No abstract available

[en] For radionuclides with half-lives greater than a thousand years, mass spectrometers equipped with atom counting detection systems offer greater sensitivity than is possible by radiometric techniques. However, conventional mass spectrometers are ultimately limited by interferences from isobars, molecules or neighbouring abundant isotopes.The techniques employed in accelerator mass spectrometry have the potential to eliminate or greatly reduce these interferences. Accelerator mass spectrometry can achieve isotopic abundance sensitivities of 10^-15 and detection limits of less than 1 fg. This technique has been widely used for radiocarbon dating and related applications. In this paper, we address its application to detection of nuclear signatures at trace levels, in support of international nuclear safeguards, focusing on the utilization of the radionuclides ¹²⁹I and ²³⁶U. (author)

[en] Improvements in the safeguards system in the last ten years have focussed on verifying the completeness of declarations made by States. Environmental sampling has been introduced, for samples taken within and around declared facilities. To provide assurance of the absence of undeclared facilities, the concept of wide area environmental sampling (WAES) has been proposed and studied. WAES is based on the assumption that operation of undeclared nuclear facilities will result in the routine or accidental release small amounts of radionuclides, for example, certain long-lived fission and activation products. Key signatures of reprocessing include ⁸⁵Kr, ⁹⁰Sr, ^134,137Cs, ¹²⁹I and Pu isotopes. Although not recognised in earlier work, ²³⁶U also has great potential, as it has higher yield in uranium irradiation than any fission product. The expected levels of the WAES signature radionuclides are very low and pose considerable challenges to sampling and detection techniques. In the cases of ¹²⁹I and ²³⁶U, Accelerator Mass Spectrometry (AMS) offers the most sensitive detection method available. While ¹²⁹I has been measured for a number of years by AMS, at ANSTO we have pioneered the measurement of ²³⁶U in environmental materials by this method

[en] The release of plutonium (Pu) from the 2011 Fukushima accident has raised questions on how prevalent it is in the environment and how its cycling into the biosphere compares with that from the previous Nagasaki and global-fallout sources. Here, we report on systematic sampling and analysis of soils, earthworms, and wild boar as markers of Pu in the deposition areas near the Fukushima Daiichi Nuclear Power Station (FDNPS) and Nagasaki. Highly-sensitive Accelerator Mass Spectrometry (AMS) methods were used to distinguish the Pu sourced from the FDNPS accident, and Nagasaki-detonation, from worldwide fallout Pu. We primarily used 241Pu/239Pu atom ratios, as the other typically-used Pu measures (240Pu/239Pu atom ratios, activity concentrations) were less sensitive and did not distinguish the FDNPS Pu from background in most study samples. Near the FDNPS, results indicate that five years after the accident, 0.4% – 2% of the Pu in the local soils (0-5cm) had originated from the FDNPS releases, the remainder being from global fallout. The trace amounts of FDNPS Pu (e.g., 0.02-0.04 Bq kg-1239Pu estimated in local ~3km deposition) contrasted sharply with the 134+137Cs content which was about 106 times greater than background in the same samples. The accident also contributed new Pu of ~0.3% – 3% in earthworms and ~1% – 10% in wild boar near the FDNPS. The soil and wild boar data from across the study sites consistently indicate only low levels of new accident-Pu and do not support the concept of a substantial undiscovered deposit of Pu near the FDNPS. Unlike sparsely-taken individual soil samples that might miss a Pu hotspot, the wild boar samples represent the integration of uptake throughout their entire foraging areas. Near Nagasaki, our measurements in 2016 show a lasting legacy of Pu sourced from the 1945 detonation (~93% soils, ~88% earthworm, ~96% boar in samples <5km from the Nagasaki hypocentre; the remainder from global fallout). Even with these high percentages arising from the 1945 detonation, the Pu amounts at all study sites in Japan are comparable to background fallout levels elsewhere and are orders of magnitude lower than what remains near Chernobyl. At the study areas, the dose rates from Pu to organisms, as well as to potential human consumers of wild boar meat, have been only slightly elevated above background and are orders of magnitude lower than the dose potentials from the 134,137Cs in samples from near the FDNPS. The results demonstrate progress in increasing the sensitivity of AMS methods, including the use of 241Pu/239Pu atom ratios, to compare recent and past nuclear contamination events and suggest that the Nagasaki-detonation Pu will be distinguishable in the environment long after the FDNPP-accident Pu is not.

[en] AMS studies in earth sciences at ANTARES, ANSTO created a need for the processing of mineral and ice samples for ¹⁰Be, ²⁶Al and ³⁶Cl target preparation. Published procedures have been adapted to our requirements and improved upon where necessary. In particular, new methods to isolate Be with reproducible, high recoveries in the presence of excess Al and Ti were achieved. An existing elution scheme for a cation exchange column procedure was modified to incorporate the use of a 0.25 M H₂SO₄+0.015% H₂O₂ washing step to elute the Ti peroxide complex formed. Problems with dust contamination in ice contributing to measured ¹⁰Be signals are also addressed and a procedure developed for its removal

[en] Highlights: •Sulphur hexafluoride is investigated as a stripper gas in tandem accelerators. •For heavy ions at low terminal voltage, mean charge states are found to be up to 1 charge unit higher than with argon gas. •Charge state distributions are found to be broader than with argon gas. •For charge states above the mean charge state, yields are typically doubled using SF₆. •Using SF₆ stripper gas, the efficiency of actinides AMS analysis is doubled. -- Abstract: We have investigated sulphur hexafluoride as a stripper gas in tandem accelerators by using the ANTARES accelerator system at ANSTO to measure charge state distributions for this gas. Results are reported at 4 MV terminal voltage for injected negative ions ranging from carbon to uranium oxide. For iodine and thorium the distributions are extended across a range of energies of practical use for accelerator mass spectrometry, ion beam analysis and other accelerator applications. Charge state distributions using sulphur hexafluoride are found to have mean charge states up to 1 charge unit higher than, and to be broader than, corresponding distributions for argon gas, except in the case of carbon beams. As a result, SF₆ is shown to provide significantly higher yields for charge states of heavy ions above the mean charge state. We now perform actinide AMS measurements with 9% yield to the 5+ charge state, compared to 4–5% achieved previously with argon gas

[en] The International Atomic Energy Agency (IAEA) applies a range of ultra-sensitive detection techniques to provide assurance that Member States are in compliance with their safeguards agreements. Environmental samples are collected which can contain minute traces of nuclear material or other evidence. Careful analysis of these samples reveals the nature of the activities undertaken in the vicinity of the sampling point. This paper reviews the analytical techniques that are being applied. To ensure that the IAEA has access to the best available methods, samples are distributed to a group of qualified laboratories around the world for analysis. The Accelerator Mass Spectrometry facility at the Australian Nuclear Science and Technology Organisation (ANSTO) is part of this select group of laboratories, and is the only AMS facility currently accredited with the IAEA. AMS provides the highest sensitivity available for detection of particularly useful signature radioisotopes, including ¹²⁹I,²³⁶U and plutonium isotopes

[en] The application of Accelerator Mass Spectrometry (AMS) to the measurement of the radionuclides ¹⁴C, ¹⁰Be, ²⁶Al and ³⁶Cl has dramatically increased our understanding of factors that affect climate and has led to a greater understanding of natural processes. Using the ANTARES AMS facility at ANSTO we are able to analyse samples containing as few as 10⁵ atoms of these radionuclides. Cosmogenic radionuclides produced by the interaction of cosmic rays with the upper atmosphere and exposed surface rocks are stored in natural archives. By measuring small variations in the concentrations of these isotopes over time, information can be inferred about the systems governing these changes. Over the last four years we have undertaken a broad range of climate change and environmental studies, based on the ultra-sensitive technique of accelerator mass spectrometry (AMS). Some specific examples of projects investigating the ice sheet at Law Dome, Antarctica and minerals extracted from geological surface formations will be given

[en] The long-lived uranium isotope ²³⁶U (T_1/2=23.4 Ma) is produced by ²³⁵U neutron capture and builds up to high levels in nuclear fuel. It has been distributed in the environment as a result of nuclear activities including nuclear explosions, accidents at nuclear plants, dumping of nuclear waste and releases from nuclear facilities. ²³⁶U is a potentially useful tracer of irradiated uranium for nuclear safeguards or other applications, due to its virtual absence in natural samples (²³⁶U:²³⁸U ratio ∼10^-10 in uranium ore). We have measured ²³⁶U in soil and sediment reference materials (IAEA 375, 135 and 300) by accelerator mass spectrometry (AMS). The AMS system on the ANTARES accelerator has been upgraded to make such measurements possible. The system, including sample preparation procedures, is described and the results discussed

[en] Over the past three years, the Australian National Tandem for Applied Research (ANTARES) AMS facility at ANSTO has been expanding its sample preparation and measurement capability, particularly for ¹⁰Be, ²⁶Al and ³⁶Cl. During this time, ANSTO has continued its collaboration with the AAD and CSIRO Atmospheric Research on the measurement of cosmogenic isotopes from Law Dome, Antarctica. This research program has been supported by the construction of a dedicated geochemistry laboratory for the processing of ice and rock samples for the preparation of AMS targets. Here we present our first results for ¹⁰Be concentrations measured in ice cores from three sites at Law Dome and describe the sample processing protocol and aspects of the AMS measurement procedure. These sites are characterised by an eightfold difference in accumulation rate with a common precipitation source. In combination with an established ice chronology, this has enabled some preliminary findings concerning the relationship between the snow accumulation rate and the measured ¹⁰Be concentration for Law Dome during recent times. Additionally, we present ⁷Be and ¹⁰Be/⁷Be measurements made for a few surface snow samples from Law Dome and Australia