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Register a new userLifetimes and moments measurements to investigate the structure of midheavy nuclei
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Mid-heavy nuclei offer unique opportunities to study the collective and single-particle aspects of nuclear structure. This mass regime is a dynamic area where protons and neutrons generally occupy different orbitals, giving rise to complex structures with a wide variety of shapes, shape evolution and shape coexistence. To that end, measurements of nuclear lifetimes and electromagnetic moments ($mu$,$Q$) can be invaluable. Recent experimental activities of the NuSTRAP group in Athens have focused on $gamma$-spectroscopy studies employing the RoSPHERE array in Magurele, Romania. In recent studies [1,2], the neutron-rich $^{144-146}$Ba isotopes have exhibited octupole degrees of freedom. Similar questions exist for the lighter $^{140}$Ba isotope, which is located at the onset of octupole collectivity. In addition, understanding the structure of heavier, even-even hafnium isotopes requires more data regarding shape coexistence and shape evolution. Preliminary results on lifetimes in this area pave the way to understand dynamical phenomena prior to studying more exotic species in the future.
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We present mass excesses (ME) of neutron-rich isotopes of Ar through Fe, obtained via TOF-$Brho$ mass spectrometry at the National Superconducting Cyclotron Laboratory. Our new results have significantly reduced systematic uncertainties relative to a prior analysis, enabling the first determination of ME for $^{58,59}{rm Ti}$, $^{62}{rm V}$, $^{65}{rm Cr}$, $^{67,68}{rm Mn}$, and $^{69,70}{rm Fe}$. Our results show the $N=34$ subshell weaken at Sc and vanish at Ti, along with the absence of an $N=40$ subshell at Mn. This leads to a cooler accreted neutron star crust, highlighting the connection between the structure of nuclei and neutron stars.
Inelastic ${}^{6}$Li scattering at 100 MeV/u on ${}^{12}$C and ${}^{93}$Nb have been measured with the high-resolution magnetic spectrometer Grand Raiden. The magnetic-rigidity settings of the spectrometer covered excitation energies from 10 to 40 MeV and scattering angles in the range $0^circ < theta_{text{lab.}}< 2^circ$. The isoscalar giant monopole resonance was selectively excited in the present data. Measurements free of instrumental background and the very favorable resonance-to-continuum ratio of ${}^{6}$Li scattering allowed for precise determination of the $E0$ strengths in ${}^{12}$C and ${}^{93}$Nb. It was found that the monopole strength in ${}^{12}$C exhausts $52 pm 3^text{(stat.)} pm 8 ^text{(sys.)}$% of the energy-weighted sum rule (EWSR), which is considerably higher than results from previous $alpha$-scattering experiments. The monopole strength in ${}^{93}$Nb exhausts $92 pm 4^text{(stat.)} pm 10 ^text{(sys.)}$% of the EWSR, and it is consistent with measurements of nuclei with mass number of $Aapprox90$. Such comparison indicates that the isoscalar giant monopole resonance distributions in these nuclei are very similar, and no influence due to nuclear structure was observed.
To test the predictive power of ab initio nuclear structure theory, the lifetime of the second 2+ state in neutron-rich 20O, tau(2+_2 ) = 150(+80-30) fs, and an estimate for the lifetime of the second 2+ state in 16C have been obtained, for the first time. The results were achieved via a novel Monte Carlo technique that allowed us to measure nuclear state lifetimes in the tens-to-hundreds femtoseconds range, by analyzing the Doppler-shifted gamma-transition line shapes of products of low-energy transfer and deep-inelastic processes in the reaction 18O (7.0 MeV/u) + 181Ta. The requested sensitivity could only be reached owing to the excellent performances of the AGATA gamma-tracking array, coupled to the PARIS scintillator array and to the VAMOS++ magnetic spectrometer. The experimental lifetimes agree with predictions of ab initio calculations using two- and three-nucleon interactions, obtained with the valence-space in-medium similarity renormalization group for 20O, and with the no-core shell model for 16C. The present measurement shows the power of electromagnetic observables, determined with high-precision gamma spectroscopy, to assess the quality of first-principles nuclear structure calculations, complementing common benchmarks based on nuclear energies. The proposed experimental approach will be essential for short lifetimes measurements in unexplored regions of the nuclear chart, including r-process nuclei, when intense ISOL-type beams become available.
item[Background] Ground-state spins and magnetic moments are sensitive to the nuclear wave function, thus they are powerful probes to study the nuclear structure of isotopes far from stability. item[Purpose] Extend our knowledge about the evolution of the $1/2^+$ and $3/2^+$ states for K isotopes beyond the $N = 28$ shell gap. item[Method] High-resolution collinear laser spectroscopy on bunched atomic beams. item[Results] From measured hyperfine structure spectra of K isotopes, nuclear spins and magnetic moments of the ground states were obtained for isotopes from $N = 19$ up to $N = 32$. In order to draw conclusions about the composition of the wave functions and the occupation of the levels, the experimental data were compared to shell-model calculations using SDPF-NR and SDPF-U effective interactions. In addition, a detailed discussion about the evolution of the gap between proton $1d_{3/2}$ and $2s_{1/2}$ in the shell model and {it{ab initio}} framework is also presented. item[Conclusions] The dominant component of the wave function for the odd-$A$ isotopes up to $^{45}$K is a $pi 1d_{3/2}^{-1}$ hole. For $^{47,49}$K, the main component originates from a $pi 2s_{1/2}^{-1}$ hole configuration and it inverts back to the $pi 1d_{3/2}^{-1}$ in $^{51}$K. For all even-$A$ isotopes, the dominant configuration arises from a $pi 1d_{3/2}^{-1}$ hole coupled to a neutron in the $ u 1f_{7/2}$ or $ u 2p_{3/2}$ orbitals. Only for $^{48}$K, a significant amount of mixing with $pi 2s_{1/2}^{-1} otimes u (pf)$ is observed leading to a $I^{pi}=1^{-}$ ground state. For $^{50}$K, the ground-state spin-parity is $0^-$ with leading configuration $pi 1d_{3/2}^{-1} otimes u 2p_{3/2}^{-1}$.
It is proposed here to investigate three major properties of the nuclear force that influence the amplitude of shell gaps, the nuclear binding energies as well as the nuclear $beta$-decay properties far from stability, that are all key ingredients for modeling the r-process nucleosynthesis. These properties are derived from experiments performed in different facilities worldwide, using several various state-of-the-art experimental techniques including transfer and knockout reactions. Expected consequences on the r process nucleosynthesis as well as on the stability of super heavy elements are discussed.
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