ANNUAL REPORT 2011 - Instituto de Estructura de la Materia

oppositely aligned spins. This pairing gives rise to properties similar to superfluid helium and superconductivityresulting from paired electrons in condensed matter physics. In the structure that has been identified in the palladiumisotope 92 Pd, the neutrons and protons couple into a system of spin-aligned pairs, leading to completely differentproperties compared with what is found in ”normal” atomic nuclei. The interpretation of the experimental data hasbeen provided with the aid of microscopic nuclear shell model calculations. These calculations suggest a completelydifferent picture for the heavy self-conjugate nuclei to what has been proposed in earlier theoretical models ofneutron-proton pairing. The results of this experiment have been published in 2011 in Nature (B. Cederwall et al.).MAGNETIC MOMENT MEASUREMENTS WITH STABLE (UNILAC, GSI) AND EXOTIC, NEUTRON-RICH (REX-ISOLDE) ION BEAMSMagnetic moments are a sensitive probe of single-particle properties in nuclei. They can be measured by observingthe perturbation introduced by a magnetic field on the angular correlation of gamma rays emitted in the de-excitationof the nuclear state of interest. Magnetic moment measurements were of paramount importance in the developmentof nuclear structure, and they were and are still essential to test the predictive power of the nuclear shell model. In2011 we published the results of state-of-the-art measurements of excited state magnetic moments in stable tinisotopes using the transient field technique (TF) in combination with Coulomb excitation in inverse kinematics (A.Jungclaus et al., PLB; J. Walker et al., PRC; J. Walker, PhD thesis, Universidad Autónoma de Madrid, May 2011).The g factors of the first excited 2 + states in a series of Sn isotopes, determined with unprecedented precision,allowed to discriminate between a number of different modern shell model approaches and thus shed light on thedevelopment of collectivity in the semi-magic chain of Sn isotopes.While the TF technique is well established in conjunction with stable ion beams the development of radioactivebeams, while extending the domain of investigation to exotic nuclear realms very far from the line of stability, posenew experimental challenges. Our group accepted these challenges and intended to overcome certain inherentlimitations thanks to the construction of a new, purpose built, state-of-the-art reaction chamber, which wasdeveloped and built at IEM-CSIC. The new chamber harbors the external coils and yoke to produce the 80 mT fieldrequired to magnetize the ferromagnetic Gd target layer and a liquid-nitrogen cooling circuit to keep its temperaturewell below the Curie temperature. In November 2011 A. Jungclaus, A. Illana, A. Perea, J. A. Briz and R. Orlandi, incollaboration with researchers from other European research institutes, run the first part of the experiment IS483 atREX-ISOLDE, CERN, aimed at measuring the magnetic moment of the first excited 2 + state in 72 Zn. The obtaineddata are currently being analyzed by A. Illana in the frame of his PhD work and the experiment will be completedwith the remaining shifts in 2012. This pioneering measurement should permit to test the best available large-scaleshell model calculations which describe this region of the nuclear chart.REFLECTION ASYMMETRY IN LIGHT-ACTINIDE NUCLEI USING THE GAMMA-RAY TRACKINGARRAY AGATAReflection asymmetry and octupole deformation are well established nuclear properties and have been observed inmedium to heavy nuclei across the whole nuclear chart. On this topic, we participated in an experiment at theLaboratori Nazionali di Legnaro (Legnaro (PD), Italy) in March 2011 to characterize new structures observed in theoctupole-deformed light-actinide nuclei 220 Ra and 222 Th. The main goal of the experiment was to measure the linearpolarization of gamma transitions linking new, unpublished rotational bands to the ground-state band. Thisexperiment made use of the AGATA demonstrator, an array of five triple cluster highly segmented germaniumdetectors, which offers unprecedented sensitivity, resolution and efficiency by exploiting its tracking capabilities.The gamma-ray interaction positions within the AGATA detectors can be used to reconstruct the scattering angles ofthe transitions of interest, and hence determine their linear polarization. In this context it is also worth mentioningthat in 2011 the AGATA collaboration, the experimental nuclear physics group of the IEM is member of, reviewedall the achievements made in the AGATA project in recent years including all the necessary infrastructure to operateand support the spectrometer in an extensive (33 pages) article published in Nuclear Instruments and Methods A.R+D FOR THE R3B EXPERIMENT AT THE FUTURE FAIR FACILITYThe experiment R 3 B is part of the first phase of the FAIR facility. The experimental configuration of R 3 B isdesigned for the detection in complete kinematics of the products emitted in reactions at relativistic energies. TheCALorimeter for In-Flight gammA detection (CALIFA) will be situated around the reaction target and should beable to reach the necessary angular and energy resolution to detect gamma cascades of 1 to 10 MeV with a strongDoppler broadening and Lorenz boost as well as to detect protons of up to 300 MeV energy.Our R&D work concentrates on developments for the CALIFA gamma and proton spectrometer. We have within thecollaboration concluded the design of the Barrel part of the CALIFA calorimeter and a Technical Design Report(TDR) was, in December 2011, handed in to the FAIR management for evaluation.58