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Register a new userHigh-precision mass measurement of $^{56}$Cu and the redirection of the rp-process flow
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We report the mass measurement of $^{56}$Cu, using the LEBIT 9.4T Penning trap mass spectrometer at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of $^{56}$Cu is critical for constraining the reaction rates of the $^{55}$Ni(p,$gamma$)$^{56}$Cu(p,$gamma$)$^{57}$Zn($beta^+$)$^{57}$Cu bypass around the $^{56}$Ni waiting point. Previous recommended mass excess values have disagreed by several hundred keV. Our new value, ME=$-38 626.7(6.4)$ keV, is a factor of 30 more precise than the suggested value from the 2012 atomic mass evaluation [Chin. Phys. C {bf{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 {bf{41}}, 030003 (2017)]. The new experimental average was used to calculate the astrophysical $^{55}$Ni(p,$gamma$) and $^{57}$Zn($gamma$,p) reaction rates and perform reaction network calculations of the rp-process. These show that the rp-process flow redirects around the $^{56}$Ni waiting point through the $^{55}$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.
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The low-lying energy levels of proton-rich $^{56}$Cu have been extracted using in-beam $gamma$-ray spectroscopy with the state-of-the-art $gamma$-ray tracking array GRETINA in conjunction with the S800 spectrograph at the National Superconducting Cyclotron Laboratory at Michigan State University. Excited states in $^{56}$Cu serve as resonances in the $^{55}$Ni(p,$gamma$)$^{56}$Cu reaction, which is a part of the rp-process in type I x-ray bursts. To resolve existing ambiguities in the reaction Q-value, a more localized IMME mass fit is used resulting in $Q=639pm82$~keV. We derive the first experimentally-constrained thermonuclear reaction rate for $^{55}$Ni(p,$gamma$)$^{56}$Cu. We find that, with this new rate, the rp-process may bypass the $^{56}$Ni waiting point via the $^{55}$Ni(p,$gamma$) reaction for typical x-ray burst conditions with a branching of up to $sim$40$%$. We also identify additional nuclear physics uncertainties that need to be addressed before drawing final conclusions about the rp-process reaction flow in the $^{56}$Ni region.
The masses of very neutron-deficient nuclides close to the astrophysical rp- and nu p-process paths have been determined with the Penning trap facilities JYFLTRAP at JYFL/Jyvaskyla and SHIPTRAP at GSI/Darmstadt. Isotopes from yttrium (Z = 39) to palladium (Z = 46) have been produced in heavy-ion fusion-evaporation reactions. In total 21 nuclides were studied and almost half of the mass values were experimentally determined for the first time: 88Tc, 90-92Ru, 92-94Rh, and 94,95Pd. For the 95Pdm, (21/2^+) high-spin state, a first direct mass determination was performed. Relative mass uncertainties of typically $delta m / m = 5 times 10^{-8}$ were obtained. The impact of the new mass values has been studied in nu p-process nucleosynthesis calculations. The resulting reaction flow and the final abundances are compared to those obtained with the data of the Atomic Mass Evaluation 2003.
Reactions on the proton-rich nuclides drive the nucleosynthesis in Core-Collapse Supernovae (CCSNe) and in X-ray bursts (XRBs). CCSNe eject the nucleosynthesis products to the interstellar medium and hence are a potential inventory of p-nuclei, whereas in XRBs nucleosynthesis powers the light curves. In both astrophysical sites the Ni-Cu cycle, which features a competition between $^{59}$Cu(p,$alpha$)$^{56}$Ni and $^{59}$Cu(p,$gamma$)$^{60}$Zn, could potentially halt the production of heavier elements. Here, we report the first direct measurement of $^{59}$Cu(p,$alpha$)$^{56}$Ni using a re-accelerated $^{59}$Cu beam and cryogenic solid hydrogen target. Our results show that the reaction proceeds predominantly to the ground state of $^{56}$Ni and the experimental rate has been found to be lower than Hauser-Feshbach-based statistical predictions. New results hint that the $ u p$-process could operate at higher temperatures than previously inferred and therefore remains a viable site for synthesizing the heavier elements.
The beta-decay half-life of 62Ga has been studied with high precision using on-line mass separated samples. The decay of 62Ga which is dominated by a 0+ to 0+ transition to the ground state of 62Zn yields a half-life of T_{1/2} = 116.19(4) ms. This result is more precise than any previous measurement by about a factor of four or more. The present value is in agreement with older literature values, but slightly disagrees with a recent measurement. We determine an error weighted average value of all experimental half-lives of 116.18(4) ms.
The ground-state-to-ground-state $beta$-decay $Q$-value of $^{135}textrm{Cs}(7/2^+)to,^{135}textrm{Ba}(3/2^+)$ was directly measured for the first time utilizing the Phase-Imaging Ion-Cyclotron Resonance (PI-ICR) technique at the JYFLTRAP Penning-trap setup. It is the first direct determination of this $Q$-value and its value of 268.66(30),keV is a factor of three more precise than the currently adopted $Q$-value in the Atomic Mass Evaluation 2016. Moreover, the $Q$-value deduced from the $beta$-decay endpoint energy has been found to deviate from our result by approximately 6 standard deviations. The measurement confirms that the first-forbidden unique $beta^-$-decay transition $^{135}textrm{Cs}(7/2^+)to,^{135}textrm{Ba}(11/2^-)$ is a candidate for antineutrino-mass measurements with an ultra-low $Q$-value of $0.44(31)$ keV. This $Q$-value is almost an order of magnitude smaller than in any presently running or planned direct (anti)neutrino-mass experiment.
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