Continuous transport of Pacific-derived anthropogenic radionuclides towards the Indian Ocean

Unusually high concentrations of americium and plutonium have been observed in a sediment core collected from the eastern Lombok Basin between Sumba and Sumbawa Islands in the Indonesian Archipelago. Gamma spectrometry and accelerator mass spectrometry data together with radiometric dating of the core provide a high-resolution record of ongoing deposition of anthropogenic radionuclides. A plutonium signature characteristic of the Pacific Proving Grounds (PPG) dominates in the first two decades after the start of the high yield atmospheric tests in 1950’s. Approximately 40–70% of plutonium at this site in the post 1970 period originates from the PPG. This sediment record of transuranic isotopes deposition over the last 55 years provides evidence for the continuous long-distance transport of particle-reactive radionuclides from the Pacific Ocean towards the Indian Ocean

In-situ production of natural ²³⁶U in groundwaters and ores in high-grade uranium deposits

In nature, primordial ²³⁶U has long since decayed to concentrations below detection. However, measurement of ²³⁶U produced in-situ by neutron capture on ²³⁵U in high-grade uranium deposits is made possible by recent advances in accelerator mass spectrometry (AMS). The detection of appreciable quantities of ²³⁶U in groundwaters may reflect local uranium mineralisation, and thus prove useful in uranium exploration and potential age and ore grade estimations. Nine mineralised sediments from the South Australian Beverley North sandstone-hosted uranium deposits have ²³⁶U/²³⁸U ratios ranging from (1.57±0.43)×10⁻¹² to (9.09±0.55)×10⁻¹², and U concentrations that vary by almost three orders of magnitude, ranging from 78.9 to 24,200μg/g. Overall, the samples with the highest [U] have higher ²³⁶U/²³⁸U ratios, consistent with the generation of higher neutron fluxes with one notable exception with anomalously high [U] and a relatively low ²³⁶U/²³⁸U ratio. The observed variability in the ²³⁶U/²³⁸U ratio both within the deposits themselves, and between deposits may reflect heterogeneous mineralogy, elemental composition and water contents, which can affect the neutron flux generated within the samples. A single groundwater sampled within mineralisation from the Pepegoona West deposit yielded a ²³⁶U/²³⁸U ratio of (6.57±2.97)×10⁻¹². This is the first published data detecting natural, non-anthropogenic ²³⁶U in groundwater in contact with a uranium deposit. The ²³⁶U/²³⁸U isotopic composition of the single groundwater sample is indistinguishable from that of the mineralised sediments from the same deposit. This is interpreted to reflect isotopic equilibration between the mineralisation and groundwater, rather than the in-situ production of ²³⁶U by neutron capture on dissolved ²³⁵U in the waters due to the low [U] typical of these highly reducing groundwaters. ²³⁶U appears to have limited mobility in the Pepegoona West groundwater system, as evidenced by the lack of signature in groundwaters sampled from nearby wells in low-grade and un-mineralised portions of the deposit. This suggests that the detection of ²³⁶U in the highly reducing groundwaters prevalent in this area may not be applicable as a proxy for uranium mineralisation. However, use of this technique as a potential exploration tool may have greater success in other areas with different hydrogeological conditions, specifically where the groundwaters are oxidising and uranium has a greater solubility as U(VI) complexes.10 page(s

Chronology, sedimentation, accumulation rates, and artificial radionuclides of sediment core GeoB10065-9

Unusually high concentrations of americium and plutonium have been observed in a sediment core collected from the eastern Lombok Basin between Sumba and Sumbawa Islands in the Indonesian Archipelago. Gamma spectrometry and accelerator mass spectrometry data together with radiometric dating of the core provide a high-resolution record of ongoing deposition of anthropogenic radionuclides. A plutonium signature characteristic of the Pacific Proving Grounds (PPG) dominates in the first two decades after the start of the high yield atmospheric tests in 1950's. Approximately 40?70% of plutonium at this site in the post 1970 period originates from the PPG. This sediment record of transuranic isotopes deposition over the last 55 years provides evidence for the continuous long-distance transport of particle-reactive radionuclides from the Pacific Ocean towards the Indian Ocean

ColPuS, a new multi-isotope plutonium standard for Accelerator Mass Spectrometry

A new multi-isotope plutonium standard for isotopic ratio measurements with Accelerator Mass Spectrometry (AMS) was created by gravimetric mixing of different single-isotope standards provided by IRMM (Pu-239, Pu-240, Pu-242, Pu-244). This standard material has been measured at the AMS facilities at Canberra (Australia), Cologne (Germany), Caserta (Italy), Sevilla (Spain) and Zurich (Switzerland). Additionally, the material was characterized using a Neptune MC-ICPMS (multi-collector inductively coupled plasma mass spectrometry) at the joint Cologne-Bonn isotope facility. The results of this laboratory intercomparison are presented and consensus values for the isotope compositions of the standard material are proposed

Reaction cross sections 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl at keV neutron energies investigated by Accelerator Mass Spectrometry

Typical neutron energies for the astrophysical s-process follow the Maxwell-Boltzmann distribution in the keV energy range. Neutron capture cross sections highly relevant for modelling the s-process can be experimentally determined by using the Time-of-Flight (ToF) method [1] or by the activation technique. If the reaction product is a long-lived radionuclide (t1/2 ~ yr -100 Myr), the cross section can be determined by activation with a quasi-stellar neutron distribution (typically kT = 25 keV) and a subsequent accelerator mass spectrometry (AMS) measurement of the reaction product [2]. Comparison of a number of such neutron capture cross sections shows a systematic bias, i.e. AMS data being lower than the ToF data [3, 4]. To investigate this discrepancy, we repeated experiments for two reactions that allow for highly precise AMS data: Maxwellian-averaged cross sections for the reactions 54Fe(n,γ)55Fe and 35Cl(n,γ)36Cl were investigated with dedicated activations at the Frankfurt Neutron Source (FRANZ) in Germany [5] and AMS measurements at two independent facilities. Analogously to previous activations, a quasi-stellar neutron spectrum of kT = 25 keV was produced via the 7Li(p,n) reaction, but at a different neutron-producing facility. Furthermore, to complement existing ToF and AMS data, an additional neutron activation of 54Fe and 35Cl at a proton energy of 2 MeV was performed, yielding data in the not-yet explored kT = 90 keV region. The irradiated metallic Fe foil and NaCl pellet (both of natural isotopic composition) were chemically processed and converted to AMS targets (Fe2O3 and AgCl) together with non-irradiated blanks. The subsequent AMS measurements of both radionuclides, 36Cl and 55Fe, were performed at two complementary AMS facilities, the Heavy Ion Accelerator Facility (HIAF) at the Australian National University [6] and at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany [7]. AMS allows a direct measurement of the 55Fe/54Fe and 36Cl/35Cl conversion ratios that result from the irradiation. The cross section is then deduced from the isotope ratio and the neutron fluence, which is determined using Au monitor foils. The new experiment was designed to produce highly accurate data and, owing to the two independent AMS measurements, it minimizes unrecognized sources of uncertainties in the AMS technique. The new preliminary data obtained in this work seem to confirm the previous AMS results. Consequently, the systematic discrepancy between AMS and ToF data remains unresolved. [1] Guber, K.H., et al., Phys. Rev. C 65, 058801 (2002). [2] Györky, Gy., et al., Eur. Phys. J. A 55, 41 (2019). [3] Capote, R., et al., Nucl. Data Sheets 163 (2020): 191. [4] Slavkovská, Z., et al., EPJ Web Conf. Vol. 232, p.02005, EDP Sciences, 2020. [5] Reifarth, R., et al., Publ. Astron. Soc. Aust. 26.3 (2009): 255. [6] Fifield, L.K., et al. Nucl. Instr. Meth. B: 268 (2010): 858. [7] Rugel, G., et al., Nucl. Instr. and Meth. in Phys. Res. B 370 (2016) 94.for RADIAT