Nuclear ground state properties including mass, charge radii, spins and moments can be
determined by applying atomic physics techniques such as Penning-trap based mass
spectrometry and laser spectroscopy. The MATS and LaSpec setups at the low-energy
beamline at FAIR will allow us to extend the knowledge of these properties further into the
region far from stability.
The mass and its inherent connection with the nuclear binding energy is a fundamental
property of a nuclide, a unique “fingerprint”. Thus, precise mass values are important for a
variety of applications, ranging from nuclear-structure studies like the investigation of shell
closures and the onset of deformation, tests of nuclear mass models and mass formulas, to
tests of the weak interaction and of the Standard Model. The required relative accuracy ranges
from 10-5 to below 10-8 for radionuclides, which most often have half-lives well below 1 s.
Substantial progress in Penning trap mass spectrometry has made this method a prime choice
for precision measurements on rare isotopes. The technique has the potential to provide high
accuracy and sensitivity even for very short-lived nuclides. Furthermore, ion traps can be used
for precision decay studies and offer advantages over existing methods.
With MATS (Precision Measurements of very short-lived nuclei using an Advanced
Trapping System for highly-charged ions) at FAIR we aim to apply several techniques to very
short-lived radionuclides: High-accuracy mass measurements, in-trap conversion electron and
alpha spectroscopy, and trap-assisted spectroscopy. The experimental setup of MATS is a
unique combination of an electron beam ion trap for charge breeding, ion traps for beam
preparation, and a high precision Penning trap system for mass measurements and decay
studies.
For the mass measurements, MATS offers both a high accuracy and a high sensitivity. A
relative mass uncertainty of 10-9 can be reached by employing highly-charged ions and a nondestructive
Fourier-Transform Ion-Cyclotron-Resonance (FT-ICR) detection technique on
single stored ions. This accuracy limit is important for fundamental interaction tests, but also
allows for the study of the fine structure of the nuclear mass surface with unprecedented
accuracy, whenever required. The use of the FT-ICR technique provides true single ion
sensitivity. This is essential to access isotopes that are produced with minimum rates which
are very often are the most interesting ones. Instead of pushing for highest accuracy, the high
charge state of the ions can also be used to reduce the storage time of the ions, hence making
measurements on even shorter-lived isotopes possible.
Decay studies in ion traps will become possible with MATS. Novel spectroscopic tools for
in-trap high-resolution conversion-electron and charged-particle spectroscopy from carrierfree
sources will be developed, aiming e.g. at the measurements of quadrupole moments and
E0 strengths. With the possibility of both high-accuracy mass measurements of the shortestlived
isotopes and decay studies, the high sensitivity and accuracy potential of MATS is
ideally suited for the study of very exotic nuclides that will only be produced at the FAIR
facility.
Laser spectroscopy of radioactive isotopes and isomers is an efficient and modelindependent
approach for the determination of nuclear ground and isomeric state properties.
Hyperfine structures and isotope shifts in electronic transitions exhibit readily accessible
information on the nuclear spin, magnetic dipole and electric quadrupole moments as well as
root-mean-square charge radii. The dependencies of the hyperfine splitting and isotope shift on the nuclear moments and mean square nuclear charge radii are well known and the
theoretical framework for the extraction of nuclear parameters is well established. These
extracted parameters provide fundamental information on the structure of nuclei at the limits
of stability. Vital information on both bulk and valence nuclear properties are derived and an
exceptional sensitivity to changes in nuclear deformation is achieved. Laser spectroscopy
provides the only mechanism for such studies in exotic systems and uniquely facilitates these
studies in a model-independent manner.
The accuracy of laser-spectroscopic-determined nuclear properties is very high.
Requirements concerning production rates are moderate; collinear spectroscopy has been
performed with production rates as few as 100 ions per second and laser-desorption resonance
ionization mass spectroscopy (combined with β-delayed neutron detection) has been achieved
with rates of only a few atoms per second.This Technical Design Report describes a number of complementary experimental devices
for laser spectroscopy, which will provide a complete system with respect to the physics and
isotopes that can be studied. Since MATS and LaSpec require high-quality low-energy beams,
the two collaborations have a common beamline to stop the radioactive beam of in-flight
produced isotopes and prepare them in a suitable way for transfer to the MATS and LaSpec
setups, respectively.
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