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Register a new userFirst direct measurement of $^{59}$Cu(p,$alpha$)$^{56}$Ni: A step towards constraining the Ni-Cu cycle in the Cosmos
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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.
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Angle-integrated cross-section measurements of the $^{56}$Ni(d,n) and (d,p) stripping reactions have been performed to determine the single-particle strengths of low-lying excited states in the mirror nuclei pair $^{57}$Cu-$^{57}$Ni situated adjacent to the doubly magic nucleus $^{56}$Ni. The reactions were studied in inverse kinematics utilizing a beam of radioactive $^{56}$Ni ions in conjunction with the GRETINA $gamma$-array. Spectroscopic factors are compared with new shell-model calculations using a full $pf$ model space with the GPFX1A Hamiltonian for the isospin-conserving strong interaction plus Coulomb and charge-dependent Hamiltonians. These results were used to set new constraints on the $^{56}$Ni(p,$gamma$)$^{57}$Cu reaction rate for explosive burning conditions in x-ray bursts, where $^{56}$Ni represents a key waiting point in the astrophysical rp-process.
A sample of Lambdas produced in 2 A*GeV Ni + Cu collisions has been obtained with the EOS Time Projection Chamber at the Bevalac. Low background in the invariant mass distribution allows for the unambiguous demonstration of Lambda directed flow. The transverse mass spectrum at mid-rapidity has the characteristic shoulder-arm shape of particles undergoing radial transverse expansion. A linear dependence of Lambda multiplicity on impact parameter is observed, from which a total Lambda + Sigma^0 production cross section of $112 +/- 24 mb is deduced. Detailed comparisons with the ARC and RVUU models are made.
An inelastic $alpha$-scattering experiment on the unstable $N=Z$, doubly-magic $^{56}$Ni nucleus was performed in inverse kinematics at an incident energy of 50 A.MeV at GANIL. High multiplicity for $alpha$-particle emission was observed within the limited phase-space of the experimental setup. This observation cannot be explained by means of the statistical-decay model. The ideal classical gas model at $kT$ = 0.4 MeV reproduces fairly well the experimental momentum distribution and the observed multiplicity of $alpha$ particles corresponds to an excitation energy around 96 MeV. The method of distributed $malpha$-decay ensembles is in agreement with the experimental results if we assume that the $alpha$-gas state in $^{56}$Ni exists at around $113^{+15}_{-17}$ MeV. These results suggest that there may exist an exotic state consisting of many $alpha$ particles at the excitation energy of $113^{+15}_{-17}$ MeV.
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.
The existence of the miscibility gap in the Cu-Ni system has been an issue in both computational and experimental discussions for half a century [Chakrabarti et al., Phase diagrams of binary nickel alloys, ASM, 1991]. Here we propose a new miscibility gap in the Cu-Ni system measured in nano-layered thin films by Secondary Neutral Mass Spectrometry. The maximum of the symmetrical miscibility curve is around 800~K at Cu50%Ni50%. To our best knowledge, this is the first experiment determining the miscibility from the measurement of the atomic fraction of Copper and Nickel in the whole composition range. Needless to say that Ni, Cu and its alloys are important in various fields, accordingly this result affects different areas to understand materials sciences.
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