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LIONTRAP Experiment

LIONTRAP stack of various cylindrical Penning traps
Figure 1: LIONTRAP stack of various cylindrical Penning traps

The LIONTRAP (Light ION TRAP) experiment, situated at Johannes Gutenberg University (JGU) in Mainz, is a dedicated mass spectrometer aiming for most precise mass measurements on various light ions.

The electron, the proton and the neutron together with their simplest combinations, such as the atomic nuclei deuteron, triton and helion are the foundational constituents of our visible universe. High-precision mass measurements of these light atomic nuclei enable sensitive tests of fundamental physics. For example, the mass of the proton is an important input parameter for hydrogen spectroscopy and thus impacts the value of the Rydberg constant. The mass of the deuteron is the starting point for the determination of the atomic mass of the neutron. Furthermore, an ultra-precise measurement of the mass difference of helium-3 and tritium is essential for a highly relevant consistency check of the (anti-)neutrino mass measurement in the KATRIN experiment. Here, unprecedented relative precisions better than 10-11 is required. At present, the most precisely measured light ion masses from spectrometers around the world show an inconsistency, which we call the light ion mass puzzle. All this brings in the necessity of independent checks of these light ion masses, which will be performed by LIONTRAP. The LIONTRAP apparatus is based on the former bound-electron g-factor experiment for highly charged ions, where amongst other things, we have performed the most stringent test of quantum electrodynamics (QED) in strong fields and we have improved the precision of the electron mass by a factor of 13 [1].

Recently, LIONTRAP has been able to determine the mass of the proton to 11 digits, the world-leading precision [2]. This success and further developments will also enable the determination of the deuteron mass and other light ions with unprecedented precision and thus to shed some light on the light ion masses.

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11.02.2021: LIONTRAP experiment currently has open positions. Please visit opportunities if you are interested.

Most Recent Publications

02.09.2020, Nature: Penning trap mass measurements of the deuteron and the HD+ molecular ion externer Link. Our recent paper presents the most precise mass value of the deuteron, measured directly in atomic mass units, with a relative uncertainty of eight parts per trillion. Furthermore, the mass of the HD+ molecular ion is also measured providing a consistency check of our results for the masses of the deuteron and the proton.

29.08.2019, PRA: High-precision mass spectrometer for light ions externer Link. We have published a new paper, explaining the measurement concept and a doubly compensated cylindrical Penning trap.

22.06.2017, PRL: High-Precision Measurement of the Proton’s Atomic Mass externer Link. The paper discusses the first measurement campaign of LIONTRAP, where the atomic mass of a single proton is measured with a precision of 32 parts-per-trillion.


[1]   S. Sturm et al., "High-precision measurement of the atomic mass of the electron" externer Link, Nature, vol. 506, no. 7489, pp. 467-70, Feb 27 2014.
[2]   F. Heiße, F. Köhler-Langes, S. Rau, J. Hou, S. Junck, A. Kracke, A. Mooser, W. Quint, S. Ulmer, G. Werth, K. Blaum, and S. Sturm, "High-Precision Measurement of the Proton's Atomic Mass" externer Link, Phys. Rev. Lett., vol. 119, no. 3, p. 033001, Jul 21 2017.