Constraining the astrophysical $p$ process: Cross section measurement of the ${}^{84}\mathrm{Kr}(p,\gamma ){}^{85}\mathrm{Rb}$ reaction in inverse kinematics

One of the biggest questions in nuclear astrophysics is understanding where the elements come from and how they are made. This work focuses on the $p$ process, a nucleosynthesis process that consists of a series of photodisintegration reactions responsible for producing stable isotopes on the proton-rich side of stability. These nuclei, known as the $p$ nuclei, cannot be made through the well-known neutron-capture processes. Currently $p$-process models rely heavily on theory to provide the relevant reaction rates to predict the final $p$-nuclei abundances and more experimental data is needed. The present work reports on an experiment performed with the SuN detector at the National Superconducting Cyclotron Laboratory, NSCL, at Michigan State University using the ReA facility to measure the ${}^{84}\mathrm{Kr}(p,\gamma ){}^{85}\mathrm{Rb}$ reaction cross section in inverse kinematics. The reverse ${}^{85}\mathrm{Rb}(\gamma ,p){}^{84}\mathrm{Kr}$ reaction is a branching point in the $p$-process reaction network that was highlighted as an important reaction in sensitivity studies in the production of the ${}^{78}\mathrm{Kr}$ $p$ nucleus. A new hydrogen gas target was designed and fabricated and a new analysis technique for background subtraction and efficiency calculations of the detector were developed. The experimental cross section is compared to standard statistical model calculations using the non-smoker and talys codes.

Figure 1

The theoretical production paths, in blue, and destruction paths, in red, for ${}^{85}\mathrm{Rb}$ [16, 17]. All reactions have been measured except for the ${}^{85}\mathrm{Rb}(\gamma ,p){}^{84}\mathrm{Kr}$ branch.

Figure 2

A diagram of the experimental set up without SuNSCREEN. Beam comes from the right and enters the setup. The beam current was measured off of the beam pipe which was used as a Faraday cup.

Figure 3

Total absorption spectra for the ${}^{84}\mathrm{Kr}(p,\gamma ){}^{85}\mathrm{Rb}$ reaction at the highest beam energy, 3.7 MeV/u. The black dotted line shows the gas cell filled with hydrogen gas, the red dotted-dashed line shows the scaled empty cell spectrum and the purple line is the fully background subtracted and Doppler corrected spectrum. The purple spectrum was used for the remaining analysis and the insert is a closer view of the sum peak above background.

Figure 4

The previously published SuN data for the ${}^{90}\mathrm{Zr}(p,\gamma ){}^{91}\mathrm{Nb}$ reaction are displayed as green circles [22], the non-smoker calculations as a black line, and the cross section values calculated with the new efficiency analysis are shown as blue stars.

Figure 5

An example of the ${\chi }^2$ minimization fits of the TAS, sum of segments and multiplicity spectra for the 3.1 MeV/u beam energy. The red dotted lines are simulations and the black lines are the experimental data.

Figure 6

The measured cross section of the ${}^{84}\mathrm{Kr}(\mathrm{p},\gamma ){}^{85}\mathrm{Rb}$ reaction are shown as black circles and the associated uncertainty. The dashed line shows the energy width of the gas cell and the data point is presented at the effective energy. The blue band shows the talys calculations and the red line shows the non-smoker calculations.

Figure 7

The mass region around ${}^{84}\mathrm{Kr}$ with the predominant ($\gamma ,n$) reactions highlighted in light blue at the top and bottom of the figure. With the ${}^{84}\mathrm{Kr}(p,\gamma ){}^{85}\mathrm{Rb}$ cross section agreeing with theory, the reaction flow would follow the blue highlighted reaction chain instead of the striped red reaction chain.