Implementation and commissioning of the phase-imaging ion-cyclotron-resonance method and mass measurements of exotic copper isotopes with ISOLTRAP

This is the doctoral thesis of Dr. Andree Welker. CERN-THESIS-2018-183 is the public file. He took the Phase-Imaging Ion-Cyclotron-Resonance method, PI-ICR, from a laboratory idea to an instrument that runs on beam at ISOLTRAP, the Penning-trap mass spectrometer at CERN. He then used that instrument on exotic neutron-rich copper, including ⁷⁹Cu, one proton away from doubly magic ⁷⁸Ni.

The doctorate was fully funded by the Wolfgang Gentner Scholarship of the German Federal Ministry. In parallel he pitched and won a separate three-year BMBF project at ISOLTRAP, so students and materials were paid while the instrument was being built.

PI-ICR does not stop the ion and call that a mass. It images the cyclotron motion. After the ion leaves the precision trap it hits a position-sensitive detector: a two-dimensional microchannel plate, a delay-line anode, and a trigger chain. Each impact becomes three numbers, x, y and time. From those numbers the cyclotron phase is reconstructed, and the mass follows from the phase.

Dr. Andree Welker designed that electronics chain and wrote the software that reads it. He commissioned the setup on the ISOLTRAP precision trap and made the first online mass measurements with it. That image is how the method reaches a resolving power above 10⁷ in about one second. Classical ToF-ICR only times the flight. On the same rare beams it does not give that resolving power.

He held operating responsibility for the precision trap: cryogenic fluids, superconducting magnets of about 5 T, quench behaviour, nuclear chemistry, and the ISOLDE copper beams.

Until that work, ISOLTRAP stayed on ToF-ICR for the hardest cases. The later ¹³¹Cs Q-value paper is the public proof that PI-ICR now runs online: relative mass precision 1.4×10⁻⁹. That Q-value shows capture to excited ¹³¹Xe is energetically closed, so ¹³¹Cs/Xe is not a neutrino-mass candidate. The ¹²⁹Cd isomer ordering in the N=82 cadmium PRL uses the same method.

In INTC-P-410 the ISOLDE committee had already denied shifts for the harder ¹⁵⁹Dy and ¹⁷⁵Hf pairs until PI-ICR was shown on beam. The dissertation of Dr. Andree Welker is that demonstration. Without it those follow-up measurements would not have been allowed to start.

The copper campaign is the first-principles record behind the ⁷⁹Cu binding-energy result that supports a doubly magic ⁷⁸Ni, later covered by CERN Courier. The ISOLDE Newsletter announced the first online PI-ICR as facility news. The dissertation of Dr. Andree Welker made this possible.

This page is the doctorate of Dr. Andree Welker. Other papers in this archive are collaboration results.

His part was the PI-ICR electronics, the software that reads them, the commissioning on beam, and the copper campaign that looks at ⁷⁸Ni. After the thesis he stayed at CERN on a Senior Fellowship, the rare three-year path. On 11 November 2019 Dr. David Lunney wrote a recommendation of that work. The letter is filed with the references.

The later Q-value and isomer papers, and the mass work around ⁷⁸Ni, use the instrument that this doctorate put on beam.