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Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy of radionuclides

Periodic Reporting for period 4 - MIRACLS (Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy of radionuclides)

Reporting period: 2021-07-01 to 2021-12-31

The MIRACLS project at ISOLDE/CERN develops and employs a novel method to perform high-resolution laser spectroscopy of exotic, short-lived radionuclides which are currently out of reach for conventional techniques. To this end, the novel approach of the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS) enhances the sensitivity of classical collinear laser spectroscopy by a factor of 20-600. It is based on an electrostatic ion beam trap (EIBT) or also called multi reflection time of flight (MR-ToF) device in which (radioactive) ions bounce back and forth between two electrostatic mirrors. This scheme allows extended observation times and hence higher experimental sensitivity. In order to preserve the high resolution of conventional collinear laser spectroscopy, a dedicated MIRACLS' MR-ToF device is designed to operate at a beam energy of 30 keV compared to a few kiloelectronvolts in contemporary MR-ToF instruments.

The MIRACLS technique has opened a path to probe exotic radionuclides located in currently (in terms of laser spectroscopy) uncharted territory of the nuclear landscape. Among others, these measurements reveal the nuclear charge radii of short-lived radionuclides which are crucial benchmarks for modern nuclear structure theory. Indeed, advances in nuclear theory have recently led to improved theoretical descriptions of nuclear charge radii along Magnesium, Calcium, Tin, or Cadmium isotopic chains. Due to the higher asymmetry in their neutron-to-proton ratio, radionuclides very far away from stability but accessible by MIRACLS expose subtile features of the nuclear force and hence represent challenging benchmarks for modern nuclear theory and generally our understanding of atomic nuclei.

While MIRACLS was developed for fluorescence-based collinear laser spectroscopy, its high potential for other applications was also recognised. Perhaps most interestingly, we have used the novel methods to precisely measure electron affinities, i.e. the released energy when an electron is attached to a neutral atom. The electron affinity of an element is an important property in chemistry (eg. reactivity) as well as atomic theory (eg. many body correlations).
After the setup of the required continuous-wave laser system, a proof-of-principle experiment has been successfully performed which demonstrated the technical feasibility and scientific potential of the novel MIRACLS approach. The technique was further advanced and systematic studies of collinear laser spectroscopy in an MR-ToF device were performed.

After the successful completion of the proof-of-principle experiment, the same apparatus was further upgraded and utilised for work with negative ions. Taking advantage of the MIRACLS concept, this represents a new approach for laser-photodetachment-threshold studies in order to measure electron affinities with high precision and/or of very small samples such as rarely produced radionuclides. This effort has been completed with a successful physics measurement which demonstrates the improved precision and sensitivity enabled by the MIRACLS approach for this type of measurements.

Moreover, the unique combination of ion traps and lasers at MIRACLS allowed the investigation of laser cooling as a previously unexplored tool to prepare cold beams of radioactive ions for subsequent experiments. For instance, it was demonstrated experimentally that the performance of MR-ToF devices for mass separation is significantly improved when ions are previously laser cooled. Other future applications, such as Penning-trap mass measurements or laser spectroscopy, have been proposed and studied via ion-optical simulations. These simulations have previously been successfully benchmarked against the obtained experimental laser-cooling data.

In parallel, an advanced MIRACLS apparatus operating at higher ion beam energy is constructed and currently commissioned. In addition to the 30-keV MR-ToF device, it also host a versatile Paul trap for optimal preparation of the radioactive ion beam. The design of this apparatus is based on extensive simulations of the ion trajectories in the MR-ToF device and Paul trap as well as on the practical lessons learned in the previously described proof-of-principle experiment.
For the first time, MR-ToF devices and collinear laser spectroscopy have been successfully combined to establish a new approach for highly sensitive, high resolution laser spectroscopy which is especially well suited in order to investigate exotic radionuclides with the lowest yields at radioactive ion beam facilities.

While initially designed for optical laser spectroscopy of radioactive ions, MIRACLS has also boosted the performance of laser-photodetachment-threshold studies in order to measure electron affinities with increased precision and/or significantly reduced sample size.

The laser-cooling methods developed within the MIRACLS apparatus promise radioactive beams at much reduced temperatures as imperative for next-generation trap and laser-spectroscopy experiments at radioactive ion beam facilities.