24Mg(p,γ)25Al
PI: David Rapagnani
Astrophysical Motivation
The 24Mg(p,γ)25Al reaction (Q = 2271.37 keV) is relevant in the context of the MgAl cycle which activates during H burning for T ≥ 80 MK. While this reaction provides only a minor contribution to the stellar energy budget, it is a key process affecting the abundances of Mg and Al isotopes in Globular Cluster stars. Spectroscopic analyses of different stars in the same cluster have revealed a Mg–Al anti-correlation similar to the widespread O-Na anti-correlation in the same clusters. This unique abundance pattern is commonly interpreted as the result of the CNO, NeNa and MgAl cycles operating in stars that were born in the early epochs of cluster formation but whose identification has proven challenging.
In addition, stardust grains that formed around AGB stars carry the signature of H burning at high temperatures, including indication of partial activation of the MgAl cycle in the form of enhanced 26Al production by 24Mg(p,γ)25Al.
To gain a further insight on Al production in stars, a detailed knowledge of all the reactions involved in the MgAl cycle is required, including 24Mg(p,γ)25Al.
Experimental Aim
Experimental knowledge on this reaction is mostly based on a comprehensive cross section measurements at Elab < 2.3 MeV [Trautvetter] and the last works performed at TUNL and at LUNA focused on the lowest energy resonances only.
At temperatures relevant for hydrogen shell burning, the reaction rate is dominated by a direct capture (DC) component, while a resonance at Elab = 222.9 keV solely dominates at higher temperatures.
At lower temperatures, a minor contribution is due to the low energy tail of the broad resonance at Elab = 823.3 keV and the uncertainty is dominated by the limited knowledge of the DC component. On the other hand, the DC component also strongly interferes with some of the higher energy resonances, which makes the analysis nontrivial.
Due to large uncertainty on DC cross sections at Ep = 823 keV and discrepancy on the Elab= 222.9 keV resonance strength ωγ previously measured, more direct data are necessary to resolve possible systematic offsets.
Experimental Setup
The LUNA400 accelerator could produce proton beam with Ep ≤ 400 keV and this give the possibility to explore the low energy region. To achieve the goal of a direct measurement of DC component below 400 keV, this experimental project, in the framework of the LUNA Collaboration, is using enriched solid target, mandatory to avoid beam-induced background from the overwhelmingly strong resonances in 25Mg(p,γ)26Al and 26Mg(p,γ)27Al at Ep = 316 and 338 keV.
Moreover, this experiment uses an high efficiency BGO detector, which is segmented for the discrimination between signal and background events.