The collinear laser spectroscopy technique provides high resolution measurements of the nuclear size and electromagnetic moments. This method relies on the excitation of electrons of an isotope from the energy levels of a low-lying hyperfine manifold to those of an excited state using continuous wave lasers, followed by detection of the subsequent de-excitation. This is achieved by counting the fluorescence emitted by the atoms, using a photomultiplier tube. The energy difference between the hyperfine levels is sensitive to the nuclear properties and thus allows for their precise measurement. The high experimental resolution is achieved by performing the experiment in a collinear geometry, meaning that the ion beam and the laser beam are propagating along the same axis. Because the ion beam is accelerated to a high velocity (beam energy of 30 keV), Doppler broadening effects are thus suppressed.
The measured nuclear properties can be used to test our understanding of the nuclear many-body problem. As the nuclear magnetic moment probes the single particle nature of the nucleus while the quadrupole moment is related to the collective behavior, these properties combined provide crucial information about the complex interactions between the protons and neutrons inside the atomic nucleus. In addition to the electromagnetic moments, the changes in the nuclear charge radius of the radioactive isotopes with respect to the stable isotope can also be extracted from the laser spectroscopy measurements. This gives access to the variation of the nuclear charge distribution along an isotopic chain and provides information on both the bulk properties of nuclear matter and the details of nucleon-nucleon interactions.
Operating at the intersection of nuclear and atomic physics, this technique can also be used for the measurement of the atomic properties such as the hyperfine parameters or the atomic field and mass shift constants.
The RAPTOR project - RIS And Purification Traps for Optimised spectRoscopy
The RAPTOR project was initiated to provide a more efficient variant of collinear laser spectroscopy, though with a trade-off some in terms of the high precision of that method. The beamline is currently (2021) under commissioning; first results are expected in the coming months.
The RAPTOR beamline is designed to accept ion beams with a variable beam energy between 1 and 10 keV. This means that the Doppler broadening is suppressed less as compared to standard collinear laser spectroscopy, but gives the device a small footprint and provides longer interaction times between the atoms and the lasers. Rather than monitoring the excitation of the atoms via the detection of their spontaneous decay, instead a step-wise laser ionization process is used. Thus, more efficient charged-particle detection can be employed. The device is thus particularly well suited to the study of isotopes produced at very low rates and/or with very short lifetimes, but where the highest precision is not required to extract nuclear information.
The fact that atoms are laser ionized inside the RAPTOR beamline means they can then be further manipulated, for example to perform an additional purification. The lab has therefore also developed the capability of injecting ions from the RAPTOR beamline directly into the double-Penning trap mass spectrometer. Thus, ultra-low background hyperfine spectra can be obtained, free from any contribution of isobaric beam contamination.
The long interaction times offered by the use of slower beams also makes it possible to perform laser-radiofrequency double resonance spectroscopy, which offers superior precision on hyperfine constants of the atomic ground state, but provides no information on the excited atomic states. Thus, it is an ideal tool to measure higher-order nuclear moments or hyperfine anomalies, but is not sensitive to changes in the nuclear charge radius.
This project was funded by the Marie Skłodowska-Curie grant agreement No 844829.
