at the high energy  emission of sodium-22 (1274.5 keV). It can be demonstrated that these deposition MDA values correspond to about 0.2–0.3 µ Bq/m for the activity concentration: very low values are perfectly adequate for monitoring purposes, but still not enough Environments 2020, 7, 12 22 22 4 of 12 for a continuous monitoring of Na in atmosphere, as at ground level the Na activity concentrations are sometimes even lower [5]. Figure 2. Plastic jar for the wet and dry deposition measurements placed on the top of the cap of a Figure 2. Plastic jar for the wet and dry deposition measurements placed on the top of the cap of a hyperpure germanium detectors (HPGe) detector. The dry residue (about 4 g) is uniformly distributed hyperpure germanium detectors (HPGe) detector. The dry residue (about 4 g) is uniformly distributed in a thin cylindrical shaped geometry. in a thin cylindrical shaped geometry. These are the standard procedures followed in the context of the Italian Environmental Therefore, in order to improve the sensitivity of the measurements, single annual samples were Radioactivity National Monitoring Network (RESORAD): they allow reaching a quite-good sensitivity assembled, simply mixing the 12 monthly samples: each monthly sample of a given year (4 g of dry for most radionuclides. For instance, the MDA (minimum detectable activity), referred to as Cs, residue) was transferred into a larger jar (Figure 3) and counted as a new composite annual sample. 2 22 2 was about 0.015 Bq/m , while for Na a slightly larger value applies, 0.025 Bq/m , due to a lower Operating in this way, a significant decrease of the MDA values for deposition is expected, as spectrometric eciency at the high energy emission of sodium-22 (1274.5 keV). It can be demonstrated can be easily calculated using the simple, classic MDA formula given by Currie in 1968 [18]: that these deposition MDA values correspond to about 0.2–0.3 Bq/m for the activity concentration: 4.66 ∙ 𝜌 very low values are perfectly adequate for monitoring purposes, but still not enough for a continuous 𝑀𝐷𝐴 = 𝐷 (4) 22 22 𝜀 ∙ 𝑟 ∙ 𝑆 ∙ √𝑡 monitoring of Na in atmosphere, as at ground level the Na activity concentrations are sometimes 𝛾 𝛾 even lower [5]. where t is the counting time, back is the standard deviation of the background,  is the photopeak Therefore, in order to improve the sensitivity of the measurements, single annual samples were efficiency of the HPGe detector at the specific radionuclide emission energy, r is the  yield of the assembled, simply mixing the 12 monthly samples: each monthly sample of a given year (4 g of dry emission and S is the surface area. From this expression, taking into account Equations (2) and (3), residue) was transferred into a larger jar (Figure 3) and counted as a new composite annual sample. the expression for the MDA activity concentrations can be written as follows: Operating in this way, a significant decrease of the MDA values for deposition is expected, as can 4.66 ∙ 𝜌 ∙ 𝜆 be easily calculated using the simple, classic MDA formula given by Currie in 1968 [18]: 𝑀𝐷𝐴 = (5) ( ) 𝜀 ∙ 𝑟 ∙ 𝑆 ∙ √𝑡 ∙ 1 − 𝑒 ∙ 𝑣 𝛾 𝛾 𝑚 4.66 back MDA = p (4) in which the sampling time  is 1 month for the standard samples and 1 year for the composite "  r  S t sample. The improvement of the MDA values for annual measurements is due to the increased value  of the quantity (1−e ), a factor that largely dominates two negative effects: (1) the decrease of the where t is the counting time,  is the standard deviation of the background, " is the photopeak back photopeak efficiency  caused by increasing of the solid angle of the counting geometry; (2) the slight eciency of the HPGe detector at the specific radionuclide emission energy, r is the yield of the emission and S is the surface area. From this expression, taking into account Equations (2) and (3), the expression for the MDA activity concentrations can be written as follows: 4.66 back MDA = p (5) "  r  S t (1 e )  v 𝜆𝜏 𝑏𝑎𝑐𝑘 𝑏𝑎𝑐𝑘 Environments 2019, 6, x FOR PEER REVIEW 5 of 12 increase of the Compton background standard deviation related to the greater size of the annual sample. In order to boost further the sensitivity performances of the  spectrometry, the counting times were also increased from the standard value (57,600 seconds) up to 200,000 seconds. In Table 1 all the factors contributing to the variation of the MDA values are summarized, with monthly measurements taken as reference. Environments 2020, 7, 12 5 of 12 Table 1. Multiplying factors affecting MDA values: monthly measurements taken as reference. in which the sampling time  is 1 month for the standard samples and 1 year for the composite sample.  Sampling Time (1−e ) @ 1274.5 keV back Counting Time Overall Factor The improvement of the MDA values for annual measurements is due to the increased value of the Monthly me  asurement 1 1 1 1 1 quantity (1e ), a factor that largely dominates two negative e ects: (1) the decrease of the photopeak Annual measurement 0.0947 1.586 1.889 0.537 0.152 eciency " caused by increasing of the solid angle of the counting geometry; (2) the slight increase of the Compton background standard deviation related to the greater sizof the annual sample. In order to Operating in this way we were able to considerably lower the MDA values down to MDAC ≈ boost further the sensitivity performances of the spectrometry, the counting times were also increased 0.05 µ Bq/m , at least a factor of 5–6 better than the previous typical values: MDAC levels of this order from the standard value (57,600 s) up to 200,000 s. In Table 1 all the factors contributing to the variation of magnitude are supposed to be adequate for the detection of the very low ground level Na activity of the MDA values are summarized, with monthly measurements taken as reference. concentrations. Figure 3. Jar containing an annual sample, made up putting together and mixing 12 monthly samples, Figure 3. Jar containing an annual sample, made up putting together and mixing 12 monthly samples, ready to be counted on the top of the cap of a HPGe detector. ready to be counted on the top of the cap of a HPGe  detector. Thus, the 12 monthly samples collected from 2014 to 2018 (five years) were put together, mixed Table 1. Multiplying factors a ecting MDA values: monthly measurements taken as reference. and counted as five annual composite samples. Unfortunately, this approach could not be extended Sampling Time (1e ) " @ 1274.5 keV  Counting Time Overall Factor back to samples older than 6–7 years, because the residual radioactivity present in the samples reaches Monthly 22 undetectable levels due to the relatively short half-life of Na (2.603 years). Therefore, to work around 1 1 1 1 1 measurement the problem, we followed a method recently proposed in [19]. The method, called the spectral Annual 0.0947 1.586 1.889 0.537 0.152 summation technique, is based on the very simple idea to build a virtual, annual spectrum simply by measurement summing channel by channel, the  rays counts recorded in each monthly spectrum: of course, the new virtual spectrum will not be affected by the Na decay issue, as each monthly spectrum was Operating in this way we were able to considerably lower the MDA values down to previously acquired shortly after the collection of the samples. The virtual reconstructed annual MDA  0.05 Bq/m , at least a factor of 5–6 better than the previous typical values: MDA levels of C C spectrum will be equivalent to an annual spectrum of the same size acquired with a much longer this order of magnitude are supposed to be adequate for the detection of the very low ground level Na activity concentrations. Thus, the 12 monthly samples collected from 2014 to 2018 (five years) were put together, mixed and counted as five annual composite samples. Unfortunately, this approach could not be extended to samples older than 6–7 years, because the residual radioactivity present in the samples reaches undetectable levels due to the relatively short half-life of Na (2.603 years). Therefore, to work Environments 2020, 7, 12 6 of 12 around the problem, we followed a method recently proposed in [19]. The method, called the spectral summation technique, is based on the very simple idea to build a virtual, annual spectrum simply by summing channel by channel, the rays counts recorded in each monthly spectrum: of course, the new virtual spectrum will not be a ected by the Na decay issue, as each monthly spectrum was previously acquired shortly after the collection of the samples. The virtual reconstructed annual spectrum will be Environments 2019, 6, x FOR PEER REVIEW 6 of 12 equivalent to an annual spectrum of the same size acquired with a much longer counting time (twelve times the typical counting time of a monthly spectrum), thereby resulting in a substantial reduction of counting time (twelve times the typical counting time of a monthly spectrum), thereby resulting in a the MDA value. substantial reduction of the MDA value. In order to avoid distortion phenomena, the summation operations should be performed with In order to avoid distortion phenomena, the summation operations should be performed with great care: only the spectra acquired with the same spectrometer and with the same channel-energy great care: only the spectra acquired with the same spectrometer and with the same channel-energy calibration data can be summed: even small calibration di erences and slight shifts can shatter the calibration data can be summed: even small calibration differences and slight shifts can shatter the final result, reducing the detection capabilities. final result, reducing the detection capabilities. The typical sensitivity performance of the virtual spectrum can be estimated by means of The typical sensitivity performance of the virtual spectrum can be estimated by means of Equation (5), inserting the proper values of all the relevant parameters. We have found thusly that Equation (5), inserting the proper values of all the relevant parameters. We have found thusly that MDA  0.07 Bq/m , a value very close to the experimental one, obtained with the composite sample C 3 MDAC ≈ 0.07 Bq/m , a value very close to the experimental one, obtained with the composite sample (Figure 3). (Figure 3). 3. Results and Discussion 3. Results and Discussion In Figures 4 and 5 the measured spectra of the annual samples 2018 and 2014 are shown. Both In Figures 4 and 5 the measured  spectra of the annual samples 2018 and 2014 are shown. Both spectra look very similar: the 1274.5 keV 22 Na emission, marked in red, is always present, while in spectra look very similar: the 1274.5 keV Na emission, marked in red, is always present, while in the 2014 spectrum the peak is considerably smaller. The only relevant di erence is the lack of the the 2014 spectrum the peak is considerably smaller. The only relevant difference is the lack of the 477.6 keV Be peak in the 2014 spectrum, because it completely disappeared due to decay. 477.6 keV Be peak in the 2014 spectrum, because it completely disappeared due to decay. Figure 4. The annual spectrum of 2018: The Na peak marked in red is evident in the higher energy part Figure 4. The annual spectrum of 2018: The Na peak marked in red is evident in the higher energy of the spectrum. The other marked peak belongs to Be, the most abundant cosmogenic radionuclide part of the spectrum. The other marked peak belongs to Be, the most abundant cosmogenic in radio atmospher nuclide. e in atmosphere. Environments 2020, 7, 12 7 of 12 Environments 2019, 6, x FOR PEER REVIEW 7 of 12 Environments 2019, 6, x FOR PEER REVIEW 7 of 12 Figure 5. The annual spectrum of 2014: The Na peak is still evident, but much smaller with respect Figure 5. The annual spectrum of 2014: The Na peak is still evident, but much smaller with respect toFigure that in5. Figur The e ann 4. ual Mor spec eover tru ,m the of 2014 Be peak : The completely Na peak is disappear still evident ed , due but to much decay small . er with respect to that in Figure 4. Moreover, the Be peak completely disappeared due to decay. to that in Figure 4. Moreover, the Be peak completely disappeared due to decay. In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the In Figures 6 and 7, the corresponding 2018 and 2014 annual spectra, reconstructed from the recorded monthly spectra by means of the spectral summation, are shown. recorded monthly  spectra by means of the spectral summation, are shown. recorded monthly  spectra by means of the spectral summation, are shown. 22 7 22 Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak 22 7 22 22 7 22 Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak Figure 6. Annual summation spectrum for 2018: Both Na and Be peaks are present. The Na peak in the composite spectrum is significantly smaller than that in the corresponding measured spectrum in the composite spectrum is significantly smaller than that in the corresponding measured spectrum in the composite spectrum is significantly smaller than that in the corresponding measured spectrum (Figure 4). (Figure 4). (Figure 4). Environments 2020, 7, 12 8 of 12 Environments 2019, 6, x FOR PEER REVIEW 8 of 12 22 7 22 7 Figure 7. Annual summation spectrum for 2014: Both Na and Be peaks are present. It is worth Figure 7. Annual summation spectrum for 2014: Both Na and Be peaks are present. It is worth noting that in the 2014 measured spectrum (see Figure 5) the Be peak is missing. noting that in the 2014 measured spectrum (see Figure 5) the Be peak is missing. The detection of the Na peaks in the reconstructed spectra obtained with the spectrum summation The detection of the Na peaks in the reconstructed spectra obtained with the spectrum technique is certainly a good achievement. However, in order to quantitively compare the results summation technique is certainly a good achievement. However, in order to quantitively compare obtained the resul with ts ob these tained two with di th ere ent se tw appr o di oaches, fferent an appro additional aches, astep n adis diti needed. onal step Actually is needed. , the Act two udeposition ally, the data two sets depo , Dsitio and n dD ata, sets show , Dvalues a and Dm that , sho di w va erlues from theach at differ other from by eabout ach other one by or der about of on magnitude, e order of as a m magnitude, as shown in Table 2. shown in Table 2. Table 2. Na annual deposition values from 2014 to 2018. Table 2. Na annual deposition values from 2014 to 2018. 2 2 22 Year Measured Da (Bq/m ) Reconstructed Dm (Bq/m ) Rain (mm) Year Measured D (Bq/m ) Reconstructed D (Bq/m ) Rain (mm) a m 2018 0.7604 ± 5.8% 0.0529 ± 10.1% 1520.9 2018 0.7604 5.8% 0.0529 10.1% 1520.9 2017 0.3977 ± 13.1% 0.1022 ± 12.0% 736.8 2017 0.3977 13.1% 0.1022 12.0% 736.8 2016 0.5429 ± 9.6% 0.0207 ± 25.3% 1122.4 2016 0.5429 9.6% 0.0207 25.3% 1122.4 2015 0.5027 9.2% 0.0268 17.9% 866 2015 0.5027 ± 9.2% 0.0268 ± 17.9% 866 2014 0.4800 15.8% 0.0253 18.1% 1655.4 2014 0.4800 ± 15.8% 0.0253 ± 18.1% 1655.4 In Table 2 the annual precipitation values (mm of rain) recorded at the sampling site (Ivrea) are In Table 2 the annual precipitation values (mm of rain) recorded at the sampling site (Ivrea) are reported as well, because it is a relevant parameter for deposition mechanisms. reported as well, because it is a relevant parameter for deposition mechanisms. The reason for these discrepancies can be explained taking into account that, for measured The reason for these discrepancies can be explained taking into account that, for measured values, values, the measured deposition quantity, i.e., the annual deposition Da, is given by the expression: the measured deposition quantity, i.e., the annual deposition D , is given by the expression: −𝜆 ⋅ (6) 𝐷 = ⋅ (1 − 𝑒 ) D =  1 e (6) where  is the downward flux of the radionuclide (Bq/(m ∙s) and a = 1 year is the sampling time, while for the reconstructed deposition Dm the following hold 2 s: where  is the downward flux of the radionuclide (Bq/(m s) and  = 1 year is the sampling time, while for the reconstructed deposition D the following holds: −𝜆 ⋅ (7) 𝐷 = ⋅ (1 − 𝑒 ) in which  = 1 month. being monthly collected deposition. The downward flux  can thus be m D =  1 e (7) evaluated independently using the two different Equations (6) and (7), using as a normalization factor  the proper (1−e ) quantity. One can argue that the comparison between the two methods should be in which  = 1 month. being monthly collected deposition. The downward flux  can thus be made, more significantly, in terms of the activity concentrations C instead of the fluxes . This is very evaluated independently using the two di erent Equations (6) and (7), using as a normalization factor easy to do: Taking into account Equation (3) the following equation (Equation (8)) can be obtained, the proper (1e ) quantity. One can argue that the comparison between the two methods should be made, more significantly, in terms of the activity concentrations C instead of the fluxes . This is very 𝜏𝑚 𝜏𝑎 Environments 2020, 7, 12 9 of 12 easy to do: Taking into account Equation (3) the following equation (Equation (8)) can be obtained, allowing a direct calculation of the atmospheric activity concentration values fromhe deposition data: C = (8) (1 e )  v in which for all data the average value v = 0.04 m/s were used [16]. The activity concentration values calculated with Equation (8) are then plotted for comparison in Figure 8. The overall uncertainty of C is largely dominated by the v component and can be estimated of the order of 20%. (a) (b) 22 22 Figure 8. Comparison of the Na activity concentration values measured and obtained with the Figure 8. Comparison of the Na activity concentration values measured and obtained with the spectral spectral summation summation technique: technique: in in the the first first panel panel ( (a a) ) ar are e displayed displayed the the raw raw data data while while in in ((b b)) the the rain rain normalized normalized ones. ones. In the panel a, the raw data are displayed: at first sight no apparent linear trend appears, while the removal of the data with the highest summation spectral value (year 2017; C 1 Bq/m ) would lead to a quite good linear correlation (R = 0.871). The exclusion of this data could be supported by the consideration that particularly low rainfall values occurred in 2017 (see Table 2): actually, it has already pointed out that the overall amount of precipitation can substantially a ect the “real” v value, a parameter that plays an essential role in the calculation of the activity concentration C. Therefore, a di erent approach can be tried, normalizing all the activity concentration data to the average precipitation rate in the period 2014–2018, as follows: C C  , where Cn are the n = i i i P normalized values, P is the average precipitation value in that period and the subscript i refers to each individual activity concentration value. The normalized results are shown in the panel b: while some discrepancies still remain, the correlation seems improved. The agreement of the two data sets could probably be improved further with a more precise and sophisticated normalization procedure, taking into account not only the overall precipitation values but also the number of precipitation events and their distribution along the year. Nevertheless, in spite of these still open issues, the obtained results show that the spectral summation technique applied to the deposition data are able to detect the low level Na traces in the atmosphere, giving values that in most cases are in fairly good agreement (within 25%–30%) with the measured ones. The reconstruction of long time series of this (and maybe others) radionuclide from old stored Figure 9. In the last few years the Na activity concentration values show an apparently spectra is therefore a realistic perspective, very promising for several environmental studies. increasingly trend. However, as an example, some preliminary interesting results can be shown. Taking as the most reliable estimation for the Na annual activity concentrations, the average of the measured and reconstructed values, and plotting them versus time, an apparent increasing trend appears (Figure 9). Environments 2019, 6, x; doi: FOR PEER REVIEW www.mdpi.com/journal/environments (a) (b) Figure 8. Comparison of the Na activity concentration values measured and obtained with the spectral summation technique: in the first panel (a) are displayed the raw data while in (b) the rain Environments 2020, 7, 12 10 of 12 normalized ones. Figure 9. In the last few years the 22 Na activity concentration values show an apparently Figure 9. In the last few years the Na activity concentration values show an apparently increasingly trend. increasingly trend. This trend is consistent with the corresponding decrease of solar activity that we experienced in those years due to the well-known 11 year solar cycle (in particular, the end of the 24th solar cycle), leading to an increase of the GCR component, and thus to greater production rates of all cosmogenic radionuclides. In Figure 10 the Na activity concentrations data available at the moment are plotted together with the periodically varying, yearly averaged sunspot numbers, taken as a proxy for solar activity [20–22]: as expected, the data clearly seem to be inversely correlated with the average sunspot numbers. The calculation by means of the summation spectra technique of the Na activity concentration in the past few years until the beginning of this century is currently on the way and will allow us to verify Environments 2019, 6, x FOR PEER REVIEW 2 of 2 these conclusions in a broader time range. Environments 2019, 6, x; doi: FOR PEER REVIEW www.mdpi.com/journal/environments Sunspot Numbers vs Na-22 250 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 Year Figure 10. Sunspot numbers (magenta) and Na (blue) ground level atmospheric activity concentrations Figure 10. Sunspot numbers (magenta) and Na (blue) ground level atmospheric activity displayed with their uncertainty bars. concentrations displayed with their uncertainty bars. Sunspot Numbers Annual Average Na-22 [microBq/m ] Environments 2020, 7, 12 11 of 12 Author Contributions: Conceptualization, M.M.; data curation, M.M., L.B., S.B., B.B., M.G. and M.C.L.; formal analysis, M.M., L.B., S.B., B.B., M.G. and M.C.L.; methodology, M.M.; software, L.B. and S.B.; supervision, M.M., L.B. and M.C.L.; writing—original draft, M.M.; writing—review and editing, M.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. References 1. LNE-LNHB/CEA Table de Radionucléides. Available online: www.nucleide.org/DDEP_WG/Nuclides (accessed on 30 September 2019). 2. Lal, D.; Peters, B. Cosmic ray produced radioactivity on the Earth. In Handbuch der Physik; Springer: Berlin, Germany, 1967; Volume 46/2, pp. 552–616. 3. Magnoni, M. Chapter 12—Environmental Radioactivity and Radioecology. In Physical Agents in the Environment and Workplaces-Noise and Vibrations, Electromagnetic Fields and Ionizing Radiation; Licitra, G., d’Amore, G., Magnoni, M., Eds.; Taylor and Francis Group; LLC: Boca Raton, USA, 2018. 4. NCRP. Report n 94, Exposure of the Population in the United States and Canada from Natural Background Radiation; NCRP: Bethesda, MD, USA, 1987. 5. Blazej, S.; Mietelski, J.W. Cosmogenic Na-22, Be-7 and terrestrial Cs-137, K-40 radionuclides in ground level air samples collected in Krakow (Poland) over years 2003–2006. J. Radioanal. Nucl. Chem. 2014, 300, 747–756. [CrossRef] [PubMed] 6. Dutkiewicz, V.A.; Husain, L. Stratospheric and tropospheric components of 7Be surface air. J. Geophys. Res. 1985, 90, 5783–5788. [CrossRef] 7. Dutkiewicz, V.A.; Husain, L. Determination of stratospheric ozone at ground level using Be/ozone ratios. Geophys. Res. Lett. 1979, 6, 171–174. [CrossRef] 8. Vieeze, W.; Singh, H.B. The distribution of beryllium-7 in the troposphere: Implication on stratospheric-tropospheric exchange. Geophys. Res. Lett. 1980, 7, 805–808. [CrossRef] 9. Yoshimori, M. Production and behaviour of beryllium-7 isotope in the upper atmosphere. Adv. Space Res. 2005, 36, 922–926. [CrossRef] 10. Ho man, I.; Lewis, B.; Chan, P. Circulation of cosmogenic Na using the global monitoring network o the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO). J. Environ. Radioact. 2018, 187, 8–15. [CrossRef] [PubMed] 11. European Radiation Dosimetry Group. Radiation Protection 85, Exposure of Air Crew to Cosmic Radiation; EURADOS report 1996-01; European Radiation Dosimetry Group: EC, Brussels, Belgium and Luxemburg, Luxemburg, 1996. 12. O’Brien, K.H. Secular variation in the production of cosmogenic isotopes in earth’s atmosphere. J. Geophys. Res. 1979, 84, 423–431. [CrossRef] 13. Damatto, S.R. Be-7 measured at ground air level and rainfall in the city of Sao Paulo. In Proceedings of the INAC 2013—International Nuclear Atlantic Conference, Recife, Brazil, 24–29 November 2013; ISBN 978-85-99141-05-2. 14. Field, C.V.; Schmidt, G.A.; Koch, D.; Salyk, C. Modeling production and climate related impacts of Be concentrations in ice cores. J. Geophys. Res. 2006, 111, D15107. [CrossRef] 15. Magnoni, M. La deposizione umida e secca: Aspetti sperimentali e teorici. In Proceedings of the Congress ‘Twenty-Five Years After Chernobyl Accident: Studies, Remarks and Recent Findings’, Udine, Italy, 10–12 March 2011. 16. Facchinelli, A.; Magnoni, M.; Gallini, L.; Bonifacio, E. 137Cs contamination from Chernobyl of soils in Piemonte (North-West Italy): Spatial distribution and deposition model. Water Air Soil Pollut. 2002, 134, 341–352. [CrossRef] 17. Giardina, M.; Bu a, P. A new approach for modeling dry deposition velocity of particles. Atmos. Environ. 2018, 180, 11–22. [CrossRef] 18. Currie, L.A. Limit of Qualitative Detection and Quantitative Determination. Anal. Chem. 1968, 40, 586–593. [CrossRef] Environments 2020, 7, 12 12 of 12 19. Ho man, I.; Lewis, B.; Chan, P.; Ungar, K. Ho man Analysis of Na using a spectral summation technique on high-volume aerosol samples. J. Environ. Radioact. 2017, 169, 151–158. [CrossRef] [PubMed] 20. SILSO, Royal Observatory of Belgium. Available online: www.sidc.be/SILSO (accessed on 14 November 2019). 21. Hoyt, D.V.; Schatten, K.H. Group sunspot numbers: A new solar activity reconstruction. Part 1. Sol. Phys. 1998, 179, 189–219. [CrossRef] 22. Hoyt, D.V.; Schatten, K.H. Group sunspot numbers: A new solar activity reconstruction. Part 2. Sol. Phys. 1998, 181, 491–512. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Environments – Multidisciplinary Digital Publishing Institute

Published: Feb 11, 2020

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