Search

Observed neutron-capture elemental abundances in metal-poor stars, along with ongoing analysis of the extremely metal-poor Eu-enriched sub-class provide new guidance for astrophysical models aimed at finding the r-process sites. The present paper emphasizes the importance of nuclear physics parameters entering in these models, particularly β -decay properties of neutron-rich nuclei. In this context, several r-process motivated β -decay experiments performed at the National Superconducting Cyclotron Laboratory (NSCL) are presented, including a summary of results and impact on model calculations.

The production of ISOL beams of hafnium is described. Radioactive Hf isotopes were produced at ISOLDE by 1.4 GeV proton-induced spallation in Ta and W foils. Chemical evaporation in form of HfF4 and mass separation in the molecular sideband HfF + after electron impact ionization provided intense and pure beams. Beams of 158−185 Hf and short-lived isomers down to 1.1 s 177m Hf were observed, but the method could be extended to reach even more exotic isotopes: down to about 154 Hf (N = 82) on the neutron-deficient side and up to neutron-rich 188 Hf.

A correct understanding of r-process nucleosynthesis requires the knowledge of nuclear-structure properties far from β-stability and a detailed description of the possible astrophysical environments. With respect to nuclear data, in recent years the main focus at CERN/ISOLDE has been put on the 132Sn region to explore the role of the N=82 shell closure and its consequences on the r-process matter flow through the A ≃ 130 Solar-System r-abundance peak.

Background: Nuclear structure plays a significant role on the rapid neutron capture process ($r$ process) since shapes evolve with the emergence of shells and subshells. There was some indication in neighboring nuclei that we might find examples of a new $N=56$ subshell, which may give rise to a doubly magic ${}_{34}^{90}$Se${}_{56}$ nucleus.Purpose: $\ensuremath{\beta}$-decay half-lives of nuclei around ${}^{90}$Se have been measured to determine if this nucleus has in fact a doubly magic character.Method: The fragmentation of a ${}^{136}$Xe beam at the National Superconducting Cyclotron Laboratory at Michigan State University was used to create a cocktail of nuclei in the $A=90$ region.Results: We have measured the half-lives of 22 nuclei near the $r$-process path in the $A=90$ region. The half-lives of ${}^{88}\phantom{\rule{-0.16em}{0ex}}$As and ${}^{90}$Se have been measured for the first time. The values were compared with theoretical predictions in the search for nuclear-deformation signatures of a $N=56$ subshell, and its possible role in the emergence of a potential doubly magic ${}^{90}$Se. The impact of such hypothesis on the synthesis of heavy nuclei, particularly in the production of Sr, Y, and Zr elements was investigated with a weak $r$-process network.Conclusions: The new half-lives agree with results obtained from a standard global QRPA model used in $r$-process calculations, indicating that ${}^{90}$Se has a quadrupole shape incompatible with a closed $N=56$ subshell in this region. The impact of the measured ${}^{90}$Se half-life in comparison with a former theoretical predication associated with a spherical half-life on the weak $r$ process is shown to be strong.

Half-life (${T}_{1/2}$) and \ensuremath{\beta}-delayed neutron branching (${P}_{n}$) values of 492(25) ms and 49(8)$%$, 350(15) ms and 72(8)$%$, and 93(13) ms and 90(10)$%$ for the $r$-process nuclei ${}^{137,138,139}$Sb, respectively, have been measured at the CERN On-Line Isotope Mass Separator (ISOLDE) facility by counting \ensuremath{\beta}-delayed neutrons. More precise ${T}_{1/2}$ and ${P}_{n}$ values of 300(15) ms and 27(4)$%$, and 273(7) ms and 50(8)$%$ for ${}^{136,137}$Sn, respectively, have also been measured. The sources were prepared by using the selective ionization of Sb or Sn with the Resonance Ionization Laser Ion Source and the high-resolution mass separator. The new data for Sb isotopes are compared with calculated ${T}_{1/2}$ and ${P}_{n}$ values for both spherical and nonspherical shapes. The data have been incorporated into parametrized nucleosynthesis calculations of the $r$ process in high-entropy winds of core-collapse supernovae in order to study the properties of the $A$ $=$ 130 solar-system $r$-process abundance peak.

Measurements of $\ensuremath{\beta}$-decay properties of $A\ensuremath{\lesssim}110$ r-process nuclei have been completed at the National Superconducting Cyclotron Laboratory at Michigan State University. $\ensuremath{\beta}$-decay half-lives for $^{105}\mathrm{Y}$, $^{106,107}\mathrm{Zr}$, and $^{111}\mathrm{Mo}$, along with $\ensuremath{\beta}$-delayed neutron emission probabilities of $^{104}\mathrm{Y}$, $^{109,110}\mathrm{Mo}$ and upper limits for $^{105}\mathrm{Y}$, $^{103\ensuremath{-}107}\mathrm{Zr}$, and $^{108,111}\mathrm{Mo}$ have been measured for the first time. Studies on the basis of the quasi-random-phase approximation are used to analyze the ground-state deformation of these nuclei.

The astrophysical r-process is responsible for synthesis of roughly half of the elements heavier than iron. In spite of this significance, there are many uncertainties regarding the site of the r-process and the neutron-rich nuclei involved. Studying these nuclei presents a challenge, as they lie far from the valley of stability. Nuclear properties such as β decay half-lives and βdelayed neutron emission probabilities are critical inputs for r-process models. The neutron rich Ge-Br isotopes are in the region just after the N=50 bottle neck in the “classical” r-process, or may serve as seed material for the high entropy neutrino-wind r-process. Neutron rich nuclei play an important role in both nuclear astrophysics and nuclear structure. 1 Central to the study of nuclear structure are the shape of a nucleus and how that shape changes from spherical to deformed. In some regions there is a smooth change in deformation as nucleons are either added or subtracted, and yet in other regions this transition is very rapid. The Ge-Br isotopes to be measured lie between the N=56 sub-shell closure and the onset of deformation at N=60, just below the Sr-Zr region, for which the most pronounced transition from spherical to deformed ground-state shapes have been observed. Neutron rich Ge-Br isotopes will be studied at the NSCL at Michigan State University. Production will be by fragmentation of a 120 MeV/u 136 Xe beam on a Be target. The A1900 fragment separator will block unwanted species produced in this reaction. The transmitted nuclei will be implanted in a dual-sided silicon detector, which is part of the Beta Counting System detector. Implanted nuclei will be identified by the ∆E-time of flight method. Beta decays will then be detected by a single-sided silicon detector, which is also part of the Beta Counting System. Beta-delayed neutrons will be detected by the NERO neutron counter, which will surround the Beta Counting System. This will provide measurements of the β decay half-lives and β-delayed neutron emission probabilities. Then, SEGA germanium detector array will be used to detect gamma rays from decays of nuclei implanted in the Beta Counting System. The first 2 + and 4 + levels will be measured, determining the deformation of these nuclei.

New levels are reported for 124,126Cd populated in the decay of 124,126Ag isomers, respectively. In addition, new data from direct population of levels in 124Cd from alpha-induced fission of 238U are reported, along with new shell-model calculations for 126Cd.

The present paper reports on several r-process motivated β-decay experiments undertaken at the National Superconducting Cyclotron Laboratory. β-decay half-lives and β-delayed neutron-emission probabilities were measured for nuclei around the r-process A = 70–80 and A = 90 – 110 mass regions. The data are discussed on the basis of quasi-random phase approximation calculations. The emphasis is made on the impact of these data upon calculations of r-process abundances.