Published version Kirsebom, O. S.; Hukkanen, M.; Kankainen, A.; Trzaska, W. H.; Strömberg, D. F.; Martínez-Pinedo, G.; Andersen, K.; Bodewits, E.; Brown, B. A.; Canete, L.; Cederkäll, J.; Enqvist, T.; Eronen, T.; Fynbo, H. O. U.; Geldhof, S.; de Groote, R., Jenkins, D. G.; Jokinen, A.; Joshi, P.; Khanam, A.; Kostensalo, J.; Kuusiniemi, P.; Langanke, K.; Moore, I.; Munch, M.; Nesterenko, D. A.; Ovejas, J. D.; Penttilä, H.; Pohjalainen, I.; Reponen, M.; Rinta-Antila, S.; Riisager, K.; de Roubin, A.; Schotanus, P.; Srivastava, P. C.; Suhonen, J.; Swartz, J. A.; Tengblad, O.; Vilen, M.; Vínals, S.; Äystö, J. Kirsebom, O. S.; Hukkanen, M.; Kankainen, A.; Trzaska, W. H.; Strömberg, D. F.; Martínez-Pinedo, G.; Andersen, K.; Bodewits, E.; Brown, B. A.; Canete, L.; Cederkäll, J.; Enqvist, T.; Eronen, T. et al. (2019). Measurement of the 2+→0+ ground-state transition in the β decay of 20F.We report the first detection of the second-forbidden, nonunique, 2 + → 0 + , ground-state transition in the β decay of 20 F. A low-energy, mass-separated 20 F + beam produced at the IGISOL facility in Jyväskylä, Finland, was implanted in a thin carbon foil and the β spectrum measured using a magnetic transporter and a plasticscintillator detector. The β-decay branching ratio inferred from the measurement is b β = [0.41 ± 0.08(stat) ± 0.07(sys)] × 10 −5 corresponding to log f t = 10.89(11), making this one of the strongest second-forbidden, nonunique β transitions ever measured. The experimental result is supported by shell-model calculations and has significant implications for the final evolution of stars that develop degenerate oxygen-neon cores. Using the new experimental data, we argue that the astrophysical electron-capture rate on 20 Ne is now known to within better than 25% at the relevant temperatures and densities.

Excited levels in 87 Br, populated in β decay of 87 Se, have been studied by means of γ-ray spectroscopy using an array of broad energy Ge detectors. 87 Se nuclei were produced by irradiating a natural Th target with 25-MeV protons. Fission products were extracted from the target chamber using the IGISOL technique, then separated on a dipole magnet and Penning trap (JYFLTRAP) setup. The scheme of excited levels of 87 Br has been significantly extended. 114 new transitions and 51 new levels were established. β feedings and log(f t) values of levels were determined. The upper limit for β feeding to the ground state of 87 Br was determined to be 23(5)%. Ground state spin and parity 5/2 − was confirmed, as suggested by previous studies. We also confirm the low-energy excited state at 6.02 keV. The ground state and two lowest excited states in 87 Br were interpreted as the (π f 5/2) 3 j, j−1, j−2 triplet produced by the so-called anomalous coupling. The 333.61-and 699.26-keV levels were interpreted as π p 3/2 and π p 1/2 single-particle excitations. The 9/2 + level reported previously as corresponding to the π g 9/2 single-particle excitation is proposed to be an isomer with half-life 20 ns. Large-scale shell-model calculations performed in this work are in good agreement with experimental results.

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Precision nuclear charge radii measurements in the light-mass region are essential for understanding the evolution of nuclear structure, but their measurement represents a great challenge for experimental techniques. At the Collinear Resonance Ionization Spectroscopy (CRIS) setup at ISOLDE-CERN, a laser frequency calibration and monitoring system was installed and commissioned through the hyperfine spectra measurement of 38-47 K. It allowed for the extraction of the hyperfine parameters and isotope shifts with better than 1 MHz precision. These results are in excellent agreement with available literature values and they demonstrate the suitability of the CRIS technique for the study of nuclear observables in light atomic systems. In addition, the spectral line shapes obtained under different conditions were systematically investigated, highlighting the importance of finding optimal conditions, under which the extracted nuclear properties remain unaffected by laser-atom interactions.

Isotope shifts from collinear laser spectroscopy of doubly charged yttrium isotopes Vormawah, L. J.; Vilén, Markus; Beerwerth, R.; Campbell, P.; Cheal, B.; Dicker, A.; Eronen, Tommi; Fritzsche, S.; Geldhof, Sarina; Jokinen, Ari; Kelly, S.; Moore, Iain; Reponen, Mikael; Rinta-Antila, Sami; Stock, S. O.; Voss, Annika Vormawah, L. J., Vilén, M., Beerwerth, R., Campbell, P., Cheal, B., Dicker, A., . . . Voss, A. (2018 Collinear laser spectroscopy has been performed on doubly charged ions of radioactive yttrium in order to study the isotope shifts of the 294.6-nm 5s 2 S 1/2 → 5p 2 P 1/2 line. The potential of such an alkali-metal-like transition to improve the reliability of atomic-field-shift and mass-shift factor calculations, and hence the extraction of nuclear mean-square radii, is discussed. Production of yttrium ion beams for such studies is available at the IGISOL IV Accelerator Laboratory, Jyväskylä, Finland. This newly recommissioned facility is described here in relation to the on-line study of accelerator-produced short-lived isotopes using collinear laser spectroscopy and application of the technique to doubly charged ions.

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An off-line ion source station has been commissioned at the IGISOL (Ion Guide Isotope Separator On-Line) facility. It offers the infrastructure needed to produce stable ion beams from three off-line ion sources in parallel with the radioactive ion beams produced from the IGISOL target chamber. This has resulted in improved feasibility for new experiments by offering reference ions for Penningtrap mass measurements, laser spectroscopy and atom trap experiments.