Speaker
Description
About 50% of the elements heavier than iron are produced in the so-called s-process, where the lifetime for neutron capture of the nuclei involved is typically longer than their β-decay lifetimes. In the modeling of the s-process, great uncertainty derives from the competition between neutron capture and β-decay, in particular in some isotopes called “branching points”. $^{85}$Kr is an important branching point of the s-process, that influences both the $^{86}$Kr/$^{82}$Kr ratio in presolar grains and the abundances of heavy Sr isotopes that are produced also by r-process. A better understanding of this branching point can be achieved only if the neutron capture cross section on $^{85}$Kr is sufficiently well constrained, but a direct measurement of this cross section is extremely complicated due to the radioactivity of the sample.
The (d,pγ) reaction has been demonstrated to be a reliable indirect probe of the (n,γ)-reaction cross section, and $^{85}$Kr can be accelerated as a pure beam. For this reason, the $^{85}$Kr(d,pγ)$^{86}$Kr reaction has been carried out at 10 MeV/u in inverse kinematics at Argonne’s ATLAS facility using the HELIOS spectrometer and the Apollo array. Excited state at energies from around 2-14 MeV in $^{86}$Kr were populated, where S$_n$=9.86 MeV, with a Q-value resolution of about 150 keV. The $2^+ \to 0^+$ and $4^+ \to 2^+$ γ-rays are clearly observed, from which the γ-ray emission probabilities as a function of excitation energy [P$_{pγ}$(E$_{ex}$)] can be determined. P$_{pγ}$ shows the characteristic behaviour with a constant value below S$_n$ and a decrease at higher excitation energies. These data are used to extract the cross sections for $^{85}$Kr(n,γ) reaction, complementing recent direct, high-precision measurements on the stable Kr isotopes. The technique has significant potential for future indirect (n,γ)-reaction studies.
