DREAMS at FZD: The new accelerator mass spectrometry facility

The installation of a new 6 MV-Tandetron accelerator opens a new topic of research at FZ Dresden-Rossendorf: Accelerator mass spectrometry (AMS). The Dresden AMS facility (DREAMS) will be mainly used for measurements of the long-lived radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, ⁴¹Ca and ¹²⁹I (T1/2 = 0.1-15.7 Ma) in the isotopic ratio range of 10^-10- 10^-16. The same accelerator will be partially also used for spatial- and depth-resolved chemical analysis using ion beam techniques such as Rutherford Backscattering Spectrometry (RBS), Elastic Recoil Detection (ERD), Particle-induced X-ray and Gamma-ray emission (PIXE/PIGE) and Nuclear Reaction Analysis (NRA). Using these methods, we are able to measure nondestructively "all natural" elements, i.e. H to U; most elements with lateral, some in 3D resolution. Additionally, materials modification via high energy ion implantation is possible.
The AMS injector consist of two Cs-sputter ion sources SO-110 with wheels for up to 200 sputter targets, a 54° electrostatic analyzer (ESA) and a 90° bouncer magnet for sequential acceleration of stable isotopes and radionuclides. In contrast to common low-energy AMS facilities, which have mainly specialized in radiocarbon analyses (¹⁴C), the FZD-AMS is the first modern type facility in the EU that will run at a terminal voltage of 6 MV. The electrostatic accelerator operates with a Cockcroft-Walton type high voltage cascade for generating the terminal voltage. The high-energy part of the system consists of a 90° analyzing magnet, a set of precise Faraday cups with energy slits for measuring the stable nuclides and stabilising the terminal voltage, a set of absorber foils for isobar suppression, a 35° ESA, and a 30° vertical magnet for suppressing interfering species. The radionuclides will be finally detected by a ΔE/E gas ionisation chamber containing four anodes. There is a main advantage of us-ing a high-energy accelerator for mass spectrometry: The background and interfering signals, resulting from molecular ions and ions with similar masses (e.g. isobars) are nearly completely eliminated. Thus, AMS provides much lower detection limits compared to conventional mass spectrometry.
The benefits from using AMS for radiation protection, nuclear safety, nuclear waste, radioecology, phytology, nutrition, toxicology, and pharmacology research are obvious and manifold: Smaller sample sizes, easier and faster sample preparation, higher sample throughput and the redundancy for radiochemistry la-boratories will largely reduce costs. Lower detection limits will widen applications to shorter and longer time scales and to sample types which could never been investigated before. Especially in environmental and geosciences, the determination of long-lived cosmogenic radionuclides like ¹⁰Be, ²⁶Al, and ³⁶Cl became more and more important within the last decades. Using these nuclides dating of suddenly occurring prehistoric mass movements, e.g. volcanic eruptions, rock valanches, tsunamis, meteor impacts, earth quakes and glacier movements, is possible. Additionally, glacier movements and data from ice cores give hints for the reconstruction of historic climate changes and providing information for the validation of climate model predicting future changes. Taking the advantage of location into account DREAMS will soon also focus on applications of radionuclides of anthropogenic origin such as plutonium isotopes and ¹²⁹I.
Investment and maintenance costs of a high-energy AMS facility are much higher compared to e.g. methods determining radionuclides via their decay. All methods including AMS need sophisticated radiochemical separation procedures. However, for several nuclides the requirements with respect to the purity of the final sample to be measured are much lower for AMS. As it is the case for most destructive analytical methods AMS chemical sample preparation takes much longer and is more expensive that the actual measurement, which takes about 10 - 60 min.