ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

The figure shows a compilation of the existing data on S34-factor along with our results (Madrid'11). 'Data pointsin colour' are taken from refs. [PRL 93(2004)262503](Weizmann'04), [PRL93 (2004)262503](LUNA06, 07), [Phys.Rev. C76 (2007)055801] (Seattle'07), [PRL 102(2009) 232502] (ERNA'09). The data points shown in grey comefrom Ref. [Rev. Mod. Phys. 83(2011)195]) and at medium energies is also included data from Parker et al. [Phys.Rev. 131(1963)2578], which do not agree with the Madrid'11 and ERNA'09. A normalized curve of the calculationsmade by T. Neff, PRL 106(2011)042502 is overlaid on the data points.The reaction 12 C(,) 16 O plays a key role in nuclear astrophysics as it determines the carbon to oxygen ratio andstrongly influences the nucleosynthesis of all heavier elements. The cross section of this reaction at helium-burningtemperatures is of the order of 10 -17 barn, which renders an experimental study unfeasible. The reaction is known tobe dominated by a sum of resonance tails and direct captures to both the ground and excited states of 16 O. Ofparticular interest are the 2 + and 1 - states located at 6917 and 7117 keV, respectively. Our group has proposed to usethe 19 F(p, ) 16 O reaction to populate the states of interest in 16 O. A first run test was performed at CMAM onDecember of 2011. The experimental setup consisted of four dE-E telescopes, composed of two 60-m DSSDs +500-m PADs placed downstream of the target, plus two 20-m SSSDs + 300-m DSSDs. A primary beam ofprotons was accelerated at energies in the range 800 to 1500 keV, and impinged upon an MgF 2 target. The outgoingparticles were detected in coincidence using the four detectors. Our EXNUC Postdoc Ana D. Becerril will analyzethe data from this run in collaboration with L. Buchmann from TRIUMF, Canada. Monte Carlo simulations will beperformed and compared to experimental results. Based on our findings an optimized measurement will be proposedat CMAM later in 2012.DIPOLE POLARIZABILITY STUDY OF lIGHT NUCLEIOne of the most important and unexpected discoveries of Nuclear Physics in recent years has been the existence ofthe neutron halo in some nuclei near or at the drip line. A halo state is basically a threshold effect resulting from thepresence of bound states near the continuum. The combination of a small binding energy together with the shortrangeof nuclear forces allows the nucleon (or a cluster of nucleons) to go far away and to have a non-negligibleprobability at distances much greater than the normal nuclear radius (= r o A 1/3 , r o = 1.2 fm).The polarizability parameter is a fundamental property of a nucleus and it is a measure of its response to an externalelectric field. In the adiabatic limit (corresponding to high excitation energies), the deviation of the elastic scatteringcross section with respect to the prediction of the Rutherford formula is determined by the strength of the dipolepolarizability. Consequently, measuring the elastic cross section of a nucleus by a heavy target at sub-Coulombenergies provides a practical method for determining the polarizability parameter of this nucleus.To determine thepolarizability in stable and standar nuclei and in the extreme case of halo nuclei we have performed experimentsboth at CMAM and in diffeent radioactive beam facilities. In the following we describe the advances done during2011.The effect of dipole polarizability should be particularly large for nuclei with halo and should manifest as areduction of the elastic cross section at large angles. So we have perfomed an experimental and theoretical study ofthe scattering of halo nuclei at energies near the Coulomb barrier. This project is carried out in collaboration withgroups from the Universities of Seville and Huelva, with whom we have associated units. We have studied twoneutron halo nuclei such as 6 He, see 2006 annual report, and in 2008 we measured 11 Li at TRIUMF (Canada) and wehave studied the one neutron halo in 11 Be at REX-ISOLDE (Eur.Phys J. A, 2009, PRL 2010 and PRC inpreparation). The analysis of the Triumf data is almost finished and forms the PhD thesis of our student (grantCSIC-Costa Rica) Mario Cubero. The result shows that the elastic differential cross section obtained for 11 Li on lead56