Measurement of the 25Mg(α,n)28Si reaction cross section at LNL

The detection of the 1809 keV emission line associated with the decay of 26Al in the interstellar medium provides a direct evidence of recent nucleosynthesis events in our galaxy. 26Al is thought to be mainly produced in massive stars, but in order to have a quantitative understanding of the 26Al distribution, the cross section of all the nuclear reactions involved in its production should be accurately known. A recent sensitivity study demonstrated that the 25Mg(α,n)28Si is the reaction with the strongest impact on the synthesis of 26Al during explosive Neon and Carbon burning [4]. In order to improve the experimental knowledge of the 25Mg(α,n)28Si cross section, a new direct measurement has been performed at Legnaro National Laboratories. The experimental setup, the data analysis and preliminary results are discussed.


Introduction
The detection of short lived radionuclides inside the Milky Way represents an observational evidence of the theory of stellar nucleosynthesis. 26 Al is one of the first radioactive isotopes detected in the interstellar medium. Its lifetime (τ ∼ 1.04 · 10 6 years) is shorter than the timescale of the chemical evolution of the galaxy (∼ 10 10 years). Therefore, the presence of 26 Al can be associated with recent nucleosynthesis events. 26 Al decays to the first excited state of 26 Mg that, in turn, de-excites emitting a characteristic 1809 keV gamma-ray. All-sky observations done by the CGRO (Compton Gamma-Ray Observatory) [1] and INTEGRAL (International Gamma-Ray Astrophysics Laboratory) [2] space-borne missions provided maps of the intensity of the 1809 keV 26 Al line in our Galaxy, and allowed for the identification of the regions where 26 Al is intensively produced. Moreover, the analysis of pre-solar grains found in pristine meteorites reveald an excess of 26 Mg. This excess can be interpreted assuming that 26  System, and gives information on the composition of the pre-solar nebula [3]. In order to explain the 26 Al abundance in the Milky Way, the main sources of 26 Al should be identified. Gamma-ray observations of our Galaxy demonstrated that the 26 Al abundance has a maximum in the galactic plane, and that it is particularly intense in the stellar-forming regions. Moreover, analyzing the Doppler shift of the 1809 keV line it has been possible to deduce that 26 Al is co-rotating with the Galaxy. All this observational evidence favour the massive stars as the main sources of 26 Al [5]. According to standard models of stellar evolution, massive stars may synthesize 26 Al in three different evolutionary phases: C/Ne shell burning, explosive C/Ne burning and, for stars more massive than 30 solar masses, core H burning [6]. In all those phases, 26 Al is mainly produced by proton capture on 25 Mg. The final abundance of 26 Al depends on the rate of all the nuclear reactions that contribute to its production or destruction. The 25 Mg(α, n) 28 Si reaction destroys the 25 Mg seeds from which 26 Al is produced, and it is the nuclear reaction with the largest impact on the production of 26 Al in explosive C/Ne burning [4]. Explosive C/Ne burning occurs at a peak temperature of 2.3 GK. At this temperature, the Gamow window of the 25 Mg(α, n) 28 Si reaction extends from 1 to 4 MeV. In the energy range E α = 1 -6 MeV, the 25 Mg(α,n) 28 Si cross section has been reported by many authors ([7] - [11]). A summary of the currently available experimental cross sections is reported in fig.  1. Below 2.5 MeV the literature data are characterized by large uncertainties mainly due to beaminduced background, and the reaction rate reported by NACRE [12] is calculated adopting the unpublished cross section reported in [10]. Above 2.5 MeV, the NACRE rate is based on Hauser-Feshbach calculations, disregarding the existing experimental cross sections even in the energy region where they are in good agreement. The 25 Mg(α,n) 28 Si reaction has been studied at Legnaro National Laboratories at beam energies between 3 and 5 MeV. As illustrated in fig. 1, above 3.6 MeV the literature data are in good agreement while between 3 and 3.5 MeV the discrepancy is as high as a factor of 2.

Experimental setup
A sketch of the experimental setup is shown in fig. 2. A pulsed alpha beam with a repetition period of 333 ns and an integrated beam current of about 200 nA was delivered by the CN Van de Graaf accelerator. The beam current was measured with the backscattering technique, using two Si detectors placed at 150 degrees with respect to the beam direction.
Our targets were made of 95.75% enriched 25 MgO (70 µg/cm 2 ) evaporated on a 1 mg/cm 2 gold backing. Reaction neutrons were detected with ten BC501 liquid scintillators from the RIPEN array [13], positioned at 2 m from the target and covering the angular range from 17.5 to 106 degrees with respect to the beam direction. The neutron energy was measured with the time-of-fligth (TOF) technique. Two LaBr 3 :Ce detectors were placed close to the target chamber, in order to study the gamma radiation produced by the reaction.

Data analysis and preliminary results
The pulse shape analysis (PSA) technique was used to perform gamma-neutron discrimination and to reduce the background due to uncorrelated gamma rays ( fig. 3). The neutron TOF is determined with respect to the prompt gamma radiation emitted from the reaction. Measuring the neutron energy, it is possible to determine the contribution to the cross section of different 28 Si excited states, and to identify background neutrons produced by (α,n) reactions on light contaminants (mainly 13 C, 18 O and 19 F) possibly accumulated on the target. The differential cross section has been evaluated independently for each detector, in order to determine the angular distribution of neutrons. Preliminary results at 5 MeV beam energy are shown in fig. 4.
The data analysis is still ongoing. In particular, Rutherford backscattering spectrometry measurements to evaluate the precise target thickness are planned for the end of 2013 at the AN2000 accelerator of Legnaro National Laboratories.