Exotic Radioisotope Samples for Nuclear Science Experiments
Exotic radioisotopes like those of the actinide elements are essential for various applications in the nuclear sciences. Our own most prominent use is that as target isotopes for the production of the superheavy elements (SHE) in heavy ion induced fusion reactions. Due to our expertise in purifying actinidies, producing tailor-made samples and characterize these, we provide a multitude of samples to various international collaborations. The studying topics range from targets for laser and mass spectroscopic investigations, over myon-based experiments for the determination of nuclear charge radii to recoil sources where the daughter nuclides are to be used for atomic and nuclear physics or for chemical experiments. Our targets are also involved in a bunch of experiments related to the exotic low-energy isomer 229mTh, which has the capability to be used in the future in the construction of a "nuclear clock", as well as for experiments for neutrino research, like the ECHo project.
High Performance Targets for SHE Experiments
Every accelerator-based experiment requires a target that is irradiated with highly intense ion beams (1012-1013 projectiles per second), often for extended periods of time. The Nuclear Chemistry section of the department of chemistry at the University of Mainz provides the facilities and expertise to produce high-quality targets also from the most exotic and often highly radioactive transuranium isotopes including 242,244Pu, 243Am, 248Cm, 249Bk, and 249Cf. The targets are generally produced by molecular plating (MP), a reliable, electrochemical deposition method, providing homogeneous layers with high yields.
Production of Actinide Targets
Targets with specific requirements regarding thickness, homogeneity, (radio-)chemical purity and mechanical stability are also of interest for a number of nuclear physics and chemistry experiments in other research areas. To fulfill these requirements, and to offer a broad range of possible target specifications, various target production techniques have been studied over the past decades.
The longest used technique is an electrochemical deposition called molecular plating (MP). The desired actinide compound is dissolved in an alcoholic solution and deposited on a backing connected to the cathode of the electrochemical cell. By applying a constant current density of < 1 mA/cm², which corresponds often to high voltages of a few hundred Volts, for several hours, a homogeneous deposition of the dissolved material on the backing is achieved with yields of up to more than 90%. Major drawbacks of the method are, that the maximum achievable film thickness is < 1000 µg/cm² and that one does not get a chemically well defined layer, but ends up with a mixture of oxides, hydroxides, carbonates and formiates.
Beside the classical MP, we started to investigate other electrochemical approaches known from lanthanide chemistry. The aim is to adopt these approaches to actinides, namely the use of anhydrous organic f-metal compounds like tosylates or triflates in water-free solvents like N-N’-dimethylformamide (DMF). First experiments show the potential of these methods to form homogeneous layers with areal densities of more than 1 mg/cm², the typical limit for MP targets, and a good stability under heavy ion beam irradiation.
In addition to the electrochemical procedures, other production methods like “Drop-on-Demand” (DoD) printing, “Self-adsorption” (SA) and “Spincoating” (SC) are employed.
Self-adsorption processes are based on the chemisorption of chemical species onto functional groups on the surface of a substrate, e.g., passivated titanium. The major advantage of SA sources is the possibility to get monolayers of the target material, providing ideal conditions for recoil ion sources.
The DoD system combines a commercial nanoliter dispenser (Biofluidix PipeJet) with a x-y-translation stage, enabling to print aqueous as well as organic solvents on any kind of substrate, e.g., metal, polymer or carbon foils. The wetting of the substrates surface by the droplets is affected by surface parameters, like roughness or passivation, as well as by solvent properties, like polarity, viscosity and pH value. The wetting behavior is directly related to the size and homogeneity of the remaining deposition after evaporation of the droplets. The DoD printing is applicable for a wide range of targets.
Picture of an assembled Drop-on-Demand system, with the dispenser pipe connected to a reservoir, mounted above a x-y-translation stage. Photo/©: R. Haas / Univ. Mainz
Working principle of the Drop-on-Demand system. Link
Spincoating is another approach targeting the production of thin layers of rare isotopes. A volume of an appropriate solution is poured onto a substrate mounted on a rotating stage. Depending on the rotational speed, the centrifugal forces cause the solution to be spread over the whole substrate, causing an uniform wetting of the surface. The method works best by using special precursor solutions with high viscosity. Polymer assisted deposition (PAD) and metal-oxid decomposition (MOD) are two approaches to be mentioned, which can yield the desired target layers after post-processing by heat treatment in an oven. First tests with a self-constructed spincoater revealed promising results with Ce and Dy as target material. Further development for application of actinides has to be done.
Our group participates in a number of international collaborations by providing necessary actinide and lanthanide targets tailored to the specific requirements of the individual experiments. Examples of ongoing and future experiments using targets produced in Mainz are:
Our group is part of the "Electron Capture in Holmium" (ECHo) collaboration, aiming for neutrino mass determination. We provide Ho and Dy microsamples for high-precision mass measurements with PENTATRAP at MPI-K Heidelberg, Germany. Furthermore, our group is responsible for the production of radiochemically pure samples of 163Ho. Sufficient amounts of Ho are produced at the high-flux reactor at Institut Laue Langevin in Grenoble, France, and then processed and prepared in joint activities with the Institute of Physics of the University of Mainz.
Our group is involved in a number of experiments related to the exotic isomer 229mTh. This isomer has an extraordinary low-energy excited state of 8.28(17) eV, which corresponds to a wave length of 149.7 nm. This allows the construction of a UV laser system for the direct excitation of a nuclear state, which is unique among all known nuclear isomers. This possibility opens a wide field of interest in the physics community and also might have practical applications in the future, like the construction of a "nuclear clock". Up to know, the only experimental access to 229mTh is via the decay of the mother nuclide 233U. Therefore, we provided many 233U and 229Th targets to different international groups in the last year, e.g. at UCLA Los Angeles, USA, JILA Boulder, USA, and KU Leuven, Belgium. Also the first experimental proove of the existance of the low-energy state of 229mTh was achieved at LMU Munich, Germany, in 2016, using a 233U recoil source from Mainz. In the meanwhile, we are also part of the TACTICa collaboration at the JGU, involving the group of Prof. Dima Budker, HIM, and Prof. Ferdinand Schmidt-Kaler, Department of Physics. In the frame of TACTICa, we want to capture Th ions in a Paul trap, in order to perform high-precision spectroscopy on the trapped ions. One goal is to capture and study 229mTh in detail, but we are also aiming at other available Th isotopes available at the nuclear chemistry in Mainz, where modern methods such as quantum logic spectroscopy can provide tests of the standard model and of possible variations of the fundamental constants.
Analytical Methods for Target Characterization
For the analytical investigations of the produced targets, atomic force microscopy (AFM), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), radiographic imaging (RI), as well as alpha- and gamma-spectroscopy are available at JGU Mainz. Neutron activation in the TRIGA Mainz research reactor is used for the production of radioactive tracers as well as for yield determination. In addition, we are in close contact to the target laboratory and the material research group at GSI in Darmstadt, to the JRC Karlsruhe, as well as with the radiochemistry group at the HS Mannheim University of Applied Sciences, in order to extend our analytical capacities, e.g., to Raman spectroscopy or contact angle measurements.