β-delayed γ decay of 20Mg and the Ne 19 ( p , γ ) 20 Na 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 15 O( α , γ ) 19 Ne(p, γ ) 20 Na and the 19 Ne(p, γ ) 20 Na reaction rate is expected to be dominated by a single resonance at 457 keV above the proton threshold in 20 Na. The resonance strength and, hence, reaction rate depends strongly on whether this 20 Na state at an excitation energy of 2647 keV has spin and parity of 1 + or 3 + . Previous 20 Mg ( J π = 0 + ) β + decay experiments have relied almost entirely on searches for β -delayed proton emission from this resonance in 20 Na to limit the log ft value and, hence, J π . However there is a non-negligible γ -ray branch expected that must also be limited experimentally to determine the log ft value and constrain J π . We have measured the β -delayed γ decay of 20 Mg to complement previous β -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.