Laser spectroscopy of short-lived radionuclides in an ion trap: MIRACLS’ proof-of-principle experiment

Since 1978 Collinear Laser Spectroscopy is done at COLLAPS [1], which is located at ISOLDE,CERN’sfacilityforsynthesizingradioactiveions,toexplorethenuclearshell structure of the most exotic atomic nuclides far away from stability. At COLLAPS a laser beam is overlapped with a radioactive ion beam. If the wavelength of the laser corresponds to the energy difference of the electronic transitions, the laser excites the ions. The excited ions decay back to the ionic ground state and emit fluorescence photons that can be detected with photomultiplier tubes (PMTs). By measuring the hyperfine structure of the involved ionic states one obtains information about the nuclear spin, the nuclear magnetic dipole moment and the nuclear electric quadrupole moment. This hyperfine splitting is caused by the interaction of the bound electrons withtheatomicnucleus. Theelectronsinduceanelectromagneticfieldattheplaceof the nucleus that interacts with the electromagnetic nuclear moments and the nuclear spin. By calculating the difference in fine structure splitting of two different isotopes of the sameelementwegettheisotopeshiftthatisrelatedtothenuclearmeansquarecharge radius. A change in the neutron number leads to a different size of a nuclide’s charge distribution which results in a shift in the energy of the (ionic) fine structure state. Due to its high precision and accuracy, Collinear Laser Spectroscopy remains a very important experimental technique to access nuclear spins, magnetic dipole moments, electric quadrupole moments and mean square charge radii of short-lived nuclides far from stability and to obtain knowledge about the shell structure of the most exotic nuclides[1]. However, COLLAPS is limited to nuclides with production yields of typically more than a few 1000 ions/s delivered from ISOLDE. To measure the most exotic nuclides far from stability, that can already be produced with low production yields, novel experimental techniques have to be envisioned. One shortcoming of COLLAPS is that the radioactive ions only interact with the laser in the time scale of some µs, whereas the half-life of the ions is between tens of milliseconds to seconds. To make use of the rare isotopes over their entire lifetime and therefore enhance the sensitivity of COLLAPS by a factor of 20-800, the concept of a Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy of radionuclides (MIRACLS) was introduced. The general setup of MIRACLS can be seen in figure 1. A bunched ion beam is trapped between two electrostatic mirrors and the laser interacts with the ion bunch during each revolution. Therefore the observation time is extended and only limited by the half-life of the ions or by the maximal trapping time. At the moment a proof-of-principle experiment is carried out with an existing electrostatic ion beam trap EIBT, also called multi-reflection time-of-flight MR-ToF apparatus, which was built at the University of Greifswald for the purpose of mass measurements in a student lab. This proof-of-principle experiment is performed with ionbeamenergiesofabout1.3keV,whereastherealexperimentshouldrunat30keV in order to minimise the Doppler broadening. At the proof-of-principle experiment (see figure 1) the ion beam formed in an offline ion source is bent by 90 degrees by a quadrupole bender before the ions enter the Paul trap, where we obtain a bunched beam with low energy spread. After their release from the Paul trap, the ions aretrapped for several revolutions inside the MR-ToF and interact with the laser beam during each revolution. The emitted fluorescence photons are detected by PMTs. This report will focus on improvements of the sensitivity of MIRACLS by optimizing the offline ion source (chapter 2.1), reducing the scattered laser light (chapter 2.2) and checking and improving the EIBT alignment (chapters 2.3 and 2.4) .