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Register a new userHalf Life of the Doubly-magic r-Process Nucleus 78Ni
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Nuclei with magic numbers serve as important benchmarks in nuclear theory. In addition, neutron-rich nuclei play an important role in the astrophysical rapid neutron-capture process (r-process). 78Ni is the only doubly-magic nucleus that is also an important waiting point in the r-process, and serves as a major bottleneck in the synthesis of heavier elements. The half-life of 78Ni has been experimentally deduced for the first time at the Coupled Cyclotron Facility of the National Superconducting Cyclotron Laboratory at Michigan State University, and was found to be 110 (+100 -60) ms. In the same experiment, a first half-life was deduced for 77Ni of 128 (+27 -33) ms, and more precise half-lives were deduced for 75Ni and 76Ni of 344 (+20 -24) ms and 238 (+15 -18) ms respectively.
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Low-energy excited states in 71,73Ni populated via the {beta} decay of 71,73Co were investigated in an experiment performed at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). Detailed analysis led to the construction of level schemes of 71,73Ni, which are interpreted using systematics and analyzed using shell model calculations. The 5/2- states attributed to the the f5/2 orbital and positive parity 5/2+ and 7/2+ states from the g9/2 orbital have been identified in both 71,73Ni. In 71Ni the location of a 1/2- {beta}-decaying isomer is proposed and limits are suggested as to the location of the isomer in 73Ni. The location of positive parity cluster states are also identified in 71,73Ni. Beta-delayed neutron branching ratios obtained from this data are given for both 71,73Co.
Many properties of the atomic nucleus, such as vibrations, rotations and incompressibility, can be interpreted as due to a two component quantum liquid of protons and neutrons. Electron scattering measurements on stable nuclei demonstrate that their central densities are saturated, as for liquid drops. In exotic nuclei near the limits of mass and charge, with large imbalances in their proton and neutron numbers, the possibility of a depleted central density, or a bubble structure, has been discussed in a recurrent manner since the 1970s. Here we report first experimental evidence that points to a depletion of the central density of protons in the short-lived nucleus 34Si. The proton-to-neutron density asymmetry in 34Si offers the possibility to place constraints on the density and isospin dependence of the spin--orbit force-on which nuclear models have disagreed for decades-and on its stabilizing effect towards limits of nuclear existence.
High resolution experiments have recently lead to a complete identification (energy, spin, and parity) of 151 nuclear levels up to an excitation Energy of Ex= 6.20 MeV in 208Pb. We present a thorough study of the fluctuation properties in the energy spectra of the unprecedented set of nuclear bound states. In a first approach we grouped states with the same spin and parity into 14 subspectra, analyzed standard statistical measures for short- and long-range correlations and then computed their ensemble average. Their comparison with a random matrix ensemble which interpolates between Poisson statistics expected for regular systems and the Gaussian Orthogonal Ensemble (GOE) predicted for chaotic systems shows that the data are well described by the GOE. In a second approach, following an idea of Rosenzweig and Porter we considered the complete spectrum composed of the independent subspectra. We analyzed their fluctuation properties using the method of Bayesian inference involving a quantitative measure, called the chaoticity parameter f, which also interpolates between Poisson (f=0) and GOE statistics (f=1). It turns out to be f~0.9. This is so far the closest agreement with GOE observed in spectra of bound states in a nucleus. The same analysis has also been performed with spectra computed on the basis of shell model calculations with different interactions (SDI, KB, M3Y). While the simple SDI exhibits features typical for nuclear many-body systems with regular dynamics, the other, more realistic interactions yield chaoticity parameters f close to the experimental values.
A half-life of 2.2 $pm$ 0.2 s has been deduced for the ground-state $beta$ decay of $^{84}$Mo, more than 1$sigma$ shorter than the previously adopted value. $^{84}$Mo is an even-even N = Z nucleus lying on the proton dripline, created during explosive hydrogen burning in Type I X-ray bursts in the rapid proton capture ($rp$) process. The effect of the measured half-life on $rp$-process reaction flow is explored. Implications on theoretical treatments of nuclear deformation in $^{84}$Mo are also discussed.
Half-life values are widely used in nuclear chemistry to model the exponential decay of the quantified radionuclides. The analysis of existing data reveals a general lack of information on the performed experiments and an almost complete absence of uncertainty budgets. This is the situation for 31Si, the radionuclide produced via neutron capture reaction recently used to quantify trace amounts of 30Si in a sample of the silicon material highly enriched in 28Si and used for the determination of the Avogadro constant. In order to improve the quality of the now recommended 157.36(26) min value, we carried out repeated observations of the 31Si decay rate via gamma-ray spectrometry measurements. This paper reports the result we obtained, including details of the experiment and the evaluation of the uncertainty.
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