Remodelling a multi-anode ionisation chamber detector for accelerator mass spectrometry of 53Mn

Accelerator Mass Spectrometry (AMS) is a single-atom counting technique that measures the abundance of rare, long-lived radioisotopes using only milligrams of sample. The astrophysical radioisotopes 53Mn and 60Fe have been utilised for many applications including meteoritics, exposure dating, and the search for near-Earth supernovae. 53Mn measurements at the ANU have been limited to sensitivities above 10^-13 by insufficient suppression of the stable isobar, 53Cr. To expand the applications accessible to 53Mn analysis, a new detector was commissioned that will improve the available sensitivity. This thesis covers the implementation of the new Flexible Anti-Scatter Multi-Anode (FASMA) detector. Simulations were conducted to determine the optimal placement of the detector inside the gas-filled magnet, and to assist with the design of a new multi-anode configuration. The FASMA detector was successfully tested and full spectra were recorded. These preliminary results indicate an improvement in the achievable sensitivity, even without the suppression of scattered particles. With further work, the FASMA detector should reach a sensitivity at or below 10^-14, which is competitive with the best reported level in the field. Long-lived radionuclides, such as 53Mn and 60Fe, are important for extracting the exposure history of meteorites, both in space and on Earth, as well helping to identify their origin. In light of this, cosmogenic 53Mn and 60Fe ratios were measured in ten meteorite samples. Since the available data on live 53Mn and 60Fe is scarce, these measurements will improve the constraints on current production rate models for meteorites

Analysis methods to determine the bound-state beta-decay half-life of Thallium-205

Bound-state β−-decay is an exotic decay mode that produces temperature-dependent stability in nuclei. A striking example is 205Tl, in part because of its impact on the 205Pb/204Pb cosmochronometer—a short-lived ra-dionuclide clock that can provide unique constraints on s process material in the early solar system. The bound-state β−-decay of 205Tl was measured at GSI, where fully stripped 205Tl81+ ions were produced and stored in the Experimental Storage Ring. Decay occurred during storage producing increased 205Pb daughters with increased storage time. This contribution briefly outlines the experiment and describes analytical corrections required to extract the half-life

New and upgraded ionization chambers for AMS at the Australian National University

Three state-of-the-art ionization chambers have recently been designed at the Australian National University (ANU) to further enhance the isotopic and isobaric separation capabilities of the AMS setup at the Heavy Ion Accelerator Facility (HIAF). Two compact split-anode ionization chambers with low noise preamplifiers allow for measurements of actinides and other radioisotopes that do not have atomic isobar interferences. Both assemblies sit inside Dependex-100-Tees and are constructed such that they are retractable from the ion beam under vacuum. A high energy resolution version will be used for Pu, 129I and 55Fe. The second detector serves as the final energy detector in a time-of-flight setup and is employed for 236U and 210Pb detection. The new Flexible-AntiScattering Multi-Anode (FASMA) detector is a 7-anode ionization chamber optimized for the discrimination of 53Mn from 53Cr in the gas-filled magnet setup with the ENGE-split-pole spectrograph. The entire setup was simulated with Raytrace and SRIM. Finally, an existing 8-anode ionization chamber has been upgraded and now provides a 93Zr-93Nb isobar suppression of 5 × 104 resulting in the best limit of detection for 93Zr of any AMS-facility. This article details the various features of these detectors and compares first experimental results to performance values expected from modelling

⁶⁰Fe deposition during the late Pleistocene and the Holocene echoes past supernova activity

Nuclides synthesized in massive stars are ejected into space viastellar winds and supernova explosions. The solar system (SS)moves through the interstellar medium and collects these nucle-osynthesis products. One such product is 60Fe, a radio nuclide witha half-life of 2.6 My that is predominantly produced in mas-sive stars and ejected in supernova explosions. Extraterres-trial60Fe has been found on Earth, suggesting close-by supernovaexplosions∼2to3and∼6Ma.Here,wereportonthedetectionof a continuous interstellar 60Fe influx on Earth over the past∼33,000 y. This time period coincides with passage of our SSthrough such interstellar clouds, which have a significantly largerparticle density compared to the local average interstellar me-dium embedding our SS for the past few million years. The inter-stellar 60Fe was extracted from five deep-sea sediment samplesand accelerator mass spectrometry was used for single-atomcounting. The low number of 19 detected atoms indicates a con-tinued but low influx of interstellar60Fe. The measured 60Fe time profile over the 33 ky, obtained with a time resolution of about±9 ky, does not seem to reflect any large changes in the inter-stellar particle density during Earth’s passage through local in-terstellar clouds, which could be expected if the local cloudrepresented an isolated remnant of the most recent supernovaejecta that traversed the Earth∼2 to 3 Ma. The identified 60 Fe influx may signal a late echo of some million-year-old supernovaewith the 60Fe-bearing dust particles still permeating the interstellar mediumWe thank the Antarctic Marine Geology ResearchFacility, Florida State University (C. Sjunneskog) for providing the sedimentcores. This work was funded by Austrian Science Fund project AI00428,through the European Science Foundation Collaborative Research ProjectCoDustMas; Australian Research Council projects DP140100136,DP180100495, and DP180100496; German Academic Exchange Service proj-ect 56266169; and The Group of Eight Australia–Germany Joint ResearchCooperation Scheme. J.F. acknowledges a stipend from the University ofVienna and R.G. support from the European Cooperation in Science andTechnology“ChETEC”Action (CA16117). We also acknowledge financialsupport from the Australian Government for the Heavy Ion Accelerator Fa-cility at ANU through the National Collaborative Research InfrastructureStrategy. ICP-MS measurements were performed by S. Beutner (HZDR