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with its angular momentum (spin). Experimentally, the spin direction in an external magnetic field can be determined via the so-called Stern-Gerlach effect by precisely measuring a frequency of motion. Irradiated microwaves induce the transition between the two different spin orientations. The g factor results from the associated resonance frequency. Max Planck Institute for Nuclear Physics Address: Saupfercheckweg 1 69117 Heidelberg Germany Postal address: PO Box 103980 69029 Heidelberg Germany Phone: 06221 5160 Fax: 06221 516601 Left: A highly charged ion with only one electron stored in a cylindrical Penning trap. Right: Schematical illustration of the quantum electrodynamical effects of an electron (blue), orbiting around a nucleus (green) and interacting with a strong external magnetic field. E-mail:[email protected] Internet:http://www.mpi-hd.mpg.de In the framework of quantum electrodynamics (QED), the g factor can be calculated very accurately. Thus, precise measurements validate these calculations and, therefore, provide a test of the predictive power of the standard model. The MPIK is leading in g factor experiments focusing on the electron, in case it is bound in highly charged ions and hence exposed to very strong electromagnetic fields. Indeed, measurements on highly charged silicon ions presently represent the most precise test of QED for bound states. On the other hand, a comparison between measurement and theory also yields the electron mass, that is a parameter in the calculations. Using this method, we recently succeeded to determine the electron mass with the highest accuracy to date. In future, another g factor experiment called ALPHATRAP will be operated at the MPIK. Goals of this experiment are further tests of QED in extremest fields and the highly precise determination of the fundamental fine structure constant α using very heavy highly charged ions up to hydrogen-like lead. The MPIK contributes to further g factor experiments which aim at the determination of the magnetic moment of the proton and the antiproton as a test of CPT symmetry (located at University Mainz and CERN). Prof. Dr. Klaus Blaum Phone: +49 6221 516850 E-mail:[email protected] Contact: Dr. Sven Sturm Phone: +49 6221 516447 E-mail:[email protected] Internet:www.mpi-hd.mpg.de/blaum/ Penning Traps Precision Measurements on Single Ions Penning Traps Precision Measurements on Single Ions How are elements formed in the cosmos? Why is iron much more abundant on Earth than most of the other elements? What is the mass of an electron? Finding answers to these questions requires very accurate investigation of the basic building blocks of nature. The determination of atomic properties to the highest precision requires the observation of single, isolated atoms. For charged atoms (ions), the so-called Penning trap has proven to be an ideal tool for this purpose. Principles of Penning Traps The storage of single ions is possible due to their high sensitivity – because of their charge – to electric and magnetic fields. The first component of the Penning trap + V is a homogeneous magnetic field. The ion is forced on a circular orbit around the axis of the magnetic field by the Lorentz force. This two-dimensional harmonic motion is called cyclotron motion and has a frequency of fc = q∙B/(2p∙m), which is proportional to the outer magnetic field strength B as well as to the charge-to-mass ratio q/m of the ion. However, the magnetic field does not confine ions along the field axis. This is achieved by an electrical quadrupole field superimposed on the magnetic field by means of suitably shaped trap electrodes. The combination of electric and magnetic fields thus leads to a three-dimenoverall motion sional confinement of the ion. The motion of the ion in the resulting total potential is now a complex orbit composed of three independent magnetron motion harmonic eigenmotions. Two in principle different methods are used to determine fc: The first cyclotron one is called “time-of-flight motion ion-cyclot ron-resona nce axial and magnetron axial motion motion method” and enables the B Signal of a single stored and cooled ion in a Penning trap as a minimum in the noise spectrum of a superconducing oscillator circuit connected to the trap. direct determination of the cyclotron frequency. To apply this method, an ion is resonantly excited and ejected from the trap. A single such measurement can be very fast, so this method is especially suited for short-lived nuclides. The alternative method comprises the measurement of the image current (~10 –15 A) induced in the trap electrodes by the oscillating ion. This allows to determine the frequency by a single measurement and only one ion is required. Further, due to lower temperatures a higher precision is reached. Therefore, the imagecurrent method is applied preferentially for stable nuclides. Nuclear Masses Based on Einstein‘s principle E = mc2, highly precise mass measurements provide important information about the binding energies in nuclei that are of interest in many fields of modern physics. For example, they enable tests of mass models or deliver input parameters for the description of nucleosynthesis in astrophysics. Penning traps are ideal tools to determine nuclear masses very precisely. Nowadays, short-lived radioactive nuclides are measured by the time-of-flight method with a relative precision of a few parts in 10 –9, but stable nuclides even down to 10 –10. application of the mirror-current detection allows relative precisions better than 10 –11. This means that Penning-trap measurements give the most accurate mass values at all. PENTATRAP is a new high-precision mass spectrometer under construction at the MPIK. It is intended to determine the mass ratios of long-lived, highly charged nuclides up to hydrogen-like uranium, thereby seeking for a relative accura- cy of some parts in 1012. Neutrino physics is one of the physics fields to which the measurements will be applied. Measurements of the decay energies of certain b transitions will contribute to the determination of the mass of the electron neutrino. Furthermore, precise tests of the theory of quantum electrodynamics as well as a test of E = mc2 itself will be performed. PENTATRAP will be feeded The PENTATRAP trap tower with highly charged ions with electronics. from two electron beam ion sources (EBITs). The heart of the experiment consists of five identical traps which enable simultaneous frequency measurements in several individual traps. This will minimize the limitations of the measurement accuracy imposed by temporal fluctuations of the magnetic field. The THe-TRAP project is another nuclear mass experiment at the MPIK. It aims at measuring extremely precisely the mass ratio of 3He and 3H (= T, tritium) with a relative precision better than 10 –11. This value will contribute to the determination of the mass of the electron antineutrino by the Karlsruhe Tritium Neutrino Experiment (KATRIN). This will be achieved by a Penning trap mass spectrometer with an extremely stable magnet system that originally had been designed and constructed at the University of Washington. In 2008, it was moved to the MPIK where it is operated in a special laboratory. Further, the MPIK is involved in a number of Penning trap mass spectromenter experiments with access to exotic nuclides at external accelerator facilities (GSI: SHIPTRAP, CERN: ISOLTRAP) or nuclear reactors (University Mainz: TRIGA-Trap). Magnetic Moments Another basic property that can be measured with high precision using Penning traps, is the strength of the magnetic dipole moment of the ions, which is determined by the so-called g factor. This is a dimensionless constant connecting the strength of the magnetic moment of a particle