129I, 236U, 239Pu and 240Pu profiles in a peatbog from the Southern Hemisphere

129I, 236U, 239Pu and 240Pu profiles in a peatbog from the Southern Hemisphere Elena Chamizo1, Jose María López-Gutierrez1, Elis Holm2 and Rafael García-Tenorio1 1 Spanish National Accelerator Centre, Sevilla, Spain 2 Gothenburg University, Sweden Abstract 129I, 236U, 239Pu and 240Pu isotopes have been analyzed in the different layers of a peatbog core collected in Madagascar Island (South Hemisphere) and representing at least the last 100 years of atmospheric deposition. The determination of ultra-low levels of these isotopes has been possible by applying the Accelerator Mass Spectrometric Technique (AMS) by using the 1 MV Tandetrom AMS facility located at CNA (Seville, Spain). The elements of interest were extracted and isolated previous to the measurements by applying validated radiochemical procedures. In this contribution, the behavior along the core of the different isotopes under analysis will be discussed, evaluating in particular the magnitude of their post-depositional retention/migration. In this sense, it will be highlighted the great mobility of the 129I with a near uniform profile along the core, in opposition for example with the preservation for the Pu isotopes of the fallout bomb peak. In the case of 236U, the 236U/239Pu atomic ratios determined in a total of eight layers of the core are ranging in the interval 0.02 – 0.29 with an average value of 0.15. As far as we know, these are the first 236U results seeing the light that were obtained in deposits as peatbogs collected in the southern hemisphere. Experimental Set-up and Objectives The main objective of the work was to obtain the profiles for several artificial radionuclides (239Pu, 240Pu, 129I and 236U) in a peatbog collected in the Southern Hemisphere (Madagascar) by Accelerator Mass Spectrometry (AMS). These determinations hardly can be done by radiometric techniques due to the low levels expected for the analyzed radionuclides. The behavior of the mentioned radionuclides in the peatbog core has been analyzed, and information (scarce in the literature) about 240Pu/239Pu, 236U/238U and 236U/239Pu atom ratios in the environment of the southern hemisphere are given and briefly discussed. The study have been done by using the SARA (Spanish Accelerator Radionuclide Analyses) system of 1 MV (LEAMS) located at the Spanish National Accelerator Centre in Seville, Spain. A schematic representation of the set-up is shown in Figure 1. Radiochemical and measurement procedures used at CNA with SARA for the radionuclides involved in this work, can be found in the following references: Santos et al. (2006), Chamizo et al. (2008), Gomez-Guzmán et al. (2012), Chamizo et al. (2015) and Scognamiglio et al. (20169. In addition, and in aliquots of the different layers of the profile, 238U activity concentrations have been determined by alpha-particle spectrometry after applying a radiochemical method for isolation and conditioning for the measurement (Lehritani et al., 2012). T T T T T T T T Figure 1.- Results The 129 I, 2 3 analyzed p and 236 U/ 2 3 The corre s ratios, in discussed i CODE T OT – 2 T OT – 5 T OT – 7 T OT – 9 T OT – 10 T OT – 11 T OT – 12 T OT – 13 Table 1.- 1 2 peatbog Schematic r 3 9 Pu, 236 U a n p eatbog are 3 9 Pu atom r a s ponding 12 9 the Figure i n the follo w DEPTH (cm) 2 -4 cm 8 – 10 c m 12 – 14 c m 16 – 18 c m 18 – 20 c m 20 – 22 c m 22 – 24 c m 24 – 26 c m 2 9 I, 239 Pu, 2 r epresenta t n d 238 U co n compiled i n a tios. 9 I and 236 U 2, are als o w ing sectio n ( a (11.4 7 m (10.3 6 m (7.2 5 m (3.4 9 m (4.6 3 m (2.0 9 m (2.3 8 m (3.4 4 36 U and 23 8 t ion of the S n centration s n Table 1, w U profiles, i n o represen t n . 129I a toms/g) 7 ± 0.81)106 6 ± 2.42)106 5 ± 2.11)106 9 ± 0.25)106 3 ± 1.29)106 9 ± 0.12)106 8 ± 0.86)106 4 ± 0.34)106 8 U concent r S ARA LEA M s , expresse d w hile in T a n the Figur t ed. All th e 23 9 (ato m (264.5 ± (187.9 ± (28.6 ± (27.1 ± (163.9 ± (28.9 ± (12.2 ± (6.5 ± r ations (at o M S system d in atoms/g a ble 2 are c o e 1, and th e set of re 9 Pu m s/g) ± 5.7)106 ± 5.2)106 ± 1.7)106 ± 1.6)106 ± 4.3)106 ± 1.5)106 ± 1.0)106 0.7)106 o ms/g) in t h , in a total o o mpiled th e e 240 Pu/ 239 P sults are b 236U (atoms/ g (59.9 ± 3.4 ) (35.0 ± 1.9 ) (4.9 ± 1.1) (2.5 ± 0.6) (3.7 ± 0.6) (3.4 ± 0.8) (1.3 ± 0.4) (1.9 ± 0.4) h e different o f eight lay e e 240 Pu/ 239 P P u and 236 U riefly com m g ) ( ) 106 (16 . ) 106 (35 . 106 (19 . 106 (26 . 106 (22 . 106 (25 . 106 (24 . 106 (22 . layers of t h e rs from th e u, 236 U/ 238 U U/ 239 Pu ato m m ented an d 238U atoms/g) . 3 ± 1.2)1014 . 4 ± 2.1)1014 . 3 ± 1.4)1014 . 7 ± 1.6)1014 . 3 ± 1.6)1014 . 5 ± 1.6)1014 . 3 ± 1.2)1014 . 3 ± 0.7)1014 h e analyze d e U m d d CODE DEPTH (cm) 240Pu/239Pu 236U/238U236U/239Pu TOT – 2 2 -4 cm 0.150 ± 0.007 (3.67 ± 0.34)10-8 0.23 ± 0.01 TOT – 5 8 – 10 cm 0.171 ± 0.010 (0.99 ± 0.09)10-8 0.19 ± 0.01 TOT – 7 12 – 14 cm 0.173 ± 0.024 (0.25 ± 0.06)10-8 0.17 ± 0.04 TOT – 9 16 – 18 cm 0.100 ± 0.018 (0.09 ± 0.02)10-8 0.09 ± 0.02 TOT – 10 18 – 20 cm 0.069 ± 0.005 (0.17 ± 0.04)10-8 0.02 ± 0.01 TOT – 11 20 – 22 cm 0.091 ± 0.014 (0.13 ± 0.03)10-8 0.12 ± 0.03 TOT - 12 22 – 24 cm 0.105 ± 0.025 (0.05 ± 0.02)10-8 0.11 ± 0.04 TOT - 13 24 – 26 cm 0.105 ± 0.037 (0.09 ± 0.02)10-8 0.29 ± 0.07 Table 2.- 240Pu/239Pu, 236U/238U and 236U/239Pu atom ratios in the different layers of the analyzed peatbog. Figure 1.- 129I and 236U profiles in the analyzed core. Figure 2.- 239+240Pu (mBq/g), 240Pu/239Pu and 236U/239Pu atom ratios profiles Discussion The set of obtained results deserve the following comments: -The 129I deposited historically is not preserved without translocation in the different layers of the peatbog, moving along the entire core. The levels of 129I are on the other hand low, indicating the possible no accumulation of all the 129I atmospherically deposited over time. -The 239Pu profile shows enhanced concentrations of this radionuclide in the uppermost layers indicating an upward mobilization of a fraction of the historical record of deposited 239Pu over time. However, a clear peak is observed around 18-20 cm, which can be associated to the maximum deposition of global fallout in 1962-63. The chronology established by the 210Pb method ratifies the mentioned age-depth association. -The 236U profile indicates that this radionuclide in the peatbog suffers its remobilization, being not preserved any historical information about its deposition. Its mobility is clearly higher than the observed one for Pu. -The mean 240Pu/239Pu atom ratio is lower than the value for global fallout, but in a good number of the uppermost layers, the results are consistent with the average ratio of 0.17 ± 0.03 for the southern hemisphere. In the deepest layers, associated to the years 1950-1970, even lower values have been found, fact that can be an evidence of the deposition by local fallout of some 239Pu with origin in the disintegration over the collection area of the SNAP-9A satellite at the beginning of the 60´s. NEVERTHELESS, MORE EVIDENCES ARE NEEDED. -The 236U/238U atom ratios are at least three orders of magnitude higher than natural background . 236U anthropogenic origin of this radionuclide in the southern hemisphere is global fallout, as 236U is produced by nuclear or thermonuclear bombs. -The values of the 236U/238U atom ratio are on the other hand clearly lower than the found one in the vicinities of reprocessing plants and nuclear facilities, because another fraction of anthropogenic 236U in the environment is originated in the mentioned facilities. Generation of similar profiles in other peatbog cores collected in the area are now under development with special emphasis in the analysis of the actinide nuclides (236U, 239Pu and 240Pu) by AMS and the determination of 238Pu by alpha-particle spectrometry. The objective is to evaluate the magnitude of possible local fallout existing in the area with origin in the accident of the SNAP-9A satellite. Acknowledgements Work supported by the Spanish Ministry MINECO project FIS2015-69673-P. References E. Chamizo, S.M. Enamorado, M.García-León, M. Sutter and L. Wacker Plutonium Measurements on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) Nuclear Instruments and Methods B 266 (2008) 4948-4954 E.Chamizo, M. Christi and L.K.Fifield Measurement of 236U on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) Nuclear Instruments and Methods B 358 (2015) 45-51 J.M.Gomez-Guzmán, J.M.Lopez-Gutierrez, A. R. Pinto, E.Holm 129I measurements on the 1MV AMS facility at the Centro Nacional de Aceleradores (CNA, Spain) Applied Radiation and Isotopes 70 (2012) 263-268 M. Lehritani, J. Mantero, N. Casacuberta, P. Masqué and R. García-Tenorio Comparison of two sequential separation methods for U and Th determination in environmental samples by alpha-particle spectrometry Radiochimica Acta 100 (2012) 431-438 F.J. Santos, J.M.Lopez-Gutierrez, E. Chamizo, M. García-León and H. Synal Advances on the determination of 129I by accelerator mass spectrometry (AMS) Nuclear Instruments and Methods B 249 (2006) 772-775 G. Scognamiglio, E. Chamizo, J.M.Lopez-Gutierrez, A.M.Muller, S. Padilla, F.J. Santos, M. López-Lora, C.Vivo-Vilches and M. García-León Recent developments of the 1MV AMS facility at the Centro Nacional de Aceleradores Nuclear Instruments and Methods B 375 (2016) 17-25