First Direct Measurement of an Astrophysical $p$-Process Reaction Cross Section Using a Radioactive Ion Beam

We have performed the first direct measurement of the ${}^{83}\mathrm{Rb}(p,\gamma )$ radiative capture reaction cross section in inverse kinematics using a radioactive beam of ${}^{83}\mathrm{Rb}$ at incident energies of 2.4 and $2.7A\mathrm{MeV}$. The measured cross section at an effective relative kinetic energy of $E_{\mathrm{cm}}=2.393\mathrm{MeV}$, which lies within the relevant energy window for core collapse supernovae, is smaller than the prediction of statistical model calculations. This leads to the abundance of ${}^{84}\mathrm{Sr}$ produced in the astrophysical $p$ process being higher than previously calculated. Moreover, the discrepancy of the present data with theoretical predictions indicates that further experimental investigation of $p$-process reactions involving unstable projectiles is clearly warranted.

Figure 1

(a) Time difference between events observed in the TIGRESS $\gamma$-ray array and the focal plane of the EMMA recoil mass spectrometer, following the ${}^{84}\mathrm{Kr}(p,\gamma )$ reaction. (b) Energies of $\gamma$ rays observed in coincidence with $A=85$ recoils from the timing peak shown in panel (a).

Figure 2

Observed $\gamma$-ray energies in the TIGRESS array, during the measurement of the ${}^{83}\mathrm{Rb}(p,\gamma )$ reaction, as a function of TIGRESS-EMMA correlation time. A vertical cluster of counts indicates the observation of correlated primary and secondary $\gamma$ rays up to high energies, corresponding to ${}^{83}\mathrm{Rb}(p,\gamma )$ events.

Figure 3

Gamma rays observed in the eight TIGRESS detectors centered at 90° with respect to the beam axis in coincidence with $A=84$ recoils, following the ${}^{83}\mathrm{Rb}(p,\gamma )$ reaction.

Figure 4

Total cross section of the ${}^{83}\mathrm{Rb}(p,\gamma )$ reaction inferred from the measured partial cross section for populating the 793 keV state in ${}^{84}\mathrm{Sr}$ in comparison with statistical model predictions [13]. The shaded region indicates the approximate location of the Gamow window [14] for the ${}^{83}\mathrm{Rb}(p,\gamma )$ reaction in CCSN ($2\mathrm{GK}<T<3.5\mathrm{GK}$), the experimental points are centred on the effective relative kinetic energies, and the dashed horizontal bars indicate the energies covered in each measurement. The measured point at $E_{\mathrm{cm}}=2.259\mathrm{MeV}$ is a 90% confidence level upper limit.