Direct low-energy measurement of the ²⁵Mg(α,n)²⁸Si reaction via neutron spectroscopy

During core helium burning in massive stars, neutrons are produced mainly by the ²²Ne(α,n)²⁵Mg reaction. These neutrons are then captured onto heavy seed nuclei, resulting in the slow production of many of the elements between masses 60 < A < 90. One of the main challenges in modeling this weak s-process nucleosynthesis is determining the overall neutron flux.

This requires constraining the reaction rate of the primary as well as the secondary neutron sources. The ²⁵Mg nuclei produced from the ²²Ne(α,n)²⁵Mg reaction affect the neutron flux through the ²⁵Mg(α,n)²⁸Si reaction, which serves as an additional neutron source for the weak s-process in massive stars. Past attempts to measure the ²⁵Mg(α,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(α,n)²⁸Si reaction at low energies. Measurements of the cross-section have been performed down to E_{α =}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.