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RIS And Purification Traps for Optimised spectRoscopy (RAPTOR)

Periodic Reporting for period 1 - RAPTOR (RIS And Purification Traps for Optimised spectRoscopy (RAPTOR))

Reporting period: 2019-04-01 to 2021-03-31

The study of radioactive isotopes remains a frontier of nuclear physics research. This project studies the properties of refractory elements, which are difficult to produce due to their chemical properties, using beyond state-of-the-art laser spectroscopy and ion manipulation techniques. Laser spectroscopy methods are versatile tools to measure a set of nuclear properties, nuclear electromagnetic moments, spins and charge radii, and can do so in a nuclear model-independent way. These observables provide information on the size and shape of nuclei, and can be used to probe the nuclear wavefunction. By unambiguously measuring the nuclear spins, key information on the ordering of nuclear shell model orbits can be deduced while anchor points can be provided for nuclear decay studies. The nuclear charge radius provides important insight into shell and subshell effects , correlations and mixing, and can be related to nuclear deformation and related topics. The radioactive refractory isotopes of Fe-Ni, Tc-Pd and W-Os have eluded study with optical methods for many decades, a problem which has been identified as the next major challenge for laser spectroscopy. This lack of data leaves large gaps in our understanding of ground-state properties and nuclear structure in these regions. This project seeks to finally address the experimental deficiencies that have so far prevented measurements on many refractory species, and thus makes a significant and unique contribution to the field. It furthermore aims at a closer integration of the laser spectroscopy and ion trapping programmes present in the IGISOL laboratory in Jyväskylä, Finland.

This project enhances the capabilities of the laser spectroscopy programme at the IGISOL facility, and is divided into two main objectives.
1. The design, construction and commissioning of a new beam line tailored to enhance the CRIS method,
2. The measurement of the spins, electromagnetic moments and changes in mean-squared charge radii of 108-120Pd.
This project couples of a CRIS beamline to the existing double-Penning trap. This will provide significant advantages to both the CRIS and the Penning trap measurements.
• The Penning traps can be used as a powerful tool for beam purification , removing all remaining collisional or non-resonantly laser-ionized background from the measurements. Furthermore, the traps can be used to disentangle ground- and isomeric state contributions to the observed hyperfine structure spectra.
• Conversely, selective laser-ionization can be used to create isomerically pure beams, which allows the Penning traps to e.g. measure low-lying isomeric states without hindrance of the (potentially much more intense) beam of ground-state nuclides or other contaminants.

The novel RAPTOR (RIS And Purification Traps for Optimised spectRoscopy) concept thus represents a considerable expansion of the capabilities of the IGISOL laboratory, and opens up research avenues for the long-term future in Jyväskylä and elsewhere.
A new experimental device, called RAPTOR, was designed, simulated and constructed. This new apparatus relies on multi-laser ionization to do efficient studies of atomic hyperfine structure. The goal is to use this device to study very short-lived isotopes, which are difficult to produce and study using current experimental capabilities. The new RAPTOR device was tailored to using relatively low-energy beams produced with a very narrow time-focus. This means the RAPTOR device can be very compact, so that it can even fit in front of the existing, powerful mass-separator Penning trap in the laboratory. The construction of the device has been finalized, and tests commenced towards the end of the project, and are still ongoing.

The new beamline is furthermore designed to enable higher-precision hyperfine structure measurements my combining laser spectroscopy with radiofrequency and microwave spectroscopy techniques. In preparation for this, first proof-of-principle experiments were performed in an offline test bench, which was used to measure the magnetic octupole moment of the stable 45Sc. This result is already available on the arXiv article repository, and is currently undergoing peer review.

An important first step which must be taken prior to performing collinear laser spectroscopy or collinear resonance ionization spectroscopy on palladium is to neutralize it. This is because no suitable atomic transitions exist for singly-charged palladium. This neutralization can be performed using a charge exchange cell. This project has done two important contributions to the field related to this. First, an investigation of the charge exchange efficiency of palladium, and the distribution of final state population among metastable states, was performed. This was used to test theoretical calculations and to guide subsequent experiments on radioactive isotopes of palladium. Efficient optical transitions were found, indicating good neutralization efficiency, which enabled the measurements of electromagnetic moments, spin and nuclear charge radii of radioactive isotopes in the mass range 98-118Pd. The charge exchange studies have been published, and a first article on the measurements on the radioactive isotopes is nearing completion. Secondly, a new charge exchange cell was designed, simulated and constructed, to be used with the new RAPTOR apparatus. This cell was optimized to enable high neutralization efficiency, while minimizing adverse effects on the quality of the quality of the vacuum of the RAPTOR beamline.
Several important developments which go beyond the state of the art were made.

A new experimental beamline, called RAPTOR, was constructed at the accelerator laboratory in Finland. Based on collinear resonance ionization spectroscopy, the beamline will allow the study of very short-lived nuclei which are otherwise inaccessible. A proposal to study the ms-lifetime high-spin isomers in bismuth was already approved by the physics advisory committee of the accelerator laboratory. Furthermore, the RAPTOR is designed to allow injection of laser-ionized radioactive ions into the JYLFTRAP double-Penning trap mass spectrometer. Thus, isomerically purified beams can be delivered for mass measurements. Conversely, the high mass resolving power of this spectrometer can be used to remove unwanted contamination from the beam, which will serve to dramatically improve the signal-to-background of the measurements. These capabilities will be demonstrated soon using stable ions at first, before moving to radioactive ions.

In addition, during the RAPTOR project important first steps were made to bring laser-radiofrequency methods to the radioactive ion beam laboratory. Using these methods, higher-precision spectroscopy of hyperfine constants becomes possible, which will allow first measurements of hyperfine anomalies and octupole moments of selected radioactive isotopes. The new RAPTOR beamline includes a radiofrequency interaction region; thus, collinear laser-radiofrequency spectroscopy will be possible in the near future.

The new RAPTOR beamline thus considerably expands on the capabilities of the laboratory, and on the state-of-the-art of the field in general.

The project has already yielded the first technical results. Commissioning of the new RAPTOR device has already begun, and will be continued throughout 2021. First results on radioactive isotopes are expected late 2021 or early 2022.
Artistic rendition of the design drawings of the RAPTOR beamline.