Z. Ge

A Multi-Reflection Time-of-Flight Mass Spectrometer (MR-ToF-MS) has been commissioned at the Ion-Guide Isotope Separator On-Line (IGISOL) facility. It consists of six electrode pairs that form a nearly energy-isochronous potential and a pulsed drift-tube to trap the ions between the electrodes. Time-of-flight peak widths down to 22 ns full-width at half-maximum and mass-resolving powers of $\approx 1.5 \times 10^5$ within 20 ms have been demonstrated. The obtained time-focus and mass-resolving power depend sensitively on the trapping energy, energy spread and the number of revolutions of the ions. The mass-resolving power is affected by the temporal and energy spread of the ions entering the MR-ToF-MS, and fluctuations in the electrode voltages due to temperature variations. The longitudinal emittance corresponding to the temporal and energy spread of $^{39}$K is estimated to be 175 eVns based on the data, close to the expected 186(10) eVns. The time-of-flight temperature sensitivity is found to be -5.55(30) ppm/K. In addition to atomic mass measurements of short-lived exotic nuclides, the MR-ToF-MS can be used as a fast mass separator for various other experiments at IGISOL and as an ion counter for laser spectroscopy and yield measurements.

The MORA experimental setup is designed to measure the triple-correlation D parameter in nuclear beta decay. The D coefficient is sensitive to possible violations of time-reversal invariance. The experimental configuration consists of a transparent Paul trap surrounded by a detection setup with alternating beta and recoil-ion detectors. The octagonal symmetry of the detection setup optimizes the sensitivity of positron-recoil-ion coincidence rates to the D correlation, while reducing systematic effects. MORA utilizes an innovative in-trap laser polarization technique. The design and performance of the ion trap, associated beamline elements, lasers and beta and recoil-ion detectors, are presented. Recent progress towards the polarization proof-of-principle is described.

The mass of the electron-antineutrino can be determined in dedicated measurements of the $β$ spectral shape near the $β$ endpoint of a $β^-$ transition, with a low $Q$ value enhancing the sensitivity of the measurement. One such low-$Q$-value candidate is the transition between the $6^+$ isomer of $^{110}$Ag and the $5^+_2$ state in $^{110}$Cd ($Q^{\ast}_{β,m}=-0.12(131)$ keV). To reduce the uncertainty of the $Q$ value, we have used the phase-imaging ion-cyclotron-resonance technique with the JYFLTRAP double Penning trap and performed a high-precision atomic-mass measurement of $^{109}$Ag with $^{110}$Cd as a reference. Combined with the known spectroscopic data, we obtain a re-evaluated value $Q^{\ast}_{β,m}=405(135)$ eV, for the $^{110}\text{Ag}(6^+_\text{m}) \rightarrow {^{110}\text{Cd}}(5^+_2)$ transition. This represents the lowest $Q_β$ value for any allowed transition observed to date. In order to estimate the partial half-life ($t_{1/2}$) and branching ratio (Br) of the transition, nuclear shell-model calculations were performed using the $jj45pnb$ Hamiltonian in combination with state-of-the-art atomic calculations. The computed values $t_{1/2} = 2.23^{+5.24}_{-1.28} \times 10^7$ years and $\textrm{Br} = 3.07^{+4.16}_{-2.15} \times 10^{-8}$, along with the thermal-neutron capture on stable $^{109}$Ag as a viable production method, make $^{110}\textrm{Ag}^m$ a promising candidate for future antineutrino-mass measurements.

The masses of the ground and isomeric states in $^{124,125}$Ag have been measured using the phase-imaging ion-cyclotron-resonance technique at the JYFLTRAP double Penning trap mass spectrometer. The ground states of $^{124}$Ag and $^{125}$Ag were found to be 30(250) keV and 250(430) keV less bound but 36 and 110 times more precise than in the Atomic Mass Evaluation 2020, respectively. The excitation energy of $^{124}$Ag$^{m}$, ${E_x = 188.2(25)}$ keV, was determined for the first time. The new precise mass values have been utilised to study the evolution of nuclear structure via two-neutron separation energies. The impact on the astrophysical rapid neutron capture process has been investigated via neutron-capture reaction rate calculations. The precision measurements indicate a more linear trend in two-neutron separation energies and reduce the mass-related uncertainties for the neutron-capture rate of $^{124}$Ag$(n,γ)^{125}$Ag by a factor of around 100. The new mass values also improve the mass of $^{123}$Pd, previously measured using $^{124}$Ag as a reference.

We report on high-precision atomic mass measurements of $^{148\text{-}153}$La and $^{151}$Ce performed with the JYFLTRAP double Penning trap using the Phase-Imaging Ion-Cyclotron-Resonance technique. The masses of $^{152,153}$La were experimentally determined for the first time. We confirm the sharp kink in the two-neutron separation energies at the neutron number ${N=93}$ in the cerium (${Z=58}$) isotopic chain. Our precision mass measurements of the most exotic neutron-rich lanthanum (${Z=57}$) isotopes reveal a sudden increase in two-neutron separation energies from ${N=92}$ to ${N=93}$. Unlike in the cerium isotopic chain, the kink is not sharp but extends to ${N=94}$ forming a prominent bump. The gain in energy is about 0.4 MeV, making it one of the strongest changes in two-neutron separation energies over the whole chart of nuclides, away from nuclear shell closures. The results call for further studies to elucidate the structure of neutron-rich lanthanum isotopes.

We have performed high-precision mass measurements of neutron-rich rare-earth Tb, Dy and Ho isotopes using the Phase-Imaging Ion-Cyclotron-Resonance technique at the JYFLTRAP double Penning trap. We report on the first experimentally determined mass values for $^{169}$Tb, $^{170}$Dy and $^{171}$Dy, as well as the first high-precision mass measurements of $^{169}$Dy and $^{169\text{-}171}$Ho. For $^{170}$Ho, the two long-lived ground and isomeric states were resolved and their mass measured, yielding an isomer excitation energy of $E_\text{exc}=150.8(54)$~keV. In addition, we have performed independent crosschecks of previous Penning-trap values obtained for $^{167\text{,} 168}$Tb and $^{167\text{,} 168}$Dy. We have extended the systematics of two-neutron separation energies to the neutron midshell at $N=104$ in all of the studied isotopic chains. Our updated and new mass measurements provide better mass-related constraints for the neutron-capture reaction rates relevant to the astrophysical rapid neutron capture (r) process. The r-process abundances calculated with the new mass values seem to produce a steeper minimum at A=170 and differ by around 15-30\% from the abundances computed with the Atomic Mass Evaluation 2020 values.

The masses of $^{84}$Br, $^{105}$Mo, $^{115,119,121}$Pd, $^{122}$Ag, $^{127,129}$In, $^{132}$Sb and their respective isomeric states have been measured with the JYFLTRAP Penning trap mass spectrometer using the phase-imaging ion-cyclotron-resonance technique. The excitation energies of the isomeric states in $^{132}$Sb and $^{119}$Pd were experimentally determined for the first time, while for $^{84}$Br, $^{115}$Pd and $^{127,129}$In, the precision of the mass values was substantially improved. In $^{105}$Mo and $^{121}$Pd there were no signs of a long-lived isomeric state. The ground-state measurements of $^{119}$Pd and $^{122}$Ag indicated that both are significantly more bound than the literature values. For $^{122}$Ag, there was no indication of a proposed third long-lived state. The results for the $N=49$ nucleus $^{84}$Br and isomers close to doubly magic $^{132}$Sn have been compared to the shell-model and the microscopic quasiparticle-phonon model calculations.

Precision mass measurements of $^{104}$Y, $^{106}$Zr, $^{104,104m,109}$Nb, and $^{111,112}$Mo have been performed with the JYFLTRAP double Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line facility. The order of the long-lived states in $^{104}$Nb was unambiguously established. The trend in two-neutron separation energies around the $N=66$ neutron midshell appeared to be steeper with respect to the Atomic Mass Evaluation 2020 extrapolations for the $_{39}$Y and $_{40}$Zr isotopic chains and less steep for the $_{41}$Nb chain, indicating a possible gap opening around $Z=40$. The experimental results were compared to the BSkG2 model calculations performed with and without vibrational and rotational corrections. All of them predict two low-lying minima for $^{106}$Zr. While the unaltered BSkG2 model fails to predict the trend in two-neutron separation energies, selecting the more deformed minima in calculations and removing the vibrational correction, the calculations are more in line with experimental data. The same is also true for the $2^+_1$ excitation energies and differences in charge radii in the Zr isotopes. The results stress the importance of improved treatment of collective corrections in large-scale models and further development of beyond-mean-field techniques.

We report on the first direct mass measurements of the $^{118,119}$Cd and $^{117-119}$In isotopes performed at the Ion Guide Isotope Separator On-Line facility using the JYFLTRAP double Penning trap mass spectrometer. The masses of $^{117}$In and $^{118}$Cd isotopes are in agreement with the literature, while $^{118,119}$In and $^{119}$Cd differ from literature by 49, 13 and 85 keV (6.1, 1.9 and 2.1 standard deviations), respectively. The excitation energy of the $^{118}$In first isomeric state, $E_x = 40.3(25)$ keV, was determined for the first time. The updated mass values removed the fluctuations observed in the two-neutron separation energies and lead to a smoother linear decrease of both isotopic chains. The $\log(ft)$ value for the $^{118}$Cd decay is also found to increase from 3.93(6) to 4.089(8). The reported results indicate an absence of significant structural changes around $N=70$.

We report on the precise mass measurement of the $^{173}$Hf isotope performed at the Ion Guide Isotope Separator On-Line facility using the JYFLTRAP double Penning trap mass spectrometer. The new mass-excess value, ${\mathrm{ME} = -55390.8(30)}$ keV, is in agreement with the literature while being nine times more precise. The newly determined $^{173}$Hf electron-capture $Q$ value, $Q_{EC} = 1490.2(34)$ keV, allows us to firmly reject the population of an excited state at 1578 keV in $^{173}$Lu and 11 transitions tentatively assigned to the decay of $^{173}$Hf. Our refined mass value of $^{173}$Hf reduces mass-related uncertainties in the reaction rate of $^{174}$Hf$(γ,n)^{173}$Hf. Thus, the rate for the main photodisintegration destruction channel of the $p$ nuclide $^{174}$Hf in the relevant temperature region for the $γ$ process is better constrained.

The $β^-$ decay $Q$-value of $^{136}$Cs ($J^π= 5^+$, $t_{1/2} \approx 13$~days) was measured with the JYFLTRAP Penning trap setup at the Ion Guide Isotope Separator On-Line (IGISOL) facility of the University of Jyväskylä, Finland. The mono-isotopic samples required in the measurements were prepared with a new scheme utilised for the cleaning, based on the coupling of dipolar excitation with Ramsey's method of time-separated oscillatory fields and the phase-imaging ion-cyclotron-resonance (PI-ICR) technique. The $Q$ value is determined to be 2536.83(45) keV, which is $\sim$4 times more precise and 11.4(20) keV ($\sim$ 6$σ$) smaller than the adopted value in the most recent Atomic Mass Evaluation AME2020. The daughter, $^{136}$Ba, has a 4$^+$ state at 2544.481(24) keV and a $3^-$ state at 2532.653(23) keV, both of which can potentially be ultralow $Q$-value end-states for the $^{136}$Cs decay. With our new ground-to-ground state $Q$ value, the decay energies to these two states become -7.65(45) keV and 4.18(45) keV, respectively. The former is confirmed to be negative at the level of $\sim$ 17$σ$, which verifies that this transition is not a suitable candidate for neutrino mass determination. On the other hand, the slightly negative $Q$ value makes this transition an interesting candidate for the study of virtual $β$-$γ$ transitions. The decay to the 3$^{-}$ state is validated to have a positive low $Q$ value which makes it a viable candidate for neutrino mass determination. For this transition, we obtained a shell-model-based half-life estimate of $2.1_{-0.8}^{+1.6}\times10^{12}$ yr.

Phase-imaging ion-cyclotron-resonance technique has been implemented at the Penning-trap mass spectrometer JYFLTRAP and is routinely employed for mass measurements of stable and short-lived nuclides produced at IGISOL facility. Systematic uncertainties that impose limitations on the accuracy of measurements are discussed. It was found out that the phase evolution of the radial motion of ions in a Penning trap during the application of radio-frequency fields leads to a systematic cyclotron frequency shift when more than one ion species is present in the trap during the cyclotron frequency measurement. An analytic expression was derived to correctly account for the shift. Cross-reference mass measurements with carbon-cluster ions have been performed providing the mass-dependent and residual uncertainties.

The absolute mass value of $^{168}$Yb has been directly determined with the JYFLTRAP Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line (IGISOL) facility. A more precise value of the mass of $^{168}$Yb is needed to extract possible signatures of beyond standard model physics from high-precision isotope shift measurements of Yb atomic transition frequencies. The measured mass-excess value, ME($^{168}$Yb) = $-$61579.846(94) keV, is 12 times more precise and deviates from the Atomic Mass Evaluation 2016 value by 1.7$σ$. The impact on precision isotope shift studies of the stable Yb isotopes is discussed.