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Poster Summaries Part E: 1 – LUNA; 2 – The 25Mg(α,n)28Si Reaction

March 14, 2013

Below are some additional interesting contributions from the poster session participants.

1 – Rosanna Depalo: Towards the Study of the 22Ne(p,g)23Na Reaction at LUNA in Gran Sasso

The 22Ne(p,g)23Na reaction takes part in the NeNa cycle of hydrogen burning (Fig.1).

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This cycle plays an important role in the Red Giant Branch and Asymptotic Giant Branch phases of stellar evolution, as well as in novae explosions.

In these scenarios, the NeNa cycle is particularly important for its contribution to the synthesis of the elements between 20Ne and 27Al.

Furthermore, the proton capture on 22Ne contributes to the production of 23Na during the pre-explosion simmering in type Ia supernovae. The 23Na abundance is directly connected to the 56Ni production, and hence to the characteristic light curve of the supernova explosion. Indeed, electron capture on 23Na is one of the main processes reducing the electron abundance in the white dwarf progenitor star.

The 22Ne(p,g)23Na reaction rate is highly uncertain because of the contribution of a large number of resonances lying in the astrophysically relevant energy region (Fig. 2). Most of these resonances have never been populated in either direct or indirect experiments, and only upper limits exist for their strengths.

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[Figure 2 : Level scheme of 23Na with resonance energies of the 22Ne+p reaction in the center of mass system.  The existence of the highlighted levels at 8862, 8894 and 9000 keV is still uncertain. The resonance at 178keV has been observed  for the first time in a 12h test run at LUNA]

A measurement of the  22Ne(p,g)23Na cross section is on-going at the Laboratory for Underground Nuclear Astrophysics (LUNA), located at Laboratori Nazionali del Gran Sasso.

The test runs performed at LUNA demonstrated that, thanks to the low background, it will be possible to observe some 22Ne+p resonances for the first time, and to significantly improve the current knowledge of the reaction cross section.

2 – Antonio Caciolli: The 25MG(α,n)28Si Reaction Studied at LNL

The 26Al is one of the most interesting cases to understand the stellar nucleosynthesis. The 1.8 MeV gamma line, associated with its decay, makes the observation of 26Al easier with respect to other radio elements and it has been the subject of many investigations during the years until the recent observations of the INTEGRAL satellite of the ESA. INTEGRAL obtained a gamma map of the Milky Way and observed the 26Al around the Galaxy disk. This observation is a clear hint of recent nucleosynthesis, due to the short lifetime (t ~ 1 My) of this isotope. Moreover, the distribution of 26Al is a robust parameter to control the predictions of stellar evolution models.

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The explanation of the 26Al amount is still elusive  and one of the most interesting puzzle of cosmochemestry. In particular, the composition of the most primitive Solar System condensates (derived from the excess of 26Mg produced in its decay) claims  the existence of 26Al in the first phases of the Solar System formation and requests a local origin which is still difficult to explain. Stellar models able to predict the experimental observations are necessary to deduce the astrophysical conditions in the early time of the Solar System and its evolution during the first millions of years. These models use as input, not only the parameters of stellar structure, but also the nuclear reaction cross sections.

The 25Mg(α,n)28Si is responsible for the destruction of the 25Mg, the seed for the 26Al production. Up to now its experimental cross section is the most important nuclear source of uncertainty in the in massive stars models.

The data in literature are still not in accord between themselves and new experimental efforts are required.

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In the experiment performed at the Laboratori Nazionali di Legnaro (LNL) the alpha beam, produced at the CN accelerator, passes through a cold trap before reaching the target (MgO enriched in 25Mg) in order to suppress any contamination on the target  surface. Two silicon detectors are used to determine the beam current and to check the target stability. 10 RIPEN detectors (BC501 liquid scintillator) are installed at 2 m distance from the target to measure the neutrons produced by using the Time of Flight (TOF) technique which allows to perform neutron spectroscopy. The gamma neutron discrimination is obtained by using pulse shape analysis reducing the uncorrelated gamma background by a factor almost 100.

The differential cross section has been measured in a beam energy from 3 MeV up to 5 MeV and the data are under analysis.

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