β-Decay Half-Lives of 110 Neutron-Rich Nuclei across theN=82Shell Gap: Implications for the Mechanism and Universality of the AstrophysicalrProcess

Abstract

The β-decay half-lives of 110 neutron-rich isotopes of the elements from 37 Rb to 50 Sn were measured at the Radioactive Isotope Beam Factory. The 40 new half-lives follow robust systematics and highlight the persistence of shell effects. The new data have direct implications for r-process calculations and reinforce the notion that the second (A ≈ 130) and the rare-earth-element (A ≈ 160) abundance peaks may result from the freeze-out of an ðn; γÞ ⇄ ðγ; nÞ equilibrium. In such an equilibrium, the new half-lives are important factors determining the abundance of rare-earth elements, and allow for a more reliable discussion of the PRL 114, 192501 (2015) P H Y S I C A L R E V I E W L E T T E R S week ending 15 MAY 2015 0031-9007=15=114(19)=192501 (7) 192501-1 © 2015 American Physical Society r process universality. It is anticipated that universality may not extend to the elements Sn, Sb, I, and Cs, making the detection of these elements in metal-poor stars of the utmost importance to determine the exact conditions of individual r-process events. Introduction.-The origin of the heavy elements from iron to uranium is one of the main open questions in science. The slow neutron-capture (s) process of nucleosynthesis [1,2], occurring primarily in helium-burning zones of stars, produces about half of the heavy element abundance in the universe. The remaining half requires a more violent process known as the rapid neutron-capture (r) process [3][4][5][6]. During the r process, in environments of extreme temperatures and neutron densities, a reaction network of neutron captures and β decays synthesizes very neutron-rich isotopes in a fraction of a second. These isotopes, upon exhaustion of the supply of free neutrons, decay into the stable or semistable isotopes observed in the solar system. However, none of the proposed stellar models, including explosion of supernovae [7][8][9][10][11][12] and merging neutron stars [13][14][15][16], can fully explain abundance observations. The mechanism of the r process is also uncertain. At temperatures of one billion degrees or more, photons can excite unstable nuclei which then emit neutrons, thus, counteracting neutron captures in an ðn; γÞ ⇄ ðγ; nÞ equilibrium that determines the r process. These conditions may be found in the neutrino-driven wind following the collapse of a supernova core and the accreting torus formed around the black hole remnant of merging neutron stars. Alternatively, recent r-process models have shown that the r process is also possible at lower temperatures or higher neutron densities where the contribution from ðγ; nÞ reactions is minor. These conditions are expected in supersonically expanding neutrino-driven outflow in low-mass supernovae progenitors (e.g., 8 − 12 M ⊙ ) or prompt ejecta from neutron star mergers [17]. The final abundance distribution may also be dominated by postprocessing effects such as fission of heavy nuclei (A ≳ 280) possibly produced in merging neutron stars [18].New clues about the r process have come from the discovery of de...

Excited levels in the multishaped Pd117 nucleus studied via β decay of Rh117

“…[5]. The new region of oblate deformation above N = 70, discussed in previous sections would be consistent with the expectation that the r-process path reaches the N = 82 shell closure at Z ≈ 45 [62]. In this section we will verify the information on the β − -decay rates in this region, especially on the fast, allowed transitions.…”

βdecay of semi-magicCd130: Revision and extension of the level scheme ofIn130

“…4. The resulting β feeding as well as the corresponding log f t values, calculated taking into account a half-life of T 1/2 = 127(2) ms [3] and a β-decay energy of Q β = 8350(160) keV [18], are listed in Table II. When calculating the direct feeding to the excited states, a significant imbalance was found for the 388 keV state. The sum of the intensities of the five transitions populating this state (1732, 3144, 3328, 3366, and 3655 keV) amounts to 150(15)% of the intensity of the 388 keV γ ray (see Table I).…”

“…Figure 1 shows the γ -ray spectrum observed in prompt coincidence with the first decay event after the implantation of a 130 Cd ion in WAS3ABi. The time window between the implantation and the decay was limited to 381 ms, corresponding to three times the half-life of 130 Cd, T 1/2 = 127(2) ms [3]. Only those events were considered in which the decay occurred in the same Si detector in which the ion was implanted, and the distance between implantation and decay in the two directions perpendicular to the beam axis was smaller than or equal to 1 mm.…”

“…In 2011 the same group reported the more precise value of T 1/2 = 162(7) ms [2] obtained using the same technique. In 2015 a much shorter half-life of T 1/2 = 127(2) ms was reported from an experiment performed at RIKEN in which the observation of the decay events in coincidence with γ transitions in the daughter nucleus 130 In allowed for an unequivocal ion identification [3]. Very recently this halflife was confirmed by a measurement performed with the GRIFFIN spectrometer at TRIUMF in which a value of T 1/2 = 126(4) ms was deduced [4].…”

Impact of new data for neutron-rich heavy nuclei on theoretical models forr-process nucleosynthesis

“…There are currently many active programs to measure β-decay rates for nuclei near the rprocess path (Dillmann et al 2003;Pfeiffer et al 2001;Hosmer et al 2005;Montes et al 2006;Hosmer et al 2010;Quinn et al 2012;Madurga et al 2012;Benlliure et al 2012;Benzoni et al 2012;Domingo-Pardo et al 2013;Kurtukian-Nieto et al 2014;Morales et al 2014;Lorusso et al 2015). Even so, not all half-lives of nuclei most relevant to the r-process have been measured in the vicinity of either the N = 50 or N = 82 closed shells.…”