Atomic physics

By applying atomic-physics techniques like mass, laser, and radiofrequency spectroscopy, key information can be obtained on properties of nuclear ground and isomeric states. These are atomic masses, the nuclear spin I, the magnetic moment μI, the hyperfine anomaly 1Δ2 between two isotopes (indicated by the subscripts 1 and 2) of an element, the spectroscopic nuclear quadrupole moment Qs, and changes in the mean-square nuclear charge radius δ<r2>A,A’ between isotopes with mass numbers A and A’. But also the opposite relation between atomic and nuclear physics holds true, as quantum electrodynamics (QED) can be tested in the strong-field limit of highly-charged ions.
Atomic-physics techniques can profit from the extended beam times at ISOL@MYRRHA: high sensitivity and/or precision is needed for, e.g., studying weakly-produced beams or observing weak effects. This can only be accomplished if extended systematic studies can be performed and the necessary statistics can be acquired. Both conditions can only be fulfilled when ample beam time is available.

Measuring charge radii of light nuclei and hyperfine anomalies

Investigations of charge radii in very neutron-rich and light isotopes present stringent tests for ab-initio calculations as they probe aspects of the interactions that are less prevalent in nuclei closer to stability. By fluorescence detection in a MOT trap, resonance-ionization detection using two-photon excitation, and collinear laser spectroscopy, high-precision laser spectroscopy has been performed already on, respectively, 6,8He [1,2], 8,9,11Li [3,4], and 7,9,10,11Be [5,6]. However, the preparation for new elements is very time consuming (many years), which can be minimized if frequent on-line tests can be performed.

  fluorescence signal of a single trapped metastable helium-6 atom

The fluorescence signal of a single trapped metastable 6He atom (picture from [1]).

If sufficient precision is reached, also hyperfine anomalies can be investigated by combining laser and radiofrequency techniques. Systematic data from several isotopes of one element can be compared to nuclear models. Moreover, there is an increased interest for hyperfine anomaly since theoretical work has shown the possibility to connect the hyperfine anomaly to the change in neutron radius δ<rN2>, as compared to the change in the charge radius δ<rC2>. Except for a few cases, no systematic measurements exist yet.

[1] L.-B. Wang et al., Phys. Rev. Lett. 93 (2004) 142501
[2] P. Mueller et al., Phys. Rev. Lett. 99 (2007) 252501
[3] G. Ewald et al., Phys. Rev. Lett. 93 (2004) 113002
[4] R. Sánchez et al., Phys. Rev. Lett. 96 (2006) 033002
[5] W. Nörtershäuser et al., Phys. Rev. Lett. 102 (2009) 062503
[6] K. Okada et al., Phys. Rev. Lett. 101 (2008) 212502


(Collinear) Resonance-ionization spectroscopy

Resonance-ionization spectroscopy (RIS) is a sensitive technique due to the high detection efficiency of the ions produced by photo-ionization and due to background reduction by mass-analyzing the photo-ions and/or observing their nuclear decay. On the other hand, the resolution is poor (typically a few GHz), but sufficient to study heavy isotope chains. With an excellent sensitivity and high selectivity, one can afford to run long beam times. Currently, developments combining the best of collinear laser spectroscopy and the RIS methods, using collinear resonant laser ionization spectroscopy (CRIS) on bunched accelerated beams, are underway and will result in excellent laser resolution revealing ultimate sensitivity and selectivity.


Picture of the RILIS setup at CERN-ISOLDE 

Polarized beams

The availibilty of polarized radioactive beams will be crucial for several research projects. Examples are β-NMR in condensed-matter research (8Li) and biosciences (elements like Cu), and correlation measurements for weak-interaction studies. Polarized radioactive beams are easily achieved using, e.g., resonant collinear laser excitation with circularly polarized laser light, as illustrated in the figure.

  ISAC neutralizer / polarizer / ionizer

The polarizer beam line layout at ISAC [1] 

[1] C.D.P. Levy et al., Nucl. Instr. Meth. B 204 (2003) 689

Laser ion source trap (LIST)

The obtainable isobaric selectivity and, therefore, the applicability of a conventional resonance ionization laser ion source (RILIS) is limited by the competing process of surface ionization within the target/ion source combination of the ISOL system. This limiting effect can be avoided by geometrically decoupling the process of evaporation and ionization and by repelling unwanted surface ions by a corresponding repelling potential (as illustrated in the figure) [1]. Such a laser ion source trap (LIST) suppresses isobaric surface ion contaminations by many orders of magnitude and, consequently, enhances the selectivity of the ion source drastically. Furthermore, like the RF ion cooler and buncher, the system features high beam quality and allows ion bunching if wanted.

laser ion source trap

Picture from [1]

[1] Picture from K. Blaum et al., Nucl. Instr. Meth. B 204 (2003) 331 

QED tests

Highly charged ions provide a unique testing ground for quantum electrodynamics (QED) in very strong electric and magnetic fields, which is experimentally not accessible otherwise. The figure illustrates the electric-field strength experienced by the 1s-electron in H-like ions as a function of atomic number Z. It shows that the electric field is about 106 times larger in U91+ compared to H+.

Once the QED corrections are tested and verified by such measurements, nuclear ground-state properties of radioactive species as available at ISOL@MYRRHA can be determined with much higher accuracy than achievable by experiments on neutral atoms or moderately charged ions. Presently, the accuracy of atomic calculations in complex multi-electron systems is very much limited by correlations between the electrons.  Such effects are of no concern in ions with only one bound electron. Therefore, absolute nuclear charge radii, nuclear magnetic moments and the Bohr-Weisskopf effect can be determined with very high accuracy in the isotope series of heavy elements since the hyperfine fields of hydrogen-like ions can be safely treated by the atomic theory once QED corrections are under control.

QED tests

Picture from H.-J. Kluge, presented at the BriX workshop 2008, SCK•CEN, Mol, Belgium