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 the 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 and have to diffuse through a solid 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 solild target material. They are thermalized in a carbon monoxidecontaining 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, with the aim to extract isotopes having half-lives of at least several seconds. 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.

A cold electron-impact ion source driven by a photo-cathode – New opportunities for the delivery of radioactive molecular beams?

“…We have previously proposed a parameter study to increase the ion source efficiency to ca. 1 %, which would allow a wide range of experiments with exotic molecules [12]. A major efficiency increase requires the modification of the ion source geometry to resolve the space-charge limitation implied by the 1.5 mm laser beam spot on the cathode surface.…”

“…The new ion source will enable extraction and ionization of molecules at ISOLDE which are not stable at ultra-high temperatures. Development and implementation of this method at ISOLDE will expand its radioactive ion beam capabilities to refractory elements [12] and help to serve the…”

“…We investigated the operation of the VADIS at ambient temperature by exploiting laserinduced electron release. We have previously reported on a VADIS operated at ambient temperature in which electrons were released from the tantalum cathode by a femto-second laser [12]. We were able to show that the fragile carrier molecule Mo(CO) 6 can be ionized with efficiencies in the same order of magnitude as the noble gas krypton in the same setup.…”

“…To increase the variety of volatile carrier compounds, the temperatures of the target and ion source need to be significantly reduced. Cold target concepts have been proposed, which exploit the fast diffusion in nano-materials [11] or fully avoid diffusion and exploit the recoil momentum of nuclear reaction products [12]. However, the ion sources typically used at ISOLDE are not well suited for the ionization of more delicate volatile compounds [13].…”