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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 $\beta$-decay lifetimes. In the modeling of the s-process, great uncertainty derives from the competition between neutron capture and $\beta$-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 challenging due to the radioactivity of the sample (T$_{1/2}$ = 10.7 yr). However, $^{85}$Kr can be accelerated as a pure beam, and the (d,p$\gamma$) reaction has been demonstrated to be a reliable indirect probe of the (n,$\gamma$)-reaction cross section.
The $^{85}$Kr(d,p$\gamma$)$^{86}$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. Neutron excitations 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 coupling between Apollo and HELIOS allows to observe the $\gamma$-rays in coincidence with the protons, to determine the $\gamma$-ray emission probabilities as a function of excitation energy [P$_{p\gamma}$(E$_{ex}$)]. The $2^+ \to 0^+$ and $4^+ \to 2^+$ $\gamma$-rays are clearly observed, showing the characteristic constant value of P$_{p\gamma}$ below S$_n$ and a decrease above S$_n$. These data are used to extract the cross sections for $^{85}$Kr(n,$\gamma$) reaction, complementing recent direct, high-precision measurements on the stable Kr isotopes. This technique demonstrates significant potential for future indirect studies of the (n,$\gamma$) reaction.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics, under Contract No. DE-AC02- 06CH11357 and by the National Science Foundation, USA under Grant No. PHY-2012522 (Florida State University’s John D. Fox Laboratory). This research used resources of ANL’s ATLAS facility, which is a DOE Office of Science User Facility.
