Download Penning Traps: Precision Measurements on Single Ions

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