Offline investigation of thermodynamic and chemical processes in targets for radioactive beam production

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The irradiation of ISOL (isotope separation on-line) targets with high-energy proton beams for the production of rare isotope beams (RIB) is a well-established method at ISAC/TRIUMF. Some of the most challenging projects in subatomic physics research are in need of exotic elements with extreme numbers of protons and neutrons to gain insight in the stability of matter and the formation of the elements. In order to provide cutting-edge research with very short-lived RIB, the development of better targets to accommodate their efficient release is essential. Even with recent major improvements, such as the development of composite ceramic uranium carbide targets, it becomes increasingly difficult to accommodate requests of high-profile experiments for RIB, e.g. very neutron-rich Ca and Be beams. While relatively little can be done about in-target production rates, the effusion times of isotopes to the ion source could be decreased significantly by steering thermodynamic and chemical processes into the desired direction, thus, increasing the yields especially of short-lived isotopes. However, the understanding of these processes in ISOL targets with operating temperatures as high as 2300° C, is limited. The nuclear reactions induced by the proton beam lead to the production of every element on the periodic table lighter than the target material itself. The effusion process is unique for every element and depends on its volatility, temperature, chemical reactions and stoichiometric ratios. Some initial on-line investigations, for example, steering the formation of fluorides into the desired direction with CF4, were promising, but not conclusive due to the limited availability of online testing time. I propose a research project to investigate thermodynamic and chemical processes offline under realistic operating conditions (temperature, pressure, etc.) where they can be controlled much more accurately and where the processing of many samples is not limited by scarce and expensive beam time. The experiments will be performed with a table-top apparatus based on a commercial quadrupole mass spectrometer (QMS) with an ionizer unit and a high-temperature vacuum reaction column which serves as a simplified model for an online target. Solid samples, volatiles and/or reaction gases will be introduced into the column via a fast injection for the investigation of volatilization and retention times. The development of such an apparatus will combine my expertise with atomic beam sources and mass spectrometry and the know-how at TRIUMF on high-temperature furnaces and target fabrication, e. g. e-beam welding of tantalum. Such a high-temperature furnace mass spectrometer (HTF-MS) offers the opportunity for graduate students with a physics or/and chemistry major to work on a variety of interesting questions, aiming at the volatilization of refractive species from an ISOL target. In particular the volatilization of the neutron-rich Ca and Be isotopes has a high priority. They are best produced with sufficient rates from actinide oxide or carbide targets with the disadvantage that Be or Ca are retained within the target matrix as relatively immobile oxides or carbides. The challenge is to investigate the optimal conditions for the fast volatilization of Ca and Be as fluorides. Additionally, a multitude of other molecules will be investigated as well.