  Neutrons produced by the carbon fusion reaction 12C(12C,n)23Mg play an
important role in stellar nucleosynthesis. However, past studies have shown
large discrepancies between experimental data and theory, leading to an
uncertain cross section extrapolation at astrophysical energies. We present the
first direct measurement that extends deep into the astrophysical energy range
along with a new and improved extrapolation technique based on experimental
data from the mirror reaction 12C(12C,p)23Na. The new reaction rate has been
determined with a well-defined uncertainty that exceeds the precision required
by astrophysics models. Using our constrained rate, we find that 12C(12C,n)23Mg
is crucial to the production of Na and Al in Pop-III Pair Instability
Supernovae. It also plays a non-negligible role in the production of weak
s-process elements as well as in the production of the important galactic
gamma-ray emitter 60Fe.
