Stellar Neutron Sources for the Slow Neutron Capture Nucleosynthesis

The interplay between the 22Ne(a,g)26Mg and the competing 22Ne(a,n)25Mg reactions determines the efficiency of the latter as a neutron source at the temperatures of stellar helium burning. In both cases, the rates are dominated by the alpha-cluster resonance at 830 keV. This resonance plays a particularly important role in determining the strength of the neutron flux for both the weak and main s-process as well as the n-process. Recent experimental studies based on transfer reactions suggest that the neutron and gamma-ray strengths for this resonance are approximately equal.

In this work, the 22Ne(a,n)25Mg resonance strength has been remeasured and found to be similar to the previous direct studies. This reinforces an 830 keV resonance strength that is approximately a factor of three larger for the 22Ne(a,n)25Mg reaction than for the 22Ne(a,g)26Mg reaction.

The reaction rate of the 22Ne(a,g)26Mg is dominated by the low energy resonances at E_{alpha,lab} = 650 and 830 keV respectively. The E_{alpha,lab} = 830 keV resonance has been measured previously, but there are some uncertainties in the previous measurements. We confirmed the measurement of the E_{alpha,lab} = 830 keV resonance using implanted 22Ne targets. We obtained a resonance strength of 35 +/- 4 micro eV, and provide a weighted average of the present and previous measurements of 35 +/- 2 micro eV with reduced uncertainties compared to previous studies. We also attempted to measure the strength of the predicted resonance at E_{alpha,lab} = 650 keV directly for the first time and found an upper limit of 0.15 micro eV for the strength of this resonance.

The 25Mg nuclei produced from the 22Ne(a,n)25Mg reaction affect the overall neutron flux by the 25Mg(a,n)28Si reaction which serves as an additional neutron source for the weak s-process during the neon burning phase in massive stars. Past attempts to measure the 25Mg(a,n)28Si reaction using neutron counters have been greatly hindered by low Z background reactions. In the present work, neutron spectroscopy has been accomplished using deuterated liquid scintillator detectors combined with spectrum unfolding to make an improved measurement of the 25Mg(a,n)28Si reaction at low energies. Measurements of the cross section have been preformed down to E_alpha ~ 1.5 MeV with an unprecedented level of background rejection. In addition, the separation of the different background contributions gives further insight into the results of previous counter measurements.