International Conference on Electromagnetic Isotope Separators and Related Topics (EMIS 2015)

The extraction of 229Th3+ from a buffer-gas stopping cell

14 May 2015, 09:50

20m

Pentlind Ballroom

Oral Presentation Session 12

Speaker

Mr Lars von der Wense (Ludwig-Maximilians-University Munich)

Description

Among all known nuclear isotopes, 229Th possesses the lowest nuclear excitation, revealing an energy of just 7.6 eV above the ground state. This low energy is equivalent to a wavelength of the corresponding gamma-ray of about 160 nm, which is set in the VUV region and conceptually allows for the application of laser-spectroscopic methods to the nucleus. The excited state is predicted to be isomeric with a half-life of up to several hours, which would lead to an extremely sharp linewidth, making the isomeric ground-state transition of 229Th an ideal candidate for a nuclear-based frequency standard. So far, however, the energy of the isomeric state has not been measured precisely enough to allow for a direct optical access to the transition. Further, the direct observation of the isomeric ground-state decay has not been conclusively proven, so that, besides the exact energy, also the half-life and the gamma branching-ratio have to be stated as unknown. In our approach to directly detect the isomeric ground-state transition, 229Th is populated via a 2 % decay branch in the alpha-decay of 233U. 229(m)Th recoil ions, as produced by a 233U source, are stopped within ultrapure He in a buffergas stopping cell strictly designed to UHV standards. The ions are then extracted with the help of electric guiding fields and a supersonic Laval nozzle. Successively, a sub-mm diameter ion beam is formed by an RFQ guiding structure. Having formed an ion beam allows for a mass-purification of the extracted recoil ions with the help of a customized quadrupole mass separator, designed for high transmission. In this way, possible decays from short-lived daughters in the decay chain are highly suppressed. A search for photons and electrons originating from the isomeric deexcitation is performed behind the QMS. This concept has the advantage that a measurement of the transition can be performed within an environment of ultra-low background, which opens up the way for an unambiguous identification of the isomeric decay. Obviously, a large combined extraction and mass-purification efficiency of 229Th ions is of major importance for this concept. Measurements were performed to quantify this efficiency. Surprisingly, a large extraction efficiency of 10 % is obtained in the 3+ charge state. This is the first time, that a significant extraction rate in the 3+ charge state could be observed from a buffer-gas stopping cell. This work was supported by DFG (Th956/3-1).