High-Precision Mass Measurement of ${}^{56}\mathrm{Cu}$ and the Redirection of the $rp$-Process Flow

We report the mass measurement of ${}^{56}\mathrm{Cu}$, using the LEBIT 9.4 T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of ${}^{56}\mathrm{Cu}$ is critical for constraining the reaction rates of the ${}^{55}\mathrm{Ni}(p,\gamma )$ ${}^{56}\mathrm{Cu}(p,\gamma )$ ${}^{57}\mathrm{Zn}({\beta }^{+})$ ${}^{57}\mathrm{Cu}$ bypass around the ${}^{56}\mathrm{Ni}$ waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, $\mathrm{ME}=-38626.7(7.1)\mathrm{keV}$, is a factor of 30 more precise than the extrapolated value suggested in the 2012 atomic mass evaluation [Chin. Phys. C 36, 1603 (2012)], and more than a factor of 12 more precise than values calculated using local mass extrapolations, while agreeing with the newest 2016 atomic mass evaluation value [Chin. Phys. C 41, 030003 (2017)]. The new experimental average, using our new mass and the value from AME2016, is used to calculate the astrophysical ${}^{55}\mathrm{Ni}(p,\gamma )$ and ${}^{56}\mathrm{Cu}(p,\gamma )$ forward and reverse rates and perform reaction network calculations of the $rp$ process. These show that the $rp$-process flow redirects around the ${}^{56}\mathrm{Ni}$ waiting point through the ${}^{55}\mathrm{Ni}(p,\gamma )$ route, allowing it to proceed to higher masses more quickly and resulting in a reduction in ashes around this waiting point and an enhancement to higher-mass ashes.

Figure 1

A sample 50-ms ${{}^{56}\mathrm{Cu}}^{+}$ time-of-flight ion cyclotron resonance used for the determination of the frequency ratio of ${\nu }_{\mathrm{ref}}^{\mathrm{int}}({\mathrm{C}}_4{\mathrm{H}}_7^{+})/{\nu }_c({{}^{56}\mathrm{Cu}}^{+})$. The solid red curve represents a fit of the theoretical profile [28].

Figure 2

Measured cyclotron frequency ratios $R={\nu }_{\mathrm{ref}}^{\mathrm{int}}/{\nu }_c({{}^{56}\mathrm{Cu}}^{+})$ relative to the average value $\overline{R}$; the gray bar represents the $1\sigma$ uncertainty in $\overline{R}$.

Figure 3

Difference of measured $R$ values of ${}^{41}\mathrm{K}$ relative to the value calculated from AME2016 [6]. The gray bar represents the average $R$ value and its $1\sigma$ uncertainty; the uncertainty of the AME2016 value, $1.5\times {10}^{-10}$, is not visible on this graph.

Figure 4

Rate for the ${}^{55}\mathrm{Ni}(p,\gamma ){}^{56}\mathrm{Cu}$ reaction and $1\sigma$ uncertainties for AME2012 (black band) and Tu et al. masses (red band) and using the experimental mass (blue band). The prior (dashed lines) and new reverse rates (dashed blue line) are also shown.

Figure 5

Fractional difference of abundance by mass number of this work (solid blue line) compared to that using the masses from AME2012 [16] and the same fractional difference using the mass from Tu et al. [14] (dashed red line).