Low-energy measurement of the 25 Mg (𝛼,𝑛) 28 Si reaction via neutron spectroscopy
During core helium and carbon burning in massive stars, neutrons are produced mainly by the 22 Ne (𝛼,𝑛) 25 Mg reaction. Some of these released neutrons are captured by heavy seed nuclei from previous nucleosynthesis events, resulting in the slow production of many of the elements between masses 60 ≤ 𝐴 ≤ 90 via the weak 𝑠 process. Determining the overall neutron flux available in this environment is one of the main challenges in modeling its contributions to heavy element production. Not only must the reaction rate be well defined for the primary neutron source, but the rates of neutron poison and recycling reactions must also be well determined. One critical reaction in the simulation network is 25 Mg (𝛼,𝑛) 28 Si. This reaction, together with 25 Mg (𝑛,𝛾) 26 Mg and 25 Mg (𝛼,𝛾) 29 Si, determine how 25 Mg nuclei affect the available neutron flux. Past attempts to measure the 25 Mg (𝛼,𝑛) 28 Si cross section using neutron moderator counters have been greatly hindered by lower-𝑍 background reactions. Here, in the present work, neutron spectroscopy with deuterated liquid scintillator detectors has been used. The experimental spectra have been analyzed by applying spectrum unfolding techniques to achieve improved background discrimination for the 25 Mg (𝛼,𝑛) 28 Si reaction at low energies, down to 𝐸 𝛼 = 1.75 MeV. The separation of the different background contributions gives further insight into the results of previous moderator counter measurements and the measurements lead to a revised and more reliable determination of the reaction rate.