β-Decay Half-Lives of Very Neutron-Rich Kr to Tc Isotopes on the Boundary of ther-Process Path: An Indication of Fastr-Matter Flow

Abstract

The β-decay half-lives of 38 neutron-rich isotopes from 36 Kr to 43 Mo and 116,117 Tc are reported here for the first time. These results when compared to previous standard models indicate an overestimation in the predicted half-lives by a factor of two or more in the A ≈ 110 region. A revised model based on the second generation gross theory of β decay better predicts the measured half-lives and suggests a more rapid flow of the rapid neutron-capture process (r-matter flow) through this region than previously predicted.About half of the elements heavier than Fe are thought to be produced in rapid neutron-capture process (rprocess) nucleosynthesis, a sequence of neutron-capture and β-decay processes. Although the astronomical site and the mechanism of the r-process are not yet fully understood, it is generally agreed that the process must occur in environments with extreme neutron densities. The study of the elemental distribution along the r-process path requires sensitive β-decay related information such as β-decay half-lives, β-delayed neutron-emission probabilities, and nuclear masses. In particular, determination of the timescale that governs matter flow from the r-process "seeds" to the heavy nuclei, as well as the distribution in the r-process peaks, depends sensitively on decay half-lives [1,2].Isotopes with extreme neutron-to-proton ratios in the mass region A = 110 − 125 have attracted special attention since theoretical r-process yields are found to underestimate isotopic abundances observed in the predicted global abundances by an order of magnitude or more [1,3,4]. This discrepancy has been investigated using numerous mass formulae that differ mainly in the strength of the nuclear shell closures [5,6]. The results indicate that considerable improvements in the global abundances of the isotopes can be achieved by assuming a quenching of the N = 82 shell gap. The properties of most of these crucial r-process nuclei are, however, currently unknown due to their extremely low production yields in the laboratory.A number of experimental studies on nuclei around neutron-rich krypton to technetium have been performed to investigate the region of the r-process path near N = 82 [7][8][9]. In the current work, we report on a first systematic study of the β-decay properties of very exotic, neutron-rich 36 Kr to 43 Tc nuclides that contribute to the r-process.Decay spectroscopy of very neutron-rich nuclei around A = 110 was performed at the recently-commissioned RIBF facility at RIKEN. A secondary beam, comprised of a cocktail of neutron-rich nuclei, was produced by inflight fission of a 345-MeV/nucleon 238 U beam in a 550-mg/cm 2 Be target. The primary beam was produced by the RIKEN cyclotron accelerator complex with a typical intensity ∼ 0.3 pnA at the production target posi-

Improvement to the gross theory of β decay by inclusion of change in parity

“…In some cases, the half-lives calculated by the gross theory are consistent with experimental trends, and an example of this is shown for neutron-rich nuclei in Ref. [8].…”

Half-Life Systematics across theN=126Shell Closure: Role of First-Forbidden Transitions in theβDecay of Heavy Neutron-Rich Nuclei

“…First-forbidden contributions are incorporated only macroscopically, using the statistical gross theory as a perturbation of the main Gamow-Teller strength. Calculated half-lives exceed the measured values for nuclei near the A ∼ 80 and 130 waiting points by a factor of 2 or more, suggesting that the description of the first-forbidden strength is not optimal [18][19][20]. Larger discrepancies are expected for the A ∼ 195 r-process nuclei [7], where first-forbidden transitions may significantly influence the low-energy spectra due to the opening of the intruder π0i 13=2 state near the Fermi surface.…”

“…But at extreme values of isospin, theoretical predictions may be biased by microscopic structural effects that modify the shape of the β-strength function, such as nuclear shell quenching or deformation [12,13]. Until now, such theories have only been tested with information on β decay around the first two waiting points at N ¼ 50 and 82 [13][14][15][16][17][18][19][20]. However, no results are available for the β decays near the A ∼ 195 r-abundance peak.…”

High-Kisomers in neutron-rich zirconium isotopes

“…[2][3][4][5][6][7][8][9]), but only Xu et al [2], albeit with a more phenomenological approach, have given detailed predictions of isomeric states in the neutron-rich even-even zirconium isotopes. Now, with the success of the RIKEN RIBF accelerator complex, major experimental advances have been possible, extending knowledge of these isotopes [10,11]. One of the notable nuclear structure features was the discovery [11] of an isomer in 108 Zr, but not in 106 Zr or 104 Zr.…”