COLLAPS is a small experiment located at the “isotope factory” ISOLDE at CERN. Its aim is the investigation of ground state properties of exotic, short lived nuclei, such as spins, electro-magnetic moments and charge radii.
All these observables contribute widely to our understanding of the nuclear force – they give valuable information about the coupling between nucleons, about symmetry of the nuclear wave-functions and thus about the symmetry of the nuclear interaction itself. In this way, for example, the discovery that the nuclei can possess a spectroscopic nuclear moment gave the decisive proof for the existence of non-central parts of the nucleon-nucleon force. Another example is the light halo nucleus 11Li, whose spin and magnetic moment have been measured for the first time by our group. These results confirmed the halo-structure, and excluded a stong deformation, which was also considered before. In the same way, in 2005, we determined the spin of 31Mg, which turns out to lie in an interesting region of “island of inversion.”

COLLAPS combines expertise in atomic and nuclear physics, because it mainly uses the hyperfine interaction to obtain information about nuclei by manipulating the atomic electrons. This is indicated already by its name: COLLAPS stands for COLlinear Laser Spectroscopy, i.e. we use the laser light to induce electron-transitions in atoms or ions, and from the hyperfine splitting (HFS) or isotope shifts (IS), we get the ground state properties of the nuclei. Presently, we also use another method: beta-NMR technique, which can give very precise values of the magnetic dipole moments and electric quadrupole moments.

Technique & set-up

In collinear laser spectroscopy atomic hyperfine spectra are measured which give access to the isotope shifts and hyperfine splitting. These effects arise from the interaction between the nucleus and its surrounding electron cloud, so by measuring them very precisely, we gain access to nuclear information:

  • The nuclear moments (magnetic dipole moment and electrostatic quadrupole moment) interact with the magnetic and electric fields generated by the electrons which causes a splitting of the fine structure atomic levels.

  • The different mass and size of isotopes of the same element cause a small shift in frequency of the spectral line when going from one isotope to another, the so-called isotope shift. From this shift, the difference in mean-square charge radii of the different isotopes can be determined.


On the following COLLAPS pages you can find more details about our experimental techniques:

All the other methods are based on the principle of optical pumping:

  • Detection of the optical resonance by making use of the change in the population of states with big differences in ionization energy - the reionization method
  • Detection of the population change by observing the change in nuclear polarization with the ß-NMR technique.


The first collinear experiments of the Mainz group have been performed at the Mainz reactor in 1978, yielding IS and HFS for a number of n-rich Rb and Cs isotopes. In 1979-1980 the setup was improved and moved to CERN, where is it still located at the moment. Since this time it has undergone many modifications and improvements.
The first measurements at CERN were devoted to Ba and Yt isotopes. In 1986 a new, sensitive detection scheme was proposed: collisional ionization It was shown to be successful on Kr isotopes (NIMB17,354). One year later a first publication appeared devoted to measurements on 11Li, performed with beta-NMR combined with optical pumping in a collinear configuration (PLB197,311). In 1988 a new non-optical detection method was developed and used on Sr at COLLAPS (PRL60,25). A report on a successful application of resonance ionization on a fast Yt beam came in 1991 (JPB,24,4831). In 1995, the group reported on a collinear experiment using collisional ionization and alpha-detection (ENAM95Proc.,133). Then, in 1998-2001 experiments on Ne were performed, which used a collinear-anticollinear configuration to calibrate the acceleration voltage. For more information about the more recent experiments, see our publication page.

For our current research program, you are kindly invited to visit our News and Publications pages, where all the recent and future activities, as well as articles and experiment-proposals, have been placed.

Information about ISOLDE can be accessed at www.cern.ch/isolde.

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