beta-delayed gamma decay of Mg-20 and the Ne-19(p, gamma) Na-20 breakout reaction in Type I X-ray bursts

摘要

Certain astrophysical environments such as thermonuclear outbursts on accreting neutron stars (Type-I X-ray bursts) are hot enough to allow for breakout from the Hot CNO hydrogen burning cycles to the rapid proton capture (rp) process. An important breakout reaction sequence is O-15(alpha,gamma) Ne-19(p,gamma) 20Na and the Ne-19(p,beta) Na-20 reaction rate is expected to be dominated by a single resonance at 457 keV above the proton threshold in Na-20. The resonance strength and, hence, reaction rate depends strongly on whether this Na-20 state at an excitation energy of 2647 keV has spin and parity of 1(+) or 3(+). Previous Mg-20 (J(pi) = 0(+)) beta(+) decay experiments have relied almost entirely on searches for beta-delayed proton emission from this resonance in Na-20 to limit the log ftvalue and, hence, J(pi). However there is a non-negligible gamma-ray branch expected that must also be limited experimentally to determine the log ftvalue and constrain J(pi). We have measured the beta-delayed gamma decay of Mg-20 to complement previous beta-delayed proton decay work and provide the first complete limit based on all energetically allowed decay channels through the 2647 keV state. Our limit confirms that a 1(+) assignment for this state is highly unlikely. (C) 2018 The Authors. Published by Elsevier B.V.