The Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique has been commissioned at the JYFLTRAP double Penning trap mass spectrometer. This technique is based on projecting the ion motion in the Penning trap onto a position-sensitive multichannel-plate ion detector. Mass measurements of stable 85 Rb + and 87 Rb + ions with well-known mass values show that relative uncertainties ∆m/m ≤ 7 · 10 −10 are possible to reach with the PI-ICR technique at JYFLTRAP. The significant improvement both in resolving power and in precision compared to the conventional Time-of-Flight Ion Cyclotron Resonance technique will enable measurements of close-lying isomeric states and of more exotic isotopes as well as ultra-high precision measurements required, e.g., for neutrino physics. In addition, a new phase-dependent cleaning method based on the differences in the accumulated cyclotron motion phases has been demonstrated with short-lived 127 In + and 127m In + ions.

The rare-earth peak in the r-process abundance pattern depends sensitively on both the astrophysical conditions and subtle changes in nuclear structure in the region. This work takes an important step elucidating the nuclear structure and reducing the uncertainties in r-process calculations via precise atomic mass measurements at the JYFLTRAP double Penning trap. 158 Nd, 160 Pm, 162 Sm, and 164−166 Gd have been measured for the first time and the precisions for 156 Nd, 158 Pm, 162,163 Eu, 163 Gd, and 164 Tb have been improved considerably. Nuclear structure has been probed via twoneutron separation energies S2n and neutron pairing energy metrics Dn. The data do not support the existence of a subshell closure at N = 100. Neutron pairing has been found to be weaker than predicted by theoretical mass models. The impact on the calculated r-process abundances has been studied. Substantial changes resulting in a smoother abundance distribution and a better agreement with the solar r-process abundances are observed.

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.

Publisher's PDF Rakopoulos, V.; Lantz, M.; Solders, A.; Al-Adili, A.; Mattera, A.; Canete, Laetitia; Eronen, Tommi; Gorelov, Dmitry; Jokinen, Ari; Kankainen, Anu; Kolhinen, Veli; Moore, Iain; Nesterenko, Dmitrii; Penttilä, Heikki; Pohjalainen, Ilkka; Rinta-Antila, Sami; Simutkin, V.; Vilén, Markus; Voss, Annika; Pomp, S.Rakopoulos, V., Lantz, M., Solders, A., Al-Adili, A., Mattera, A., Canete, L., . . . Pomp, S. (2018). First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments. Physical Review C, 98 (2) We report the first experimental determination of independent isomeric yield ratios using direct ion counting with a Penning trap, which offered such a high resolution in mass that isomeric states could be separated. The measurements were performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyväskylä. The isomer production ratios of 81 Moreover, based on the isomeric yield ratios, the root-mean-square angular momenta (J rms ) of the fission fragments after scission were estimated using the TALYS code. The experimentally determined isomeric yield ratios, and consequently the deduced J rms , for 130 Sn are significantly lower compared to 128 Sn for both fissioning systems. This can be attributed to the more spherical shape of the fragments that contribute to the formation of 130 Sn, due to their proximity to the N = 82 shell closure. The values of J rms for 129 Sb are higher than 128 Sn for both reactions, despite the same neutron number of both nuclides (N = 78), indicating the odd-Z effect where fission fragments with odd-Z number tend to bear larger angular momentum than even-Z fragments. The isomer production ratio for the isotopes of Sn is more enhanced in the nat U(p, f ) reaction than in 232 Th(p, f ). The opposite is observed for 96 Y and 97 Y. These discrepancies might be associated to different scission shapes of the fragments for the two fission reactions, indicating the impact that the different fission modes can have on the isomeric yield ratios.

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.