Speaker
Description
Carbon burning is the third stage of stellar evolution, determining the fate of both massive stars and low-mass stars in binary systems.
Only stars with a mass larger than a critical value M∗ up ∼ 10M⊙, can ignite Carbon in non-degenerate conditions and proceed to the next advanced burning stages up to the formation of a gravitationally unstable iron core.
Various final destinies are possible, among which a direct collapse into a black hole or the formation of a neutron star followed by the violent ejection of the external layers (type II SN).
Less massive stars M < Mup ∼ 7M⊙, never attain the conditions for C ignition and will evolve into CO White Dwarfs.
The values of M∗ up and Mup depend on the 12C + 12C reaction rate, which remains uncertain at astrophysical energies.
Stellar Carbon burning occurs mainly through the 12C(12C, α)20Ne and 12C(12C, p)23Na reactions.
These cross-sections can be measured either by detecting the emitted charged particles
or the γ-rays produced by the decay excited states of 20Ne and 23Na.
The 12C + 12C fusion reactions were investigated across a broad energy range.
However the lowest energy reached by direct measurement is 2.1 MeV, still above astrophysical energies.
Indirect data obtained with methods such as the Trojan Horse approach are available at astrophysical energies,
but suffer from uncertainties in the renormalization.
Direct measurements are thus essential for both stellar evolution models and the interpretation of indirect data.
In this context, a direct study is currently being carried out by the LUNA collaboration at the Bellotti ion beam facility in the deep underground laboratories of the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, where intense carbon beams with energies up to 7 MeV and excellent energy resolution and stability are available.
The objective of this measurement is to directly determine the 12C+12C cross-section at astrophysical energies by γ spectroscopy.
The detection setup is made of several NaI scintillators surrounding a 150% HPGe in a compact configuration, covering
~3.5π steradians.
This configuration ensures high efficiency and preserves the
HPGe resolution (1.2 keV at 1.33 MeV).
The NaI setup will also act as an active veto for Compton, environmental, and beam-induced background.
The detectors are placed in side a 2cm copper shielding surrounded by a 25cm lead shielding,
which will further reduce the environmental background at LNGS of over two orders of magnitude.
This setup will allow us to achieve unprecedented sensitivity,
with an expected background about four orders of magnitude lower than the previous direct measurements that reached the lowest energies.
With this setup, we'll be able to shed light on the level density of 24Mg through the de-excitation of 20Ne and 23Na nuclei.
This will allow us to explore the possible cluster structures of the 24Mg nucleus.
In particular, we'll be able to examine the E_cm = 1.5 MeV - 3.5 MeV energy
window (15.44 MeV to 17.44 MeV considering the Q-value), where the cluster states could be found.
Those states will determine the rate of the 12C+12C at astrophysical energies.
With my contribution I will present details of recent results setup development and installation,
together with Geant4 simulations and a detailed characterization of the HPGe detector active volume.
I will also present preliminary results of the first beam-on-target devoted to
the direct measurement of the 12C+12C reaction at energies E_cm > 2 MeV .