22Ne(α,n)25Mg
PI: Andreas Best
Astrophysical Motivation
The reaction 22Ne(α,n)25Mg is one of the most important reactions for nuclear astrophysics. It is the main neutron source for the weak s process, which is responsible for the creation of elements in the mass range 50 < A < 90. In addition it provides a secondary neutron burst during the later stages of the main s process, modifying the abundances around the so called “branch points” of s process nucleosynthesis, which are important markers for the astrophysical conditions during this phase. Not least, together with its competitor channel 22Ne(α,γ)26Mg it directly impacts the abundances of the Mg isotopes, which can be directly observed in stellar atmospheres.
The reaction has a negative Q value and only starts to operate at energies above ca. 600 keV, which lies in the energy range of interest for above mentioned scenarios. The cross section needs to be know from the neutron threshold up to and including a strong resonance at 832 keV, which is the only resonance measured so far in this range. From indirect data we know of the existence of a number of positive parity states below this resonance, but their strengths are not well constrained by nuclear data, leading to massive uncertainties in nucleosynthesis predictions.
Experimental Aims
The goal of the underground measurement of 22Ne(α,n)25Mg is the first determination of resonance strengths in the astrophysically important energy range and the remeasurement of the 832 keV resonance. This experiment is being carried out in the framework of the ERC starting grant SHADES (#852016). By using a novel hybrid neutron detector array we will be able to detect neutrons with a relatively high efficiency while keeping some energy sensitivity, which is crucial for identifying possible beam induced background or external neutron sources. With the high-intensity alpha beam from the new MV accelerator at the Bellotti IBF, the new detection array, a recirculating gas target of > 99.99% enriched 22Ne and the drastic reduction of the natural neutron background provided by the deep underground location we will achieve a sensitivity increase by over two orders of magnitude over the state of the art, aiming at discovery measurement of so far unseen low energy resonances.
