An advanced radio-frequency quadrupole ion cooler for Accelerator Mass Spectrometry

Dr Markus Schiffer1,2, Oscar Machhart2, Susan Herb1, Martin Martschini2, Robin Golser2, Erik Strub3, Tibor Dunai4, Alfred Dewald1

1University of Cologne, Institute for Nuclear Physics, , Germany, 2University of Vienna, Faculty of Physics, Isotope Physics, , Austria, 3University of Cologne, Institute for Nuclear Chemistry, , Germany, 4University of Cologne, Institute for Geology and Mineralogy, , Germany

Ion Laser InterAction Mass Spectrometry (ILIAMS) has demonstrated an extraordinarily high isobar suppression capability for a variety of radionuclides which are important for accelerator mass spectrometry. It uses selective laser photodetachment of decelerated ion beams in a gas-filled radio-frequency quadrupole (RFQ) cooler for the suppression of interfering isobars [1]. Furthermore, the admixture of O₂ gas (≈3%) to the helium buffer gas has revealed an even higher isobar suppression, larger than 10⁵ in the case of ⁹⁰Sr/⁹⁰Zr, at the Vienna Environmental Research Accelerator (VERA), without the use of a laser.

Therefore, we started to develop a RFQ cooler designed for the deceleration and trapping of ion beams with high beam emittance like heavy molecular anions, e.g. ⁹⁰SrF₃. The system will be finally used at the Cologne 6 MV accelerator after commissioning and test measurements at VERA. The ion cooler will use gas reactions with the option of adding a laser in a later phase.

The new RFQ design intends to solve technical challenges by a self-aligned structure with the possibility of readjustment without breaking the vacuum. The vacuum chamber of the RFQ can be opened at the top for easy maintenance and for changes in the experimental setup. This allows changes of the quadrupole itself, like the installation of higher-order multipole segments at the entrance of the radio-frequency device providing higher acceptance for divergent ion beams. Based on the design of the injection unit of ISOLTRAP [2] an elliptical injection electrode was developed that should increase the theoretical transmission in comparison to ring or conical electrodes. It will also allow to slow down the ions far away from the central region where the buffer gas is leaking out of the RFQ trough the central aperture.

A new and easy to manufacture guide field assembly was developed to reduce the electric multipole order, which is naturally induced by inclined DC electrodes. For this purpose, diagonally split cylindrical electrodes are capacitively coupled to a core rod that is carrying the RF signal. Consequently, only low DC voltages are needed to create a gradually changing potential in longitudinal direction. This design is a simplification of the diagonally split hyperbolical RF electrode assembly of BECOLA [3]. In this contribution, we will compare different guiding field structures by the calculation of multipole expansion coefficients.

Additionally, a remote-controlled radio-frequency resonance tuning and impedance matching system for heavy radionuclide applications will be presented, which is able to drive the RFQ with frequencies in the range of 1 MHz with RF peak-to-peak voltages of 250 V.

[1] M. Martschini et al., The ILIAMS project – An RFQ ion beam cooler for selective laser photodetachment at VERA, NIM B 456 (2019) 213-217.

[2] A. Kellerbauer et al., Buffer gas cooling of ion beams, NIM A 469 (2001) 276–285.

[3] B.R. Barquest et al., RFQ beam cooler and buncher for collinear laser spectroscopy of rare isotopes, NIM A 866 (2017) 18-28.


Biography:

Education
2012–2018 Doctoral Studies, Dr. rer. nat., Institute for Nuclear Physics, University of Cologne.
2003–2012 Diploma Experimental Physics, Institute for Nuclear Physics, University of Cologne.
Scientific Career
Since 07/2020 Research Associate, Institute for Nuclear Physics, University of Cologne.
01/20–06/20 Research Associate, University of Vienna, Isotope Physics, Austria.
01/13–12/19 Research Associate, Institute for Nuclear Physics, University of Cologne.

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Date

Nov 17 2021

Time

WEDNESDAY
8:45 am - 9:40 am