The 13 C(α, n) 16 O (Q =2215 keV) reaction is the main neutron source for the s process,
which is responsible for the nucleosynthesis of about half of the heavy ( A > 58 ) nuclei
in the universe. This process takes place in the interiors of low mass AGB stars, in a
stellar environment with temperatures between 90 and 100 MK, corresponding to a
Gamow window between 140 and 230 keV (center of mass energies).
The state of the art refers to several direct and indirect measurements performed dur-
ing the last decades.
Direct measurements present in literature are affected by large systematic uncertain-
ties, up to 50%, due to an high surface environmental background, to the difficulty to
keep under control the target degradation during beam irradiation.
For this reason it is still mandatory to perform a low overall uncertainty direct mea-
surement to constrain the cross section.
A new measurement has been performed in the frame of the LUNA (Laboratory for
Underground Nuclear Astrophysics) experiment scientific program in the first part of
The low background environment of Gran Sasso National Laboratory and a high inten-
sity alpha beam ( I ̄ = 200 μA ) provided by the LUNA400 accelerator permitted to go at
lower energies permitted for the first time to measure the 13 C(α, n) 16 O absolute cross
section down to E cm = 260 keV with an overall unprecedented uncertainty of 15%.
A Geant4 code simulation was devoted to maximize the efficiency of a custom neutron
detector array based on 3 He counters with low intrinsic background embedded in a
A long characterization procedure on 99% enriched 13 C targets evaporated on tanta-
lum backings has been performed at MTA Atomki: an innovative analysis method was
used to quantify the target modification under an intense alpha beam bombardment,
and keep under control one of the main sources of systematic uncertainties.
I will present the preliminary results of the absolute cross section of the 13 C(α, n) 16 O
reaction approaching to the Gamow window.