Approaching the Gamow Window with Stored Ions: Direct Measurement of ${}^{124}\text{X}\text{e}(p,\gamma )$ in the ESR Storage Ring

We report the first measurement of low-energy proton-capture cross sections of ${}^{124}\mathrm{Xe}$ in a heavy-ion storage ring. ${{}^{124}\mathrm{Xe}}^{54+}$ ions of five different beam energies between 5.5 and 8 AMeV were stored to collide with a windowless hydrogen target. The ${}^{125}\mathrm{Cs}$ reaction products were directly detected. The interaction energies are located on the high energy tail of the Gamow window for hot, explosive scenarios such as supernovae and x-ray binaries. The results serve as an important test of predicted astrophysical reaction rates in this mass range. Good agreement in the prediction of the astrophysically important proton width at low energy is found, with only a 30% difference between measurement and theory. Larger deviations are found above the neutron emission threshold, where also neutron and $\gamma$ widths significantly impact the cross sections. The newly established experimental method is a very powerful tool to investigate nuclear reactions on rare ion beams at low center-of-mass energies.

Figure 1

The figure shows the experimental setup at the ESR from the gas target to the next dipole magnet. Three Ge x-ray detectors are placed around the interaction region at angles of 35°, 60°, and 90°. The DSSSD is positioned in the last quarter of the dipole to intercept the produced ${}^{125}\mathrm{Cs}$ ions, which are separated from the circulating ${}^{124}\mathrm{Xe}$ beam due to their magnetic rigidity.

Figure 2

The ion hit distribution measured with the DSSSD is shown. On top of a broad background of elastically scattered ${}^{124}\mathrm{Xe}$ ions a narrow cluster of ${}^{125}\mathrm{Cs}$ ions can be identified as products of the ($p,\gamma$) reaction.

Figure 3

Depicted is the spectrum of x-ray radiation recorded by the 90° detector at the target with a beam energy of 8 AMeV. The K-REC peak at 45 keV is used for luminosity normalization. Various other atomic processes are visible through their characteristic lines. For details see text.

Figure 4

Comparison of experimental and predicted cross sections: the red solid line shows the predictions obtained with the same input as used in the reaction rate libraries for astrophysics; the green dashed line shows a similar calculation but with the proton width increased by 30% and the neutron width increased by a factor of 2.5 (see text for details).