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Coulomb Dissociation of 27P

In this work the astrophysical 26Si(p,γ)27P reaction is studied using the Coulomb dissociation technique. We performed a 27P Coulomb Dissociation experiment at GSI, Darmstadt (28 May-5 June 2007) using the ALADIN-LAND setup which allows complete-kinematic studies. A secondary 27P beam at 498 AMeV impinging a 515mg/cm2 Pb target was used. The relative energy of the outgoing system (26Si+p) is measured obtaining the resonant states of the 27P. Preliminary results show four resonant states measured at 0.36±0.07, 0.88±0.09, 1.5±0.2, 2.3±0.3 MeV and evidence of a higher state at around 3.1 MeV. The preliminary total cross section obtained for relative energies between 0 and 3 MeV has been measured and yields 55±7 mb.


Introduction
Nuclear Physics inputs are of great importance in the building of a coherent picture of the production and evolution of the constituents of the Universe. In particular, the production and evolution of stars is an important topic throughout the history of the science. The success of the theories by Eddington (1920) [1] and Hans Bethe (1939) [2] explaining nuclear fusion processes involved in stars lead to the development of nucleosynthesis theories as a key part of the understanding of the Universe.
The famous review paper by Burbidge, Fowler and Hoyle in 1957 [3] proposed the stars as the seat of the origin of the elements as previously suggested in 1946 by Hoyle [4]. Previous theories assumed that nuclides were built in a primordial fireball (for which there was no evidence) at the beginning of the Universe. Those models were very attractive for the scientific community and managed to explain many observable features. However, they failed in the explanation of the observational fact that not all the stars exhibit the same surface composition. Primordial theories explained the distribution of material in a cosmic scale as spatially uniform and independent of time, and thus the composition should be uniform. However, the variation of the universal abundance distribution of the elements, requires their distribution after the formation by ejecting material in several phases of the evolution of the stars (red giants, supernovae, novae...). This fact supports the stellar synthesis theories. In particular, in this work we are interested in the proton rich side of the nuclide chart, whose nuclei evolve mainly due to the rapid proton capture process (rp-process). This process consists of consecutive proton captures producing heavier nuclei. Particularly, we are interested in the region of formation of 26 Al. The measurement of populations of this element in stellar scenarios served as a proof of the ongoing nucleosynthesis in stars. This nucleus has a life time of 1.05×10 6 years, much shorter than the age of the Universe. Thus, the detection of this nucleus is a direct evidence of nucleosynthesis as an ongoing process. Nucleosynthesis of 26 Al is complicated by the presence of a short-lived 26 Al m (τ 9.15 s) spin isomer. The only way to synthesise the long-lived 26 Al g isotope in nova explosions is through proton capture reactions on 25 Mg, which can yield both the 26 Al ground and isomeric states [5]. A subset of the nuclide chart with the ions involved in this process is shown in figure 1. The isomeric state decays predominantly to the ground state of 26 Mg, while the ground state decays to the first excited of 26 Mg giving the γ -ray of 1.809 MeV, (the one detected in galactic measurements). The β -decay of 26 Si mainly populates the 26 Al(g.s.), and the production of the 26 Si comes from the competition of the β -decay 25 Al and the reaction 25 Al(p,γ) 26 Si that is destructed in the 26 Si(p,γ) 27 P. This reaction is the small part of the puzzle which is addressed in this work.

Coulomb Dissociation technique
The Coulomb dissociation technique [6] [7] is proposed as a very suitable method for the investigation of electromagnetic transitions between a bound state of two particles and resonant states at small relative energies between the interacting particles. In this method, the nuclear Coulomb field is used as a source for the photodisintegration process, so instead of the direct  [9], so the experimental measurement of the Coulomb breakup can be used to obtain the corresponding γ-induced cross section and, by using the detailed balance theorem, the related direct (p,γ) reaction cross section.

Experiment S223 at GSI
The experiment was performed at the German laboratory Hemholtzzentrum fur Schwerionenforschung (GSI) located in Darmstadt, in particular in the ALADIN-LAND experimental setup. A 36 Ar primary beam was accelerated and produced a secondary 27 P beam at 498 AMeV by nuclear fragmentation on a Be target. This secondary beam was selected in-flight in the Fragment Separator (FRS) [10] [11] [12].
After the selection of the secondary beam in the FRS, the exotic nuclei are transmitted to the ALADIN-LAND setup in Cave C, that comprises the following parts: This ensemble of detectors allows the kinematically complete measurement of the reaction of interest.

Experimental results
The relative energy spectrum is a very convenient observable which helps to study the resonant states of the incoming nucleus and the different electromagnetic decay modes. This spectrum is constructed from the measured momenta as follows: where m i,j are the masses of the outgoing fragments, γ i,j the Lorentz factor, β i,j their velocity in terms of the speed of light, ϑ ij the relative angle between both fragments, E γ the excitation energy of the projectile and m proj the mass of the projectile. Figure 2 shows the preliminary histogram obtained for our measured data after the selection of the reaction channel. A first view of the spectrum clearly shows the evidence of the existence of resonant states that are actually predicted by the theoretical model of S. Typel [13] and measured in a previous Coulomb Dissociation experiment in RIKEN at much lower energy (57 AMeV) [14] [15] [16] A preliminary scheme of the measured excited states in this experiment is represented in figure 3 in comparison with the previous measurements of RIKEN [14]   . Preliminary level scheme for the 27 P excited states measured in this experiment, compared to the previous measurements in RIKEN [14][15] [16], the mirror nucleus 27 Mg, USDB calculations [17] and ANC extrapolation [18] [18] and also to theoretical USDB expectations and the comparison to the mirror nucleus states ( 27 Mg). Our measurements show good agreement with the others within the resolution. The preliminary total cross section for the Coulomb dissociation was evaluated and amounts 55± 7 mb for relative energies smaller than 3 MeV. The considered error sources are the statistical errors and the contribution from the efficiency calculation. S. Typel's calculation [13] predicted a value of 98 mb for energies up to 3 MeV which does not reproduce the experimental result accurately even though the order of magnitude is correct. Further analysis will allow to calculate the partial cross section for the different resonant states and direct capture component

Conclusions
The reaction 26 Si(p,γ) 27 P was studied by using the virtual photon theory in a Coulomb Dissociation experiment in inverse kinematics and preliminary results are shown. The relative energy spectrum of the outgoing particles was obtained, showing the resonant states of the 27 P. Four states are reported at energies 1.22±0.07, 1.75±0.09, 2.4±0.3, 3.1±0.4 MeV and evidence of a resonant state at around 3.9±0.4 MeV. All of them are in good agreement with previous measurements in RIKEN within the resolution. The fourth state was first evidenced in the RIKEN experiment with low statistics; with our measurement we find evidence of the higher state at 3.9±0.4 MeV. The total cross section for energies up to 3 MeV is 55±7 mb about 1.8 times smaller than the theoretical prediction [13].