Precision experiments with stored ions and antimatterMax Planck Institute for Nuclear PhysicsUniversity of HeidelbergEuropean Research Council
Ultracold Ions and Antimatter Research
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High-precision mass measurements on exotic nuclides

According to the equivalence of mass and energy, which was postulated by Albert Einstein in 1905, the mass of any bound system, such as an atom or molecule, depends on the binding forces that act between the constituent particles. Conversely, sufficiently precise mass measurements allow a direct and quantitative access to the nuclear (and even atomic or molecular) binding energies. Depending on the studied systems, this information is crucial for many different fields of physics, ranging from the nuclear mass surface to astrophysics to fundamental interactions.

high-precision mass measurement
The equivalence of mass and energy means that a high-precision mass measurement will yield information on the binding energies of a system.

The measurement of the masses of single ions in Penning traps has revolutionized the field of mass spectrometry in recent years. Previously unheard-of relative mass precisions of up to 10−11 in the case of stable nuclides and up to 10−8 for short-lived radionuclides have been achieved. In Penning trap mass spectrometry experiments, the mass of a stored ion is determined by the time-of-flight (TOF) cyclotron resonance technique.
Using a radiofrequency excitation near the cyclotron frequency of the trapped ion, its radial motions are coupled and converted in such a way as to maximize the rotational energy. After ejection of the particle from the trap, the magnetic-field gradient in the inhomogeneous-field region exerts an accelerating force on the ion, and the ions that have been resonantly excited reach the detector after a shorter time of flight than those which were left undisturbed. The TOF spectrum of a large number of ions is recorded on a detector and the cyclotron frequency is determined from the minimum in the TOF distribution.
The ISOLTRAP experiment at CERN's ISOLDE facility in Geneva, Switzerland, was the first such system installed at a radioactive-nuclear-beam facility in the 1990s and is unsurpassed to this day in terms of efficiency, versatility and precision. It has until now been used to measure the mases of more than 300 radionuclides, many of them for the first time.

ISOLTRAP's precision Penning trap
The mass measurements on exotic nuclides take place in ISOLTRAP's precision Penning trap, in the field of a 6-T superconducting magnet.

ISOLTRAP is approved for data taking until 2012, and the ISOLTRAP Collaboration, which consists of about 25 scientists from 8 institutes, conducts roughly 3–5 data taking periods per year. In addition to the ongoing physics program, current activities are aimed at further improving the efficiency and precision of the apparatus, such as by installing a more efficient particle detector and by reducing fluctuations in the temperature and pressure of the liquid helium contained in the Dewar vessel of the superconducting magnet. Furthermore, an alternative excitation scheme based on the Ramsey method has been successfully studied.

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