Investigations on states of 20Mg and spallation reaction effects for constraining nuclear physics inputs for X-ray bursts

Abstract

We present the first observation of a resonance state in the proton drip-line nucleus [superscript 20]Mg. The resonance state was populated via inelastic scattering of [superscript 20]Mg with a solid deuteron target using the IRIS facility stationed at TRIUMF, Canada. Together with the ground state and first excited state, a new resonance state is observed at excitation energy of 3.68 [plus or minus] 0.04 MeV. Based on a comparison of the measured angular distributions to distorted wave Born approximation calculations, the first excited state is consistent with L=2 excitation confirming its spin to be 2+. Similar comparison for new resonance suggests a spin possibility of either (4+) or (2+). The new resonance state lies in the Gamow window and hence puts constraint on the [superscript 18]Ne(2p, gamma)[superscript 20]Mg reaction rate, a possible breakout reaction from hot CNO cycles in X-ray bursts. The new resonance state is higher in energy than expectations based on mirror symmetry to [superscript 20]O. The inferred reaction rate of [superscript 19]Na(p, gamma)[superscript 20]Mg is lower than the previous predictions. The net rate of the [superscript 18]Ne(2p, gamma)[superscript 20]Mg is found to be competitive to beta decay of [superscript 18]Ne only at high densities for it to be a viable breakout path. A comparison to theoretical predictions show that the calculations based on chiral interactions and NN+3N forces fails to explain the observed resonance state. The new data will therefore serve as guidance to benchmark the nuclear structure models and interactions at the drip-line. In a related study of CNO cycles in X-ray bursts, the spallation of the accreted material in the atmosphere of a neutron star has been modelled considering a full cascading destruction process. The results show that the replenishment of CNO elements in a cascading process is minuscule and the CNO abundances are reduced to negligible values. The impact of reduced CNO metallicity on X-ray burst ignition conditions are discussed.