Titanium carbide-carbon porous nanocomposite materials for radioactive ion beam production

Abstract

The Isotope Separator OnLine (ISOL) technique is used at the ISOLDE - Isotope Separator OnLine DEvice facility at CERN, to produce radioactive ion beams for physics research. At CERN protons are accelerated to 1.4 GeV and made to collide with one of two targets located at ISOLDE facility. When the protons collide with the target material, nuclear reactions produce isotopes which are thermalized in the bulk of the target material grains. During irradiation the target is kept at high temperatures (up to 2300 °C) to promote diffusion and effusion of the produced isotopes into an ion source, to produce a radioactive ion beam. Ti-foils targets are currently used at ISOLDE to deliver beams of K, Ca and Sc, however they are operated at temperatures close to their melting point which brings target degradation, through sintering and/or melting which reduces the beam intensities over time. For the past 10 years, nanostructured target materials have been developed and have shown improved release rates of the produced isotopes, due to the short diffusion distances and high porosities. In here a new Ti-based refractory material is developed to replace the currently used Ti-foils. Since nanometric TiC can't be maintained at high temperatures (T>1200 °C) due to sintering, a processing route was developed to produce TiC-C nanocomposites where the carbon allotropes used were either graphite, carbon black or multi wall carbon nanotubes (MWCNT). The developed nanocomposites sinterability was tested up to 1800 °C and they were characterized according to dimensional changes, relative density, mass losses, surface area, TiC particle size and microstructure morphology. All carbon allotropes had a significant effect on the stabilization of the nanometric TiC where the best result was obtained for a 1:1 volume ratio of TiC:carbon black at 1800 °C where TiC crystallite sizes were of 76 nm (from 51 nm in green) and density of 55 %, followed by TiC:MWCNT with TiC of 138 nm (58 % dense). The processing introduced a ZrO2 contamination from the milling media, forming ZrC that solubilizes in the TiC phase, increasing its lattice parameter. TiC sintering kinetics were studied through the master sintering curve and the activation energy determined for sintering, 390 kJ/mol, were close to the ones obtained in the literature. Using the same method, the calculated activation energy for TiC-carbon black was 555 kJ/mol resulting from the carbon which reduces the TiC sintering, reducing its coordination number. The nanocomposites referred (and the TiC) were irradiated and studied in terms of isotope (Be, Na, Mg, K, Sc and Ca) release, where the nanocomposite with the highest isotope released fraction, TiC-carbon black was selected for the final target material. To produce a full target the processing was scaled up and a target prototype was build and tested at ISOLDE. Li, Na and K isotope intensities and release time-structure were measured from the target prototype, where in comparison with Ti-based materials, Na and Li intensities were higher, K were slightly lower and Ca were lower. The target presents an apparently longer release time structure when comparing with standard materials, as seen in other nanomaterial targets, which is likely related with effusion of the isotopes in the material porosity. Furthermore, contrarily to the Ti-foil targets, the obtained intensities were stable over the full operation time. At the end of this thesis suggestions for a future work which include a second iteration of the TiC-C nanocomposite (already developed) and further modeling.