New Approach for Measuring Properties of $rp$-Process Nuclei

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture ($rp$) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the $\gamma$-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, ${}^{33}\mathrm{Ar}$, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated ${}^{32}\mathrm{Cl}(p,\gamma ){}^{33}\mathrm{Ar}$ rate that is critical to the modeling of the $rp$ process. This approach has the potential to measure key properties of almost all interesting nuclei on the $rp$-process path.

Figure 1

The upper panel shows the Doppler-corrected $\gamma$-ray spectrum coincident with ${}^{33}\mathrm{Ar}$ ions detected in the S800 focal plane. The lower panel shows the Doppler-corrected $\gamma$-ray spectrum in coincidence with $\gamma$ rays at 2460 keV. Clear evidence for a coincidence between the 1359 and 2460 keV $\gamma$ rays is seen.

Figure 2

The ${}^{33}\mathrm{Ar}$ energy level diagram of this work. Vertical arrows represent observed electromagnetic transitions. Dashed lines and arrows denote weak evidence. Spin and parity ($J^{\pi }$) was determined by analogy with the ${}^{33}\mathrm{P}$ analog nucleus. Intensities of the transitions relative to the 1359 keV transition are given by a percentage at the top of each arrow. Prior to this work only the 1359 and 1789 keV levels had been observed. $S_p$ denotes the proton separation energy [25, 26].

Figure 3

The astrophysical ${}^{32}\mathrm{Cl}(p,\gamma ){}^{33}\mathrm{Ar}$ ground state capture reaction rates as a function of temperature, $T_9$, in GK. Contributions from individual resonances and the sum of all direct capture contributions (nonresonant) are shown.

Figure 4

The total astrophysical ${}^{32}\mathrm{Cl}(p,\gamma ){}^{33}\mathrm{Ar}$ ground state capture rate based on this work (black) and Ref. 7 (grey). Only uncertainties from excitation energies are shown. The grey shaded region shows the possible range of rates based solely on the shell model assuming a typical uncertainty in excitation energy of 100 keV. The region in black is the rate uncertainty based on the measured excitation energies presented in this Letter.