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28.05.2014 | Trapped: Measuring the proton’s magnetic field

Photo: Dr. Holger Kracke/Helmholtz-Institut Mainz

The double Pennig trap in which the proton was kept for thirteen months.


For the first time scientists have measured the magnetic field of a single proton. For this purpose they kept it in a trap for thirteen months—a new record. The magnetic momentum is essential for the scientists who look for the reason of the imbalance of antimatter and matter in the universe.

A very precisely measured amount of energy is added to the proton in its trap. Was it enough to reverse the polarity of the proton’s magnetic field? At the University of Mainz an elaborate measuring equipment helps finding it out: a double Penning trap. With the help of the 6.5 centimeter long tube the scientists have now measured the magnetic moment as precisely as never before. „Our measurements’ result is a magnetic moment of 2,792847350(9) in units of the so called nuclear magneton. This is three times more precise than the previous value which is based on a measurement of several protons, parts of hydrogen atoms, taking place in 1972“, explains Dr. Wolfgang Quint, atomic physicist at GSI, who built the experiment together with scientists of the University of Mainz, the Helmholtz Institute Mainz and the Max Planck Institute for Nuclear Physics in Heidelberg. „It took 42 years to develop a better measuring method. The difficulty is caused by the very small magnetic moment of a single proton.“

The magnetic field, or the magnetic moment, is created by the spin, the proton’s intrinsic angular momentum. To calculate the magnetic moment’s value the scientists look for the amount of energy that is able to flip the magnetic poles. The precise result is owed to the double Penning trap technique. In one part of the trap the scientists create an inhomogeneous magnetic field to determine the direction of the proton’s magnetic field. In the other part a uniform magnetic field keeps the proton, but leaves its spin almost undisturbed to avoid falsified results. „There we systematically add amounts of energy. Then we transport the proton back into the inhomogeneous measuring part and test whether the spin flipped“, Quint explains. These steps were repeated consecutively for thirteen months adjusting the amount of energy more and more precisely. In total the proton travelled the distance between the two traps several thousand times.

Within this year an identical measuring equipment at CERN is supposed to measure an antiproton in the same way. Comparing the two values could solve the riddle of the imbalance of antimatter and matter in the universe. A difference between the magnetic moment of an antiproton and a proton could explain why after the big bang there was more matter left over than antimatter. Eventually this could solve why planets and stars were able to form at all.

Apart from the Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz, the Max Planck Institute for Nuclear Physics in Heidelberg und GSI scientists of the Ulmer Initiative Research Unit at RIKEN in Japan and of the Ruprecht Karls University Heidelberg were involved.

More information

Original scientific paper in Nature: Nature 509, 596–599 (29 May 2014), doi:10.1038/nature13388

The double Pennig trap in which the proton was kept for thirteen months.
Photo: Dr. Holger Kracke/Helmholtz-Institut Mainz