Recent experiments at the JYFLTRAP Penning trap

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Recent experiments at the JYFLTRAP Penning trap © The Authors, 2020 Published version Kankainen, Anu; Eronen, Tommi; Nesterenko, Dmitrii; de Roubin, Antoine; Vilén, Markus Kankainen, A., Eronen, T., Nesterenko, D., de Roubin, A., & Vilén, M. (2020). Recent experiments at the JYFLTRAP Penning trap. Hyperfine Interactions, 241(1), Article 43. https://doi.org/10.1007/s10751-020-01711-5 2020 Hyperfine Interactions (2020) 241:43 https://doi.org/10.1007/s10751-020-01711-5 Recent experiments at the JYFLTRAP Penning trap Anu Kankainen1·Tommi Eronen1·Dmitrii Nesterenko1·AntoinedeRoubin 1,2 · Markus Vil´ en1,3 ©The Author(s) 2020 Abstract The JYFLTRAP double Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line (IGISOL) facility offers excellent possibilities for high-precision mass measurements of radioactive ions. Around 400 atomic masses, including around 50 isomeric states, have been measured since JYFLTRAP became operational. JYFLTRAP has also been used as a high-resolution mass separator for decay spectroscopy experiments as well as an ion counter for fission yield studies. In this contribution, an overview of recent activities at the JYFLTRAP Penning trap is given, with a focus on nuclei discussed in the PLATAN2019 meeting. Keywords Penning trap ·Atomic mass ·Nuclear binding energy ·Isomers 1 Introduction Atomic masses can be measured with the highest achievable precision using Penning trap mass spectrometers. Very high precision (≤1keV/c 2) mass measurements are important for fundamental physics [1,2], such as for weak-interaction studies, testing the isospin symmetry or the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Mass measurements with a good precision of ≤10 keV/c2are also needed to study subtle changes in nuclear structure [2,3] or to provide accurate inputs for astrophysical reaction network calculations [4]. In this contribution, we review the recent activities at the JYFLTRAP double Penning trap mass spectrometer [5–7] located at the Ion Guide Isotope Separator This article is part of the Topical Collection on Proceedings of PLATAN 2019, 1st International Conference, Merger of the Poznan Meeting on Lasers and Trapping Devices in Atomic Nuclei Research and the International Conference on Laser Probing, Mainz, Germany 19-24 May 2019 Edited by Krassimira Marinova, Michael Block, Klaus D.A. Wendt and Magdalena Kowalska Anu Kankainen [email protected] 1Department of Physics, University of Jyv¨ askyl¨ a, P.O. Box 35 (YFL), FI-40014, Jyv¨ askyl¨ a, Finland 2Present address: Centre d’Etudes Nucl´ eaires de Bordeaux Gradignan, Universite de Bordeaux, CENBG-CS 10120, F-33175 Gradignan Cedex, France 3Present address: CERN, CH-1211, Geneva, Switzerland 43 Page 2 of 14 Hyperfine Interactions (2020) 241:43 40 50 60 70 80 90 100 30 35 40 45 50 55 60 65 AME16 exp. stable JYFLTRAP PLATAN 2019 Proton number Z Neutron number N Z=N Fig. 1 JYFLTRAP measurements presented in PLATAN 2019 (in red) together with previous studies (in blue) for Z=26 −67. Nuclei with experimental values in AME16 are also shown On-Line (IGISOL) facility [8,9] in the JYFL Accelerator Laboratory of the University of Jyv¨ askyl¨ a, Finland. The JYFLTRAP double Penning trap has been operational more than 15 years. During these years, around 400 atomic masses, including around 50 isomeric states, have been measured with JYFLTRAP. Most of the studied nuclei have been neutron-rich (around 2/3), produced by protonor deuteron-induced fission on nat U or Th targets. Around third of the measurements have been performed on the neutron-deficient side, where the ions of interest are produced via fusion-evaporation reactions. Figure 1highlights in red the recent JYFLTRAP mass measurements presented in the PLATAN 2019 conference. What is not shown in Fig. 1, is that many of the measured nuclides have long-living isomeric states which have also been measured with JYFLTRAP. These provide important information on level structure far from stability. The recent ground and isomericstate mass measurements at JYFLTRAP will be discussed in Section 3and its subsections. JYFLTRAP has also been widely used for selecting isotopes or even isomers of interest for decay spectroscopy, or as an ion counter for fission yield studies. Recent post-trap and fission-yield publications will be shortly reviewed in Section. 4. 2 JYFLTRAP double Penning trap at IGISOL JYFLTRAP [5–7] is a cylindrical double Penning trap mass spectrometer located inside a 7 T superconducting solenoid at the IGISOL facility [9]. The continuous radioactive ion beam from IGISOL is first mass-separated using a dipole magnet, which is usually sufficient to select the mass number of interest A. In addition to IGISOL, ion beams can be delivered from an offline ion source station [10], housing an electric discharge ion source and a surface ion source. There is also a space reservation for a laser ablation ion source. The continuous ion beam is cooled and bunched in a radiofrequency quadrupole (RFQ) [11], and the ion bunches are further injected into the first trap of JYFLTRAP, also known Hyperfine Interactions (2020) 241:43 Page 3 of 14 43 as the preparation trap. In the first trap, the ions of interest are selected using the buffer-gas cooling technique [12], and sent further to the second trap for precision mass measurements, or to post-trap decay spectroscopy station after the traps. Usually the Time-of-Flight Ion Cyclotron Resonance (TOF-ICR) technique [13,14] has been employed in the second trap, also known as the measurement trap, to determine the ion’s cyclotron resonance frequency νc=1 2π q mB,whereBis the magnetic field strength, and qand mare the charge and the mass of the ion, respectively. The magnetic field strength is calibrated by using reference ions with well-known mass values. A new technique to determine the ion’s cyclotron resonance frequency, the Phase-Imaging Ion Cyclotron Resonance (PI-ICR) technique [15], was commissioned at JYFLTRAP in 2017, see Ref. [7]. A position-sensitive MCP ion detector with a delay-line anode (DLD40) from RoentDek GmbH [16] was installed behind the traps, and the extraction optics was modified accordingly. Since then, the new technique has been increasingly utilized for high-precision mass measurements at JYFLTRAP. The method gives consistent results with the conventional TOF-ICR technique, as demonstrated with the Q-value measurement for the neutrinoless double electron-capture on 102Pd [17], but a higher precision can be achieved with the PI-ICR technique. Further examples of TOF-ICR and PI-ICR measurements are given in Section. 3. 3 Recent mass measurements with JYFLTRAP Since the last LASER meeting held in Poznan in May 2016, several publications have come out from JYFLTRAP experiments. The QEC value of the superallowed β+emitter 42Sc has been measured to be 6426.350(53) keV [18], the isobaric multiplet mass equation studied for the quintet at A=52 [19] and a general review on ion traps given in [3]. These were already discussed in Poznan. Here we present recent activities at JYFLTRAP with a focus on the PLATAN2019 contributions. 3.1 Nuclei in the vicinity of 78Ni Masses of nuclei close to 78Ni are important for several reasons. Firstly, the evolution of the Z=28 and N=50 shell closures and the magicity of 78Ni can be probed via mass measurements. Secondly, the masses of neutron-rich nuclei close to N=50, i.e. the strength of the N=50 shell closure, are relevant to understand in detail the core collapse phase of supernovae where electron captures on nuclei play a key role [20,21]. In a recent JYFLTRAP experiment, several neutron-rich Ni, Cu and Zn isotopes were measured to study these phenomena [22]. We also investigated nuclides below 68Ni (Z=28, N=40), namely 69,70Co and 67Fe. 68Ni has some doubly magic features, such as high excitation energy of the first 2+state and low transition strength B(E2;0g.s.→2+ 1)[23,24]. Previous mass measurements, however, have not shown a strong subshell closure at N=40 in the region [25–29]. The new JYFLTRAP measurements support the conclusion that the N=40 subshell closure is rather weak, and gets weaker below nickel [30,31]. The identification of the isomeric states in the cobalt isotopes is crucial for accurate empirical shell-gap energies and also for the investigation of a possible island of inversion below 68Ni [32,33]. At JYFLTRAP, only one state was observed for 70Co, whereas for 69Co the data indicate that there are two states present. The mass measurements of 69Co and 67Fe are also important for accurate neutron separation energies needed to calculate neutron-capture and their inverse photodisintegration 43 Page 4 of 14 Hyperfine Interactions (2020) 241:43 rates in the astrophysical rapid neutron capture process (rprocess) [34,35]. Sensitivity studies [36]haveshownthat67Fe(n, γ )68Fe and 68Co(n, γ )69Co have a strong impact on the calculated r-process abundances. With the new JYFLTRAP measurements, we could reduce the mass-related uncertainties in these impactful neutron-capture rates. Moreover, the photodisintegration rates are much higher than previously considered. 3.2 Neutron-rich silver isotopes Nuclei close to the doubly magic 132Sn are important both for nuclear structure and for the r-process. Mass measurements in this region provide information on the evolution of the Z=50 and N=82 shell closures, oneand two-neutron separation energies and pairing effects in the region. Recent studies have shown that the masses of the nuclei close to 132Sn have the highest impact on the calculated r-process abundances for different astrophysical scenarios [37]. Recently neutron-rich silver isotopes 113−124Ag were investigated at JYFLTRAP. Previously, 112,114−124Ag have been studied at the ISOLTRAP Penning trap at CERN [38] and 125,126Ag at the ESR storage ring at GSI [39]. Of these, the ESR measurements have uncertainties of 200-300 keV which are not adequate for the r-process modeling or detailed studies of nuclear structure. In addition, the 126Ag mass obtained at ESR is 730(370) keV lower than the extrapolated value in the Atomic Mass Evaluation 2016 [40]. The ISOLTRAP measurements using the TOF-ICR technique were hampered by the existence of low-lying isomeric states and difficulties to identify the measured state. In these measurements the state for 115Ag and 119Ag could not be assigned as the ground state or isomer, and 121−124Ag were assumed to be an admixture of the states which increased the uncertainty of the measurements. 120Ag was assigned as the ground state in [38], however, a decay spectroscopy study performed at Holifield Radioactive Ion Beam Facility (HRIBF) showed that 120Ag actually has three long-living (>10 ms) states [41]. Therefore, it is unclear which state was measured at ISOLTRAP, the ground state or the first isomeric state. The mass measurements of silver isotopes at JYFLTRAP were performed with the PIICR technique [7,15]. Phase accumulation times in the measurement trap had to be selected for each case individually depending on the excitation energies of the isomers to fully separate them. Stable 133Cs+ions were used for the calibration of the magnetic field. The measured states in the studied silver isotopes are shown in Table 1. The excitation energies of the 119m,120m,122n,123m,124mAg isomeric states were measured for the first time. The precision for several ground-state mass values was improved, since the isomeric states were separated in most of the cases. In 121Ag only one state was observed, while the low-lying isomeric state is additionally known in literature [42]. In 122Ag two states with different half-lives were observed, while three long-living states are known in literature [42]. The ground state and the first isomeric state with an unknown excitation energy in 122Ag have similar half-lives and could not be distinguished. The excitation energies of the isomeric states in 113−118Ag are known with a good accuracy (sub-keV) from spectroscopic measurements [42] and can be used to cross-check our mass measurements. Figure 2shows the projection of ion cyclotron motion in the measurement trap onto the position-sensitive MCP detector for one of the PI-ICR measurements of 120Ag. 3.3 Neutron-rich rare-earth isotopes Neutron-rich rare-earth isotopes have been studied in two recent mass measurement campaigns at JYFLTRAP [43,44]. Altogether 22 nuclides have been measured, of which 14 Hyperfine Interactions (2020) 241:43 Page 5 of 14 43 Table 1 States in silver isotopes studied at JYFLTRAP and their properties State JπT1/2E∗,keV 113gAg 1/2−5.37(5) h 113mAg 7/2+68.7(16) s 43.5(1) 114gAg 1+4.6(1) s 115gAg 1/2−20.0(5) m 115mAg 7/2+18.0(7) s 41.16(10) 116gAg (0−) 3.83(8) m 116mAg (3+) 20(1) s 47.90(10) 116nAg (6−) 9.3(3) s 129.8(22) 117gAg 1/2−# 73.6(14) s 117mAg 7/2+# 5.34(5) s 28.6(2) 118gAg 1−3.76(15) s 118nAg 4(+) 2.0(2) s 127.63(10) 119gAg 1/2−#6.0(5)s 119nAg 7/2+# 2.1(1) s 20#(20#) 120gAg 4(+) 1.52(7) s 120mAg (0−,1 −) 940(100) ms 0#(50#) 120nAg 7(−) 384(22) ms 203.0(2) 121xAg g: 7/2+#; m: 1/2−# g: 780(20) ms; m: 200# ms m: 20#(20#) 122xAg g: (3+); m: (1−) g: 529(13) ms; m: 550(50) ms m: 80#(50#) 122nAg 9(−) 200(50) ms 80#(50#) 123gAg 7/2+# 300(5) ms 123mAg 1/2−# 100# ms 20#(20#) 124gAg (2−) 177.9(26) ms 124mAg (8−) 144(20) ms 0#(100#) Jπ,T1/2and E∗are the spin with the parity, the half-life and the excitation energy of the isomeric state, correspondingly, taken from NUBASE2016 [42]. The values estimated from systematic trends in neighboring nuclides are marked by #. The ground states are indicated with g, the first isomeric state and the second isomeric state are indicated with mand n, correspondingly. The states, where the ground state and the isomeric state could not be distinguished, are referred with x for the first time. These include the first measurements of 158Nd, 160,161Pm, 162,163Sm, 164,165Eu, 164−167Gd, and 165,167,168Tb. The new mass values agree with the extrapolations of AME16 [40] in most of the cases. Typically, the JYFLTRAP values were somewhat higher than predicted by the extrapolations [44]. The largest deviations between the new JYFLTRAP mass values and AME16 were found for 154Nd, 220(60) keV, and 156Nd, 260(200) keV. Both mass values have been previously based on beta-decay end-point energies [45,46] which tend to underestimate the Q values, and thus the masses. Indeed, the JYFLTRAP mass value for 154Nd agrees with the recent mass value from the Canadian Penning Trap (CPT) [47]. In the latter experimental campaign, the masses of 162Eu and 163Gd were remeasured using the TOF-ICR technique with a 1600 ms quadrupolar excitation time in the second trap 43 Page 6 of 14 Hyperfine Interactions (2020) 241:43 Fig. 2 Projection of the cyclotron motion of 120Ag+ions onto the position-sensitive detector in PI-ICR method. The three detected ion spots on the detector correspond to the three long-living states in 120Ag to resolve the ground and isomeric states from each other. In addition, the PI-ICR technique was applied for a detailed study of the ground and isomeric states in 162Eu, see Ref. [44]. The rare-earth masses are important for understanding the formation of the rare-earth abundance peak at A=165 in the rprocess. It has been proposed to form via fission cycling [48] or during the freezeout when matter is decaying toward the stability. In the latter scenario, a kink in the neutron separation energies could funnel the flow toward the midshell [49,50]. Spectroscopic data indicate that there is an onset of deformation at N=88−90: the 2+energies drop dramatically and the E(4+)/E(2+)ratios increase suddenly. Moreover, it has been suggested that there would be a subshell closure at N=100 based on the observed small kink in the 2+energies at N=100. In the JYFLTRAP campaign we wanted to investigate whether there is a kink in oneor two-neutron separation energies that could funnel the r-process flow or would support a subshell closure at N=100. The new JYFLTRAP data do not introduce significant changes in the trends of twoneutron separation energies (see Fig. 3). No significant kinks supporting the proposed subshell closure or a change in the nuclear structure, are observed for the studied isotopic chains. Interestingly, neutron pairing energies were found to be lower than predicted by the commonly used theoretical models when approaching the midshell at N=104 [43,44]. The impact of the JYFLTRAP mass values on the calculated r-process abundances was studied for a representative dynamical ejecta trajectory for a 1.35 solar-mass neutron-star merger from Ref. [51], with a very low initial electron fraction Ye=0.016 and low entropy per baryon s/kB=8. A simple asymmetric split [52] was assumed for fission fragment distributions to ensure that the rare-earth peak forms entirely via the dynamical formation mechanism of Refs. [49,50]. Compared to a baseline study employing experimental mass values from AME16 [40] and theoretical mass values from the Finite-Range Droplet Model Hyperfine Interactions (2020) 241:43 Page 7 of 14 43 95 100 105 10 11 12 13 14 15 16 Neutron number N S2n (MeV) Ce (Z=58) Pr (Z=59) Nd (Z=60) Pm (Z=61) Sm (Z=62) Eu (Z=63) Gd (Z=64) Tb (Z=65) Dy (Z=66) Ho (Z=67) Fig. 3 Two-neutron separation energies for the studied isotopic chains in the rare-earth region. The black lines are based on AME16 values [40], and the values affected by the new JYFLTRAP measurements are highlighted in red 2012 (FRDM2012) [53], the new JYFLTRAP values produce a smoother abundance pattern and a better agreement with the observed r-process abundances is achieved [43,44]. 3.4 Q -value measurements of rare weak decays Decays through weak interaction offer a possibility to determine the mass of a neutrino. In ordinary βdecays, the mass of a neutrino manifests itself as a distorted shape of the emitted beta spectrum close to the endpoint energy (i.e., Q-value). Such an experiment is KATRIN that uses tritium, whose βendpoint energy is 18.6 keV [54]. To detect a distortion in the beta spectrum near the end-point energy, a Q-value as small as possible is desirable. This has prompted a survey to map rare weak decays [55,56]thatareβ+,β−or EC decays of parent ground states to excited states in the daughter nucleus with a small Q-value (i.e., <1keV). The shape of the beta spectrum near the endpoint is expected to be relatively simple. These decays are also suitable for theoretical modeling of atomic effects in nuclear decay. Several candidates for rare weak decays have been identified, all with well-known excitation energies in the daughter nucleus. However, the uncertainties in the ground-stateto-ground-state Q-values remain too high to extract a precise Q-value for a decay to an excited state. A direct parent-daughter mass ratio measurement with a relative precision of around 10−9would allow extraction of Q-values at the uncertainty level of a few hundred eV. It is imperative to know whether the Q-value is positive to find out the decays that could be feasible for determining the mass of a neutrino. At JYFLTRAP, Q-values of four potential rare weak β-decay candidates were measured recently, summarized in Table 2. As the nuclei of interest are close to stability, they were produced relatively easily. Out of the studied nuclei, 135Cs was produced using proton-induced fission of natural uranium while the others were produced using protonor deuteron-induced fusion reactions. The analysis of the collected data is ongoing. 43 Page 8 of 14 Hyperfine Interactions (2020) 241:43 Table 2 Cases, whose ground-state-to-ground-state Q-values were measured at JYFLTRAP Parent (Jπ) Daughter (Jπ)E∗,keV Q,keV 111In (9/2+)111Cd (7/2+) 853.94 ±0.07 6.36 ±3.0 111Cd (3/2+) 855.6 ±1.0 4.6 ±3.2 111Cd (3/2+) 864.8 ±0.3 −4.5 ±3.0 131I(7/2 +)131Xe (9/2+) 971.22 ±0.13 −0.4 ±0.6 155Eu (5/2+)155Gd (9/2−) 251.7056 ±0.0010 0.1 ±0.9 135Cs (7/2+)135Ba (11/2−) 268.218 ±0.020 0.7 ±1.0 The first column shows the decaying parent state, the second column the excited state in the daughter nucleus, third column the excitation energy of the excited state in the daughter and the fourth column the Q-value to the excited state in the daughter. The tabulated spins and parities (Jπ) of the states, as well as the excitation energies and Q-values are based on Refs. [42,57] 3.5 Nuclei close to the N = Z line in the A =80 −90 mass region Heavier neutron-deficient nuclei close to the N=Zline have been recently studied using an upgraded version of the heavy-ion ion-guide, HIGISOL [58], at IGISOL. The first online experiment with the upgraded system employed 222-MeV 36Ar8+ions impinging into anat Ni target [59]. High-precision mass measurements were performed with JYFLTRAP for 82Zr, 84Nb, 86Mo, 88Tc, 88Tcmand 89Ru [59,60]. Two of the masses, 88Tcmand 89Ru, were measured for the first time and the precisions of 82Zr, 84Nb and 88Tc were improved significantly. Additionally, the mass of 86Mo given in AME16 [40] was verified. The effect of the new data on the mass surface was studied, and similar behaviour as reported in the literature [40] was observed. 88Tc was studied at JYFLTRAP already in 2008 [61] but at the time the isomeric state 88Tcmcould not be resolved from the more abundant ground state. With the PI-ICR technique, the mass of the isomer could be determined for the first time. The isomer was measured against the ground state using the PI-ICR technique, yielding an excitation energy of Ex=70.4(31)keV [59]. The mass of the dominantly produced ground state was measured using the TOF-ICR technique, which together with the excitation energy yielded also a mass value for the isomer. The order of the three lowest states in 88Tc was studied based on the obtained excitation energy for the isomeric state, Weisskopf estimates for the three states, and available spectroscopic data from literature. The most likely energies and spin-parities of the first three states in 88Tc are (Ex,Jπ)=(0keV,2+),(70.4 keV,6+)and (95 keV,4+). Shell-model calculations were also performed for comparison but those were highly sensitive to the used model space and interaction model (for details, see Ref. [59]). In addition to the six atomic masses that were directly measured, the masses of 82Mo and 86Ru (Tz=−1) were determined using theoretical mirror displacement energies (MDEs) and the directly measured masses of their respective mirror partners 82Zr and 86Mo (Tz= +1) [59]. The resulting mass-excess values predict more tightly bound nuclei than literature [40], by more than 500 keV, and reduce the uncertainties of the predicted masses. The JYFLTRAP mass measurements of 82Zr and 84Nb showed that mass values measured at the CSRe storage ring [62] deviate from the corresponding Penning-trap measurements, CSRe results being typically around 20 keV smaller. In order to investigate whether this is a more general feature, published results from the CSRe storage ring were gathered and compared to available Penning-trap results. A total of 17 nuclides were available for