A concept for the extraction of the most refractory elements at CERN-ISOLDE as carbonyl complex ions

Abstract

We introduce a novel thick-target concept tailored to the extraction of refractory 4d and 5d transition metal radionuclides of molybdenum, technetium, ruthenium and tungsten for radioactive ion beam production. Despite the more than 60-year old history of thick-target ISOL mass-separation facilities like ISOLDE, the extraction of these most refractory elements as radioactive ion beam has so far not been successful. In ordinary thick ISOL targets, their radioisotopes produced in the target are stopped within the condensed target material. Here, we present a concept which overcomes limitations associated with this method. We exploit the recoil momentum of nuclear reaction products for their release from the solid target material. They are thermalized in a carbon monoxide-containing atmosphere, in which volatile carbonyl complexes form readily at ambient temperature and pressure. This compound serves as volatile carrier for transport to the ion source. Excess carbon monoxide is removed by cryogenic gas separation to enable low pressures in the source region, in which the species are ionized and hence made available for radioactive ion beam formation. The setup is operated in batch mode. Initially, we investigate the feasibility of the approach with isotopes of more than 35s half-life. At the cost of reduced efficiency, the concept could also be applied to isotopes with half-lives of at least one to 10s. We report parameter studies of the key processes of the method, which validate this concept and which define the parameters for the setup. This would allow for the first time the extraction of radioactive molybdenum, tungsten and several other transition metals at thick-target ISOL facilities.

Opportunities for fundamental physics research with radioactive molecules

“…In commonly used Versatile Arc Discharge Ion Sources (VADIS) (Penescu et al 2010), electrons to induce ionization in collision with neutrals are thermionically released from a cathode that is heated to around 2000 • C. The introduction of a photo-cathode that releases electrons by the photo-electric effect enables operation at ambient temperature. A photo-cathode driven ion source was proposed in Ballof et al (2022b). The feasibility of the approach is supported by a proof-of-concept experiment (Ballof et al 2022a), however, further development is required to reach higher ionization efficiencies.…”

Opportunities for Fundamental Physics Research with Radioactive Molecules

“…In commonly used Versatile Arc Discharge Ion Sources (VADIS) (Penescu et al, 2010), electrons to induce ionization in collision with neutrals are thermionically released from a cathode that is heated to around 2000 • C. The introduction of a photo-cathode that releases electrons by the photo-electric effect enables operation at ambient temperature. A photo-cathode driven ion source was proposed in (Ballof, Chrysalidis, Düllmann, Fedosseev, Granados, Leimbach, Marsh, Ramos, Ringvall-Moberg, Rothe, Stora, Wilkins and Yakushev, 2022). The feasibility of the approach is supported by a proof-of-concept experiment (Ballof, Au, Barbero, Chrysalidis, Düllmann, Fedosseev, Granados, Heinke, Marsh, Owen, Rothe, Stora and Yakushev, 2022), however, further development is required to reach higher ionization efficiencies.…”

Target Development towards First Production of High-Molar- Activity 44gSc and 47Sc by Mass Separation at CERN-MEDICIS

“…Refractory-element release from the target units (e.g., Si, B, C, Sc, Ti, V, and transition metals Nb, Mo, Tc, W, Re, Os) can be promoted with in situ chemical reactions in the target containers and ion sources by creating more volatile molecules and can be extracted from target units as molecular-ion species [ 27 ]. Molecular-beam formation is achieved by either injecting reactive gas, such as CF and SF [ 28 ], CO [ 29 ], or evaporating salts (e.g., CCl , AgCl, BF ) from a separate container within the target unit. Molecular-beam formation can also be observed from impurities in raw target materials or from its chemical composition (e.g., LaF , Ce S , CeS) [ 30 , 31 ]; however, in such cases, the reaction rate cannot be controlled during operations.…”